spring-boot-maven-plugin插件是将springboot的应用程序打包成fat jar的插件。首先我们说一下啥叫fat jar。fat jar 我们暂且叫他胖jar吧,实在是找不到官方叫法了。我们一般的jar，里面放的是.class文件已经resources目录下的东西，但是fat jar 它可以把jar作为内容包含进去。也就是说，spring boot 借助spring-boot-maven-plugin将所有应用启动运行所需要的jar都包含进来，从逻辑上将具备了独立运行的条件。
我们将普通插件maven-jar-plugin生成的包和spring-boot-maven-plugin生成的包unzip，比较一下他们直接的区别，发现使用spring-boot-maven-plugin生成的jar中主要增加了两部分，第一部分是lib目录，这里存放的是应用的Maven依赖的jar包文件,第二部分是spring boot loader相关的类，这个我们下一节再说spring boot 的加载流程。
在项目中需要先加入spring-boot-maven-plugin。
org.springframework.boot
spring-boot-maven-plugin
1.3.2.RELEASE
test.ApplicationMain
repackage
默认是在package阶段执行spring-boot-maven-plugin repackage这个目标。我们看一下RepackageMojo的关键方法execute
@Override
public void execute() throws MojoExecutionException, MojoFailureException {
if (this.project.getPackaging().equals("pom")) {
getLog().debug("repackage goal could not be applied to pom project.");
return;
}
if (this.skip) {
getLog().debug("skipping repackaging as per configuration.");
return;
}
//得到项目中的原始的jar，就是使用maven-jar-plugin生成的jar
File source = this.project.getArtifact().getFile();
//要写入的目标文件，就是fat jar
File target = getTargetFile();
Repackager repackager = new Repackager(source) {
//从source中寻找spring boot 应用程序入口的main方法。
@Override
protected String findMainMethod(JarFile source) throws IOException {
long startTime = System.currentTimeMillis();
try {
return super.findMainMethod(source);
}
finally {
long duration = System.currentTimeMillis() - startTime;
if (duration > FIND_WARNING_TIMEOUT) {
getLog().warn("Searching for the main-class is taking some time, "
+ "consider using the mainClass configuration "
+ "parameter");
}
}
}
};
//如果插件中指定了mainClass就直接使用
repackager.setMainClass(this.mainClass);
if (this.layout != null) {
getLog().info("Layout: " + this.layout);
repackager.setLayout(this.layout.layout());
}
//寻找项目运行时依赖的jar，过滤后
Set artifacts = filterDependencies(this.project.getArtifacts(),
getFilters(getAdditionalFilters()));
//将Artifact转化成Libraries
Libraries libraries = new ArtifactsLibraries(artifacts, this.requiresUnpack,
getLog());
try {
LaunchScript launchScript = getLaunchScript();
//进行repackage
repackager.repackage(target, libraries, launchScript);
}
catch (IOException ex) {
throw new MojoExecutionException(ex.getMessage(), ex);
}
if (this.classifier != null) {
getLog().info("Attaching archive: " + target + ", with classifier: "
+ this.classifier);
this.projectHelper.attachArtifact(this.project, this.project.getPackaging(),
this.classifier, target);
}
else if (!source.equals(target)) {
this.project.getArtifact().setFile(target);
getLog().info("Replacing main artifact " + source + " to " + target);
}
}
基本上重要的步骤都有注释，应该不难理解的。再来看下面，当然也不是重点，看看就行。
public void repackage(File destination, Libraries libraries,
LaunchScript launchScript) throws IOException {
if (destination == null || destination.isDirectory()) {
throw new IllegalArgumentException("Invalid destination");
}
if (libraries == null) {
throw new IllegalArgumentException("Libraries must not be null");
}
if (alreadyRepackaged()) {
return;
}
destination = destination.getAbsoluteFile();
File workingSource = this.source;
//如果源jar与目标jar的文件路径及名称是一致的
if (this.source.equals(destination)) {
//将源jar重新命名为原名称+.original,同时删除原来的源jar
workingSource = new File(this.source.getParentFile(),
this.source.getName() + ".original");
workingSource.delete();
renameFile(this.source, workingSource);
}
destination.delete();
try {
//将源jar变成JarFile
JarFile jarFileSource = new JarFile(workingSource);
try {
repackage(jarFileSource, destination, libraries, launchScript);
}
finally {
jarFileSource.close();
}
}
finally {
if (!this.backupSource && !this.source.equals(workingSource)) {
deleteFile(workingSource);
}
}
}
这一步所做的是清理工作，如果源jar同目标文件路径名称等一致，将源jar重命名，原来的文件删除。为目标文件腾位置。下面的重点来了。
private void repackage(JarFile sourceJar, File destination, Libraries libraries,
LaunchScript launchScript) throws IOException {
JarWriter writer = new JarWriter(destination, launchScript);
try {
final List unpackLibraries = new ArrayList();
final List standardLibraries = new ArrayList();
libraries.doWithLibraries(new LibraryCallback() {
@Override
public void library(Library library) throws IOException {
File file = library.getFile();
if (isZip(file)) {
if (library.isUnpackRequired()) {
unpackLibraries.add(library);
}
else {
standardLibraries.add(library);
}
}
}
});
//按照规则写入manifest文件
writer.writeManifest(buildManifest(sourceJar));
Set seen = new HashSet();
writeNestedLibraries(unpackLibraries, seen, writer);
//写入源jar中的内容
writer.writeEntries(sourceJar);
//写入标准的jar,依赖的jar
writeNestedLibraries(standardLibraries, seen, writer);
if (this.layout.isExecutable()) {
//写入spring boot loader的类
writer.writeLoaderClasses();
}
}
finally {
try {
writer.close();
}
catch (Exception ex) {
// Ignore
}
}
}
上面就是一通写，将所需要的内容全部写入到目标文件中。然后就有了我们的fat jar。