如何判断是否当前线程持有锁

背景

有时候我们必须知道是否当前线程持有锁，怎么知道

• 如果用synchronized，用Thread.holdsLock(lockObj) 获取
• 如果使用 Lock（juc下的），则用 lock.isHeldByCurrentThread() (不能用 Thread.holdsLock(lockObj))

注意 Lock 接口本身是没有 isHeldByCurrentThread 接口的，必须使用ReentrantLock、ReentrantWriteLock这些实现类。

注意

• ReentrantReadLock 没有 isHeldByCurrentThread()。因为ReentrantReadLock作为读锁根本就不需要获得锁从而没有这个方法

• ReentrantReadWriteLock 也没有这个方法

代码验证

package com.wyf.test.is_cur_thread_hold_lock;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 判断当前线程是否持有锁
 * 结论:
 * 只有使用 synchronized 的时候,这个 Thread.holdsLock(lockObj) 才能否正确判断,在 Lock 里总是返回false(而且你也不知道holdsLock的入参传什么,holdsLock的
 * 参数其实就是synchronized的锁对象,所以Lock里调用 Thread.holdsLock(lockObj) 根本就讲不通
 */
public class IsCurThreadHoldLock {
    public static void main(String[] args) {
        testSynchronized();// 通过 Thread.holdsLock(lockObj) 检查当前线程是否持有锁对象
        //testLock();// Lock 不能通过 Thread.holdsLock(lockObj)
        //testLockByIsHeldByCurrentThread();// 可以通过 ReentrantLock#isHeldByCurrentThread() 来判断
        //testLockByIsHeldByCurrentThread2();// 可以通过 ReentrantWriteLock#isHeldByCurrentThread() 来判断
    }


    private static void testSynchronized() {
        Object lockObj = new Object();
        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + ",before:" + Thread.holdsLock(lockObj));
            synchronized (lockObj) {
                System.out.println(Thread.currentThread().getName() + ",in:" + Thread.holdsLock(lockObj));
                sleep(5);
            }
            System.out.println(Thread.currentThread().getName() + ",after:" + Thread.holdsLock(lockObj));
        }, "t0").start();

        sleep(1);

        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + ",before:" + Thread.holdsLock(lockObj));
            sleep(1);
        }, "t1").start();

    }


    private static void testLock() {
        Lock lock = new ReentrantLock();
        new Thread(() -> {
            // 传 lock 到 Thread.holdsLock 方法里是很荒谬的
            System.out.println(Thread.currentThread().getName() + ",before:" + Thread.holdsLock(lock));
            try {
                lock.lock();
                System.out.println(Thread.currentThread().getName() + ",in:" + Thread.holdsLock(lock));
                sleep(5);
            } finally {
                lock.unlock();
            }
            System.out.println(Thread.currentThread().getName() + ",after:" + Thread.holdsLock(lock));
        }, "t0").start();

        sleep(1);

        new Thread(() -> {
            try {
                System.out.println(Thread.currentThread().getName() + ",before:" + Thread.holdsLock(lock));
                sleep(1);
            } finally {

            }

        }, "t1").start();
    }


    private static void testLockByIsHeldByCurrentThread() {
        // 必须使用具体的实现类, Lock 接口里没有 isHeldByCurrentThread 方法
        ReentrantLock lock = new ReentrantLock();
        new Thread(() -> {
            // 传 lock 到 Thread.holdsLock 方法里是很荒谬的
            System.out.println(Thread.currentThread().getName() + ",before:" + lock.isHeldByCurrentThread());
            try {
                lock.lock();
                System.out.println(Thread.currentThread().getName() + ",in:" + lock.isHeldByCurrentThread());
                sleep(5);
            } finally {
                lock.unlock();
            }
            System.out.println(Thread.currentThread().getName() + ",after:" + lock.isHeldByCurrentThread());
        }, "t0").start();

        sleep(1);

        new Thread(() -> {
            try {
                System.out.println(Thread.currentThread().getName() + ",before:" + lock.isHeldByCurrentThread());
                sleep(1);
            } finally {

            }

        }, "t1").start();
    }

    private static void testLockByIsHeldByCurrentThread2() {
        // 必须使用具体的实现类, Lock 接口里没有 isHeldByCurrentThread 方法
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        ReentrantReadWriteLock.ReadLock readLock = lock.readLock();// 没有 isHeldByCurrentThread接口,读锁不需要获得锁,所以就没有
        ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();
        new Thread(() -> {
            // 传 lock 到 Thread.holdsLock 方法里是很荒谬的
            System.out.println(Thread.currentThread().getName() + ",before:" + writeLock.isHeldByCurrentThread());
            try {
                writeLock.lock();
                System.out.println(Thread.currentThread().getName() + ",in:" + writeLock.isHeldByCurrentThread());
                sleep(5);
            } finally {
                writeLock.unlock();
            }
            System.out.println(Thread.currentThread().getName() + ",after:" + writeLock.isHeldByCurrentThread());
        }, "t0").start();

        sleep(1);

        new Thread(() -> {
            try {
                System.out.println(Thread.currentThread().getName() + ",before:" + writeLock.isHeldByCurrentThread());
                sleep(1);
            } finally {

            }

        }, "t1").start();
    }

    private static void sleep(int secs) {
        try {
            TimeUnit.SECONDS.sleep(secs);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}