Add a BuildInfo task for generating build.properties with Gradle

The commit adds a new BuildInfo task that can be used to generate
a build.properties file, intended for inclusion in the Actuator's
info endpoint.

A default instance of the task can be configure using the plugin's
DSL:

springBoot {
	buildInfo()
}

Additional properties can also be configured using the DSL:

springBoot {
	buildInfo {
		additionalProperties = [
			'foo': 'bar'
		]
	}
}

When configured via the DSL, the Java plugin's classes task is
configured to depend on the build info task. Alternatively, if more
control is required, the task can be declared and configured manually:

task buildInfo(type: org.springframework.boot.gradle.buildinfo.BuildInfo) {
	additionalProperties = [
		'foo': 'bar'
	]
}

classes {
	dependsOn buildInfo
}

See gh-2559

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

@@ -0,0 +1,146 @@
+/*
+ * 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.loader.tools;
+
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Map;
+import java.util.Properties;
+
+/**
+ * A {@code BuildPropertiesWriter} writes the {@code build.properties} for consumption by
+ * the Actuator.
+ *
+ * @author Andy Wilkinson
+ * @author Stephane Nicoll
+ */
+public final class BuildPropertiesWriter {
+
+	private final File outputFile;
+
+	/**
+	 * Creates a new {@code BuildPropertiesWriter} that will write to the given
+	 * {@code outputFile}.
+	 *
+	 * @param outputFile the output file
+	 */
+	public BuildPropertiesWriter(File outputFile) {
+		this.outputFile = outputFile;
+	}
+
+	public void writeBuildProperties(ProjectDetails projectDetails) throws IOException {
+		Properties properties = createBuildInfo(projectDetails);
+		createFileIfNecessary(this.outputFile);
+		FileOutputStream outputStream = new FileOutputStream(this.outputFile);
+		try {
+			properties.store(outputStream, "Properties");
+		}
+		finally {
+			try {
+				outputStream.close();
+			}
+			catch (IOException ex) {
+				// Continue
+			}
+		}
+	}
+
+	private void createFileIfNecessary(File file) throws IOException {
+		if (file.exists()) {
+			return;
+		}
+		File parent = file.getParentFile();
+		if (!parent.isDirectory() && !parent.mkdirs()) {
+			throw new IllegalStateException("Cannot create parent directory for '"
+					+ this.outputFile.getAbsolutePath() + "'");
+		}
+		if (!file.createNewFile()) {
+			throw new IllegalStateException("Cannot create target file '"
+					+ this.outputFile.getAbsolutePath() + "'");
+		}
+	}
+
+	protected Properties createBuildInfo(ProjectDetails project) {
+		Properties properties = new Properties();
+		properties.put("build.group", project.getGroup());
+		properties.put("build.artifact", project.getArtifact());
+		properties.put("build.name", project.getName());
+		properties.put("build.version", project.getVersion());
+		properties.put("build.time", formatDate(new Date()));
+		if (project.getAdditionalProperties() != null) {
+			for (Map.Entry<String, String> entry : project.getAdditionalProperties()
+					.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);
+	}
+
+	/**
+	 * Build-system agnostic details of a project.
+	 */
+	public static final class ProjectDetails {
+
+		private final String group;
+
+		private final String artifact;
+
+		private final String name;
+
+		private final String version;
+
+		private final Map<String, String> additionalProperties;
+
+		public ProjectDetails(String group, String artifact, String version, String name,
+				Map<String, String> additionalProperties) {
+			this.group = group;
+			this.artifact = artifact;
+			this.name = name;
+			this.version = version;
+			this.additionalProperties = additionalProperties;
+		}
+
+		public String getGroup() {
+			return this.group;
+		}
+
+		public String getArtifact() {
+			return this.artifact;
+		}
+
+		public String getName() {
+			return this.name;
+		}
+
+		public String getVersion() {
+			return this.version;
+		}
+
+		public Map<String, String> getAdditionalProperties() {
+			return this.additionalProperties;
+		}
+
+	}
+}