java中的多线程Reetrant锁
我不熟悉java中的多线程。我试着用锁。这是我的代码示例
package com;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class UsingLocks {
Lock lock = new ReentrantLock();
public static void main(String[] args) {
// TODO Auto-generated method stub
UsingLocks job = new UsingLocks();
Thread [] threads= new Thread[5];
for(int i=0;i<5;i++){
threads[i]= new Thread(new LockTask(job));
}
for(int i=0;i<5;i++){
threads[i].start();
}
}
public void lockingJob() {
System.out.println("Thread "+Thread.currentThread().getName()+" trying to Acquire lock");
try {
lock.tryLock();
//lock.lock(); //When I use this, code works fine
int time=new Random().nextInt(10)+3;
System.out.println("Thread "+Thread.currentThread().getName()+" Acquired lock for "+time+" seconds.");
TimeUnit.SECONDS.sleep(time);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Now releasing lock "+Thread.currentThread().getName());
lock.unlock();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("After Unlock "+Thread.currentThread().getName());
}
}
class LockTask implements Runnable{
UsingLocks job;
public LockTask(UsingLocks job) {
// TODO Auto-generated constructor stub
this.job=job;
}
@Override
public void run() {
// TODO Auto-generated method stub
job.lockingJob();
}
}
下面是我使用tryLock()时的输出
Thread Thread-1 trying to Acquire lock
Thread Thread-0 trying to Acquire lock
Thread Thread-2 trying to Acquire lock
Thread Thread-1 Acquired lock for 12 seconds.
Thread Thread-2 Acquired lock for 3 seconds.
Thread Thread-0 Acquired lock for 8 seconds.
Thread Thread-3 trying to Acquire lock
Thread Thread-3 Acquired lock for 9 seconds.
Thread Thread-4 trying to Acquire lock
Thread Thread-4 Acquired lock for 6 seconds.
Now releasing lock Thread-2
Exception in thread "Thread-2" java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:155)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1260)
at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:460)
at com.UsingLocks.lockingJob(UsingLocks.java:37)
at com.LockTask.run(UsingLocks.java:66)
at java.lang.Thread.run(Thread.java:745)
Now releasing lock Thread-4
Exception in thread "Thread-4" java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.ReentrantLock$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 com.UsingLocks.lockingJob(UsingLocks.java:37)
at com.LockTask.run(UsingLocks.java:66)
at java.lang.Thread.run(Thread.java:745)
现在,根据我的理解,当第一个线程执行tryLock()时,它应该获得锁,而其他线程不应该能够获得锁。但正如输出所示。线程1获得锁后,线程2也获得锁,依此类推。这怎么可能呢。请告诉我这里缺少什么。 提前谢谢
# 1 楼答案
ReentrantLock#tryLock()
-仅当调用时锁未被另一个线程持有时才获取锁&;如果成功,则返回true,否则返回false如果我们看到堆栈跟踪-
更清楚的是,无法获得锁的线程在尝试释放锁时引发异常
# 2 楼答案
调用tryLock()时,只返回一个布尔值,它在下一行/语句(如果有)是否执行时具有无方向性“。 这就是为什么tryLock()应该被用作检查执行某些活动/代码块的原因,这些活动/代码块需要同步。希望这能回答你的问题
# 3 楼答案
原因是,如果锁已经被另一个线程持有,tryLock从不阻塞
下面是关于tryLock()的文档
仅当调用时锁未被另一个线程持有时才获取锁
如果锁未被另一个线程持有,则获取锁,并立即返回值true,将锁持有计数设置为1。即使将该锁设置为使用公平排序策略,调用tryLock()也会立即获取该锁(如果该锁可用),无论当前是否有其他线程正在等待该锁。这种“讨价还价”行为在某些情况下是有用的,即使它破坏了公平性。如果您希望遵守此锁的公平性设置,则使用几乎相等的tryLock(0,TimeUnit.SECONDS)(它还检测中断)
如果当前线程已经持有该锁,那么持有计数将增加1,并且该方法返回true
如果锁由另一个线程持有,则此方法将立即返回值false强>
# 4 楼答案
我对你的代码做了一些修改,并亲自尝试了一下
我找到了那个可重入锁。不管我做了什么,tryLock都会抛出一个非法的MonitorStateException。我认为这不合适
# 5 楼答案
试着做些这样的事情,让它更清楚