From e513fe4666acb99c97fdbedd85afb85849bb753f Mon Sep 17 00:00:00 2001
From: Phillip Webb <pwebb@pivotal.io>
Date: Wed, 22 Jan 2020 01:07:55 -0800
Subject: [PATCH] Add layertools jarmode

Add a new `spring-boot-layertools` module which provides jarmode support
for working with layers. The module works with both classic fat jars,
as well as layered jars.

Closes gh-19849
---
 eclipse/spring-boot-project.setup             |   2 +-
 settings.gradle                               |   1 +
 .../spring-boot-dependencies/build.gradle     |   1 +
 .../spring-boot-layertools/build.gradle       |  21 ++
 .../boot/layertools/Command.java              | 335 ++++++++++++++++++
 .../boot/layertools/Context.java              |  93 +++++
 .../boot/layertools/ExtractCommand.java       | 107 ++++++
 .../boot/layertools/HelpCommand.java          | 105 ++++++
 .../boot/layertools/ImplicitLayers.java       |  83 +++++
 .../boot/layertools/IndexedLayers.java        | 102 ++++++
 .../boot/layertools/LayerToolsJarMode.java    |  89 +++++
 .../boot/layertools/Layers.java               |  55 +++
 .../boot/layertools/ListCommand.java          |  46 +++
 .../boot/layertools/package-info.java         |  20 ++
 .../main/resources/META-INF/spring.factories  |   2 +
 .../boot/layertools/CommandTests.java         | 166 +++++++++
 .../boot/layertools/ContextTests.java         | 108 ++++++
 .../boot/layertools/ExtractCommandTests.java  | 138 ++++++++
 .../boot/layertools/HelpCommandTests.java     |  63 ++++
 .../boot/layertools/ImplicitLayersTests.java  |  83 +++++
 .../boot/layertools/IndexedLayersTests.java   |  85 +++++
 .../layertools/LayerToolsJarModeTests.java    |  71 ++++
 .../boot/layertools/ListCommandTests.java     |  48 +++
 .../boot/layertools/TestPrintStream.java      |  76 ++++
 .../boot/layertools/help-extract-output.txt   |   7 +
 .../boot/layertools/help-output.txt           |   7 +
 .../boot/layertools/list-output.txt           |   4 +
 27 files changed, 1917 insertions(+), 1 deletion(-)
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/build.gradle
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Command.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Context.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ExtractCommand.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/HelpCommand.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ImplicitLayers.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/IndexedLayers.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/LayerToolsJarMode.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Layers.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ListCommand.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/package-info.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/resources/META-INF/spring.factories
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/CommandTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ContextTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ExtractCommandTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/HelpCommandTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ImplicitLayersTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/IndexedLayersTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/LayerToolsJarModeTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ListCommandTests.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/TestPrintStream.java
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-extract-output.txt
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-output.txt
 create mode 100644 spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/list-output.txt

diff --git a/eclipse/spring-boot-project.setup b/eclipse/spring-boot-project.setup
index b940e2276f..608ecf8a84 100644
--- a/eclipse/spring-boot-project.setup
+++ b/eclipse/spring-boot-project.setup
@@ -127,7 +127,7 @@
         name="spring-boot-tools">
       <predicate
           xsi:type="predicates:NamePredicate"
-          pattern="spring-boot-(tools|antlib|configuration-.*|loader|.*-tools|.*-plugin|autoconfigure-processor|cloudnativebuildpack)"/>
+          pattern="spring-boot-(tools|antlib|configuration-.*|loader|.*-tools|*.layertools|.*-plugin|autoconfigure-processor|cloudnativebuildpack)"/>
     </workingSet>
     <workingSet
         name="spring-boot-starters">
diff --git a/settings.gradle b/settings.gradle
index b04b4ca821..13c5353026 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -45,6 +45,7 @@ include 'spring-boot-project:spring-boot-tools:spring-boot-cloudnativebuildpack'
 include 'spring-boot-project:spring-boot-tools:spring-boot-configuration-metadata'
 include 'spring-boot-project:spring-boot-tools:spring-boot-configuration-processor'
 include 'spring-boot-project:spring-boot-tools:spring-boot-gradle-plugin'
+include 'spring-boot-project:spring-boot-tools:spring-boot-layertools'
 include 'spring-boot-project:spring-boot-tools:spring-boot-loader'
 include 'spring-boot-project:spring-boot-tools:spring-boot-loader-tools'
 include 'spring-boot-project:spring-boot-tools:spring-boot-maven-plugin'
diff --git a/spring-boot-project/spring-boot-dependencies/build.gradle b/spring-boot-project/spring-boot-dependencies/build.gradle
index 0b37761d8d..0b60838002 100644
--- a/spring-boot-project/spring-boot-dependencies/build.gradle
+++ b/spring-boot-project/spring-boot-dependencies/build.gradle
@@ -1425,6 +1425,7 @@ bom {
 				'spring-boot-configuration-metadata',
 				'spring-boot-configuration-processor',
 				'spring-boot-devtools',
+				'spring-boot-layertools',
 				'spring-boot-loader',
 				'spring-boot-loader-tools',
 				'spring-boot-properties-migrator',
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/build.gradle b/spring-boot-project/spring-boot-tools/spring-boot-layertools/build.gradle
new file mode 100644
index 0000000000..9ffb23e381
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/build.gradle
@@ -0,0 +1,21 @@
+plugins {
+	id 'java-library'
+	id 'org.springframework.boot.conventions'
+	id 'org.springframework.boot.deployed'
+	id 'org.springframework.boot.internal-dependency-management'
+}
+
+description = 'Spring Boot Layers Tools'
+
+dependencies {
+	api platform(project(':spring-boot-project:spring-boot-parent'))
+
+	implementation project(':spring-boot-project:spring-boot-tools:spring-boot-loader')
+	implementation project(':spring-boot-project:spring-boot')
+	implementation 'org.springframework:spring-core'
+
+	testImplementation "org.assertj:assertj-core"
+	testImplementation "org.junit.jupiter:junit-jupiter"
+	testImplementation "org.mockito:mockito-core"
+}
+
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Command.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Command.java
new file mode 100644
index 0000000000..49623002fe
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Command.java
@@ -0,0 +1,335 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Deque;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+
+/**
+ * A command that can be launched from the layertools jarmode.
+ *
+ * @author Phillip Webb
+ */
+abstract class Command {
+
+	private final String name;
+
+	private final String description;
+
+	private final Options options;
+
+	private final Parameters parameters;
+
+	/**
+	 * Create a new {@link Command} instance.
