Add jarmode support to the loader code

Update the `Launcher` class to allow a packaged jar to be  launched in
a different mode. The launcher now checks for a `jarmode` property and
attempts to find a `JarMode` implementation using the standard
`spring.factories` mechanism.

Closes gh-19848

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/ExplodedURLClassLoader.java

@@ -0,0 +1,47 @@
+/*
+ * 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.loader;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+/**
+ * {@link URLClassLoader} used for exploded archives.
+ *
+ * @author Phillip Webb
+ */
+class ExplodedURLClassLoader extends URLClassLoader {
+
+	ExplodedURLClassLoader(URL[] urls, ClassLoader parent) {
+		super(urls, parent);
+	}
+
+	@Override
+	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
+		try {
+			Class<?> result = findClass(name);
+			if (resolve) {
+				resolveClass(result);
+			}
+			return result;
+		}
+		catch (ClassNotFoundException ex) {
+		}
+		return super.loadClass(name, resolve);
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/LaunchedURLClassLoader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * 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.
@@ -75,6 +75,17 @@ public class LaunchedURLClassLoader extends URLClassLoader {
 
 	@Override
 	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
+		if (name.startsWith("org.springframework.boot.loader.jarmode.")) {
+			try {
+				Class<?> result = findClass(name);
+				if (resolve) {
+					resolveClass(result);
+				}
+				return result;
+			}
+			catch (ClassNotFoundException ex) {
+			}
+		}
 		Handler.setUseFastConnectionExceptions(true);
 		try {
 			try {

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/Launcher.java

@@ -19,7 +19,6 @@ package org.springframework.boot.loader;
 import java.io.File;
 import java.net.URI;
 import java.net.URL;
-import java.net.URLClassLoader;
 import java.security.CodeSource;
 import java.security.ProtectionDomain;
 import java.util.ArrayList;
@@ -41,6 +40,8 @@ import org.springframework.boot.loader.jar.JarFile;
  */
 public abstract class Launcher {
 
+	private static final String JAR_MODE_LAUNCHER = "org.springframework.boot.loader.jarmode.JarModeLauncher";
+
 	/**
 	 * Launch the application. This method is the initial entry point that should be
 	 * called by a subclass {@code public static void main(String[] args)} method.
@@ -52,7 +53,9 @@ public abstract class Launcher {
 			JarFile.registerUrlProtocolHandler();
 		}
 		ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator());
-		launch(args, getMainClass(), classLoader);
+		String jarMode = System.getProperty("jarmode");
+		String launchClass = (jarMode != null && !jarMode.isEmpty()) ? JAR_MODE_LAUNCHER : getMainClass();
+		launch(args, launchClass, classLoader);
 	}
 
 	/**
@@ -94,19 +97,19 @@ public abstract class Launcher {
 		if (supportsNestedJars()) {
 			return new LaunchedURLClassLoader(urls, getClass().getClassLoader());
 		}
-		return new URLClassLoader(urls, getClass().getClassLoader());
+		return new ExplodedURLClassLoader(urls, getClass().getClassLoader());
 	}
 
 	/**
 	 * Launch the application given the archive file and a fully configured classloader.
 	 * @param args the incoming arguments
-	 * @param mainClass the main class to run
+	 * @param launchClass the launch class to run
 	 * @param classLoader the classloader
 	 * @throws Exception if the launch fails
 	 */
-	protected void launch(String[] args, String mainClass, ClassLoader classLoader) throws Exception {
+	protected void launch(String[] args, String launchClass, ClassLoader classLoader) throws Exception {
 		Thread.currentThread().setContextClassLoader(classLoader);
-		createMainMethodRunner(mainClass, args, classLoader).run();
+		createMainMethodRunner(launchClass, args, classLoader).run();
 	}
 
 	/**

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/MainMethodRunner.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * 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.
@@ -45,6 +45,7 @@ public class MainMethodRunner {
 	public void run() throws Exception {
 		Class<?> mainClass = Thread.currentThread().getContextClassLoader().loadClass(this.mainClassName);
 		Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
+		mainMethod.setAccessible(true);
 		mainMethod.invoke(null, new Object[] { this.args });
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jarmode/JarMode.java

