[Java] lock、tryLock和lockInterruptibly的差別

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在Java中的Lock介面有lock、tryLock和lockInterruptibly三個功能很相近的method，看完javadoc後還是搞不太清楚它們之間的差異性，終於找到一篇解釋清楚的文章了。

  • lock()：若lock被thread A取得，thread B會進入block狀態，直到取得lock。
  • tryLock()：若當下不能取得lock，thread就會放棄。
  • lockInterruptibly()：跟lock()情況一下，但是thread B可以透過interrupt被喚醒處理InterruptedException。

範例：

  • lock()：
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import java.util.Date;
import java.util.concurrent.locks.ReentrantLock;

public class LockDemo {
	private final ReentrantLock	lock;

	public LockDemo() {
		lock = new ReentrantLock();
	}

	public static void main(String[] args) throws InterruptedException {
		LockDemo lockDemo = new LockDemo();
		Runnable runnable = new Runnable() {
			@Override
			public void run() {
				lockDemo.lock.lock();
				System.out.println(String.format("%s %s locked", new Date(System.currentTimeMillis()), Thread.currentThread().getName()));
			}
		};
		Thread threadA = new Thread(runnable, "Thread A");
		Thread threadB = new Thread(runnable, "Thread B");

		threadA.start();
		threadB.start();
	}
}

可以發現ThreadB呈現block狀態，一直在等待Thread A釋放lock。

  • tryLock()：
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import java.util.Date;
import java.util.concurrent.locks.ReentrantLock;

public class TryLockDemo {
	private final ReentrantLock	lock;

	public TryLockDemo() {
		lock = new ReentrantLock();
	}

	public static void main(String[] args) throws InterruptedException {
		TryLockDemo lockDemo = new TryLockDemo();
		Runnable runnable = new Runnable() {
			@Override
			public void run() {
				if (lockDemo.lock.tryLock()) {
					System.out.println(String.format("%s %s locked", new Date(System.currentTimeMillis()), Thread.currentThread().getName()));
				}
			}
		};
		Thread threadA = new Thread(runnable, "Thread A");
		Thread threadB = new Thread(runnable, "Thread B");

		threadA.start();
		threadB.start();
	}
}

若一開始lock被Thread A取得，Thread B透過tryLock()當下若沒有取得到lock，就會放棄。

  • lockInterruptibly()：
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import java.util.Date;
import java.util.concurrent.locks.ReentrantLock;

public class LockInterruptiblyDemo {
	private final ReentrantLock	lock;

	public LockInterruptiblyDemo() {
		lock = new ReentrantLock();
	}

	public static void main(String[] args) throws InterruptedException {
		LockInterruptiblyDemo lockDemo = new LockInterruptiblyDemo();

		Runnable runnable = new Runnable() {
			@Override
			public void run() {
				try {
					lockDemo.lock.lockInterruptibly();
					System.out.println(String.format("%s %s locked", new Date(System.currentTimeMillis()), Thread.currentThread().getName()));
				}
				catch (InterruptedException e) {
					System.out.println(String.format("%s %s interrupted", new Date(System.currentTimeMillis()), Thread.currentThread().getName()));
				}							
			}
		};
		Thread threadA = new Thread(runnable, "Thread A");
		Thread threadB = new Thread(runnable, "Thread B");

		threadA.start();
		Thread.sleep(1000);		
		threadB.start();
		threadB.interrupt();
	}
}

Thread A先取得lock，Thread B無法取得lock進入block狀態，可以透過發出interrupt方式喚醒Thread B。

Lock和synchronized幾乎是大同小異，但是Lock可以做更細微的同步方式。

參考資料：

用通俗的語言說說lock和lockInterruptibly的區別