+	 * @param name the name of the command
+	 * @param description a description of the command
+	 * @param options the command options
+	 * @param parameters the command parameters
+	 */
+	Command(String name, String description, Options options, Parameters parameters) {
+		this.name = name;
+		this.description = description;
+		this.options = options;
+		this.parameters = parameters;
+	}
+
+	/**
+	 * Return the name of this command.
+	 * @return the command name
+	 */
+	String getName() {
+		return this.name;
+	}
+
+	/**
+	 * Return the description of this command.
+	 * @return the command description
+	 */
+	String getDescription() {
+		return this.description;
+	}
+
+	/**
+	 * Return options that this command accepts.
+	 * @return the command options
+	 */
+	Options getOptions() {
+		return this.options;
+	}
+
+	/**
+	 * Return parameters that this command accepts.
+	 * @return the command parameters
+	 */
+	Parameters getParameters() {
+		return this.parameters;
+	}
+
+	/**
+	 * Run the command by processing the remaining arguments.
+	 * @param args a mutable deque of the remaining arguments
+	 */
+	final void run(Deque<String> args) {
+		List<String> parameters = new ArrayList<>();
+		Map<Option, String> options = new HashMap<>();
+		while (!args.isEmpty()) {
+			String arg = args.removeFirst();
+			Option option = this.options.find(arg);
+			if (option != null) {
+				options.put(option, option.claimArg(args));
+			}
+			else {
+				parameters.add(arg);
+			}
+		}
+		run(options, parameters);
+	}
+
+	/**
+	 * Run the actual command.
+	 * @param options any options extracted from the arguments
+	 * @param parameters any parameters extracted from the arguements
+	 */
+	protected abstract void run(Map<Option, String> options, List<String> parameters);
+
+	/**
+	 * Static method that can be used to find a single command from a collection.
+	 * @param commands the commands to search
+	 * @param name the name of the command to find
+	 * @return a {@link Command} instance or {@code null}.
+	 */
+	static Command find(Collection<? extends Command> commands, String name) {
+		for (Command command : commands) {
+			if (command.getName().equals(name)) {
+				return command;
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * Parameters that the command accepts.
+	 */
+	protected static final class Parameters {
+
+		private final List<String> descriptions;
+
+		private Parameters(String[] descriptions) {
+			this.descriptions = Collections.unmodifiableList(Arrays.asList(descriptions));
+		}
+
+		/**
+		 * Return the parameter descriptions.
+		 * @return the descriptions
+		 */
+		List<String> getDescriptions() {
+			return this.descriptions;
+		}
+
+		@Override
+		public String toString() {
+			return this.descriptions.toString();
+		}
+
+		/**
+		 * Factory method used if there are no expected parameters.
+		 * @return a new {@link Parameters} instance
+		 */
+		protected static Parameters none() {
+			return of();
+		}
+
+		/**
+		 * Factory method used to create a new {@link Parameters} instance with specific
+		 * descriptions.
+		 * @param descriptions the parameter descriptions
+		 * @return a new {@link Parameters} instance with the given descriptions
+		 */
+		protected static Parameters of(String... descriptions) {
+			return new Parameters(descriptions);
+		}
+
+	}
+
+	/**
+	 * Options that the command accepts.
+	 */
+	protected static final class Options {
+
+		private final Option[] values;
+
+		private Options(Option[] values) {
+			this.values = values;
+		}
+
+		private Option find(String arg) {
+			if (arg.startsWith("--")) {
+				String name = arg.substring(2);
+				for (Option candidate : this.values) {
+					if (candidate.getName().equals(name)) {
+						return candidate;
+					}
+				}
+			}
+			return null;
+		}
+
+		/**
+		 * Return if this options collection is empty.
+		 * @return if there are no options
+		 */
+		boolean isEmpty() {
+			return this.values.length == 0;
+		}
+
+		/**
+		 * Return a stream of each option.
+		 * @return a stream of the options
+		 */
+		Stream<Option> stream() {
+			return Arrays.stream(this.values);
+		}
+
+		/**
+		 * Factory method used if there are no expected options.
+		 * @return a new {@link Options} instance
+		 */
+		protected static Options none() {
+			return of();
+		}
+
+		/**
+		 * Factory method used to create a new {@link Options} instance with specific
+		 * values.
+		 * @param values the option values
+		 * @return a new {@link Options} instance with the given values
+		 */
+		protected static Options of(Option... values) {
+			return new Options(values);
+		}
+
+	}
+
+	/**
+	 * An individual option that the command can accepts. Can either be an option with a
+	 * value (e.g. {@literal --log debug}) or a flag (e.g. {@literal
+	 * --verbose}).
+	 */
+	protected static final class Option {
+
+		private final String name;
+
+		private final String valueDescription;
+
+		private final String description;
+
+		private Option(String name, String valueDescription, String description) {
+			this.name = name;
+			this.description = description;
+			this.valueDescription = valueDescription;
+		}
+
+		/**
+		 * Return the name of the option.
+		 * @return the options name
+		 */
+		String getName() {
+			return this.name;
+		}
+
+		/**
+		 * Return the description of the expected argument value or {@code null} if this
+		 * option is a flag/switch.
+		 * @return the option value description
+		 */
+		String getValueDescription() {
+			return this.valueDescription;
+		}
+
+		/**
+		 * Return the name and the value description combined.
+		 * @return the name and value description
+		 */
+		String getNameAndValueDescription() {
+			return this.name + ((this.valueDescription != null) ? " " + this.valueDescription : "");
+		}
+
+		/**
+		 * Return a description of the option.
+		 * @return the option description
+		 */
+		String getDescription() {
+			return this.description;
+		}
+
+		private String claimArg(Deque<String> args) {
+			return (this.valueDescription != null) ? args.removeFirst() : null;
+		}
+
+		@Override
+		public boolean equals(Object obj) {
+			if (this == obj) {
+				return true;
+			}
+			if (obj == null || getClass() != obj.getClass()) {
+				return false;
+			}
+			return this.name.equals(((Option) obj).name);
+		}
+
+		@Override
+		public int hashCode() {
+			return this.name.hashCode();
+		}
+
+		@Override
+		public String toString() {
+			return this.name;
+		}
+
+		/**
+		 * Factory method to create a flag/switch option.
+		 * @param name the name of the option
+		 * @param description a description of the option
+		 * @return a new {@link Option} instance
+		 */
+		protected static Option flag(String name, String description) {
+			return new Option(name, null, description);
+		}
+
+		/**
+		 * Factory method to create value option.
