如何判断是否当前线程持有锁

绝地灬酷狼 2022-11-18 01:57 160阅读 0赞

# 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();
            }
        }
    }