偏向锁在什么条件下会升级为轻量级锁？

问题描述

synchronized锁升级时，偏向锁在什么条件下会升级为轻量级锁？

背景信息

Java版本为java11。

代码

▼
(java)
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import org.openjdk.jol.info.ClassLayout;

public class BiasedLockThreadIdCheck {

    static final Object lock = new Object();

    public static void main(String[] args) throws Exception {

        System.out.println("Before any lock:");
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());

        Thread t1 = new Thread(() -> {
            System.out.println("T1 ID: " + Thread.currentThread().getId());
            synchronized (lock) {
                System.out.println("T1 获得锁后:");
                System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            }
        });

        t1.start();
        t1.join();

        // 注释下列代码前后，t2获得的锁类型不同！
        // Thread.sleep(5000);

        Thread t2 = new Thread(() -> {
            System.out.println("T2 ID: " + Thread.currentThread().getId());
            synchronized (lock) {
                System.out.println("T2 获得锁后:");
                System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            }
        });

        t2.start();
        t2.join();
    }
}

结果

中间没有sleep，t2获取到的是偏向锁，但是偏向的仍然是线程t1：

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text
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Before any lock:
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

T1 ID: 27
T1 获得锁后:
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000001fea4a2e005 (biased: 0x000000007fa928b8; epoch: 0; age: 0)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

T2 ID: 28
T2 获得锁后:
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000001fea4a2e005 (biased: 0x000000007fa928b8; epoch: 0; age: 0)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

中间加了sleep之后（sleep时间相对要长一些），t2获取到的是轻量级锁：

▼
text
复制代码
Before any lock:
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

T1 ID: 27
T1 获得锁后:
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000001487e9b1005 (biased: 0x00000000521fa6c4; epoch: 0; age: 0)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

T2 ID: 28
T2 获得锁后:
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00000011a29ff080 (thin lock: 0x00000011a29ff080)
  8   4        (object header: class)    0x00001000
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

困惑

很奇怪啊，就在写这个贴子的时候，中间没有sleep，有两次t2获取到的也是轻量级锁。所以偏向锁升级为轻量级锁的过程究竟是怎样的。