+		 * @param name the name of the option
+		 * @param valueDescription a description of the expected value
+		 * @param description a description of the option
+		 * @return a new {@link Option} instance
+		 */
+		protected static Option of(String name, String valueDescription, String description) {
+			return new Option(name, valueDescription, description);
+		}
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Context.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Context.java
new file mode 100644
index 0000000000..8995e305dc
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Context.java
@@ -0,0 +1,93 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.nio.file.Paths;
+
+import org.springframework.boot.system.ApplicationHome;
+import org.springframework.util.Assert;
+
+/**
+ * Context for use by commands.
+ *
+ * @author Phillip Webb
+ */
+class Context {
+
+	private final File jarFile;
+
+	private final File workingDir;
+
+	private String relativeDir;
+
+	/**
+	 * Create a new {@link Context} instance.
+	 */
+	Context() {
+		this(new ApplicationHome().getSource(), Paths.get(".").toAbsolutePath().normalize().toFile());
+	}
+
+	/**
+	 * Create a new {@link Context} instance with the specified value.
+	 * @param jarFile the source jar file
+	 * @param workingDir the working directory
+	 */
+	Context(File jarFile, File workingDir) {
+		Assert.state(jarFile != null && jarFile.isFile() && jarFile.exists()
+				&& jarFile.getName().toLowerCase().endsWith(".jar"), "Unable to find source JAR");
+		this.jarFile = jarFile;
+		this.workingDir = workingDir;
+		this.relativeDir = deduceRelativeDir(jarFile.getParentFile(), this.workingDir);
+	}
+
+	private String deduceRelativeDir(File sourceFolder, File workingDir) {
+		String sourcePath = sourceFolder.getAbsolutePath();
+		String workingPath = workingDir.getAbsolutePath();
+		if (sourcePath.equals(workingPath) || !sourcePath.startsWith(workingPath)) {
+			return null;
+		}
+		String relativePath = sourcePath.substring(workingPath.length() + 1);
+		return (relativePath.length() > 0) ? relativePath : null;
+	}
+
+	/**
+	 * Return the source jar file that is running in tools mode.
+	 * @return the jar file
+	 */
+	File getJarFile() {
+		return this.jarFile;
+	}
+
+	/**
+	 * Return the current working directory.
+	 * @return the working dir
+	 */
+	File getWorkingDir() {
+		return this.workingDir;
+	}
+
+	/**
+	 * Return the directory relative to {@link #getWorkingDir()} that contains the jar or
+	 * {@code null} if none relative directory can be deduced.
+	 * @return the relative dir ending in {@code /} or {@code null}
+	 */
+	String getRelativeJarDir() {
+		return this.relativeDir;
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ExtractCommand.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ExtractCommand.java
new file mode 100644
index 0000000000..67b8dfc20c
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ExtractCommand.java
@@ -0,0 +1,107 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.nio.file.Files;
+import java.util.List;
+import java.util.Map;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipInputStream;
+
+import org.springframework.util.StreamUtils;
+import org.springframework.util.StringUtils;
+
+/**
+ * The {@code 'extract'} tools command.
+ *
+ * @author Phillip Webb
+ */
+class ExtractCommand extends Command {
+
+	static final Option DESTINATION_OPTION = Option.of("destination", "string", "The destination to extract files to");
+
+	private final Context context;
+
+	private final Layers layers;
+
+	ExtractCommand(Context context) {
+		this(context, Layers.get(context));
+	}
+
+	ExtractCommand(Context context, Layers layers) {
+		super("extract", "Extracts layers from the jar for image creation", Options.of(DESTINATION_OPTION),
+				Parameters.of("[<layer>...]"));
+		this.context = context;
+		this.layers = layers;
+	}
+
+	@Override
+	protected void run(Map<Option, String> options, List<String> parameters) {
+		try {
+			File destination = options.containsKey(DESTINATION_OPTION) ? new File(options.get(DESTINATION_OPTION))
+					: this.context.getWorkingDir();
+			for (String layer : this.layers) {
+				if (parameters.isEmpty() || parameters.contains(layer)) {
+					mkDirs(new File(destination, layer));
+				}
+			}
+			try (ZipInputStream zip = new ZipInputStream(new FileInputStream(this.context.getJarFile()))) {
+				ZipEntry entry = zip.getNextEntry();
+				while (entry != null) {
+					if (!entry.isDirectory()) {
+						String layer = this.layers.getLayer(entry);
+						if (parameters.isEmpty() || parameters.contains(layer)) {
+							write(zip, entry, new File(destination, layer));
+						}
+					}
+					entry = zip.getNextEntry();
+				}
+			}
+		}
+		catch (IOException ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
+	private void write(ZipInputStream zip, ZipEntry entry, File destination) throws IOException {
+		String path = StringUtils.cleanPath(entry.getName());
+		File file = new File(destination, path);
+		if (file.getAbsolutePath().startsWith(destination.getAbsolutePath())) {
+			mkParentDirs(file);
+			try (OutputStream out = new FileOutputStream(file)) {
+				StreamUtils.copy(zip, out);
+			}
+			Files.setAttribute(file.toPath(), "creationTime", entry.getCreationTime());
+		}
+	}
+
+	private void mkParentDirs(File file) throws IOException {
+		mkDirs(file.getParentFile());
+	}
+
+	private void mkDirs(File file) throws IOException {
+		if (!file.exists() && !file.mkdirs()) {
+			throw new IOException("Unable to create folder " + file);
+		}
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/HelpCommand.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/HelpCommand.java
new file mode 100644
index 0000000000..29aa98ef66
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/HelpCommand.java
@@ -0,0 +1,105 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.PrintStream;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+
+/**
+ * Implicit {@code 'help'} command.
