ReentrantLock中tryLock的使用

[microsoftq]

请各位帮忙看看，以下是代码

        static ReentrantLock lock = new ReentrantLock();
	
	public static void main(String[] args) {
		ExecutorService es = Executors.newCachedThreadPool();
		
		es.execute(new Runnable() {
			
			@Override
			public void run() {
				foo("thread1");
			}
		});
		
		es.execute(new Runnable() {
			
			@Override
			public void run() {
				foo("thread2");
			}
		});
	}
	
	private static void foo(String threadName) {
		try {
			lock.tryLock();
			System.out.println(lock.getHoldCount());
			System.out.println(threadName);
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		} finally {
			lock.unlock();
			System.out.println(lock.getHoldCount());
		}
	}

输出信息是：
1
thread1
（此处无等待）
0
thread2
0
Exception in thread "pool-1-thread-2" java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:155)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1260)
at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:460)
at test.reentrantlock.Test.foo(Test.java:43)
at test.reentrantlock.Test.access$0(Test.java:31)
at test.reentrantlock.Test$2.run(Test.java:26)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

为什么线程2会立刻获得锁，而且获得锁之后的HoldCount是0，最后抛出的异常是因为线程1准备释放锁时，它已经不是锁持有者了。

[microsoftq]

各位帮忙看看啊

[deng129253]

你得仔细看看API，你对ReentrantLock理解有问题 private static void foo(String threadName) { if (lock.tryLock()) {//仅在调用时锁为空闲状态才获取该锁。 //如果锁可用，则获取锁，并立即返回值 true。 //如果锁不可用，则此方法将立即返回值 false。 System.out.println(lock.getHoldCount()); System.out.println(threadName); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock();//释放锁在这，不是下面 System.out.println(lock.getHoldCount() + "**"); } } } 线程池也没有关闭，es.shutdown();//必须关闭线程池

[若鱼1919]

改成这个你再试一下： ExecutorService es = Executors.newFixedThreadPool(1);

[若鱼1919]

public int getHoldCount() Queries the number of holds on this lock by the current thread. 因为锁是可重入的，同一个线程可多次获取同一把锁。getHoldCount（）说的就是线程获取了几次锁了

[deng129253]

可以试试用lockInterruptibly方法，它会在无法获取锁时阻塞 lock.lockInterruptibly(); System.out.println(lock.getHoldCount()); System.out.println(threadName); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); System.out.println(lock.getHoldCount() + "**"); }

[若鱼1919]

import java.util.concurrent.*;
import java.util.concurrent.locks.*;
public class ReentrantLockTest{
	static ReentrantLock lock = new ReentrantLock();
     
    public static void main(String[] args) {
        ExecutorService es = Executors.newCachedThreadPool();
         
        es.execute(new Runnable() {
             
            @Override
            public void run() {
                foo("thread1");
            }
        });
         
        es.execute(new Runnable() {
             
            @Override
            public void run() {
                foo("thread2");
            }
        });
    }
     
    private static void foo(String threadName) {
    	while(true){
        	if(lock.tryLock()){
        		try{
	            	System.out.println(Thread.currentThread().getName()+":"+lock.getHoldCount());
		            System.out.println(Thread.currentThread().getName()+":"+threadName);
		            try {Thread.sleep(2000);} catch (InterruptedException e) { e.printStackTrace();}
		            break;
           		}finally {
		            lock.unlock();
		            System.out.println(Thread.currentThread().getName()+":"+lock.getHoldCount());
		        }
    		} 
    	}
    }
}

[qq_26572043]

trylock不锁线程，使用lock就可以了

[O_森_O]

tryLock 方法返回一个boolean值表示是否获取锁成功，你没有对这个返回值做任何判断就直接执行后面的代码，当然会出问题。 改进方法如6L.