通過方法暴露Servlet的靜態字段 - 需要數據完整性

Question

我已經寫了一個名爲AbcServlet.java Servlet中，在此我有一個Static String field called cacheSeller。它正在填充在小服務程序的方法init和doGet中。它是由一個CacheSellerClearThread.java清除如下圖所示：

AbcServlet.java

public class AbcServlet extends HttpServlet { 

    private static String cacheSeller = null; 

    @Override 
    public void init() throws ServletException { 
     super.init(); 
     cacheSeller = populateCacheSeller(); 
    } 

    /** 
    * As soon as multiple requests are coming doGet is being called 
    */ 
    @Override 
    public void doGet(HttpServletRequest req, HttpServletResponse resp) 
      throws ServletException, IOException { 
     synchronized (this.getClass()) { 
      if (cacheSeller == null) { 
       cacheSeller = populateCacheSeller(); 
      } 
     } 

    } 

    private String populateCacheSeller() { 
     String fetchItFromSomewhere = "";// some logic to fetch the sting 
     return fetchItFromSomewhere; 
    } 

    public static synchronized void clearCacheSeller() { 
     cacheSeller = null; 
    } 
} 

CacheSellerClearThread.java

/** 
* This thread is clearing the string field cacheSeller of AbcServlet 
* 
*/ 
public class CacheSellerClearThread extends Thread { 

    public void run() { 
     while (true) { 
      try { 
       Thread.sleep(2000L); 
      } catch (InterruptedException e) { 
       e.printStackTrace(); 
      } 
      //Here it is clearing the static string field cacheSeller of AbcServlet 
      AbcServlet.clearCacheSeller(); 
     } 
    } 
} 

，如果你看一下代碼，正在讀cacheSeller值並在doGet方法中進行修改。所有請求線程將執行doGet，同時CacheSellerClearThread將在每2秒後清除其值。所以爲了保持cacheSeller值的數據完整性，我使用同步。

我需要的是有沒有其他的方式來實現這一目標，而不使用在clearCacheSeller方法顯式同步，並沒有在Servlet中應用明確的類級鎖的建議。 （我的意思是說使用任何先進的併發API，如原子參考或任何其他你可以建議。）

我已經寫了新的實現使用AtomicReference，請建議，我需要在任何地方使用同步。

public class AbcServlet extends HttpServlet { 

    private static AtomicReference<String> cacheSeller = new AtomicReference<String>(); 

    @Override 
    public void init() throws ServletException { 
     super.init(); 
     cacheSeller.set(populateCacheSeller()); 
    } 

    /** 
    * As soon as multiple requests are coming doGet is being called 
    */ 
    @Override 
    public void doGet(HttpServletRequest req, HttpServletResponse resp) 
      throws ServletException, IOException { 
     if (cacheSeller.get() == null) { 
      cacheSeller.compareAndSet(null, populateCacheSeller()); 
     } 
     resp.getWriter().print(cacheSeller.get()); 
    } 

    private String populateCacheSeller() { 
     String fetchItFromSomewhere = "";// some logic to fetch the sting 
     return fetchItFromSomewhere; 
    } 

    public static void clearCacheSeller() { 
     cacheSeller.set(null); 
    } 
} 

Answer 1

您可以使用類似Google Guava創建緩存。

例如：

LoadingCache<String, String> cache = 
CacheBuilder.newBuilder() 
.expireAfterWrite(2, TimeUnit.MINUTES) 
.build(loader); 

有這樣的事情，你可以避開你的輔助線程來清除緩存。

此實現是一個簡單的內存中緩存，它也是線程安全的。

問候

Answer 2

我會使用Executors框架建議。

Executors.newSingleThreadScheduledExecutor().scheduleAtFixedRate(new Runnable() { 
      @Override 
      public void run() { 
       AbcServlet.clearCacheSeller(); 
      } 
     }, 2L,2L,TimeUnit.SECONDS); 

而在你doGet，僅同步初始化：

@Override 
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
      String localCacheSeller = cacheSeller; 
      if (cacheSeller == null) { 
       synchronized (this.getClass()) { 
        if (cacheSeller == null) { //need this because cacheSeller may already have been initialized while we waited 
         cacheSeller = populateCacheSeller(); 
         localCacheSeller = cacheSeller; //because the executor may clear it after we initialized 
        } 
       } 
      } 
    }