+ *
+ * @author Phillip Webb
+ */
+class HelpCommand extends Command {
+
+	private final Context context;
+
+	private final List<Command> commands;
+
+	HelpCommand(Context context, List<Command> commands) {
+		super("help", "Help about any command", Options.none(), Parameters.of("[<command]"));
+		this.context = context;
+		this.commands = commands;
+	}
+
+	@Override
+	protected void run(Map<Option, String> options, List<String> parameters) {
+		run(System.out, options, parameters);
+	}
+
+	void run(PrintStream out, Map<Option, String> options, List<String> parameters) {
+		Command command = (!parameters.isEmpty()) ? Command.find(this.commands, parameters.get(0)) : null;
+		if (command != null) {
+			printCommandHelp(out, command);
+			return;
+		}
+		printUsageAndCommands(out);
+	}
+
+	private void printCommandHelp(PrintStream out, Command command) {
+		out.println(command.getDescription());
+		out.println();
+		out.println("Usage:");
+		out.println("  " + getJavaCommand() + " " + getUsage(command));
+		if (!command.getOptions().isEmpty()) {
+			out.println();
+			out.println("Options:");
+			int maxNameLength = getMaxLength(0, command.getOptions().stream().map(Option::getNameAndValueDescription));
+			command.getOptions().stream().forEach((option) -> printOptionSummary(out, option, maxNameLength));
+		}
+	}
+
+	private void printOptionSummary(PrintStream out, Option option, int padding) {
+		out.println(String.format("  --%-" + padding + "s  %s", option.getNameAndValueDescription(),
+				option.getDescription()));
+	}
+
+	private String getUsage(Command command) {
+		StringBuilder usage = new StringBuilder();
+		usage.append(command.getName());
+		if (!command.getOptions().isEmpty()) {
+			usage.append(" [options]");
+		}
+		command.getParameters().getDescriptions().forEach((param) -> usage.append(" " + param));
+		return usage.toString();
+	}
+
+	private void printUsageAndCommands(PrintStream out) {
+		out.println("Usage:");
+		out.println("  " + getJavaCommand());
+		out.println();
+		out.println("Available commands:");
+		int maxNameLength = getMaxLength(getName().length(), this.commands.stream().map(Command::getName));
+		this.commands.forEach((command) -> printCommandSummary(out, command, maxNameLength));
+		printCommandSummary(out, this, maxNameLength);
+	}
+
+	private int getMaxLength(int minimum, Stream<String> strings) {
+		return Math.max(minimum, strings.mapToInt(String::length).max().orElse(0));
+	}
+
+	private void printCommandSummary(PrintStream out, Command command, int padding) {
+		out.println(String.format("  %-" + padding + "s  %s", command.getName(), command.getDescription()));
+	}
+
+	private String getJavaCommand() {
+		return "java -Djarmode=layertools -jar " + this.context.getJarFile().getName();
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ImplicitLayers.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ImplicitLayers.java
new file mode 100644
index 0000000000..c709d0445e
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ImplicitLayers.java
@@ -0,0 +1,83 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.zip.ZipEntry;
+
+/**
+ * {@link Layers} implementation that uses implicit rules to slice the application.
+ *
+ * @author Phillip Webb
+ */
+class ImplicitLayers implements Layers {
+
+	private static final String DEPENDENCIES_LAYER = "dependencies";
+
+	private static final String SNAPSHOT_DEPENDENCIES_LAYER = "snapshot-dependencies";
+
+	private static final String RESOURCES_LAYER = "resources";
+
+	private static final String APPLICATION_LAYER = "application";
+
+	private static final List<String> LAYERS;
+	static {
+		List<String> layers = new ArrayList<>();
+		layers.add(DEPENDENCIES_LAYER);
+		layers.add(SNAPSHOT_DEPENDENCIES_LAYER);
+		layers.add(RESOURCES_LAYER);
+		layers.add(APPLICATION_LAYER);
+		LAYERS = Collections.unmodifiableList(layers);
+	}
+
+	private static final String[] CLASS_LOCATIONS = { "", "BOOT-INF/classes/" };
+
+	private static final String[] RESOURCE_LOCATIONS = { "META-INF/resources/", "resources/", "static/", "public/" };
+
+	@Override
+	public Iterator<String> iterator() {
+		return LAYERS.iterator();
+	}
+
+	@Override
+	public String getLayer(ZipEntry entry) {
+		return getLayer(entry.getName());
+	}
+
+	String getLayer(String name) {
+		if (name.endsWith("SNAPSHOT.jar")) {
+			return SNAPSHOT_DEPENDENCIES_LAYER;
+		}
+		if (name.endsWith(".jar")) {
+			return DEPENDENCIES_LAYER;
+		}
+		if (!name.endsWith(".class")) {
+			for (String classLocation : CLASS_LOCATIONS) {
+				for (String resourceLocation : RESOURCE_LOCATIONS) {
+					if (name.startsWith(classLocation + resourceLocation)) {
+						return RESOURCES_LAYER;
+					}
+				}
+			}
+		}
+		return APPLICATION_LAYER;
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/IndexedLayers.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/IndexedLayers.java
new file mode 100644
index 0000000000..582e6d349a
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/IndexedLayers.java
@@ -0,0 +1,102 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+import java.util.jar.JarFile;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+import java.util.zip.ZipEntry;
+
+import org.springframework.util.Assert;
+import org.springframework.util.StreamUtils;
+
+/**
+ * {@link Layers} implementation backed by a {@code BOOT-INF/layers.idx} file.
+ *
+ * @author Phillip Webb
+ */
+class IndexedLayers implements Layers {
+
+	private static final String APPLICATION_LAYER = "application";
+
+	private static final String SPRING_BOOT_APPLICATION_LAYER = "springbootapplication";
+
+	private static final Pattern LAYER_PATTERN = Pattern.compile("^BOOT-INF\\/layers\\/([a-zA-Z0-9-]+)\\/.*$");
+
+	private List<String> layers;
+
+	IndexedLayers(String indexFile) {
+		String[] lines = indexFile.split("\n");
+		this.layers = Arrays.stream(lines).map(String::trim).filter((line) -> !line.isEmpty())
+				.collect(Collectors.toCollection(ArrayList::new));
+		Assert.state(!this.layers.isEmpty(), "Empty layer index file loaded");
+		if (!this.layers.contains(APPLICATION_LAYER)) {
+			this.layers.add(0, SPRING_BOOT_APPLICATION_LAYER);
+		}
+	}
+
+	@Override
+	public Iterator<String> iterator() {
+		return this.layers.iterator();
+	}
+
+	@Override
+	public String getLayer(ZipEntry entry) {
+		String name = entry.getName();
+		Matcher matcher = LAYER_PATTERN.matcher(name);
+		if (matcher.matches()) {
+			String layer = matcher.group(1);
+			Assert.state(this.layers.contains(layer), "Unexpected layer '" + layer + "'");
+			return layer;
+		}
+		return this.layers.contains(APPLICATION_LAYER) ? APPLICATION_LAYER : SPRING_BOOT_APPLICATION_LAYER;
+	}
+
+	/**
+	 * Get an {@link IndexedLayers} instance of possible.
+	 * @param context the context
+	 * @return an {@link IndexedLayers} instance or {@code null} if this not a layered
+	 * jar.
+	 */
+	static IndexedLayers get(Context context) {
+		try {
+			try (JarFile jarFile = new JarFile(context.getJarFile())) {
+				ZipEntry entry = jarFile.getEntry("BOOT-INF/layers.idx");
+				if (entry != null) {
+					String indexFile = StreamUtils.copyToString(jarFile.getInputStream(entry), StandardCharsets.UTF_8);
+					return new IndexedLayers(indexFile);
+				}
+			}
+			return null;
+		}
+		catch (FileNotFoundException ex) {
+			return null;
+		}
+		catch (IOException ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/LayerToolsJarMode.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/LayerToolsJarMode.java
new file mode 100644
index 0000000000..4c74fd250b
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/LayerToolsJarMode.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Deque;
+import java.util.List;
+
+import org.springframework.boot.loader.jarmode.JarMode;
+
+/**
+ * {@link JarMode} providing {@code "layertools"} support.
