鎖升級過程

      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啓動，由於偏向鎖延時4秒開啓，此時尚未開啓偏向鎖，創建的對象鎖狀態為 001，對象為普通對象

    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. 對lock對象加鎖，還未開啓偏向鎖，鎖狀態為 00，輕量級鎖

    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. 線程sleep5秒，開啓偏向鎖

    Thread.sleep(5000);
   

4. 偏向鎖已開啓，此時創建的對象為匿名偏向對象，即對象的鎖狀態為101，包含了偏向鎖標誌位，但是偏向線程id為空

            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. 鎖狀態表示為是10，重量級鎖，偏向鎖直接升級為重量級鎖

    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