线程安全的易失性关键字(Volatile keyword for thread safety)

 我有以下代码： 
 
public class Foo {
  private volatile Map<String, String> map;

  public Foo() {
    refresh();
  }

  public void refresh() {
    map = getData();
  }

  public boolean isPresent(String id) {
    return map.containsKey(id);
  }

  public String getName(String id) {
    return map.get(id);
  }

  private Map<String, String> getData() {
    // logic
  }

}
 
 
 
 以上代码线程是安全的还是我需要在那里添加synchronized或互斥？ 如果它不是线程安全的，请澄清原因。 
 
 
 另外，我已经读过应该使用AtomicReference而不是这个，但是在AtomicReference类的源代码中，我可以看到用于保存该值的字段是易变的（以及一些便捷方法）。 
 
 
 
 是否有使用AtomicReference的特定原因？ 
 
 
 我已经阅读了与此相关的多个答案，但volatile的概念仍然让我感到困惑。 提前致谢。 

I have the below code:
 
public class Foo {
  private volatile Map<String, String> map;

  public Foo() {
    refresh();
  }

  public void refresh() {
    map = getData();
  }

  public boolean isPresent(String id) {
    return map.containsKey(id);
  }

  public String getName(String id) {
    return map.get(id);
  }

  private Map<String, String> getData() {
    // logic
  }

}
 
 
 
Is the above code thread safe or do I need to add synchronized or mutexes in there? If it's not thread safe, please clarify why.
 
 
Also, I've read that one should use AtomicReference instead of this, but in the source of the AtomicReference class, I can see that the field used to hold the value is volatile (along with a few convenience methods).
 
 
 
Is there a specific reason to use AtomicReference instead?
 
 
I've read multiple answer related to this but the concept of volatile still confuses me. Thanks in advance.

原文：https://stackoverflow.com/questions/45216390