通过实例解析Java class文件编译加载过程

package com.example.demo.classloader;

public class ClassLoaderScope {
  public static void main(String[] args) {
    System.out.println("-------------------Bootstrap加载类-------------------");
    String property = System.getProperty("sun.boot.class.path");
    String s = property.replaceAll(";", System.lineSeparator());
    System.out.println(s);

    System.out.println("-------------------Ext加载类-------------------");

    String property1 = System.getProperty("java.ext.dirs");
    String s1 = property1.replaceAll(";", System.lineSeparator());
    System.out.println(s1);

    System.out.println("-------------------App加载类-------------------");

    String property2 = System.getProperty("java.class.path");
    String s2 = property2.replaceAll(";", System.lineSeparator());
    System.out.println(s2);
  }
}
    /**输出结果只截取了部分*/
    //E:\JDK\jdk1.8\jre\lib\resources.jar
    //E:\JDK\jdk1.8\jre\lib\rt.jar
    //E:\JDK\jdk1.8\jre\lib\sunrsasign.jar
    //E:\JDK\jdk1.8\jre\lib\jsse.jar
    //E:\JDK\jdk1.8\jre\lib\jce.jar
    //E:\JDK\jdk1.8\jre\lib\charsets.jar
    //E:\JDK\jdk1.8\jre\lib\jfr.jar
    //E:\JDK\jdk1.8\jre\classes
    //----------------------------------------------
    //E:\JDK\jdk1.8\jre\lib\ext
    //C:\Windows\Sun\Java\lib\ext
    //----------------------------------------------
    //E:\JDK\jdk1.8\jre\lib\charsets.jar
    //E:\JDK\jdk1.8\jre\lib\deploy.jar
    //E:\JDK\jdk1.8\jre\lib\ext\access-bridge-64.jar
    //E:\JDK\jdk1.8\jre\lib\ext\cldrdata.jar
    //E:\JDK\jdk1.8\jre\lib\ext\dnsns.jar
    //E:\JDK\jdk1.8\jre\lib\ext\jaccess.jar
    //E:\JDK\jdk1.8\jre\lib\ext\jfxrt.jar

package com.example.demo.classloader;

/**
 * 验证了父加载器不是加载器的加载器
 */
public class ParentAndChild {
  public static void main(String[] args) {
    //AppClassLoader
    ClassLoader classLoader = ParentAndChild.class.getClassLoader();
    System.out.println(classLoader);

    //null 这里AppClassLoader的加载器不是ExtClassLoader 而是Bootstrap
    ClassLoader appclassLoader = ParentAndChild.class.getClassLoader().getClass().getClassLoader();
    System.out.println(appclassLoader);

    //ExtClassLoader  AppClassLoader的父加载器是ExtClassLoader
    ClassLoader parent = ParentAndChild.class.getClassLoader().getParent();
    System.out.println(parent);

    //null
    ClassLoader parentparent = ParentAndChild.class.getClassLoader().getParent().getParent();
    System.out.println(parentparent);

    //null
    ClassLoader parentparentparent = ParentAndChild.class.getClassLoader().getParent().getParent().getParent();
    System.out.println(parentparent);

    /**输出结果*/
    //sun.misc.Launcher$AppClassLoader@18b4aac2
    //null
    //sun.misc.Launcher$ExtClassLoader@23fc625e
    //null
    //Exception in thread "main" java.lang.NullPointerException at com.example.demo.classloader.ParentAndChild.main(ParentAndChild.java:22)
  }
}

package com.example.demo.classloader;

public class ClassLoaderByHand {
  public static void main(String[] args) throws ClassNotFoundException {
    Class<?> clazz = ClassLoaderByHand.class.getClassLoader().
        loadClass("com.example.demo.threaddemo.juc_002.Account");
    String name = clazz.getName();
    System.out.println(name);

  }
}

  /**
  * 输出结果
  */
  //com.example.demo.threaddemo.juc_002.Account

protected Class<?> loadClass(String name, boolean resolve)
      throws ClassNotFoundException
  {
    synchronized (getClassLoadingLock(name)) {
      /**
       * 在加载之前先调用findLoadedClass()方法查看是否已经加载过此类
       * 若加载过 返回该对象
       * 如果未加载则返回null 进行下一步
       */
      // First, check if the class has already been loaded
      Class<?> c = findLoadedClass(name);
      if (c == null) {
        long t0 = System.nanoTime();
        try {
          //判断有无父加载器 如果不为空说明还未到顶层Bootstrap递归调用loadClass()
          if (parent != null) {
            c = parent.loadClass(name, false);
          } else {
            //如果没有父加载器说明调用的加载器为Bootstrap Class Loader, 在此加载器内存中查找是否已经加载
            c = findBootstrapClassOrNull(name);
          }
        } catch (ClassNotFoundException e) {
          // ClassNotFoundException thrown if class not found
          // from the non-null parent class loader
        }
        //若以上的操作都没成功加载此类
        if (c == null) {
          // If still not found, then invoke findClass in order
          // to find the class.
          long t1 = System.nanoTime();
          //调用自己的findClass()
          c = findClass(name);

          // this is the defining class loader; record the stats
          sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
          sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
          sun.misc.PerfCounter.getFindClasses().increment();
        }
      }
      if (resolve) {
        resolveClass(c);
      }
      return c;
    }
  }