Java 同步块与 Collections.synchronizedMap [英] Java synchronized block vs. Collections.synchronizedMap

问题描述

以下代码是否设置为正确同步 synchronizedMap 上的调用?

Is the following code set up to correctly synchronize the calls on synchronizedMap?

public class MyClass {
  private static Map<String, List<String>> synchronizedMap = Collections.synchronizedMap(new HashMap<String, List<String>>());

  public void doWork(String key) {
    List<String> values = null;
    while ((values = synchronizedMap.remove(key)) != null) {
      //do something with values
    }
  }

  public static void addToMap(String key, String value) {
    synchronized (synchronizedMap) {
      if (synchronizedMap.containsKey(key)) {
        synchronizedMap.get(key).add(value);
      }
      else {
        List<String> valuesList = new ArrayList<String>();
        valuesList.add(value);
        synchronizedMap.put(key, valuesList);
      }
    }
  }
}

据我了解，我需要 addToMap() 中的同步块来防止另一个线程调用 remove() 或 containsKey()在我完成对 put() 的调用之前，但我不需要 doWork() 中的同步块，因为另一个线程无法进入 addToMap() 在 remove() 返回之前，因为我最初使用 Collections.synchronizedMap() 创建了地图.那是对的吗?有没有更好的方法来做到这一点?

From my understanding, I need the synchronized block in addToMap() to prevent another thread from calling remove() or containsKey() before I get through the call to put() but I do not need a synchronized block in doWork() because another thread cannot enter the synchronized block in addToMap() before remove() returns because I created the Map originally with Collections.synchronizedMap(). Is that correct? Is there a better way to do this?

推荐答案

Collections.synchronizedMap() 保证你想在地图上运行的每个原子操作都会被同步.

Collections.synchronizedMap() guarantees that each atomic operation you want to run on the map will be synchronized.

但是，在地图上运行两个(或更多)操作必须在一个块中同步.所以是的 - 你正在正确同步.

Running two (or more) operations on the map however, must be synchronized in a block. So yes - you are synchronizing correctly.

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