Configure buildpack to use target Java version

With this commit, the Maven `spring-boot:build-image` goal and the
Gradle `bootBuildImage` task will configure the OpenJDK buildpack
to use the same JRE version as the project's target version,
provided the buildpack Java version is not explicitly set in the
build configuration.

Fixes gh-20172

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/BuildImageMojo.java

@@ -63,6 +63,8 @@ import org.springframework.util.StringUtils;
 @Execute(phase = LifecyclePhase.PACKAGE)
 public class BuildImageMojo extends AbstractPackagerMojo {
 
+	private static final String OPENJDK_BUILDPACK_JAVA_VERSION_KEY = "BP_JAVA_VERSION";
+
 	/**
 	 * Directory containing the JAR.
 	 * @since 2.3.0
@@ -137,7 +139,7 @@ public class BuildImageMojo extends AbstractPackagerMojo {
 
 	private File getJarFile() {
 		// We can use 'project.getArtifact().getFile()' because that was done in a
-		// forked lifecyle and is now null
+		// forked lifecycle and is now null
 		StringBuilder name = new StringBuilder(this.finalName);
 		if (StringUtils.hasText(this.classifier)) {
 			name.append("-").append(this.classifier);
@@ -147,6 +149,23 @@ public class BuildImageMojo extends AbstractPackagerMojo {
 	}
 
 	private BuildRequest customize(BuildRequest request) {
+		request = customizeEnvironment(request);
+		request = customizeCreator(request);
+		return request;
+	}
+
+	private BuildRequest customizeEnvironment(BuildRequest request) {
+		if (!request.getEnv().containsKey(OPENJDK_BUILDPACK_JAVA_VERSION_KEY)) {
+			JavaCompilerPluginConfiguration compilerConfiguration = new JavaCompilerPluginConfiguration(this.project);
+			String targetJavaVersion = compilerConfiguration.getTargetMajorVersion();
+			if (StringUtils.hasText(targetJavaVersion)) {
+				return request.withEnv(OPENJDK_BUILDPACK_JAVA_VERSION_KEY, targetJavaVersion + ".*");
+			}
+		}
+		return request;
+	}
+
+	private BuildRequest customizeCreator(BuildRequest request) {
 		String springBootVersion = VersionExtractor.forClass(BuildImageMojo.class);
 		if (StringUtils.hasText(springBootVersion)) {
 			request = request.withCreator(Creator.withVersion(springBootVersion));

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/JavaCompilerPluginConfiguration.java

@@ -0,0 +1,98 @@
+/*
+ * 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.maven;
+
+import java.util.Arrays;
+
+import org.apache.maven.model.Plugin;
+import org.apache.maven.project.MavenProject;
+import org.codehaus.plexus.util.xml.Xpp3Dom;
+
+/**
+ * Provides access to the Maven Java Compiler plugin configuration.
+ *
+ * @author Scott Frederick
+ */
+class JavaCompilerPluginConfiguration {
+
+	private final MavenProject project;
+
+	JavaCompilerPluginConfiguration(MavenProject project) {
+		this.project = project;
+	}
+
+	String getSourceMajorVersion() {
+		String version = getConfigurationValue("source");
+
+		if (version == null) {
+			version = getPropertyValue("maven.compiler.source");
+		}
+
+		return majorVersionFor(version);
+	}
+
+	String getTargetMajorVersion() {
+		String version = getConfigurationValue("target");
+
+		if (version == null) {
+			version = getPropertyValue("maven.compiler.target");
+		}
+
+		return majorVersionFor(version);
+	}
+
+	private String getConfigurationValue(String propertyName) {
+		Plugin plugin = this.project.getPlugin("org.apache.maven.plugins:maven-compiler-plugin");
+		if (plugin != null) {
+			Object pluginConfiguration = plugin.getConfiguration();
+			if (pluginConfiguration instanceof Xpp3Dom) {
+				Xpp3Dom dom = (Xpp3Dom) pluginConfiguration;
+				return getNodeValue(dom, propertyName);
+			}
+		}
+		return null;
+	}
+
+	private String getPropertyValue(String propertyName) {
+		if (this.project.getProperties().containsKey(propertyName)) {
+			return this.project.getProperties().get(propertyName).toString();
+		}
+		return null;
+	}
+
+	private String getNodeValue(Xpp3Dom dom, String... childNames) {
+		Xpp3Dom childNode = dom.getChild(childNames[0]);
+
+		if (childNode == null) {
+			return null;
+		}
+
+		if (childNames.length > 1) {
+			return getNodeValue(childNode, Arrays.copyOfRange(childNames, 1, childNames.length));
+		}
+
+		return childNode.getValue();
+	}
+
+	private String majorVersionFor(String version) {
+		if (version != null && version.startsWith("1.")) {
+			return version.substring("1.".length());
+		}
+		return version;
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/test/java/org/springframework/boot/maven/JavaCompilerPluginConfigurationTests.java

