Java ReentrantLock 详解

ReentrantLock 和 synchronized 的区别：

1.ReentrantLock 不需要代码块，在开始的时候lock，在结束的时候unlock 就可以了。

但是要注意，lock 是有次数的，如果连续调用了两次lock，那么就需要调用两次unlock。否则的话，别的线程是拿不到锁的。

    /*** 很平常的用锁 没有感觉reentrantlocak 的强大** 如果当前线程已经拿到一个锁了 那么再次调用 会里面返回true* 当前线程的锁数量是 2个**/private void normalUse() {ReentrantLock reentrantLock =  new ReentrantLock();new Thread("thread1"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread1 lock");reentrantLock.lock();reentrantLock.lock();LogToFile.log(TAG,"thread1 getHoldCount:"  + reentrantLock.getHoldCount() );try {LogToFile.log(TAG,"thread1 run");Thread.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}//如果说你的线程占了两个锁 一个没有释放 其他线程都拿不到这个锁//上面lock 了两次 这里需要unLock 两次reentrantLock.unlock();reentrantLock.unlock();LogToFile.log(TAG,"thread1 unlock");}}.start();new Thread("thread2"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread2 lock");reentrantLock.lock();LogToFile.log(TAG,"thread2 run");reentrantLock.unlock();LogToFile.log(TAG,"thread2 unlock");}}.start();}

ReentrantLock trylock()

ReentrantLock trylock 会尝试去拿锁，如果拿得到，就返回true, 如果拿不到就返回false.这个都是立马返回，不会阻塞线程。
如果当前线程已经拿到锁了，那么trylock 会返回true.

    /*** trylock 不管能不能拿到锁 都会里面返回结果 而不是等待* tryLock 会破坏ReentrantLock 的 公平机制* 注意 如果没有拿到锁 执行完代码之后不要释放锁，* 否则会有exception**/private void UseTryLock() {ReentrantLock reentrantLock =  new ReentrantLock();new Thread("thread1"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread1 lock");reentrantLock.lock();try {LogToFile.log(TAG,"thread1 run");Thread.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}reentrantLock.unlock();LogToFile.log(TAG,"thread1 unlock");}}.start();new Thread("thread2"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread2 tryLock");boolean success = reentrantLock.tryLock();LogToFile.log(TAG,"thread2 run");if (success) {reentrantLock.unlock();LogToFile.log(TAG,"thread2 unlock");//    java.lang.IllegalMonitorStateException//        at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:123)//        at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1235)//        at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:429)}}}.start();}

ReentrantLock .tryLock(5, TimeUnit.SECONDS)

ReentrantLock 可以指定，尝试去获取锁，等待多长时间之后，如果还获取不到，那么就放弃。

    /*** tryLock(5, TimeUnit.SECONDS)* 如果5s之后 还拿不到锁  那么就不再堵塞线程，也不去拿锁了 执行执行下面的代码了*/private void UseTryLockWhitTime() {ReentrantLock reentrantLock =  new ReentrantLock();new Thread("thread1"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread1 lock");reentrantLock.lock();try {LogToFile.log(TAG,"thread1 run");Thread.sleep(1000 * 10);} catch (InterruptedException e) {e.printStackTrace();}reentrantLock.unlock();LogToFile.log(TAG,"thread1 unlock");}}.start();new Thread("thread2"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread2 tryLock");boolean success = false;try {success = reentrantLock.tryLock(5, TimeUnit.SECONDS);LogToFile.log(TAG,"thread2 run");if (success) {reentrantLock.unlock();LogToFile.log(TAG,"thread2 unlock");//    java.lang.IllegalMonitorStateException//        at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:123)//        at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1235)//        at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:429)}} catch (InterruptedException e) {e.printStackTrace();}}}.start();}

ReentrantLock .lock();

