同步使用條件？我怎樣才能使這個代碼的性能和線程安全？

Question

考慮下面的代碼：

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); 
    } 
} 

我怎樣才能使此代碼線程安全的？我試圖完成的是隻有一個線程來加載資源，同時任何其他線程試圖同時訪問此代碼將被丟棄（例如，遵循'else if'邏輯）直到資源被加載。可能有很多線程試圖同時訪問這些代碼，我不想同步整個方法並且有一堆線程堆積起來。

Answer 1

你正在尋找一個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

隨着double checkednon-blocking鎖定：

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，線程可以自由地高速緩存，並可能永遠不會讀取更新後的值。特別是，即使資源在第一個之後加載，第二個空檢查也將始終返回true。