Synchronized代码小测试

做本实验需要引入坐标

        <dependency>
            <groupId>org.openjdk.jol</groupId>
            <artifactId>jol-core</artifactId>
            <version>0.9</version>
        </dependency>

 

先看一下我的代码

public static void main(String[] args) throws InterruptedException {
        Object lightObject = new Object();
        try {
            Thread.sleep(6000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Object biasedLockObject = new Object();
        System.out.println("---------------------------------------加锁前---------------------------------------");
        System.out.println("偏向锁：" + ClassLayout.parseInstance(biasedLockObject).toPrintable() + "\n轻量级锁：" + ClassLayout.parseInstance(lightObject).toPrintable());
        System.out.println("---------------------------------------加锁后---------------------------------------");
//        synchronized (biasedLockObject) {
//            System.out.println("偏向锁：" + ClassLayout.parseInstance(biasedLockObject).toPrintable());
//        }
        synchronized (lightObject) {
            System.out.println("轻量级锁第一次：" + ClassLayout.parseInstance(lightObject).toPrintable());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
        }
//        System.out.println("---------------------------------------释放锁---------------------------------------");
//        System.out.println("偏向锁：" + ClassLayout.parseInstance(biasedLockObject).toPrintable() + "\n轻量级锁：" + ClassLayout.parseInstance(lightObject).toPrintable());
    
        for (int i=0;i<10;i++) {
            Thread thread = new Thread(new Runnable() {

                @Override
                public void run() {
//                    synchronized (biasedLockObject) {
//                        System.out.println("竞争偏向锁：" + ClassLayout.parseInstance(biasedLockObject).toPrintable());
//                    }
//                    
//                    System.out.println("竞争偏向锁释放：" + ClassLayout.parseInstance(biasedLockObject).toPrintable());
                  synchronized (lightObject) {
                  System.out.println("竞争轻量级锁：" + ClassLayout.parseInstance(lightObject).toPrintable());
              }
              
//              System.out.println("竞争偏向锁释放：" + ClassLayout.parseInstance(biasedLockObject).toPrintable());
              }
                
            });
            thread.start();
        }
        
        Thread.sleep(5000);
        System.out.println("轻量级锁释放后：" + ClassLayout.parseInstance(lightObject).toPrintable());
    }

代码很好懂，就是开10个线程去竞争 lightObject

  1 --------------------------------------加锁前---------------------------------------
  2 偏向锁：java.lang.Object object internals:
  3  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
  4       0     4        (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
  5       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
  6       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
  7      12     4        (loss due to the next object alignment)
  8 Instance size: 16 bytes
  9 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 10 
 11 轻量级锁：java.lang.Object object internals:
 12  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 13       0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
 14       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 15       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 16      12     4        (loss due to the next object alignment)
 17 Instance size: 16 bytes
 18 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 19 
 20 ---------------------------------------加锁后---------------------------------------
 21 轻量级锁第一次：java.lang.Object object internals:
 22  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 23       0     4        (object header)                           50 f3 9d 03 (01010000 11110011 10011101 00000011) (60683088)
 24       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 25       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 26      12     4        (loss due to the next object alignment)
 27 Instance size: 16 bytes
 28 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 29 
 30 竞争轻量级锁：java.lang.Object object internals:
 31  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 32       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 33       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 34       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 35      12     4        (loss due to the next object alignment)
 36 Instance size: 16 bytes
 37 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 38 
 39 竞争轻量级锁：java.lang.Object object internals:
 40  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 41       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 42       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 43       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 44      12     4        (loss due to the next object alignment)
 45 Instance size: 16 bytes
 46 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 47 
 48 竞争轻量级锁：java.lang.Object object internals:
 49  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 50       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 51       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 52       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 53      12     4        (loss due to the next object alignment)
 54 Instance size: 16 bytes
 55 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 56 
 57 竞争轻量级锁：java.lang.Object object internals:
 58  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 59       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 60       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 61       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 62      12     4        (loss due to the next object alignment)
 63 Instance size: 16 bytes
 64 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 65 
 66 竞争轻量级锁：java.lang.Object object internals:
 67  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 68       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 69       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 70       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 71      12     4        (loss due to the next object alignment)
 72 Instance size: 16 bytes
 73 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 74 
 75 竞争轻量级锁：java.lang.Object object internals:
 76  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 77       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 78       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 79       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 80      12     4        (loss due to the next object alignment)
 81 Instance size: 16 bytes
 82 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 83 
 84 竞争轻量级锁：java.lang.Object object internals:
 85  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 86       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 87       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 88       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 89      12     4        (loss due to the next object alignment)
 90 Instance size: 16 bytes
 91 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 92 
 93 竞争轻量级锁：java.lang.Object object internals:
 94  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
 95       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
 96       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
 97       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
 98      12     4        (loss due to the next object alignment)
 99 Instance size: 16 bytes
100 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
101 
102 竞争轻量级锁：java.lang.Object object internals:
103  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
104       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
105       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
106       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
107      12     4        (loss due to the next object alignment)
108 Instance size: 16 bytes
109 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
110 
111 竞争轻量级锁：java.lang.Object object internals:
112  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
113       0     4        (object header)                           ca 83 73 18 (11001010 10000011 01110011 00011000) (410223562)
114       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
115       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
116      12     4        (loss due to the next object alignment)
117 Instance size: 16 bytes
118 Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
119 
120 轻量级锁释放后：java.lang.Object object internals:
121  OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
122       0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
123       4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
124       8     4        (object header)                           e5 21 e4 27 (11100101 00100001 11100100 00100111) (669262309)
125      12     4        (loss due to the next object alignment)

 

结论

　　1 jvm启动的前四秒内创建的对象初始状态就是  00000001  无锁状态

　　2 第一次竞争 对应23行  01010000 　　　　　　　　　　 轻量级锁

　　3 多线程竞争开始 从30行到112行锁的类型都是重量级锁   11001010

　　4 最后锁释放后  120行                                      00000001  无锁状态

　　从实验结果能看得出来，锁是会降级的，而不是网络上好多人说的，锁不会降级。另外，再次强调下，无锁绝对不会到偏向锁。

posted on 2020-10-31 10:17  MaXianZhe  阅读(127)  评论(0)    收藏  举报

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