一旦调用了ReentrantLock .lock() ，如果你的线程被其他线程interrupt，那么你的线程并不会interrupt，而是继续等待锁。

    /*** lockInterruptibly() 和  lock 的区别是* 如果当前线程被Interrupt 了，那么lockInterruptibly 就不再等待锁了。直接会抛出来一个异常给你处理* 如果是lock 的话 就算 线程被其他的线程lockInterrupt 了，不管用，他还会继续等待锁*/private void UseLockIntercepter() {ReentrantLock reentrantLock =  new ReentrantLock();Thread thread = new Thread("thread1") {@Overridepublic void run() {super.run();LogToFile.log(TAG, "thread1 lock");reentrantLock.lock();LogToFile.log(TAG, "thread1 run");if (reentrantLock.getHoldCount() != 0) {reentrantLock.unlock();}LogToFile.log(TAG, "thread1 unlock");}};new Thread("thread2"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread2 tryLock");boolean success = false;try {reentrantLock.lock();thread.start();thread.interrupt();success = reentrantLock.tryLock(5, TimeUnit.SECONDS);LogToFile.log(TAG,"thread2 run");if (success) {reentrantLock.unlock();reentrantLock.unlock();LogToFile.log(TAG,"thread2 unlock");//    java.lang.IllegalMonitorStateException//        at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:123)//        at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1235)//        at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:429)}} catch (InterruptedException e) {e.printStackTrace();}}}.start();}

ReentrantLock .lockInterruptibly();

如果当前线程调用lockInterruptibly 这个方法去获取锁，当其他的线程把你的线程给interrupt 之后，那么你的线程就不再等待锁了，执行下面的代码。

    /*** lockInterruptibly() 和  lock 的区别是* 如果当前线程被Interrupt 了，那么lockInterruptibly 就不再等待锁了。直接会抛出来一个异常给你处理* 如果是lock 的话 就算 线程被其他的线程lockInterrupt 了，不管用，他还会继续等待锁*/private void UseLockIntercepter2() {ReentrantLock reentrantLock =  new ReentrantLock();Thread thread = new Thread("thread1") {@Overridepublic void run() {super.run();LogToFile.log(TAG, "thread1 lock");try {reentrantLock.lockInterruptibly();} catch (InterruptedException e) {e.printStackTrace();LogToFile.log(TAG, "thread1 lock InterruptedException ");}LogToFile.log(TAG, "thread1 run");if (reentrantLock.getHoldCount() != 0) {reentrantLock.unlock();}LogToFile.log(TAG, "thread1 unlock");}};new Thread("thread2"){@Overridepublic void run() {super.run();LogToFile.log(TAG,"thread2 tryLock");boolean success = false;try {reentrantLock.lock();thread.start();thread.interrupt();success = reentrantLock.tryLock(5, TimeUnit.SECONDS);LogToFile.log(TAG,"thread2 run");if (success) {reentrantLock.unlock();reentrantLock.unlock();LogToFile.log(TAG,"thread2 unlock");//    java.lang.IllegalMonitorStateException//        at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:123)//        at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1235)//        at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:429)}} catch (InterruptedException e) {e.printStackTrace();}}}.start();}

ReentrantLock newCondition()使用：

condition. 的await 相当于 object 的wait 方法，就是等待其他线程唤醒，但是调用之前必须要reentrantLock.lock(); 一下，不然会抛异常。

    /*** tryLock(5, TimeUnit.SECONDS)* 如果5s之后 还拿不到锁  那么就不再堵塞线程，也不去拿锁了 执行执行下面的代码了*/private void UseCondition() {ReentrantLock reentrantLock =  new ReentrantLock();Condition condition = reentrantLock.newCondition();Thread thread = new Thread("thread1") {@Overridepublic void run() {super.run();try {reentrantLock.lock();LogToFile.log(TAG, "thread1 wait");//await 其实是会释放锁的 其他的线程可以拿到锁 然后signal  不然的话岂不是要卡死？condition.await();LogToFile.log(TAG, "thread1 run");Thread.sleep(1000 * 3);condition.signalAll();reentrantLock.unlock();} catch (InterruptedException e) {e.printStackTrace();}LogToFile.log(TAG, "thread1 unlock");}};thread.start();Thread thread1 = new Thread("thread2") {@Overridepublic void run() {super.run();//调用signal 之前一定要lockreentrantLock.lock();//就算没有人调用await 那么signal 方法也不会有什么问题condition.signal();try {LogToFile.log(TAG, "thread2 wait");condition.await();LogToFile.log(TAG, "thread2 run");} catch (InterruptedException e) {e.printStackTrace();}reentrantLock.unlock();}};thread1.start();}

