深入理解Java虚拟机（四）

synchronized 字节码分析

我们先来看一下一个简单的方法

/**
 * @Author: cuzz
 * @Date: 2019/3/4 13:34
 * @Description:
 */
public class MyTest02 {
    int x = 0;

    public void setX(int x) {
        this.x = x;
    }
}

使用 javap -v com.cuzz.jvm.bytecode.MyTest02 命令，找到 setX 方法

public void setX(int);
  descriptor: (I)V
  flags: ACC_PUBLIC
  Code:
    stack=2, locals=2, args_size=2
       0: aload_0
       1: iload_1
       2: putfield      #2                  // Field x:I
       5: return
    LineNumberTable:
      line 12: 0
      line 13: 5
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       6     0  this   Lcom/cuzz/jvm/bytecode/MyTest02;
          0       6     1     x   I

如果我们在方法中添加 synchronzied 关键字

public class MyTest02 {
    int x = 0;

    public synchronized void setX(int x) {
        this.x = x;
    }
}

我们再反编译一下

public synchronized void setX(int);
  descriptor: (I)V
  flags: ACC_PUBLIC, ACC_SYNCHRONIZED
  Code:
    stack=2, locals=2, args_size=2
       0: aload_0
       1: iload_1
       2: putfield      #2                  // Field x:I
       5: return
    LineNumberTable:
      line 12: 0
      line 13: 5
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       6     0  this   Lcom/cuzz/jvm/bytecode/MyTest02;
          0       6     1     x   I

对比这两个反编译的结果，我们发现在 flags 中多了 ACC_SYNCHRONIZED，不会出现 monitorenter 和 monitorexit。

如果我们是在方法体重添加 synchronized 关键字

public class MyTest02 {
    String lock = "lock";
    int x = 0;

    public int getX() {
        synchronized (lock) {
            return x;
        }
    }
}

我们反编译一下 找到 getX 方法

public int getX();
  descriptor: ()I
  flags: ACC_PUBLIC
  Code:
    stack=2, locals=3, args_size=1
       0: aload_0
       1: getfield      #3                  // Field lock:Ljava/lang/String;
       4: dup
       5: astore_1
       6: monitorenter
       7: aload_0
       8: getfield      #4                  // Field x:I
      11: aload_1
      12: monitorexit
      13: ireturn
      14: astore_2
      15: aload_1
      16: monitorexit
      17: aload_2
      18: athrow
    Exception table:
       from    to  target type
           7    13    14   any
          14    17    14   any
    LineNumberTable:
      line 17: 0
      line 18: 7
      line 19: 14
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      19     0  this   Lcom/cuzz/jvm/bytecode/MyTest02;
    StackMapTable: number_of_entries = 1
      frame_type = 255 /* full_frame */
        offset_delta = 14
        locals = [ class com/cuzz/jvm/bytecode/MyTest02, class java/lang/Object ]
        stack = [ class java/lang/Throwable ]

在 6 中出现 monitorenter，在 16 中出现 moniterexit