彻底搞懂 SpringBoot jar 可执行原理

fat jar //目录结构  
├─BOOT-INF  
│  ├─classes  
│  └─lib  
├─META-INF  
│  ├─maven  
│  ├─app.properties  
│  ├─MANIFEST.MF     
└─org  
    └─springframework  
        └─boot  
            └─loader 
                ├─archive  
                ├─data  
                ├─jar  
                └─util

<plugin>
    <groupId>org.springframework.bootgroupId> 
    <artifactId>spring-boot-maven-pluginartifactId>  
    <executions>  
        <execution>  
            <goals>  
                <goal>repackagegoal>  
            goals>  
        execution>  
    executions>  
plugin>

private void repackage() throws MojoExecutionException {  
     //获取使用maven-jar-plugin生成的jar，最终的命名将加上.orignal后缀  
   Artifact source = getSourceArtifact();  
    //最终文件，即Fat jar  
   File target = getTargetFile();  
    //获取重新打包器，将重新打包成可执行jar文件 
   Repackager repackager = getRepackager(source.getFile());  
    //查找并过滤项目运行时依赖的jar  
   Set<Artifact> artifacts = filterDependencies(this.project.getArtifacts(),  
         getFilters(getAdditionalFilters()));  
    //将artifacts转换成libraries  
   Libraries libraries = new ArtifactsLibraries(artifacts, this.requiresUnpack,  
         getLog());  
   try {  
       //提供Spring Boot启动脚本  
      LaunchScript launchScript = getLaunchScript(); 
       //执行重新打包逻辑，生成最后fat jar  
      repackager.repackage(target, libraries, launchScript);  
   }  
   catch (IOException ex) {  
      throw new MojoExecutionException(ex.getMessage(), ex);  
   }  
    //将source更新成 xxx.jar.orignal文件  
   updateArtifact(source, target, repackager.getBackupFile());  
}

private Repackager getRepackager(File source) {  
   Repackager repackager = new Repackager(source, this.layoutFactory);  
   repackager.addMainClassTimeoutWarningListener(  
         new LoggingMainClassTimeoutWarningListener());  
    //设置main class的名称，如果不指定的话则会查找第一个包含main方法的类，repacke最后将会设置org.springframework.boot.loader.JarLauncher  
   repackager.setMainClass(this.mainClass);  
   if (this.layout != null) {  
      getLog().info("Layout: " + this.layout);  
       //重点关心下layout 最终返回了 org.springframework.boot.loader.tools.Layouts.Jar  
      repackager.setLayout(this.layout.layout());  
   }  
   return repackager;  
}

  
public static class Jar implements RepackagingLayout {  
   @Override  
   public String getLauncherClassName() {  
      return "org.springframework.boot.loader.JarLauncher"; 
   }  
   @Override 
   public String getLibraryDestination(String libraryName, LibraryScope scope) {  
      return "BOOT-INF/lib/";  
   }  
   @Override  
   public String getClassesLocation() {  
      return "";  
   }  
   @Override  
   public String getRepackagedClassesLocation() {  
      return "BOOT-INF/classes/";  
   }  
   @Override  
   public boolean isExecutable() {  
      return true;  
   }  
}

Manifest-Version: 1.0  
Implementation-Title: oneday-auth-server  
Implementation-Version: 1.0.0-SNAPSHOT  
Archiver-Version: Plexus Archiver  
Built-By: oneday  
Implementation-Vendor-Id: com.oneday  
Spring-Boot-Version: 2.1.3.RELEASE  
Main-Class: org.springframework.boot.loader.JarLauncher  
Start-Class: com.oneday.auth.Application  
Spring-Boot-Classes: BOOT-INF/classes/  
Spring-Boot-Lib: BOOT-INF/lib/  
Created-By: Apache Maven 3.3.9  
Build-Jdk: 1.8.0_171

public LaunchedURLClassLoader(URL[] urls, ClassLoader parent) {  
   super(urls, parent);  
}

//LaunchedURLClassLoader的实现  
protected Class loadClass(String name, boolean resolve)  
      throws ClassNotFoundException {  
   Handler.setUseFastConnectionExceptions(true); 
   try {  
      try {  
          //尝试根据类名去定义类所在的包，即java.lang.Package，确保jar in jar里匹配的manifest能够和关联               //的package关联起来  
         definePackageIfNecessary(name);  
      }  
      catch (IllegalArgumentException ex) {  
         // Tolerate race condition due to being parallel capable  
         if (getPackage(name) == null) {  
            // This should never happen as the IllegalArgumentException indicates  
            // that the package has already been defined and, therefore,  
            // getPackage(name) should not return null.  
            //这里异常表明，definePackageIfNecessary方法的作用实际上是预先过滤掉查找不到的包  
            throw new AssertionError("Package " + name + " has already been "  
                  + "defined but it could not be found");  
         }  
      }  
      return super.loadClass(name, resolve);  
   }  
   finally {  
      Handler.setUseFastConnectionExceptions(false);  
   }  
}

protected void launch(String[] args, String mainClass, ClassLoader classLoader)  
      throws Exception {  
   Thread.currentThread().setContextClassLoader(classLoader);  
   createMainMethodRunner(mainClass, args, classLoader).run();  
}

public static void main(String[] args) throws ClassNotFoundException, MalformedURLException {  
        JarFile.registerUrlProtocolHandler();  
// 构造LaunchedURLClassLoader类加载器，这里使用了2个URL，分别对应jar包中依赖包spring-boot-loader和spring-boot，使用 "!/" 分开，需要org.springframework.boot.loader.jar.Handler处理器处理  
        LaunchedURLClassLoader classLoader = new LaunchedURLClassLoader(  
                new URL[] {  
                        new URL("jar:file:/E:/IdeaProjects/oneday-auth/oneday-auth-server/target/oneday-auth-server-1.0.0-SNAPSHOT.jar!/BOOT-INF/lib/spring-boot-loader-1.2.3.RELEASE.jar!/")  
                        , new URL("jar:file:/E:/IdeaProjects/oneday-auth/oneday-auth-server/target/oneday-auth-server-1.0.0-SNAPSHOT.jar!/BOOT-INF/lib/spring-boot-2.1.3.RELEASE.jar!/")  
                },  
                Application.class.getClassLoader());  
// 加载类  
// 这2个类都会在第二步本地查找中被找出(URLClassLoader的findClass方法)  
        classLoader.loadClass("org.springframework.boot.loader.JarLauncher");  
        classLoader.loadClass("org.springframework.boot.SpringApplication");  
// 在第三步使用默认的加载顺序在ApplicationClassLoader中被找出  
   classLoader.loadClass("org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration");  

//        SpringApplication.run(Application.class, args);  
    }