为什么SpringBoot的jar可以直接运行

<build>
  <plugins>
    <plugin>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-maven-plugin</artifactId>
    </plugin>
  </plugins>
</build>

├── META-INF
│  ├── MANIFEST.MF
│  └── maven
│    └── spring.study
│      └── executable-jar
│        ├── pom.properties
│        └── pom.xml
├── lib
│  ├── aopalliance-1.0.jar
│  ├── classmate-1.1.0.jar
│  ├── spring-boot-1.3.5.RELEASE.jar
│  ├── spring-boot-autoconfigure-1.3.5.RELEASE.jar
│  ├── ...
├── org
│  └── springframework
│    └── boot
│      └── loader
│        ├── ExecutableArchiveLauncher$1.class
│        ├── ...
└── spring
  └── study
    └── executablejar
      └── ExecutableJarApplication.class

Manifest-Version: 1.0
Implementation-Title: executable-jar
Implementation-Version: 1.0-SNAPSHOT
Archiver-Version: Plexus Archiver
Built-By: Format
Start-Class: spring.study.executablejar.ExecutableJarApplication
Implementation-Vendor-Id: spring.study
Spring-Boot-Version: 1.3.5.RELEASE
Created-By: Apache Maven 3.2.3
Build-Jdk: 1.8.0_20
Implementation-Vendor: Pivotal Software, Inc.
Main-Class: org.springframework.boot.loader.JarLauncher

jar:file:/Users/format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/

/Users/format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar

META-INF/
META-INF/MANIFEST.MF
spring/
spring/study/
....
spring/study/executablejar/ExecutableJarApplication.class
lib/spring-boot-starter-1.3.5.RELEASE.jar
lib/spring-boot-1.3.5.RELEASE.jar
...

jar:file:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-starter-web-1.3.5.RELEASE.jar!/
 
jar:file:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-loader-1.3.5.RELEASE.jar!/org/springframework/boot/loader/JarLauncher.class

public static void main(String[] args) {
  // 构造JarLauncher，然后调用它的launch方法。参数是控制台传递的
  new JarLauncher().launch(args);
} 

protected void launch(String[] args) {
 try {
  // 在系统属性中设置注册了自定义的URL处理器：org.springframework.boot.loader.jar.Handler。如果URL中没有指定处理器，会去系统属性中查询
  JarFile.registerUrlProtocolHandler();
  // getClassPathArchives方法在会去找lib目录下对应的第三方依赖JarFileArchive，同时也会项目自身的JarFileArchive
  // 根据getClassPathArchives得到的JarFileArchive集合去创建类加载器ClassLoader。这里会构造一个LaunchedURLClassLoader类加载器，这个类加载器继承URLClassLoader，并使用这些JarFileArchive集合的URL构造成URLClassPath
  // LaunchedURLClassLoader类加载器的父类加载器是当前执行类JarLauncher的类加载器
  ClassLoader classLoader = createClassLoader(getClassPathArchives());
  // getMainClass方法会去项目自身的Archive中的Manifest中找出key为Start-Class的类
  // 调用重载方法launch
  launch(args, getMainClass(), classLoader);
 }
 catch (Exception ex) {
  ex.printStackTrace();
  System.exit(1);
 }
}
 
// Archive的getMainClass方法
// 这里会找出spring.study.executablejar.ExecutableJarApplication这个类
public String getMainClass() throws Exception {
 Manifest manifest = getManifest();
 String mainClass = null;
 if (manifest != null) {
  mainClass = manifest.getMainAttributes().getValue("Start-Class");
 }
 if (mainClass == null) {
  throw new IllegalStateException(
    "No 'Start-Class' manifest entry specified in " + this);
 }
 return mainClass;
}
 
// launch重载方法
protected void launch(String[] args, String mainClass, ClassLoader classLoader)
  throws Exception {
   // 创建一个MainMethodRunner，并把args和Start-Class传递给它
 Runnable runner = createMainMethodRunner(mainClass, args, classLoader);
   // 构造新线程
 Thread runnerThread = new Thread(runner);
   // 线程设置类加载器以及名字，然后启动
 runnerThread.setContextClassLoader(classLoader);
 runnerThread.setName(Thread.currentThread().getName());
 runnerThread.start();
}

@Override
public void run() {
 try {
  // 根据Start-Class进行实例化
  Class<?> mainClass = Thread.currentThread().getContextClassLoader()
    .loadClass(this.mainClassName);
  // 找出main方法
  Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
  // 如果main方法不存在，抛出异常
  if (mainMethod == null) {
   throw new IllegalStateException(
     this.mainClassName + " does not have a main method");
  }
  // 调用
  mainMethod.invoke(null, new Object[] { this.args });
 }
 catch (Exception ex) {
  UncaughtExceptionHandler handler = Thread.currentThread()
    .getUncaughtExceptionHandler();
  if (handler != null) {
   handler.uncaughtException(Thread.currentThread(), ex);
  }
  throw new RuntimeException(ex);
 }
}

private Class<?> doLoadClass(String name) throws ClassNotFoundException {
 
 // 1) Try the root class loader
 try {
  if (this.rootClassLoader != null) {
   return this.rootClassLoader.loadClass(name);
  }
 }
 catch (Exception ex) {
  // Ignore and continue
 }
 
 // 2) Try to find locally
 try {
  findPackage(name);
  Class<?> cls = findClass(name);
  return cls;
 }
 catch (Exception ex) {
  // Ignore and continue
 }
 
 // 3) Use standard loading
 return super.loadClass(name, false);
}

protected Class<?> findClass(final String name)
   throws ClassNotFoundException
{
  try {
    return AccessController.doPrivileged(
      new PrivilegedExceptionAction<Class<?>>() {
        public Class<?> run() throws ClassNotFoundException {
          // 把类名解析成路径并加上.class后缀
          String path = name.replace('.', '/').concat(".class");
          // 基于之前得到的第三方jar包依赖以及自己的jar包得到URL数组，进行遍历找出对应类名的资源
          // 比如path是org/springframework/boot/loader/JarLauncher.class，它在jar:file:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-loader-1.3.5.RELEASE.jar!/中被找出
          // 那么找出的资源对应的URL为jar:file:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-loader-1.3.5.RELEASE.jar!/org/springframework/boot/loader/JarLauncher.class
          Resource res = ucp.getResource(path, false);
          if (res != null) { // 找到了资源
            try {
              return defineClass(name, res);
            } catch (IOException e) {
              throw new ClassNotFoundException(name, e);
            }
          } else { // 找不到资源的话直接抛出ClassNotFoundException异常
            throw new ClassNotFoundException(name);
          }
        }
      }, acc);
  } catch (java.security.PrivilegedActionException pae) {
    throw (ClassNotFoundException) pae.getException();
  }
}

// 注册org.springframework.boot.loader.jar.Handler URL协议处理器
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:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-loader-1.3.5.RELEASE.jar!/")
        , new URL("jar:file:/Users/Format/Develop/gitrepository/springboot-analysis/springboot-executable-jar/target/executable-jar-1.0-SNAPSHOT.jar!/lib/spring-boot-1.3.5.RELEASE.jar!/")
    },
    LaunchedURLClassLoaderTest.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");