锁是AutoCloseable吗？

Question

Locks是自动关闭的吗？也就是说，而不是：

Lock someLock = new ReentrantLock();
someLock.lock();
try
{
    // ...
}
finally
{
    someLock.unlock();
}

...can我说：

try (Lock someLock = new ReentrantLock())
{
    someLock.lock();
    // ...
}

...in Java7？

Answer 1

不需要，Lock接口(和ReentrantLock类)都不实现AutoCloseable接口，该接口是使用新的try- with -resource语法所必需的。

如果你想让它工作，你可以写一个简单的包装器：

public class LockWrapper implements AutoCloseable
{
    private final Lock _lock;
    public LockWrapper(Lock l) {
       this._lock = l;
    }

    public void lock() {
        this._lock.lock();
    }

    public void close() {
        this._lock.unlock();
    }
}

现在，您可以像这样编写代码：

try (LockWrapper someLock = new LockWrapper(new ReentrantLock()))
{
    someLock.lock();
    // ...
}

不过，我认为您最好还是坚持使用旧的语法。让你的锁定逻辑完全可见会更安全。

Answer 2

我正在考虑自己做这件事，并做了这样的事情：

public class CloseableReentrantLock extends ReentrantLock implements AutoCloseable { 
   public CloseableReentrantLock open() { 
      this.lock();
      return this;
   }

   @Override
   public void close() {
      this.unlock();
   }
}

然后是这个类的用法：

public class MyClass {
   private final CloseableReentrantLock lock = new CloseableReentrantLock();

   public void myMethod() {
      try(CloseableReentrantLock closeableLock = lock.open()) {
         // locked stuff
      }
   }
}

Answer 3

通用ReentrantLock既不实现也不提供实现try-with-resources语句所需的AutoCloseable接口的任何内容。但是，这个概念对于Java API来说并不完全陌生，因为FileChannel.lock()提供了这个功能。

到目前为止，给出的答案都是有一些问题的解决方案，比如在每次锁调用时创建一个不必要的对象，暴露一个容易出错的API，或者在获取锁之后但进入try-finally之前有失败的风险。

Java 7解决方案：

public interface ResourceLock extends AutoCloseable {

    /**
     * Unlocking doesn't throw any checked exception.
     */
    @Override
    void close();
}

public class CloseableReentrantLock extends ReentrantLock {

    private final ResourceLock unlocker = new ResourceLock() {
        @Override
        public void close() {
            CloseableReentrantLock.this.unlock();
        }
    };

    /**
     * @return an {@link AutoCloseable} once the lock has been acquired.
     */
    public ResourceLock lockAsResource() {
        lock();
        return unlocker;
    }
}

使用lambda的更精简的Java8解决方案：

public class CloseableReentrantLock extends ReentrantLock {

    /**
     * @return an {@link AutoCloseable} once the lock has been acquired.
     */
    public ResourceLock lockAsResource() {
        lock();
        return this::unlock;
    }
}

演示：

public static void main(String[] args) {
    CloseableReentrantLock lock = new CloseableReentrantLock();

    try (ResourceLock ignored = lock.lockAsResource()) {
        try (ResourceLock ignored2 = lock.lockAsResource()) {
            System.out.println(lock.getHoldCount());  // 2
        }
    }
    System.out.println(lock.getHoldCount());  // 0
}