锁状态标识位

锁升级过程

      public static void main(String[] args) throws Exception {
            Object lock = new Object();
            System.out.println("A--->" + ClassLayout.parseInstance(lock).toPrintable());
    
            synchronized (lock) {
                System.out.println("B--->" + ClassLayout.parseInstance(lock).toPrintable());
            }
    
            Thread.sleep(5000);
    
            Object object = new Object();
            System.out.println("C--->" + ClassLayout.parseInstance(object).toPrintable());
    
            new Thread(() -> {
                synchronized (object) {
                    System.out.println("D--->" + ClassLayout.parseInstance(object).toPrintable());
                }
            }).start();
    
            synchronized (object) {
                System.out.println("E--->" + ClassLayout.parseInstance(object).toPrintable());
            }
        }

1. Jvm启动，由于偏向锁延时4秒开启，此时尚未开启偏向锁，创建的对象锁状态为 001，对象为普通对象

    Object lock = new Object();
    System.out.println("A--->" + ClassLayout.parseInstance(lock).toPrintable());
   
    A--->java.lang.Object object internals:
    OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
   
   

2. 对lock对象加锁，还未开启偏向锁，锁状态为 00，轻量级锁

    synchronized (lock) {
         System.out.println("B--->" + ClassLayout.parseInstance(lock).toPrintable());
    }
    B--->java.lang.Object object internals:
    OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           98 ff db 0c (10011000 11111111 11011011 00001100) (215744408)
          4     4        (object header)                           00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
          8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
   

3. 线程sleep5秒，开启偏向锁

    Thread.sleep(5000);
   

4. 偏向锁已开启，此时创建的对象为匿名偏向对象，即对象的锁状态为101，包含了偏向锁标志位，但是偏向线程id为空

            Object object = new Object();
    System.out.println("C--->" + ClassLayout.parseInstance(object).toPrintable());
            
            C--->java.lang.Object object internals:
    OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
   

5. 锁状态表示为是10，重量级锁，偏向锁直接升级为重量级锁

    new Thread(() -> {
       synchronized (object) {
           System.out.println("D--->" + ClassLayout.parseInstance(object).toPrintable());
       }
    }).start();
    
    D--->java.lang.Object object internals:
    OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           ea d7 02 ec (11101010 11010111 00000010 11101100) (-335357974)
          4     4        (object header)                           d8 7f 00 00 (11011000 01111111 00000000 00000000) (32728)
          8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total