Spring Boot jar可执行原理的彻底分析

<plugin>  <groupId>org.springframework.boot</groupId>  <artifactId>spring-boot-maven-plugin</artifactId>  <executions>    <execution>      <goals>        <goal>repackage</goal>      </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;}

/** * Executable JAR layout. */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: 1.0Implementation-Title: oneday-auth-serverImplementation-Version: 1.0.0-SNAPSHOTArchiver-Version: Plexus ArchiverBuilt-By: onedayImplementation-Vendor-Id: com.onedaySpring-Boot-Version: 2.1.3.RELEASEMain-Class: org.springframework.boot.loader.JarLauncherStart-Class: com.oneday.auth.ApplicationSpring-Boot-Classes: BOOT-INF/classes/Spring-Boot-Lib: BOOT-INF/lib/Created-By: Apache Maven 3.3.9Build-Jdk: 1.8.0_171

/* Extended variant of {@link java.util.jar.JarFile} that behaves in the same way but* offers the following additional functionality.* <ul>* <li>A nested {@link JarFile} can be {@link #getNestedJarFile(ZipEntry) obtained} based* on any directory entry.</li>* <li>A nested {@link JarFile} can be {@link #getNestedJarFile(ZipEntry) obtained} for* embedded JAR files (as long as their entry is not compressed).</li>**/ </ul>

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);  }

    <!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-loader -->    <dependency>      <groupId>org.springframework.boot</groupId>      <artifactId>spring-boot-loader</artifactId>      <version>2.1.3.RELEASE</version>    </dependency>    <dependency>      <groupId>org.springframework.boot</groupId>      <artifactId>spring-boot-maven-plugin</artifactId>      <version>2.1.3.RELEASE</version>    </dependency>