Upgrade to heavyweight lock, lock reentrancy will lead to lock release?

目录

前言

实验

如何验证

流程说明

main

first

reentrant

结果

总结

---

前言

近日,看到有些文章说：“产生竞争时,轻量级锁升级为重量级锁,When a thread lock CAS 操作(锁重入) 会失败,So will release lock”

首先,A lock threads might even not synchronized block completes for no release the lock,怎么想都不合理

The next experiment will certify that the above “提前释放锁” Conclusion the mistake

实验

如何验证

After lock to heavyweight lock,If it releases the lock,So lock reentrant will surely fail;So only need to verify after upgrading,Reentrant lock does not block can be

流程说明

main

    public static void main(String[] args) throws InterruptedException {
        new Thread(() -> {
            try {
                // Thread0 In method of acquiring a lock for the first time
                first();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();

        Thread.sleep(5000);

        new Thread(() -> {
            log.debug("---------------------------Thread1 The first attempt to lock---------------------------");
            synchronized (lock){
                log.debug(ClassLayout.parseInstance(lock).toPrintable());
            }
        }).start();
    }

1. 创建 Thread0,Perform include acquiring a lock operation method
2. 主线程睡眠 5s,确保 Thread0 先执行,先获取锁,And at this time for lightweight lock
3. 创建 Thread1,尝试获取锁,Produce lock competition,导致锁升级

first

    static void first() throws InterruptedException {
        log.debug("---------------------------Thread0 The first attempt to get the lock---------------------------");
        synchronized(lock){
            log.debug("Thread0 首次获取锁成功！");
            log.debug(ClassLayout.parseInstance(lock).toPrintable());
            Thread.sleep(10000);
            // 第二次获取锁（锁重入）
            reentrant();
        }
    }

1. Thread0 首次获取锁,Print lock object object head
2. 先睡眠 10s（Thead1 During this period was created and attempts to acquire lock,Causing a lock escalation）
3. 然后调用 reentrant() 方法,Into this method will perform a acquiring a lock operation again

reentrant

    static void reentrant(){
        log.debug("---------------------------Thread0 Try to lock reentrant---------------------------");
        synchronized(lock){
            log.debug("Thread0 锁重入成功！");
            log.debug(ClassLayout.parseInstance(lock).toPrintable());
        }
    }

1. Thread0 尝试再次获取锁（锁重入）
2. If not blocked,则会执行打印,To verify the conclusion of the mistake
3. 若阻塞,On behalf of the lock is released and by other threads to obtain,That conclusion is right

结果

• 红色框：表示 Thread0 的时间线
• 蓝色框：表示 Thread1 的时间线
• 紫色框：表示锁的状态

时间线：

第 26s 主线程创建 Thread0 并启动,然后进入 5s 睡眠;Thread0 Try to acquiring a lock for the first time,成功了,锁状态 00 代表轻量级锁,然后进入 10s 睡眠

5s 之后的第 31s,主线程创建 Thread1 并启动;Thread1 Try to acquiring a lock for the first time,由于 Thread0 Is still in sleep,And not releasing the lock,This time the lock competition,锁升级为重量级锁;Thread1 获取锁失败,进入阻塞

10s 之后的第 36s,Thread0 Perform lock reentrant,成功了,锁状态 10 Representatives have upgraded to a heavyweight lock;Thread0 释放锁,Thread1 Will be awakened from blocking,并且获取锁成功

总结

实验结果表明,Even if the lock has been upgraded to a heavyweight lock,Weighing a lock thread lock into the operation is not happening,So nonexistent also releases the lock you say

Lock has been upgraded,But the thread lock may do not know,本人猜测：When reentrant lock or unlock found has been upgraded to heavyweight lock,And the next is not to lock the object first perform CAS 操作,But for it to monitor 对象中的 owner 属性执行 CAS 操作

(补充：轻量级锁,Lock reentrant perform CAS 操作都会失败,But it will only generate lock records,Rather than direct releases the lock)

并发编程 synchronized (五) 重量级锁、轻量级锁_Pipi Yang233的博客-CSDN博客_synchronized轻量级锁 重量级锁