同步使用条件？我怎样才能使这个代码的性能和线程安全？

Question

考虑下面的代码：

public class SomeClass { 

    private boolean shouldBlock = false; 
    private Object resource; 

    public void handleDrawRequest(Canvas canvas) { 
    if (!shouldBlock && resource == null) 
    { 
     shouldBlock = true; 
     loadTheResource(); //which takes awhile 
     shouldBlock = false; 
    } 
    else if (shouldBlock && resrouce == null) 
    { 
     return; //another thread is taking care of the loading of the resource 
       //and its not ready yet, so just ignore this request 
    } 

    drawResourceOn(canvas); 
    } 
} 

我怎样才能使此代码线程安全的？我试图完成的是只有一个线程来加载资源，同时任何其他线程试图同时访问此代码将被丢弃（例如，遵循'else if'逻辑）直到资源被加载。可能有很多线程试图同时访问这些代码，我不想同步整个方法并且有一堆线程堆积起来。

Answer 1

你正在寻找一个AtomicBoolean

public class SomeClass { 
    // AtomicBolean defaults to the value false. 
    private AtomicBoolean loadingResource = new AtomicBoolean(); 
    private volatile Object resource; 

    public void handleDrawRequest(Canvas canvas) { 
    if (resource == null) { 
     if (loadingResource.compareAndSet(false, true)) { 
     loadTheResource(); //which takes awhile 
     } else { 
     //another thread is taking care of the loading of the resource 
     //and its not ready yet, so just ignore this request 
     return; 
     } 
    } else { 
     drawResourceOn(canvas); 
    } 
    } 
} 

Answer 2

随着double checkednon-blocking锁定：

public class SomeClass { 

    private Lock lock = new Lock(); 
    private volatile Object resource; 

    public void handleDrawRequest(Canvas canvas) { 
     if(resource==null) { 
      if(lock.tryLock()) { 
       try { 
        if(resource==null) 
         resource = loadResource(); 
       } 
       finally { 
        lock.unlock(); 
       } 
      } 
      else { 
       return; 
      } 
     } 
     drawResourceOn(canvas); 
    } 
} 

如果你不resourcevolatile，线程可以自由地高速缓存，并可能永远不会读取更新后的值。特别是，即使资源在第一个之后加载，第二个空检查也将始终返回true。