Object wait方法使用：

wait 方法调用之前也要synchronized 一个锁。

    /*** 使用wait*/private void UseWait() {ReentrantLock reentrantLock =  new ReentrantLock();Condition condition = reentrantLock.newCondition();new Thread("thread1"){@Overridepublic void run() {super.run();try {synchronized (condition) {LogToFile.log(TAG,"thread1 wait");condition.wait();}LogToFile.log(TAG,"thread1 run");Thread.sleep(1000 * 10);synchronized (condition) {condition.notifyAll();}} catch (InterruptedException e) {e.printStackTrace();}LogToFile.log(TAG,"thread1 unlock");}}.start();new Thread("thread2"){@Overridepublic void run() {super.run();synchronized (condition) {condition.notify();}try {LogToFile.log(TAG,"thread2 wait");synchronized (condition){condition.wait();}LogToFile.log(TAG,"thread2 run");} catch (InterruptedException e) {e.printStackTrace();}}}.start();}

synchronized

synchronized 锁等待的线程，当被其他线程interrupt 的时候，并不会停止。

/*** synchronized 的锁，不能被中断*/
public void testSyncInter(){Object lock = new Object();Thread thread = new Thread("thread1") {@Overridepublic void run() {super.run();synchronized (lock) {LogToFile.log(TAG, "thread1 lock");LogToFile.log(TAG, "thread1 run");}LogToFile.log(TAG, "thread1 unlock");}};new Thread("thread2"){@Overridepublic void run() {super.run();synchronized (lock) {LogToFile.log(TAG,"thread2 start");thread.start();thread.interrupt();}LogToFile.log(TAG,"thread2 run");}}.start();
}

BlockingQueue 就是使用ReentrantLock 实现的阻塞队列：

    /*** BlockingQueue  会阻塞当前的线程  等着其他线程放数据*/public void testArrayBlockQueue(){BlockingQueue<Integer> blockingQueue = new ArrayBlockingQueue<>(4);new Thread("thread1"){@Overridepublic void run() {super.run();try {LogToFile.log(TAG,"thread1 take");blockingQueue.take();LogToFile.log(TAG,"thread1 take  get");} catch (InterruptedException e) {e.printStackTrace();}}}.start();new Thread("thread2"){@Overridepublic void run() {super.run();try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}try {LogToFile.log(TAG,"thread2 put");//put 会阻塞 但是add 不会blockingQueue.put(2);} catch (InterruptedException e) {e.printStackTrace();}}}.start();}

ArrayBlockingQueue 源码解析：

下面的代码，可以看到ArrayBlockingQueue 创建了ReentrantLock ，以及两个condition

/** Main lock guarding all access */final ReentrantLock lock;/** Condition for waiting takes */private final Condition notEmpty;/** Condition for waiting puts */private final Condition notFull;public ArrayBlockingQueue(int capacity, boolean fair) {if (capacity <= 0)throw new IllegalArgumentException();this.items = new Object[capacity];lock = new ReentrantLock(fair);notEmpty = lock.newCondition();notFull =  lock.newCondition();}

当我们往里面放元素的时候，我们会对notFull 进行等待，因为队列不是空的了。

当出队列的时候，notFull.signal(); 也就是当前的队列不是满的了，可以放元素进去了。

    /*** Inserts the specified element at the tail of this queue, waiting* for space to become available if the queue is full.** @throws InterruptedException {@inheritDoc}* @throws NullPointerException {@inheritDoc}*/public void put(E e) throws InterruptedException {checkNotNull(e);final ReentrantLock lock = this.lock;lock.lockInterruptibly();try {while (count == items.length)notFull.await();enqueue(e);} finally {lock.unlock();}}private E dequeue() {// assert lock.getHoldCount() == 1;// assert items[takeIndex] != null;final Object[] items = this.items;@SuppressWarnings("unchecked")E x = (E) items[takeIndex];items[takeIndex] = null;if (++takeIndex == items.length)takeIndex = 0;count--;if (itrs != null)itrs.elementDequeued();notFull.signal();return x;}

当我们往里面取元素的时候，如果队列是空的了，那么notEmpty 就要等待，当enqueue一个元素的时候，notEmpty 不是空就唤醒，那么take 就可以执行去拿元素了。

    public E take() throws InterruptedException {final ReentrantLock lock = this.lock;lock.lockInterruptibly();try {while (count == 0)notEmpty.await();return dequeue();} finally {lock.unlock();}}private void enqueue(E x) {// assert lock.getHoldCount() == 1;// assert items[putIndex] == null;final Object[] items = this.items;items[putIndex] = x;if (++putIndex == items.length)putIndex = 0;count++;notEmpty.signal();}