synchronized方法的字节码分析javaqq43147121的博客-

首先编写一段代码将三种使用synchronized的形式都包含在里面

在反编译看看助记符代码

public class syschoriznizedbytecodetest {     Object a;     public static synchronized void test1(){         System.out.println("static synchrozied");     }     public synchronized void test2(){         System.out.println("nostatic method syschronized");     }     public void test3(){         synchronized (a){             System.out.println("code block test");         }     }   } 

我将一部分常量池和方法的一些代码删掉了，我们只看这几个测试方法    public static synchronized void test1();     descriptor: ()V     flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED     Code:       stack=2, locals=0, args_size=0          0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;          3: ldc           #3                  // String static synchrozied          5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V          8: return       LineNumberTable:         line 6: 0         line 7: 8    public synchronized void test2();     descriptor: ()V     flags: ACC_PUBLIC, ACC_SYNCHRONIZED     Code:       stack=2, locals=1, args_size=1          0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;          3: ldc           #5                  // String nostatic method syschronized          5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V          8: return       LineNumberTable:         line 9: 0         line 10: 8       LocalVariableTable:         Start  Length  Slot  Name   Signature             0       9     0  this   Lcom/nettylearn/Service/binarycode/syschoriznizedbytecodetest;    public void test3();     descriptor: ()V     flags: ACC_PUBLIC     Code:       stack=2, locals=3, args_size=1          0: aload_0          1: getfield      #6                  // Field a:Ljava/lang/Object;          4: dup          5: astore_1          6: monitorenter          7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         10: ldc           #7                  // String code block test         12: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V         15: aload_1         16: monitorexit         17: goto          25         20: astore_2         21: aload_1         22: monitorexit         23: aload_2         24: athrow         25: return       Exception table:          from    to  target type              7    17    20   any             20    23    20   any       LineNumberTable:         line 12: 0         line 13: 7         line 14: 15         line 15: 25       LocalVariableTable:         Start  Length  Slot  Name   Signature             0      26     0  this   Lcom/nettylearn/Service/binarycode/syschoriznizedbytecodetest;       StackMapTable: number_of_entries = 2         frame_type = 255 /* full_frame */           offset_delta = 20           locals = [ class com/nettylearn/Service/binarycode/syschoriznizedbytecodetest, class java/lang/Object ]           stack = [ class java/lang/Throwable ]         frame_type = 250 /* chop */           offset_delta = 4 } SourceFile: "syschoriznizedbytecodetest.java"   

我们可以发现前两种方式在字节码上面只是对方法的标示添加了synchronized这个标记，而最后一个代码块的方式我们就可以很清晰的看到它使用了monitorenter和monitorexit两个操作命令。

而且对于monitorexit还使用了两次，这是因为如果我们使用了print的io操作在io操作时可能会发生错误所以系统为我们加了一个额外的monitorexit因为如果出了错误就不会走到我们自己通过synchronized所产的那个monitorexit了，而且这里面有一个goto命令也就是说如果没有错误就会goto到return命令。