并发编程学习总结(六) ：java 显式锁ReentrantLock使用详解之测试锁与超时

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并发编程学习总结(三) ：线程的中断详解

我在上面的这篇博客中，曾经写到过线程在获取锁的过程中是无法中断的。

但是ReentrantLock提供了tryLock()、tryLock(long timeout, TimeUnit unit)、lock.lockInterruptibly()

tryLock() 方法试图申请一个锁，在成功获得锁后返回true，否则，立即返回false,而且线程可以立即离开去做其他的事情。

tryLock(long timeout, TimeUnit unit) 是一个具有超时参数的尝试申请锁的方法，阻塞时间不会超过给定的值；如果成功则返回true

lockInterruptibly() 获得锁，但是会不确定地发生阻塞。如果线程被中断，抛出一个InterruptedException异常。

如果当前获得锁的线程在做大量耗时的工作，使用lock.lock()方法申请锁的线程会一直阻塞，这样就降低了多线程的效率。而使用tryLock()方法申请锁，如果锁不可用则线程不会阻塞，转而可以去做其他工作。代码实例如下：

public class ReentrantLockTest4 {private ReentrantLock lock = new ReentrantLock();public void tryLockTest() throws InterruptedException {long beginTime = System.currentTimeMillis();while(System.currentTimeMillis() - beginTime <100) {}// 当前线程尝试获得锁，如果获得锁返回true，否则返回falseif(lock.tryLock()) {try{System.out.println(Thread.currentThread().getName() + " tryLock get lock");} finally {lock.unlock();System.out.println(Thread.currentThread().getName() + " tryLock release lock");}} else {System.out.println(Thread.currentThread().getName() + " tryLock can not get lock");}}public void lockTest() {try{// 当前线程在锁可用时直接获得锁，锁不可用时阻塞当前线程lock.lock();System.out.println(Thread.currentThread().getName() + " lock get lock");long beginTime = System.currentTimeMillis();while(System.currentTimeMillis() - beginTime <1000) {}} finally {lock.unlock();System.out.println(Thread.currentThread().getName() + " lock release lock");}}public static void main(String[] args) {// TODO Auto-generated method stubfinal ReentrantLockTest4 test = new ReentrantLockTest4();Thread tryLock = new Thread(new Runnable() {public void run() {try {test.tryLockTest();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}},"tryLock_thread");Thread lock = new Thread(new Runnable() {public void run() {test.lockTest();}},"lock_thread");tryLock.start();lock.start();}}输出结果：lock_thread lock get locktryLock_thread tryLock can not get locklock_thread lock release lock

lock方法不能被中断。如果一个线程在等待获得一个锁时被中断，中断线程在获得锁之前会一直处于阻塞状态。如果出现死锁，那么lock方法就无法被终止。但是tryLock和lockInterruptibly方法在申请锁的过程中是可以被中断的。代码如下

public class ReentrantLockTest5 {private ReentrantLock lock = new ReentrantLock();public void tryLockInterruptTest() {long beginTime = System.currentTimeMillis();while(System.currentTimeMillis() - beginTime <100) {}try {if (lock.tryLock(5000, TimeUnit.MILLISECONDS)) {try{System.out.println(Thread.currentThread().getName() + " tryLock get lock");}finally {lock.unlock();}}} catch (InterruptedException e) {// TODO Auto-generated catch blockSystem.out.println(Thread.currentThread().getName() + " was interrupted");}}public void lockTest() {try{// 当前线程在锁可用时直接获得锁，锁不可用时阻塞当前线程lock.lock();System.out.println(Thread.currentThread().getName() + " lock get lock");long beginTime = System.currentTimeMillis();while(System.currentTimeMillis() - beginTime <1000) {}} finally {lock.unlock();System.out.println(Thread.currentThread().getName() + " lock release lock");}}public static void main(String[] args) {// TODO Auto-generated method stubfinal ReentrantLockTest5 test =  new ReentrantLockTest5();Thread thread_tryLock = new Thread(new Runnable(){@Overridepublic void run() {// TODO Auto-generated method stubtest.tryLockInterruptTest();}},"tryLockInterruptTest");Thread thread_lock = new Thread(new Runnable(){@Overridepublic void run() {// TODO Auto-generated method stubtest.lockTest();}},"lockTest");thread_tryLock.start();thread_lock.start();try {Thread.sleep(200);} catch (InterruptedException e) {// TODO Auto-generated catch blockSystem.out.println("main thread was interrupted");}thread_tryLock.interrupt();}}输出结果：lockTest lock get locktryLockInterruptTest was interruptedlockTest lock release lock

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