主要通过代码，观察synchronized时，锁升级，markword的变化

借助工具

1. jol（参考：https://www.jianshu.com/p/1b370f36777f）

2. markword结构图

   
   
   markword结构图

代码

public class BiasedLock {

    public static void main(String[] args) throws Exception {
        System.out.println(org.openjdk.jol.vm.VM.current().details());

        // 必要，不然不会观察到偏向锁，去掉的话，会观察到直接由无锁到轻量级锁
        TimeUnit.SECONDS.sleep(5);

        Object obj = new Object();
        System.out.println(ClassLayout.parseInstance(obj).toPrintable());

        lock(obj);

        for (int i = 0; i < 3; i++) {
            new Thread(() -> lock(obj)).start();
        }

    }

    private static void lock(Object obj) {
        synchronized (obj) {
            System.out.println(ClassLayout.parseInstance(obj).toPrintable());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

观察结果

object header中
00000101 00000000 00000000 00000000 （偏向锁）
->
00110000 11101101 10111101 01011011 （轻量级锁）
->
11011010 11000101 00111000 01010111 （重量级锁）

# Running 64-bit HotSpot VM.
# Using compressed oop with 0-bit shift.
# Using compressed klass with 0-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

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)                           28 0f 9f 16 (00101000 00001111 10011111 00010110) (379522856)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 e0 40 02 (00000101 11100000 01000000 00000010) (37806085)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           28 0f 9f 16 (00101000 00001111 10011111 00010110) (379522856)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           30 ed bd 5b (00110000 11101101 10111101 01011011) (1539173680)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           28 0f 9f 16 (00101000 00001111 10011111 00010110) (379522856)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           da c5 38 57 (11011010 11000101 00111000 01010111) (1463338458)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           28 0f 9f 16 (00101000 00001111 10011111 00010110) (379522856)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           da c5 38 57 (11011010 11000101 00111000 01010111) (1463338458)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           28 0f 9f 16 (00101000 00001111 10011111 00010110) (379522856)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total