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/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>"));
+	}
+
+}