synchronized关键字所生成的字节码详细分析

1.synchronized 的 可重入锁和不可重入锁！！！！

• MyTest2

package jvm.bytecode;

public class MyTest2 {
    String str = "Welcome";

    private int x = 5;

    public static Integer in = 10;

    public static void main(String[] args) {
        MyTest2 myTest2 = new MyTest2();

        myTest2.setX(8);

        in = 20;
    }

    private synchronized void setX(int x) {
        this.x = x;
    }
	 //★★★★
    private void test(String str){
        synchronized (str){
            System.out.println("hello world");
        }
    }
    //★★★★
    private synchronized static void test2(){
        
    }
}

• MyTest.class文件分析：

  • 对上面的test方法进行分析：

   private void test(java.lang.String);
      descriptor: (Ljava/lang/String;)V
      flags: ACC_PRIVATE
      Code:
        stack=2, locals=4, args_size=2
           0: aload_1
           1: dup
           2: astore_2
           3: monitorenter			//★★★获取锁，进入的synchronized代码块中
           // Field java/lang/System.out:Ljava/io/PrintStream;
           4: getstatic     #10
           // String hello world
           7: ldc           #11
           // Method java/io/PrintStream.println:(Ljava/lang/String;)V
           9: invokevirtual #12                 
          12: aload_2
          13: monitorexit				//★★★正常的释放锁，出synchronized代码块
          14: goto          22
          17: astore_3
          18: aload_2
          19: monitorexit				//★★★异常的释放锁，出synchronized代码块（out流）
          20: aload_3
          21: athrow
          22: return
        Exception table:
           from    to  target type
               4    14    17   any
              17    20    17   any
        LineNumberTable:
          line 23: 0
          line 24: 4
          line 25: 12
          line 26: 22
        LocalVariableTable:
          Start  Length  Slot  Name   Signature
              0      23     0  this   Ljvm/bytecode/MyTest2;
              0      23     1   str   Ljava/lang/String;
        StackMapTable: number_of_entries = 2
          frame_type = 255 /* full_frame */
            offset_delta = 17
            locals = [ class jvm/bytecode/MyTest2, class java/lang/String, class java/lang/Object ]
            stack = [ class java/lang/Throwable ]
          frame_type = 250 /* chop */
            offset_delta = 4
  
  

  • 对test2()方法进行分析：

    • 当synchronized修饰的是静态方法的时候，实际上synchronized是给当前的MyTest2类所对应的class对象上的锁。

    • 这里仅仅是多了一个flag，在Code中是看不到：monitorenter、monitorexit

      private static synchronized void test2();
          descriptor: ()V
          flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNCHRONIZED 
          Code:
            stack=0, locals=0, args_size=0
               0: return
            LineNumberTable:
              line 30: 0