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1.偏向锁竞争

假设一种情况A线程获取偏向锁，并且A线程获取锁后并没有释放锁，此时B线程争抢偏向锁，这次算是真正的竞争锁了，那么B线程的锁状态是什么样子的？举个例子：

public class SyncSyncLockRelease1 {
    public static void main(String[] args) throws InterruptedException {
        Object obj = new Object();
        System.out.println("====A 加锁前===" + ClassLayout.parseInstance(obj).toPrintable());
        Thread A = new Thread() {
            @SneakyThrows
            @Override
            public void run() {
                synchronized (obj) {
                    System.out.println("===A 加锁中===" + ClassLayout.parseInstance(obj).toPrintable());
                    Thread.sleep(2000);
                }
            }
        };
        A.start();
        Thread.sleep(500);
        System.out.println("====B加锁前===" + ClassLayout.parseInstance(obj).toPrintable());
        Thread B = new Thread() {
            @SneakyThrows
            @Override
            public void run() {
                synchronized (obj) {
                    System.out.println("====B加锁中===" + ClassLayout.parseInstance(obj).toPrintable());
                    Thread.sleep(1000);
                }
            }
        };
        B.start();
    }
}

执行效果如下：

====A 加锁前===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

===A 加锁中===java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 98 54 20 (00000101 10011000 01010100 00100000) (542414853)
      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

====B加锁前===java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 98 54 20 (00000101 10011000 01010100 00100000) (542414853)
      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

====B加锁中===java.lang.Object object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           ea 33 b8 1b (11101010 00110011 10111000 00011011) (465056746)
      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


Process finished with exit code 0

跟之前一样，我们一块一块分析：

我们可以看到A线程加锁前锁对象是偏向锁，并且并没有记录线程id等信息，锁对象位全都是0，

当A线程进入同步代码块后，此时锁为偏向锁，并且记录了线程id。

此时A线程还处于睡眠中并没有释放锁，B线程已经启动了，B线程开始竞争前，我们发现此时锁还是偏向锁，并且记录的是A线程id。

注意此时A线程还在睡眠并没有释放锁，B线程开始真正的锁竞争，我们可以看到此时的锁对象后三位是010，代表的重量级锁！

2.总结

A线程获取偏向锁，并且A线程没有释放偏向锁（），还在syhnc的代码块里边。B线程此时过来争抢偏向锁，会直接升级为重量级锁。

并发编程（十七）Synchronized锁升级-偏向锁(下)-例3

1.偏向锁竞争

2.总结