+ *
+ * @author Phillip Webb
+ * @since 2.3.0
+ */
+public class LayerToolsJarMode implements JarMode {
+
+	@Override
+	public boolean accepts(String mode) {
+		return "layertools".equalsIgnoreCase(mode);
+	}
+
+	@Override
+	public void run(String mode, String[] args) {
+		try {
+			new Runner().run(args);
+		}
+		catch (Exception ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
+	static class Runner {
+
+		static Context contextOverride;
+
+		private final List<Command> commands;
+
+		private final HelpCommand help;
+
+		Runner() {
+			Context context = (contextOverride != null) ? contextOverride : new Context();
+			this.commands = getCommands(context);
+			this.help = new HelpCommand(context, this.commands);
+		}
+
+		private void run(String[] args) {
+			run(new ArrayDeque<>(Arrays.asList(args)));
+		}
+
+		private void run(Deque<String> args) {
+			if (!args.isEmpty()) {
+				Command command = Command.find(this.commands, args.removeFirst());
+				if (command != null) {
+					command.run(args);
+					return;
+				}
+			}
+			this.help.run(args);
+		}
+
+		static List<Command> getCommands(Context context) {
+			List<Command> commands = new ArrayList<Command>();
+			commands.add(new ListCommand(context));
+			commands.add(new ExtractCommand(context));
+			return Collections.unmodifiableList(commands);
+		}
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Layers.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Layers.java
new file mode 100644
index 0000000000..273b643056
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/Layers.java
@@ -0,0 +1,55 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.Iterator;
+import java.util.zip.ZipEntry;
+
+/**
+ * Provides information about the jar layers.
+ *
+ * @author Phillip Webb
+ * @see ExtractCommand
+ * @see ListCommand
+ */
+interface Layers extends Iterable<String> {
+
+	/**
+	 * Return the jar layers in the order that they should be added (starting with the
+	 * least frequently changed layer).
+	 */
+	@Override
+	Iterator<String> iterator();
+
+	/**
+	 * Return the layer that a given entry is in.
+	 * @param entry the entry to check
+	 * @return the layer that the entry is in
+	 */
+	String getLayer(ZipEntry entry);
+
+	/**
+	 * Return a {@link Layers} instance for the currently running application.
+	 * @param context the command context
+	 * @return a new layers instance
+	 */
+	static Layers get(Context context) {
+		IndexedLayers indexedLayers = IndexedLayers.get(context);
+		return (indexedLayers != null) ? indexedLayers : new ImplicitLayers();
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ListCommand.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ListCommand.java
new file mode 100644
index 0000000000..4a84cd847f
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/ListCommand.java
@@ -0,0 +1,46 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.PrintStream;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * The {@code 'list-layers'} tools command.
+ *
+ * @author Phillip Webb
+ */
+class ListCommand extends Command {
+
+	private Context context;
+
+	ListCommand(Context context) {
+		super("list", "List layers from the jar that can be extracted", Options.none(), Parameters.none());
+		this.context = context;
+	}
+
+	@Override
+	protected void run(Map<Option, String> options, List<String> parameters) {
+		printLayers(Layers.get(this.context), System.out);
+	}
+
+	void printLayers(Layers layers, PrintStream out) {
+		layers.forEach(out::println);
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/package-info.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/package-info.java
new file mode 100644
index 0000000000..454880d431
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/java/org/springframework/boot/layertools/package-info.java
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * JarMode support for layertools.
+ */
+package org.springframework.boot.layertools;
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/resources/META-INF/spring.factories b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/resources/META-INF/spring.factories
new file mode 100644
index 0000000000..5535966c48
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/main/resources/META-INF/spring.factories
@@ -0,0 +1,2 @@
+org.springframework.boot.loader.jarmode.JarMode=\
+org.springframework.boot.layertools.LayerToolsJarMode
\ No newline at end of file
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/CommandTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/CommandTests.java
new file mode 100644
index 0000000000..9b4f3eb987
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/CommandTests.java
@@ -0,0 +1,166 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.ArrayDeque;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import org.assertj.core.api.InstanceOfAssertFactories;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.layertools.Command.Option;
+import org.springframework.boot.layertools.Command.Options;
+import org.springframework.boot.layertools.Command.Parameters;
+
+import static org.assertj.core.api.Assertions.as;
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Command}.
+ *
+ * @author Phillip Webb
+ */
+class CommandTests {
+
+	private static final Option VERBOSE_FLAG = Option.flag("verbose", "Verbose output");
+
+	private static final Option LOG_LEVEL_OPTION = Option.of("log-level", "Logging level (debug or info)", "string");
+
+	@Test
+	void getNameReturnsName() {
+		TestCommand command = new TestCommand("test");
+		assertThat(command.getName()).isEqualTo("test");
+	}
+
+	@Test
+	void getDescriptionReturnsDescription() {
+		TestCommand command = new TestCommand("test", "Test description", Options.none(), Parameters.none());
+		assertThat(command.getDescription()).isEqualTo("Test description");
+	}
+
+	@Test
+	void getOptionsReturnsOptions() {
+		Options options = Options.of(LOG_LEVEL_OPTION);
+		TestCommand command = new TestCommand("test", "test", options, Parameters.none());
+		assertThat(command.getOptions()).isEqualTo(options);
+	}
+
+	@Test
+	void getParametersReturnsParameters() {
+		Parameters parameters = Parameters.of("[<param>]");
+		TestCommand command = new TestCommand("test", "test", Options.none(), parameters);
+		assertThat(command.getParameters()).isEqualTo(parameters);
+	}
+
+	@Test
