由于synchronized关键字锁对象的添加与释放我们并不明确,java为我们提供了一个Lock接口
Lock接口源自JDK5,其有三个直接实现类,分别是 :
ReentrantLock,ReentrantReadWriteLock.ReadLock,ReentrantReadWriteLock.WriteLock
其中后两个是ReentrantReadWriteLock的嵌套类

方法

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void lock();//获取锁
void unlock();//释放锁
void lockInterruptibly();//当前线程未中断则获取锁
Condition newCondition();//返回此Lock实例的新实例
boolean tryLock();//仅在调用时锁为空闲状态才获取锁
boolean tryLock(long time, TimeUnit unit);//若锁在给定时间空闲且当前线程未中断,获取锁
	//TimeUnit为参数的时间单位
简单案例 :
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Runnable ticket = new Runnable() {
    private int ticket = 100;
    Lock lock = new ReentrantLock();
    @Override
    public void run() {
        while (ticket > 0){
            lock.lock();
            if (ticket > 0)
                System.out.println(Thread.currentThread().getName() + "正在出售第" + ticket-- + "张票!");
            lock.unlock();
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
};
Thread thread1 = new Thread(ticket,"窗口1");
Thread thread2 = new Thread(ticket,"窗口2");
Thread thread3 = new Thread(ticket,"窗口3");
thread1.start();
thread2.start();
thread3.start();
执行结果如下 :

出现异常锁资源释放问题

由于需要程序调用unlock()方法来释放锁资源,那么如果在释放之前出现异常呢?
改动如下 :
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if (ticket > 0)
    System.out.println(Thread.currentThread().getName() + "正在出售第" + ticket-- + "张票!");
if (Thread.currentThread().getName().equals("窗口1") && ticket < 80)
    System.out.println(10 / 0);
lock.unlock();
...
thread1.start();
thread2.start();
thread3.start();
try {
    thread1.join();
} catch (InterruptedException e) {
    e.printStackTrace();
}
System.out.println(thread1.isAlive());
可以看到,由于"窗口1"被异常终止且没有释放锁,其他两个线程一直在等待锁
结果如下 : 

使用Lock实现类做同步的处理,一定要记得处理锁对象的释放问题
用try-catch-finally处理释放问题
改进如下 :
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while (ticket > 0){
    try {
        lock.lock();
        if (ticket > 0)
            System.out.println(Thread.currentThread().getName() + "正在出售第" + ticket-- + "张票!");
        if (Thread.currentThread().getName().equals("窗口1") && ticket < 80)
            System.out.println(10 / 0);
    } catch (Exception e) {
        e.printStackTrace();
        break;
    }finally {
        lock.unlock();
    }
    try {
        Thread.sleep(10);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
执行结果如下 :