Locks AutoCloseable？

Question

Locks AutoCloseable？也就是说,而不是:

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

我能说......么:

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

在Java 7中？

Answer 1

我本人正在考虑自己这样做并做了类似这样的事情:

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

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

然后这作为课程的用法:

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

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

Answer 2

不,Lock接口(也没有ReentrantLock类)都没有实现AutoCloseable接口,这是与新的try-with-resource语法一起使用所必需的.

如果你想让它工作,你可以编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();
    }
}

现在您可以编写如下代码:

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

不过,我认为你最好坚持使用旧的语法.让锁定逻辑完全可见是更安全的.

Answer 3

通用ReentrantLock既不实现也不提供任何实现AutoCloseabletry-with-resources语句所必需的接口的东西.但是,这个概念对Java API来说并不完全陌生,因为它FileChannel.lock()提供了这个功能.

到目前为止给出的答案共享具有一些问题的解决方案,例如在每次锁定调用上创建不必要的对象,暴露容易出错的API或在获取锁定之后但在输入try-finally之前风险失败.

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Leaner Java 8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;
    }
}

示范:

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
}

Answer 4

将try-with-resource为其创建和销毁时的资源运作良好try-block的离开了。它对于需要保持活动状态的资源不起作用。不会在每次使用时创建和销毁锁。它们保持活动状态，只是被锁定和解锁。这就是为什么它们不是这样AutoClosable。

正如其他人已经建议的那样，可以使用包装器来创建和销毁包装，try-with-resource并在创建和销毁时进行锁定和解锁。