+	void runWithOptionsAndParametersParsesOptionsAndParameters() {
+		TestCommand command = new TestCommand("test", VERBOSE_FLAG, LOG_LEVEL_OPTION);
+		run(command, "--verbose", "--log-level", "test1", "test2", "test3");
+		assertThat(command.getRunOptions()).containsEntry(VERBOSE_FLAG, null);
+		assertThat(command.getRunOptions()).containsEntry(LOG_LEVEL_OPTION, "test1");
+		assertThat(command.getRunParameters()).containsExactly("test2", "test3");
+	}
+
+	@Test
+	void findWhenNameMatchesReturnsCommand() {
+		TestCommand test1 = new TestCommand("test1");
+		TestCommand test2 = new TestCommand("test2");
+		List<Command> commands = Arrays.asList(test1, test2);
+		assertThat(Command.find(commands, "test1")).isEqualTo(test1);
+		assertThat(Command.find(commands, "test2")).isEqualTo(test2);
+	}
+
+	@Test
+	void findWhenNameDoesNotMatchReturnsNull() {
+		TestCommand test1 = new TestCommand("test1");
+		TestCommand test2 = new TestCommand("test2");
+		List<Command> commands = Arrays.asList(test1, test2);
+		assertThat(Command.find(commands, "test3")).isNull();
+	}
+
+	@Test
+	void parametersOfCreatesParametersInstance() {
+		Parameters parameters = Parameters.of("test1", "test2");
+		assertThat(parameters.getDescriptions()).containsExactly("test1", "test2");
+	}
+
+	@Test
+	void optionsNoneReturnsEmptyOptions() {
+		Options options = Options.none();
+		assertThat(options).extracting("values", as(InstanceOfAssertFactories.ARRAY)).isEmpty();
+	}
+
+	@Test
+	void optionsOfReturnsOptions() {
+		Option option = Option.of("test", "value description", "description");
+		Options options = Options.of(option);
+		assertThat(options).extracting("values", as(InstanceOfAssertFactories.ARRAY)).containsExactly(option);
+	}
+
+	@Test
+	void optionFlagCreatesFlagOption() {
+		Option option = Option.flag("test", "description");
+		assertThat(option.getName()).isEqualTo("test");
+		assertThat(option.getDescription()).isEqualTo("description");
+		assertThat(option.getValueDescription()).isNull();
+	}
+
+	@Test
+	void optionOfCreatesValueOption() {
+		Option option = Option.of("test", "value description", "description");
+		assertThat(option.getName()).isEqualTo("test");
+		assertThat(option.getDescription()).isEqualTo("description");
+		assertThat(option.getValueDescription()).isEqualTo("value description");
+	}
+
+	private void run(TestCommand command, String... args) {
+		command.run(new ArrayDeque<>(Arrays.asList(args)));
+	}
+
+	static class TestCommand extends Command {
+
+		private Map<Option, String> runOptions;
+
+		private List<String> runParameters;
+
+		TestCommand(String name, Option... options) {
+			this(name, "test", Options.of(options), Parameters.none());
+		}
+
+		TestCommand(String name, String description, Options options, Parameters parameters) {
+			super(name, description, options, parameters);
+		}
+
+		@Override
+		protected void run(Map<Option, String> options, List<String> parameters) {
+			this.runOptions = options;
+			this.runParameters = parameters;
+		}
+
+		Map<Option, String> getRunOptions() {
+			return this.runOptions;
+		}
+
+		List<String> getRunParameters() {
+			return this.runParameters;
+		}
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ContextTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ContextTests.java
new file mode 100644
index 0000000000..7306f2f064
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ContextTests.java
@@ -0,0 +1,108 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+/**
+ * Tests for {@link Context}.
+ *
+ * @author Phillip Webb
+ */
+class ContextTests {
+
+	@TempDir
+	File temp;
+
+	@Test
+	void createWhenSourceIsNullThrowsException() {
+		assertThatIllegalStateException().isThrownBy(() -> new Context(null, this.temp))
+				.withMessage("Unable to find source JAR");
+	}
+
+	@Test
+	void createWhenSourceIsFolderThrowsException() {
+		File folder = new File(this.temp, "test");
+		folder.mkdir();
+		assertThatIllegalStateException().isThrownBy(() -> new Context(folder, this.temp))
+				.withMessage("Unable to find source JAR");
+	}
+
+	@Test
+	void createWhenSourceIsNotJarThrowsException() throws Exception {
+		File zip = new File(this.temp, "test.zip");
+		Files.createFile(zip.toPath());
+		assertThatIllegalStateException().isThrownBy(() -> new Context(zip, this.temp))
+				.withMessage("Unable to find source JAR");
+	}
+
+	@Test
+	void getJarFileReturnsJar() throws Exception {
+		File jar = new File(this.temp, "test.jar");
+		Files.createFile(jar.toPath());
+		Context context = new Context(jar, this.temp);
+		assertThat(context.getJarFile()).isEqualTo(jar);
+	}
+
+	@Test
+	void getWorkingDirectoryReturnsWorkingDir() throws IOException {
+		File jar = new File(this.temp, "test.jar");
+		Files.createFile(jar.toPath());
+		Context context = new Context(jar, this.temp);
+		assertThat(context.getWorkingDir()).isEqualTo(this.temp);
+
+	}
+
+	@Test
+	void getRelativePathReturnsRelativePath() throws Exception {
+		File target = new File(this.temp, "target");
+		target.mkdir();
+		File jar = new File(target, "test.jar");
+		Files.createFile(jar.toPath());
+		Context context = new Context(jar, this.temp);
+		assertThat(context.getRelativeJarDir()).isEqualTo("target");
+	}
+
+	@Test
+	void getRelativePathWhenWorkingDirReturnsNull() throws Exception {
+		File jar = new File(this.temp, "test.jar");
+		Files.createFile(jar.toPath());
+		Context context = new Context(jar, this.temp);
+		assertThat(context.getRelativeJarDir()).isNull();
+	}
+
+	@Test
+	void getRelativePathWhenCannotBeDeducedReturnsNull() throws Exception {
+		File folder1 = new File(this.temp, "folder1");
+		folder1.mkdir();
+		File folder2 = new File(this.temp, "folder1");
+		folder2.mkdir();
+		File jar = new File(folder1, "test.jar");
+		Files.createFile(jar.toPath());
+		Context context = new Context(jar, folder2);
+		assertThat(context.getRelativeJarDir()).isNull();
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ExtractCommandTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ExtractCommandTests.java
new file mode 100644
index 0000000000..a2747aca2a
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ExtractCommandTests.java
@@ -0,0 +1,138 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipOutputStream;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+
+/**
+ * Tests for {@link ExtractCommand}.
