ReentrantLock中tryLock的使用

Question

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。线程

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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() + "**");
}

Answer 5

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());
        }
     } 
     }
    }
}