在ExecutorService的

Question

使用synchronized塊，我有以下的代碼片段：

public class Service<T> { 
    private ConcurrentMap<Integer, Integer> locks = new ConcurrentHashMap<Integer, Integer>(); 
    public final ExecutorService exec = Executors.newFixedThreadPool(5); 

    public Future<Boolean> foo(T o, String fileName) throws IOException { 
     return exec.submit(new Callable<Boolean>() { 
      @Override 
      public Boolean call() throws IOException { 
       File f = new File(fileName); 

//    synchronized (getCacheSyncObject(name.hashCode())) { 
//     try (OutputStream out = new FileOutputStream(f)) { 
//      //some actions 
//     } 
//    } 

       try (OutputStream out = new FileOutputStream(f)) { 
        //some actions 
       } 

       return true; 
      } 
     }); 
    } 

    private Object getCacheSyncObject(final Integer id) { 
     locks.putIfAbsent(id, id); 
     return locks.get(id); 
    } 
} 

public class Main { 
    public static void main(String[] args) throws IOException { 
     Object obj = new Object(); 

     Service<Object> service = new Service<>(); 
     Future<Boolean> result1 = service.foo(obj, "filename"); // is there a deadlock? 
     service.exec.shutdown(); 
    } 
} 

我想完成一個簡單的任務。我需要一個獨佔的文件鎖定。爲了得到這個，我把文件的文件名鎖定到同步塊。但在這種情況下，我從foo方法中沒有得到任何東西。我的程序沒有結束。

Answer 1

如果您正在尋找服務範圍內的鎖，則可以使用其字段來存儲鎖。但我建議不要使用fileName.hashCode（），因爲可能的衝突。使用全名來代替：

import java.io.File; 
import java.io.FileOutputStream; 
import java.io.IOException; 
import java.io.OutputStream; 
import java.util.concurrent.Callable; 
import java.util.concurrent.ConcurrentHashMap; 
import java.util.concurrent.ConcurrentMap; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
import java.util.concurrent.Future; 

class Service<T> { 

    private final ConcurrentMap<String, Object> locks = new ConcurrentHashMap<>(); 
    public final ExecutorService exec = Executors.newFixedThreadPool(5); 

    public Future<Boolean> foo(T o, 
           final String fileName) throws IOException { 
     return exec.submit(new Callable<Boolean>() { 
      @Override 
      public Boolean call() throws IOException { 
       File f = new File(fileName); 

       synchronized (getCacheSyncObject(fileName)) { 
        try (OutputStream out = new FileOutputStream(f)) { 
         out.write("hi".getBytes()); 
        } 
       } 

       return true; 
      } 
     }); 
    } 

    private Object getCacheSyncObject(final String name) { 
     Object result = locks.get(name); 

     if (result == null) { 
      result = new Object(); 
      Object prevLock; 
      if ((prevLock = locks.putIfAbsent(name, result)) != null) { 
       result = prevLock; 
      } 
     } 

     return result; 
    } 
} 

public class Test { 

    public static void main(String[] args) throws Exception { 
     Object obj = new Object(); 

     Service<Object> service = new Service<>(); 
     Future<Boolean> result1 = service.foo(obj, "filename"); 
     service.exec.shutdown(); 

     System.out.println("result1 = " + result1.get()); 
    } 
} 

順便說一句，即使你有hashCode（）方法的碰撞，你可以在文件中意外的輸出，但如果你沒有鎖定任何事情儘量

try (OutputStream out = new FileOutputStream(f)) { 
    out.write("hi".getBytes()); 
} 

你不應該陷入僵局。我建議檢查你的「//一些動作」