锁定为同步辅助

在 Java 5 的并发软件包引入之前，线程化程度较低。此软件包的引入提供了几个更高级别的并发编程辅助/构造。

锁是线程同步机制，其基本上用于与同步块或关键字相同的目的。

内在锁定

int count = 0; // shared among multiple threads

public void doSomething() {
    synchronized(this) {
        ++count; // a non-atomic operation
    }
}

使用锁定同步

int count = 0; // shared among multiple threads

Lock lockObj = new ReentrantLock();
public void doSomething() {
    try {
        lockObj.lock();
        ++count; // a non-atomic operation
    } finally {    
        lockObj.unlock(); // sure to release the lock without fail
    }
}

锁还具有内部锁定不提供的功能，例如锁定但保持对中断的响应，或者尝试锁定，并且在无法锁定时不阻止。

锁定，响应中断

class Locky {
    int count = 0; // shared among multiple threads

    Lock lockObj = new ReentrantLock();

    public void doSomething() {
        try {
            try {
                lockObj.lockInterruptibly();
                ++count; // a non-atomic operation
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt(); // stopping
            }
        } finally {
            if (!Thread.currentThread().isInterrupted()) {
                lockObj.unlock(); // sure to release the lock without fail
            }
        }
    }
}

只有在能够锁定时才能做某事

public class Locky2 {
    int count = 0; // shared among multiple threads

    Lock lockObj = new ReentrantLock();

    public void doSomething() {
        boolean locked = lockObj.tryLock(); // returns true upon successful lock
        if (locked) {
            try {
                ++count; // a non-atomic operation
            } finally {
                lockObj.unlock(); // sure to release the lock without fail
            }
        }
    }
}

有几种锁可供选择。有关详细信息，请参阅此处的 api 文档