如何同步多線程映射更新

Question

我在想如何同步下面的多線程，以便我仍然可以並行運行線程而不會導致保存多個對象。

public void setupRender() { 
    ExecutorService executorService = Executors.newFixedThreadPool(10); 

    final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>()); 

    for (int i = 0; i < 10; i++) { 
     executorService.execute(new Runnable() { 
      @Override 
      public void run() { 
       parse(map); 
      } 
     }); 
     System.out.println("map size " + map.size() + " loop " + i); 
    } 
} 

public void parse(Map<Integer, String> map) { 
    for (int j = 0; j < 100; j++) { 
     if (map.containsKey(j)) { 
      System.out.println("Update"); 
      //session.update(map.getKey(j)); 
     } else { 
      System.out.println("add to map " + j); 
      String obj = "test"; 
      map.put(j, obj); 
      System.out.println("save"); 
      //session.save(obj); 
     } 

     if (j % 50 == 0) { 
      System.out.println("commit"); 
      //tx.commit(); 
     } 
    } 
} 

結果，請注意同一個密鑰的多個對象添加到地圖中，但更糟糕的是保存到導致重複數據庫enteries數據庫。

map size 0 loop 0 
map size 0 loop 1 
add to map 0 
commit 
add to map 1 
add to map 0 
commit 
add to map 2 
add to map 3 
add to map 4 
add to map 2 
add to map 5 
add to map 6 
map size 1 loop 2 
add to map 7 
add to map 8 
add to map 9 
commit 
map size 10 loop 3 
commit 
add to map 5 

Answer 1

您可以使用ConcurrentHashMap和putIfAbsent來保證一致性和性能。在這種情況下，會話更新可以同時運行。

putIfAbsent: 
    - returns the previous value associated with the specified key, 
     or null if there was no mapping for the key 

public void parse(ConcurrentHashMap<Integer, String> map) { 
    for (int j = 0; j < 100; j++) {  
     String obj = "test"; 

     Object returnedValue = map.putIfAbsent(j, obj); 

     boolean wasInMap = returnedValue != null; 

     if (wasInMap) { 
      System.out.println("Update"); 
      //session.update(map.getKey(j)); 
     } else { 
      System.out.println("add to map " + j); 

      map.put(j, obj); 
      System.out.println("save"); 
      //session.save(obj); 
     } 
    } 
} 

你也可以把有這將創造條件的新實例的回調：

class Callback{ 
    abstract void saveOrUpdate(boolean wasInMap);  
} 

public void parse(Map<Integer, String> map) { 
    for (int j = 0; j < 100; j++) {  
     String obj = "test"; 

     Callback callback = (wasInMap) -> { //Java 8 syntax to be short 
      if (wasInMap) { 
       System.out.println("Update"); 
       //session.update(map.getKey(j)); 
      } else { 
       System.out.println("add to map " + j); 

       map.put(j, obj); 
       System.out.println("save"); 
       //session.save(obj); 
      } 
     } 

     Object returnedValue = map.putIfAbsent(j, callback); 

     callback.saveOrUpdate(returnedValue != null); 
    } 
} 

Answer 2

使用syncronized關鍵字來確保檢查j是否在地圖中，如果沒有，則添加它，是自動完成的。然而，它破壞了多線程的目的，因爲當一個線程正在循環時，大多數線程將被阻塞。

public void setupRender() { 
    ExecutorService executorService = Executors.newFixedThreadPool(10); 

final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>()); 

for (int i = 0; i < 10; i++) { 
    executorService.execute(new Runnable() { 
     @Override 
     public void run() { 
      parse(map); 
     } 
    }); 
    System.out.println("map size " + map.size() + " loop " + i); 
} 
} 

public void parse(Map<Integer, String> map) { 
    synchronized(map) { 
    for (int j = 0; j < 100; j++) { 
     if (map.containsKey(j)) { 
      System.out.println("Update"); 
      //session.update(map.getKey(j)); 
     } else { 
      System.out.println("add to map " + j); 
      String obj = "test"; 
      map.put(j, obj); 
      System.out.println("save"); 
      //session.save(obj); 
     } 
     } 

     if (j % 50 == 0) { 
      System.out.println("commit"); 
      //tx.commit(); 
     } 
    } 
}