鎖定為同步輔助

在 Java 5 的併發軟體包引入之前，執行緒化程度較低。此軟體包的引入提供了幾個更高階別的併發程式設計輔助/構造。

鎖是執行緒同步機制，其基本上用於與同步塊或關鍵字相同的目的。

內在鎖定

int count = 0; // shared among multiple threads

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

使用鎖定同步

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
    }
}

鎖還具有內部鎖定不提供的功能，例如鎖定但保持對中斷的響應，或者嘗試鎖定，並且在無法鎖定時不阻止。

鎖定，響應中斷

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
            }
        }
    }
}

只有在能夠鎖定時才能做某事

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
            }
        }
    }
}

有幾種鎖可供選擇。有關詳細資訊，請參閱此處的 api 文件