+ *
+ * @author Phillip Webb
+ */
+class ExtractCommandTests {
+
+	@TempDir
+	File temp;
+
+	@Mock
+	private Context context;
+
+	private File jarFile;
+
+	private File extract;
+
+	private Layers layers = new TestLayers();
+
+	private ExtractCommand command;
+
+	@BeforeEach
+	void setup() throws Exception {
+		MockitoAnnotations.initMocks(this);
+		this.jarFile = createJarFile("test.jar");
+		this.extract = new File(this.temp, "extract");
+		this.extract.mkdir();
+		given(this.context.getJarFile()).willReturn(this.jarFile);
+		given(this.context.getWorkingDir()).willReturn(this.extract);
+		this.command = new ExtractCommand(this.context, this.layers);
+	}
+
+	@Test
+	void runExtractsLayers() throws Exception {
+		this.command.run(Collections.emptyMap(), Collections.emptyList());
+		assertThat(this.extract.list()).containsOnly("a", "b", "c");
+		assertThat(new File(this.extract, "a/a/a.jar")).exists();
+		assertThat(new File(this.extract, "b/b/b.jar")).exists();
+		assertThat(new File(this.extract, "c/c/c.jar")).exists();
+	}
+
+	@Test
+	void runWhenHasDestinationOptionExtractsLayers() {
+		File out = new File(this.extract, "out");
+		this.command.run(Collections.singletonMap(ExtractCommand.DESTINATION_OPTION, out.getAbsolutePath()),
+				Collections.emptyList());
+		assertThat(this.extract.list()).containsOnly("out");
+		assertThat(new File(this.extract, "out/a/a/a.jar")).exists();
+		assertThat(new File(this.extract, "out/b/b/b.jar")).exists();
+		assertThat(new File(this.extract, "out/c/c/c.jar")).exists();
+	}
+
+	@Test
+	void runWhenHasLayerParamsExtractsLimitedLayers() {
+		this.command.run(Collections.emptyMap(), Arrays.asList("a", "c"));
+		assertThat(this.extract.list()).containsOnly("a", "c");
+		assertThat(new File(this.extract, "a/a/a.jar")).exists();
+		assertThat(new File(this.extract, "c/c/c.jar")).exists();
+	}
+
+	private File createJarFile(String name) throws IOException {
+		File file = new File(this.temp, name);
+		try (ZipOutputStream out = new ZipOutputStream(new FileOutputStream(file))) {
+			out.putNextEntry(new ZipEntry("a/"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("a/a.jar"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("b/"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("b/b.jar"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("c/"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("c/c.jar"));
+			out.closeEntry();
+			out.putNextEntry(new ZipEntry("d/"));
+			out.closeEntry();
+		}
+		return file;
+	}
+
+	private static class TestLayers implements Layers {
+
+		@Override
+		public Iterator<String> iterator() {
+			return Arrays.asList("a", "b", "c").iterator();
+		}
+
+		@Override
+		public String getLayer(ZipEntry entry) {
+			if (entry.getName().startsWith("a")) {
+				return "a";
+			}
+			if (entry.getName().startsWith("b")) {
+				return "b";
+			}
+			return "c";
+		}
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/HelpCommandTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/HelpCommandTests.java
new file mode 100644
index 0000000000..6db3a94805
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/HelpCommandTests.java
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.util.Arrays;
+import java.util.Collections;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link HelpCommand}.
+ *
+ * @author Phillip Webb
+ */
+class HelpCommandTests {
+
+	private HelpCommand command;
+
+	private TestPrintStream out;
+
+	@BeforeEach
+	void setup() {
+		Context context = mock(Context.class);
+		given(context.getJarFile()).willReturn(new File("test.jar"));
+		this.command = new HelpCommand(context, LayerToolsJarMode.Runner.getCommands(context));
+		this.out = new TestPrintStream(this);
+	}
+
+	@Test
+	void runWhenHasNoParametersPrintsUsage() {
+		this.command.run(this.out, Collections.emptyMap(), Collections.emptyList());
+		assertThat(this.out).hasSameContentAsResource("help-output.txt");
+	}
+
+	@Test
+	void runWhenHasNoCommandParameterPrintsUsage() {
+		this.command.run(this.out, Collections.emptyMap(), Arrays.asList("extract"));
+		System.out.println(this.out);
+		assertThat(this.out).hasSameContentAsResource("help-extract-output.txt");
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ImplicitLayersTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ImplicitLayersTests.java
new file mode 100644
index 0000000000..82c0836f49
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ImplicitLayersTests.java
@@ -0,0 +1,83 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.zip.ZipEntry;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ImplicitLayers}.
+ *
+ * @author Phillip Webb
+ */
+class ImplicitLayersTests {
+
+	private Layers layers = new ImplicitLayers();
+
+	@Test
+	void iteratorReturnsLayers() {
+		assertThat(this.layers).containsExactly("dependencies", "snapshot-dependencies", "resources", "application");
+	}
+
+	@Test
+	void getLayerWhenSnapshotJarReturnsSnapshotDependencies() {
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/lib/mylib-SNAPSHOT.jar")))
+				.isEqualTo("snapshot-dependencies");
+	}
+
+	@Test
+	void getLayerWhenNonSnapshotJarReturnsDependencies() {
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/lib/mylib.jar"))).isEqualTo("dependencies");
+	}
+
+	@Test
+	void getLayerWhenLoaderClassReturnsApplication() {
+		assertThat(this.layers.getLayer(zipEntry("org/springframework/boot/loader/Example.class")))
+				.isEqualTo("application");
+	}
+
+	@Test
+	void getLayerWhenStaticResourceReturnsResources() {
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/META-INF/resources/image.gif")))
+				.isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/resources/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/static/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/public/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("META-INF/resources/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("resources/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("static/image.gif"))).isEqualTo("resources");
+		assertThat(this.layers.getLayer(zipEntry("public/image.gif"))).isEqualTo("resources");
+	}
+
+	@Test
+	void getLayerWhenRegularClassReturnsApplication() {
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/com.example/App.class"))).isEqualTo("application");
+	}
+
+	@Test
+	void getLayerWhenClassResourceReturnsApplication() {
+		assertThat(this.layers.getLayer(zipEntry("BOOT-INF/classes/application.properties"))).isEqualTo("application");
+	}
+
+	private ZipEntry zipEntry(String name) {
+		return new ZipEntry(name);
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/IndexedLayersTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/IndexedLayersTests.java
new file mode 100644
index 0000000000..7f8c8674d4
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/IndexedLayersTests.java
@@ -0,0 +1,85 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.util.zip.ZipEntry;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link IndexedLayers}.
