Collect and display build information

This commit updates the Maven plugin to generate a
`META-INF/boot/build.properties` file with various build-specific
settings (group, artifact, name, version and build time). Additionally,
the plugin can be configured to write an arbitrary number of additional
properties.

A new `BuildProperties` bean is automatically exposed when such a file is
present. If that bean is present, an `InfoContributor` is automatically
created to expose that information under the `build` key.

As for the git contributor, it is possible to only display the core
settings or everything using the `management.info.build.mode` property.

See gh-2559

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

@@ -0,0 +1,126 @@
+/*
+ * Copyright 2012-2016 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.maven;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Map;
+import java.util.Properties;
+
+import org.apache.maven.plugin.AbstractMojo;
+import org.apache.maven.plugin.MojoExecutionException;
+import org.apache.maven.plugin.MojoFailureException;
+import org.apache.maven.plugins.annotations.LifecyclePhase;
+import org.apache.maven.plugins.annotations.Mojo;
+import org.apache.maven.plugins.annotations.Parameter;
+import org.apache.maven.project.MavenProject;
+
+/**
+ * Generate a {@code build.properties} file based the content of the {@link MavenProject}.
+ *
+ * @author Stephane Nicoll
+ * @since 1.4.0
+ */
+@Mojo(name = "generate-build-info", defaultPhase = LifecyclePhase.GENERATE_RESOURCES, threadSafe = true)
+public class GenerateBuildInfoMojo extends AbstractMojo {
+
+	/**
+	 * The Maven project.
+	 */
+	@Parameter(defaultValue = "${project}", readonly = true, required = true)
+	private MavenProject project;
+
+	/**
+	 * The location of the generated build.properties.
+	 */
+	@Parameter(defaultValue = "${project.build.outputDirectory}/META-INF/boot/build.properties")
+	private File outputFile;
+
+	/**
+	 * Additional properties to store in the build.properties. Each entry is prefixed by
+	 * {@code build.} in the generated build.properties.
+	 */
+	@Parameter
+	private Map<String, String> additionalProperties;
+
+	@Override
+	public void execute() throws MojoExecutionException, MojoFailureException {
+		Properties properties = createBuildInfo();
+		FileOutputStream fos = null;
+		try {
+			createFileIfNecessary(this.outputFile);
+			fos = new FileOutputStream(this.outputFile);
+			properties.store(fos, "Properties");
+		}
+		catch (FileNotFoundException e) {
+			throw new MojoExecutionException(e.getMessage(), e);
+		}
+		catch (IOException e) {
+			throw new MojoExecutionException(e.getMessage(), e);
+		}
+		finally {
+			try {
+				if (fos != null) {
+					fos.close();
+				}
+			}
+			catch (IOException e) {
+				getLog().error("Error closing FileOutputStream: " + fos);
+			}
+		}
+	}
+
+	protected Properties createBuildInfo() {
+		Properties properties = new Properties();
+		properties.put("build.group", this.project.getGroupId());
+		properties.put("build.artifact", this.project.getArtifactId());
+		properties.put("build.name", this.project.getName());
+		properties.put("build.version", this.project.getVersion());
+		properties.put("build.time", formatDate(new Date()));
+		if (this.additionalProperties != null) {
+			for (Map.Entry<String, String> entry : this.additionalProperties.entrySet()) {
+				properties.put("build." + entry.getKey(), entry.getValue());
+			}
+		}
+		return properties;
+	}
+
+	private String formatDate(Date date) {
+		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ");
+		return sdf.format(date);
+	}
+
+	private void createFileIfNecessary(File file) throws MojoExecutionException, IOException {
+		if (file.exists()) {
+			return;
+		}
+		File parent = file.getParentFile();
+		if (!parent.exists() && !parent.mkdirs()) {
+			throw new MojoExecutionException("Cannot create parent directory for '"
+					+ this.outputFile.getAbsolutePath() + "'");
+		}
+		if (!file.createNewFile()) {
+			throw new MojoExecutionException("Cannot create target file '"
+					+ this.outputFile.getAbsolutePath() + "'");
+		}
+	}
+
+}