@@ -0,0 +1,94 @@
+/*
+ * 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.maven;
+
+import java.io.IOException;
+import java.io.StringReader;
+import java.util.Arrays;
+import java.util.Properties;
+
+import org.apache.maven.model.Plugin;
+import org.apache.maven.project.MavenProject;
+import org.codehaus.plexus.util.xml.Xpp3Dom;
+import org.codehaus.plexus.util.xml.Xpp3DomBuilder;
+import org.codehaus.plexus.util.xml.pull.XmlPullParserException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link JavaCompilerPluginConfiguration}.
+ *
+ * @author Scott Frederick
+ */
+class JavaCompilerPluginConfigurationTests {
+
+	private MavenProject project;
+
+	private Plugin plugin;
+
+	@BeforeEach
+	void setUp() {
+		this.project = mock(MavenProject.class);
+		this.plugin = mock(Plugin.class);
+		given(this.project.getPlugin(anyString())).willReturn(this.plugin);
+	}
+
+	@Test
+	void versionsAreNullWithNoConfiguration() {
+		given(this.plugin.getConfiguration()).willReturn(null);
+		given(this.project.getProperties()).willReturn(new Properties());
+		JavaCompilerPluginConfiguration configuration = new JavaCompilerPluginConfiguration(this.project);
+		assertThat(configuration.getSourceMajorVersion()).isNull();
+		assertThat(configuration.getTargetMajorVersion()).isNull();
+	}
+
+	@Test
+	void versionsAreReturnedFromConfiguration() throws IOException, XmlPullParserException {
+		Xpp3Dom dom = buildConfigurationDom("<source>1.9</source>", "<target>11</target>");
+		given(this.plugin.getConfiguration()).willReturn(dom);
+		Properties properties = new Properties();
+		properties.setProperty("maven.compiler.source", "1.8");
+		properties.setProperty("maven.compiler.target", "10");
+		given(this.project.getProperties()).willReturn(properties);
+		JavaCompilerPluginConfiguration configuration = new JavaCompilerPluginConfiguration(this.project);
+		assertThat(configuration.getSourceMajorVersion()).isEqualTo("9");
+		assertThat(configuration.getTargetMajorVersion()).isEqualTo("11");
+	}
+
+	@Test
+	void versionsAreReturnedFromProperties() {
+		given(this.plugin.getConfiguration()).willReturn(null);
+		Properties properties = new Properties();
+		properties.setProperty("maven.compiler.source", "1.8");
+		properties.setProperty("maven.compiler.target", "11");
+		given(this.project.getProperties()).willReturn(properties);
+		JavaCompilerPluginConfiguration configuration = new JavaCompilerPluginConfiguration(this.project);
+		assertThat(configuration.getSourceMajorVersion()).isEqualTo("8");
+		assertThat(configuration.getTargetMajorVersion()).isEqualTo("11");
+	}
+
+	private Xpp3Dom buildConfigurationDom(String... properties) throws IOException, XmlPullParserException {
+		return Xpp3DomBuilder
+				.build(new StringReader("<configuration>" + Arrays.toString(properties) + "</configuration>"));
+	}
+
+}