How to make a singleton class threadsafe?

Question

我在Java中实现一个单例类，以确保只创建了一个类的实例。

Answer 1

The best way to make a singleton? Use an enum.

public enum Singleton {
    INSTANCE;
    public void foo(){ ... }
}

// Usage:
Singleton.INSTANCE.foo();

You get lots of help from the VM not only to avoid double instantiation, but it also helps you avoid deserialization corruption.

Answer 2

Perhaps the best way is to use an enum with a single instance. This has the added benefit of being serializable and guaranteeing singleton-ness against serialization and reflection, which no "straightforward" Singleton implementation does (private? I have reflection, I scoff derisively at your access modifiers!). It's also very simple to implement:

public enum Singleton {
    INSTANCE;

    // fields and methods go here
}

Answer 3

public class Singleton {
  public static final Singleton INSTANCE = new Singleton();
  private Singleton() { ... }
}

Using a static instance variable is preferred to the internal "Holder" class. If you like, you can also make the static member private and provide a method for accessing it, but all that does is add another method call to the stack.

Answer 4

What about lazy instanciation: getInstance() returns the singleton or create it if it is the first call.

 public class MySingleton
 {
     private static MySingleton instance;

     private MySingleton()
     {
         // construct object . . .
     }

     // For lazy initialization
     public static synchronized MySingleton getInstance()
     {
         if (instance==null)
         {
             instance = new MySingleton();
         }
         return instance;
     }

     // Remainder of class definition . . .
 } 

Answer 5

Singletons are not threadsafe per default. The common way to create a singleton is to have a factory method that handles creation of the one instance. That way you can control instance creation.

Answer 6

如果您关心并发，则不会使用共享全局状态。