字节码加载过程

双亲委托

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            //如果没有
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                    //如果父类不空 就交给父类递归
                        c = parent.loadClass(name, false);
                    } else {
                    //当某一个父类为空时就交给BootstrapClassLoad去加载
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    //加载失败就class not find 了
                    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;
        }
    }

启动类加载器（Bootstrap Classloader）

此类加载器为c++所写 native实现，负责jvm核心类的加载即加载位于 JAVA_HOME/lib/rt.jar 文件中

扩展类加载器（Extension Classloader）

扩展类加载器，负责加载 JVM 扩展类，扩展 jar 包位于 JAVA_HOME/lib/ext/*.jar 中，库名通常以 javax 开头。

AppClassLoader

加载自己程序里面的类

自定义类加载

public class MyClassLoader extends ClassLoader {
    private String dirPath;

    @Override
    public String getName() {
        return "MyClassLoader";
    }

    public MyClassLoader(String dirPath) {
        if (!dirPath.endsWith("/") && !dirPath.endsWith("\\")) {
            dirPath += "/";
        }
        this.dirPath = dirPath;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        String filePath = dirPath + name.replace('.', '/') + ".class";
        byte[] b;
        Path path;
        try {
            path = Paths.get(new URI(filePath));
            b = Files.readAllBytes(path);
            // defineClass将字节数组转换成Class对象
            return defineClass(name, b, 0, b.length);
        } catch (IOException | URISyntaxException e) {
            e.printStackTrace();
            return null;
        }
    }
}

i++

程序代码

public class Test {
    public static void main(String[] args) {
        int i=0;

        i=i++;
        System.out.println(i);
    }
}

字节码

{
  public com.shard.jdbc.service.impl.Test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: iconst_0
         1: istore_1
         2: iload_1     //这里是将局部表中值加载一份到操作栈中
         3: iinc          1, 1 这边加的是局部变量表中的值
         6: istore_1 //将操作栈中的值替换掉索引为1的值
         7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        10: iload_1
        11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
        14: return
      LineNumberTable:
        line 5: 0
        line 7: 2
        line 8: 7
        line 9: 14
}