@@ -0,0 +1,42 @@
+/*
+ * 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.loader.jarmode;
+
+/**
+ * Interface registered in {@code spring.factories} to provides extended 'jarmode'
+ * support.
+ *
+ * @author Phillip Webb
+ * @since 2.3.0
+ */
+public interface JarMode {
+
+	/**
+	 * Returns if this accepts and can run the given mode.
+	 * @param mode the mode to check
+	 * @return if this instance accepts the mode
+	 */
+	boolean accepts(String mode);
+
+	/**
+	 * Run the jar in the given mode.
+	 * @param mode the mode to use
+	 * @param args any program arguments
+	 */
+	void run(String mode, String[] args);
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jarmode/JarModeLauncher.java

@@ -0,0 +1,53 @@
+/*
+ * 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.loader.jarmode;
+
+import java.util.List;
+
+import org.springframework.core.io.support.SpringFactoriesLoader;
+import org.springframework.util.ClassUtils;
+
+/**
+ * Delegate class used to launch the fat jar in a specific mode.
+ *
+ * @author Phillip Webb
+ * @since 2.3.0
+ */
+public final class JarModeLauncher {
+
+	static final String DISABLE_SYSTEM_EXIT = JarModeLauncher.class.getName() + ".DISABLE_SYSTEM_EXIT";
+
+	private JarModeLauncher() {
+	}
+
+	public static void main(String[] args) {
+		String mode = System.getProperty("jarmode");
+		List<JarMode> candidates = SpringFactoriesLoader.loadFactories(JarMode.class,
+				ClassUtils.getDefaultClassLoader());
+		for (JarMode candidate : candidates) {
+			if (candidate.accepts(mode)) {
+				candidate.run(mode, args);
+				return;
+			}
+		}
+		System.err.println("Unsupported jarmode '" + mode + "'");
+		if (!Boolean.getBoolean(DISABLE_SYSTEM_EXIT)) {
+			System.exit(1);
+		}
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jarmode/TestJarMode.java

@@ -0,0 +1,38 @@
+/*
+ * 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.loader.jarmode;
+
+import java.util.Arrays;
+
+/**
+ * {@link JarMode} for testing.
+ *
+ * @author Phillip Webb
+ */
+class TestJarMode implements JarMode {
+
+	@Override
+	public boolean accepts(String mode) {
+		return "test".equals(mode);
+	}
+
+	@Override
+	public void run(String mode, String[] args) {
+		System.out.println("running in " + mode + " jar mode " + Arrays.asList(args));
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jarmode/LauncherJarModeTests.java

@@ -0,0 +1,86 @@
+/*
+ * 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.loader.jarmode;
+
+import java.util.Collections;
+import java.util.Iterator;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+
+import org.springframework.boot.loader.Launcher;
+import org.springframework.boot.loader.archive.Archive;
+import org.springframework.boot.testsupport.system.CapturedOutput;
+import org.springframework.boot.testsupport.system.OutputCaptureExtension;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Launcher} with jar mode support.
+ *
+ * @author Phillip Webb
+ */
+@ExtendWith(OutputCaptureExtension.class)
+class LauncherJarModeTests {
+
+	@BeforeEach
+	void setup() {
+		System.setProperty(JarModeLauncher.DISABLE_SYSTEM_EXIT, "true");
+	}
+
+	@AfterEach
+	void cleanup() {
+		System.clearProperty("jarmode");
+		System.clearProperty(JarModeLauncher.DISABLE_SYSTEM_EXIT);
+	}
+
+	@Test
+	void launchWhenJarModePropertyIsSetLaunchesJarMode(CapturedOutput out) throws Exception {
+		System.setProperty("jarmode", "test");
+		new TestLauncher().launch(new String[] { "boot" });
+		assertThat(out).contains("running in test jar mode [boot]");
+	}
+
+	@Test
+	void launchWhenJarModePropertyIsNotAcceptedThrowsException(CapturedOutput out) throws Exception {
+		System.setProperty("jarmode", "idontexist");
+		new TestLauncher().launch(new String[] { "boot" });
+		assertThat(out).contains("Unsupported jarmode 'idontexist'");
+	}
+
+	private static class TestLauncher extends Launcher {
+
+		@Override
+		protected String getMainClass() throws Exception {
+			throw new IllegalStateException("Should not be called");
+		}
+
+		@Override
+		protected Iterator<Archive> getClassPathArchivesIterator() throws Exception {
+			return Collections.emptyIterator();
+		}
+
+		@Override
+		protected void launch(String[] args) throws Exception {
+			super.launch(args);
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/META-INF/spring.factories

@@ -0,0 +1,2 @@
+org.springframework.boot.loader.jarmode.JarMode=\
+org.springframework.boot.loader.jarmode.TestJarMode