Guide users to apply dependency management plugin when resolution fails

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/SpringBootPlugin.java

@@ -78,6 +78,14 @@ public class SpringBootPlugin implements Plugin<Project> {
 			project.getPlugins().withType(action.getPluginClass(),
 					plugin -> action.execute(project));
 		}
+		UnresolvedDependenciesAnalyzer unresolvedDependenciesAnalyzer = new UnresolvedDependenciesAnalyzer();
+		project.getConfigurations().all(configuration -> configuration.getIncoming()
+				.afterResolve(resolvableDependencies -> unresolvedDependenciesAnalyzer
+						.analyze(configuration.getResolvedConfiguration()
+								.getLenientConfiguration()
+								.getUnresolvedModuleDependencies())));
+		project.getGradle().buildFinished(
+				buildResult -> unresolvedDependenciesAnalyzer.buildFinished(project));
 	}
 
 }

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/UnresolvedDependenciesAnalyzer.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2012-2017 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
+ *
+ *      http://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.gradle.plugin;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import io.spring.gradle.dependencymanagement.DependencyManagementPlugin;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.ModuleVersionSelector;
+import org.gradle.api.artifacts.UnresolvedDependency;
+import org.slf4j.LoggerFactory;
+
+/**
+ * An analyzer for {@link UnresolvedDependency unresolvable dependencies} that logs a
+ * warning suggesting that the {@code io.spring.dependency-management} plugin is applied
+ * when one or more versionless dependencies fails to resolve.
+ *
+ * @author Andy Wilkinson
+ */
+class UnresolvedDependenciesAnalyzer {
+
+	private static final org.slf4j.Logger logger = LoggerFactory
+			.getLogger(SpringBootPlugin.class);
+
+	private Set<ModuleVersionSelector> dependenciesWithNoVersion = new HashSet<>();
+
+	public void analyze(Set<UnresolvedDependency> unresolvedDependencies) {
+		this.dependenciesWithNoVersion = unresolvedDependencies.stream()
+				.map(unresolvedDependency -> unresolvedDependency.getSelector())
+				.filter(this::hasNoVersion).collect(Collectors.toSet());
+	}
+
+	void buildFinished(Project project) {
+		if (!this.dependenciesWithNoVersion.isEmpty()
+				&& !project.getPlugins().hasPlugin(DependencyManagementPlugin.class)) {
+			StringBuilder message = new StringBuilder();
+			message.append("\nDuring the build, one or more dependencies that were "
+					+ "declared without a version failed to resolve:\n");
+			this.dependenciesWithNoVersion.stream()
+					.forEach(dependency -> message.append("    " + dependency + "\n"));
+			message.append("\nDid you forget to apply the "
+					+ "io.spring.dependency-management plugin to the " + project.getName()
+					+ " project?\n");
+			logger.warn(message.toString());
+		}
+	}
+
+	private boolean hasNoVersion(ModuleVersionSelector selector) {
+		String version = selector.getVersion();
+		return version == null || version.trim().length() == 0;
+	}
+
+}

spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/plugin/DependencyManagementPluginActionIntegrationTests.java

@@ -16,11 +16,16 @@
 
 package org.springframework.boot.gradle.plugin;
 
+import java.io.File;
+import java.io.IOException;
+
+import org.gradle.testkit.runner.BuildResult;
 import org.gradle.testkit.runner.TaskOutcome;
 import org.junit.Rule;
 import org.junit.Test;
 
 import org.springframework.boot.gradle.testkit.GradleBuild;
+import org.springframework.util.FileSystemUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -50,4 +55,23 @@ public class DependencyManagementPluginActionIntegrationTests {
 						.isEqualTo(TaskOutcome.SUCCESS);
 	}
 
+	@Test
+	public void helpfulErrorWhenVersionlessDependencyFailsToResolve() throws IOException {
+		File examplePackage = new File(this.gradleBuild.getProjectDir(),
+				"src/main/java/com/example");
+		examplePackage.mkdirs();
+		FileSystemUtils.copyRecursively(new File("src/test/java/com/example"),
+				examplePackage);
+		BuildResult result = this.gradleBuild.buildAndFail("compileJava");
+		assertThat(result.task(":compileJava").getOutcome())
+				.isEqualTo(TaskOutcome.FAILED);
+		String output = result.getOutput();
+		assertThat(output).contains("During the build, one or more dependencies that "
+				+ "were declared without a version failed to resolve:");
+		assertThat(output).contains("org.springframework.boot:spring-boot-starter-web:");
+		assertThat(output).contains("Did you forget to apply the "
+				+ "io.spring.dependency-management plugin to the "
+				+ this.gradleBuild.getProjectDir().getName() + " project?");
+	}
+
 }

spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/testkit/GradleBuild.java

@@ -126,20 +126,33 @@ public class GradleBuild implements TestRule {
 
 	public BuildResult build(String... arguments) {
 		try {
+			return prepareRunner(arguments).build();
+		}
+		catch (Exception ex) {
+			throw new RuntimeException(ex);
+		}
+	}
+
+	public BuildResult buildAndFail(String... arguments) {
+		try {
+			return prepareRunner(arguments).buildAndFail();
+		}
+		catch (Exception ex) {
+			throw new RuntimeException(ex);
+		}
+	}
+
+	public GradleRunner prepareRunner(String... arguments) throws IOException {
 		Files.copy(new File(this.script).toPath(),
 				new File(this.projectDir, "build.gradle").toPath(),
 				StandardCopyOption.REPLACE_EXISTING);
-			GradleRunner gradleRunner = GradleRunner.create()
-					.withProjectDir(this.projectDir).forwardOutput();
+		GradleRunner gradleRunner = GradleRunner.create().withProjectDir(this.projectDir)
+				.forwardOutput();
 		List<String> allArguments = new ArrayList<String>();
 		allArguments.add("-PpluginClasspath=" + pluginClasspath());
 		allArguments.add("-PbootVersion=" + getBootVersion());
 		allArguments.addAll(Arrays.asList(arguments));
-			return gradleRunner.withArguments(allArguments).build();
-		}
-		catch (Exception ex) {
-			throw new RuntimeException(ex);
-		}
+		return gradleRunner.withArguments(allArguments);
 	}
 
 	public File getProjectDir() {

spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/plugin/DependencyManagementPluginActionIntegrationTests-helpfulErrorWhenVersionlessDependencyFailsToResolve.gradle

@@ -0,0 +1,16 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'java'
+apply plugin: 'org.springframework.boot'
+
+repositories {
+	mavenLocal()
+}
+
+dependencies {
+	compile 'org.springframework.boot:spring-boot-starter-web'
+}