<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 {
   //獲取使用maven-jar-plugin生成的jar，最終的命名將加上.orignal后綴
  Artifact source = getSourceArtifact();
  //最終文件，即Fat jar
  File target = getTargetFile();
  //獲取重新打包器，將重新打包成可執(zhí)行jar文件
  Repackager repackager = getRepackager(source.getFile());
  //查找并過濾項目運行時依賴的jar
  Set<Artifact> artifacts = filterDependencies(this.project.getArtifacts(),
     getFilters(getAdditionalFilters()));
  //將artifacts轉換成libraries
  Libraries libraries = new ArtifactsLibraries(artifacts, this.requiresUnpack,
     getLog());
  try {
    //提供Spring Boot啟動腳本
   LaunchScript launchScript = getLaunchScript();
    //執(zhí)行重新打包邏輯，生成最后fat jar
   repackager.repackage(target, libraries, launchScript);
  }
  catch (IOException ex) {
   throw new MojoExecutionException(ex.getMessage(), ex);
  }
  //將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());
  //設置main class的名稱，如果不指定的話則會查找第一個包含main方法的類，repacke最后將會設置org.springframework.boot.loader.JarLauncher
  repackager.setMainClass(this.mainClass);
  if (this.layout != null) {
   getLog().info("Layout: " + this.layout);
    //重點關心下layout 最終返回了 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: 1.0
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

/* 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的實現(xiàn)
protected Class<?> loadClass(String name, boolean resolve)
   throws ClassNotFoundException {
  Handler.setUseFastConnectionExceptions(true);
  try {
   try {
     //嘗試根據(jù)類名去定義類所在的包，即java.lang.Package，確保jar in jar里匹配的manifest能夠和關聯(lián)	      //的package關聯(lián)起來
     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.
       
      //這里異常表明，definePackageIfNecessary方法的作用實際上是預先過濾掉查找不到的包
      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();
// 構造LaunchedURLClassLoader類加載器，這里使用了2個URL，分別對應jar包中依賴包spring-boot-loader和spring-boot，使用 "!/" 分開，需要org.springframework.boot.loader.jar.Handler處理器處理
    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());
// 加載類
// 這2個類都會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");
// 在第三步使用默認的加載順序在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>