+ *
+ * @author Phillip Webb
+ */
+class IndexedLayersTests {
+
+	@Test
+	void createWhenIndexFileIsEmptyThrowsException() {
+		assertThatIllegalStateException().isThrownBy(() -> new IndexedLayers(" \n "))
+				.withMessage("Empty layer index file loaded");
+	}
+
+	@Test
+	void createWhenIndexFileHasNoApplicationLayerAddSpringBootApplication() {
+		IndexedLayers layers = new IndexedLayers("test");
+		assertThat(layers).contains("springbootapplication");
+	}
+
+	@Test
+	void iteratorReturnsLayers() {
+		IndexedLayers layers = new IndexedLayers("test\napplication");
+		assertThat(layers).containsExactly("test", "application");
+	}
+
+	@Test
+	void getLayerWhenMatchesLayerPatterReturnsLayer() {
+		IndexedLayers layers = new IndexedLayers("test");
+		assertThat(layers.getLayer(mockEntry("BOOT-INF/layers/test/lib/file.jar"))).isEqualTo("test");
+	}
+
+	@Test
+	void getLayerWhenMatchesLayerPatterForMissingLayerThrowsException() {
+		IndexedLayers layers = new IndexedLayers("test");
+		assertThatIllegalStateException()
+				.isThrownBy(() -> layers.getLayer(mockEntry("BOOT-INF/layers/missing/lib/file.jar")))
+				.withMessage("Unexpected layer 'missing'");
+	}
+
+	@Test
+	void getLayerWhenDoesNotMatchLayerPatternReturnsApplication() {
+		IndexedLayers layers = new IndexedLayers("test\napplication");
+		assertThat(layers.getLayer(mockEntry("META-INF/MANIFEST.MF"))).isEqualTo("application");
+	}
+
+	@Test
+	void getLayerWhenDoesNotMatchLayerPatternAndHasNoApplicationLayerReturnsSpringApplication() {
+		IndexedLayers layers = new IndexedLayers("test");
+		assertThat(layers.getLayer(mockEntry("META-INF/MANIFEST.MF"))).isEqualTo("springbootapplication");
+	}
+
+	private ZipEntry mockEntry(String name) {
+		ZipEntry entry = mock(ZipEntry.class);
+		given(entry.getName()).willReturn(name);
+		return entry;
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/LayerToolsJarModeTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/LayerToolsJarModeTests.java
new file mode 100644
index 0000000000..197bdddc36
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/LayerToolsJarModeTests.java
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.File;
+import java.io.PrintStream;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link LayerToolsJarMode}.
+ *
+ * @author Phillip Webb
+ */
+class LayerToolsJarModeTests {
+
+	private static final String[] NO_ARGS = {};
+
+	private TestPrintStream out;
+
+	private PrintStream systemOut;
+
+	@BeforeEach
+	void setup() {
+		Context context = mock(Context.class);
+		given(context.getJarFile()).willReturn(new File("test.jar"));
+		this.out = new TestPrintStream(this);
+		this.systemOut = System.out;
+		System.setOut(this.out);
+		LayerToolsJarMode.Runner.contextOverride = context;
+	}
+
+	@AfterEach
+	void restore() {
+		System.setOut(this.systemOut);
+		LayerToolsJarMode.Runner.contextOverride = null;
+	}
+
+	@Test
+	void mainWithNoParamersShowsHelp() {
+		new LayerToolsJarMode().run("layertools", NO_ARGS);
+		assertThat(this.out).hasSameContentAsResource("help-output.txt");
+	}
+
+	@Test
+	void mainWithArgRunsCommand() {
+		new LayerToolsJarMode().run("layertools", new String[] { "list" });
+		assertThat(this.out).hasSameContentAsResource("list-output.txt");
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ListCommandTests.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ListCommandTests.java
new file mode 100644
index 0000000000..09af216f29
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/ListCommandTests.java
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link ListCommand}.
+ *
+ * @author Phillip Webb
+ */
+class ListCommandTests {
+
+	private ListCommand command;
+
+	private TestPrintStream out;
+
+	@BeforeEach
+	void setup() {
+		this.command = new ListCommand(mock(Context.class));
+		this.out = new TestPrintStream(this);
+	}
+
+	@Test
+	void listLayersShouldListLayers() {
+		this.command.printLayers(new ImplicitLayers(), this.out);
+		assertThat(this.out).hasSameContentAsResource("list-output.txt");
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/TestPrintStream.java b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/TestPrintStream.java
new file mode 100644
index 0000000000..31eb1acb11
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/java/org/springframework/boot/layertools/TestPrintStream.java
@@ -0,0 +1,76 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.layertools;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.PrintStream;
+import java.nio.charset.StandardCharsets;
+
+import org.assertj.core.api.AbstractAssert;
+import org.assertj.core.api.AssertProvider;
+import org.assertj.core.api.Assertions;
+
+import org.springframework.boot.layertools.TestPrintStream.PrintStreamAssert;
+import org.springframework.util.FileCopyUtils;
+
+/**
+ * {@link PrintStream} that can be used for testing.
+ *
+ * @author Phillip Webb
+ */
+class TestPrintStream extends PrintStream implements AssertProvider<PrintStreamAssert> {
+
+	private Class<? extends Object> testClass;
+
+	TestPrintStream(Object testInstance) {
+		super(new ByteArrayOutputStream());
+		this.testClass = testInstance.getClass();
+	}
+
+	@Override
+	public PrintStreamAssert assertThat() {
+		return new PrintStreamAssert(this);
+	}
+
+	@Override
+	public String toString() {
+		return this.out.toString();
+	}
+
+	static final class PrintStreamAssert extends AbstractAssert<PrintStreamAssert, TestPrintStream> {
+
+		private PrintStreamAssert(TestPrintStream actual) {
+			super(actual, PrintStreamAssert.class);
+		}
+
+		void hasSameContentAsResource(String resource) {
+			try {
+				InputStream stream = this.actual.testClass.getResourceAsStream(resource);
+				String content = FileCopyUtils.copyToString(new InputStreamReader(stream, StandardCharsets.UTF_8));
+				Assertions.assertThat(this.actual.toString()).isEqualTo(content);
+			}
+			catch (IOException ex) {
+				throw new IllegalStateException(ex);
+			}
+		}
+
+	}
+
+}
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-extract-output.txt b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-extract-output.txt
new file mode 100644
index 0000000000..0d2f9e143c
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-extract-output.txt
@@ -0,0 +1,7 @@
+Extracts layers from the jar for image creation
+
+Usage:
+  java -Djarmode=layertools -jar test.jar extract [options] [<layer>...]
+
+Options:
+  --destination string  The destination to extract files to
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-output.txt b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-output.txt
new file mode 100644
index 0000000000..47d5f4b3ba
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/help-output.txt
@@ -0,0 +1,7 @@
+Usage:
+  java -Djarmode=layertools -jar test.jar
+
+Available commands:
+  list     List layers from the jar that can be extracted
+  extract  Extracts layers from the jar for image creation
+  help     Help about any command
diff --git a/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/list-output.txt b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/list-output.txt
new file mode 100644
index 0000000000..8f96e24d1d
--- /dev/null
+++ b/spring-boot-project/spring-boot-tools/spring-boot-layertools/src/test/resources/org/springframework/boot/layertools/list-output.txt
@@ -0,0 +1,4 @@
+dependencies
+snapshot-dependencies
+resources
+application