为什么以下代码导致死锁

Question

我有以下课程

public class LockTester implements Runnable{
 private static Locker locker = new Locker();

 public static void main(String[] args){
  for(int i=0;i<10;i++){
   Thread t = new Thread(new LockTester());
   t.start();
  }
 }

 public void run(){
   for(int i=0;i<1000;i++){
    locker.unlockFirst();//note unlocking here
    locker.lockFirst();
    locker.lockSecond();
    locker.unlockSecond();
    locker.unlockFirst();
  }
 }
}

和洛克班

public class Locker{
 private Lock lock1 = new ReentrantLock();
 private Lock lock2 = new ReentrantLock();

 public void lockFirst(){
  lock1.lock();
 }
 public void lockSecond(){
  lock2.lock();
 }
 public void unlockFirst(){
  if(lock1.tryLock()){//note change
   lock1.unlock();
  }
 }
 public void unlockSecond(){
  lock2.unlock();
 }
}

为什么运行此代码会导致死锁.

Answer 1

lock1被锁定两次:一次进入lockFirst和再次进入unlockFirst(lock1.tryLock()),但仅解锁一次unlockFirst.

ReentrantLock有一个保留计数.请参阅ReentrantLock.如果你调用tryLock,即使它已经被当前线程保持,它仍然会增加保持计数.所以,你增加两次,但只减少一次.