public class wrongsingleton {
    private static volatile wrongsingleton _instance = null;
 
    private wrongsingleton() {}
 
    public static wrongsingleton getInstance() {
 
        if (_instance == null) {
            _instance = new wrongsingleton();
        }
 
        return _instance;
    }
}
 

2. 正確寫法

public class SafeLazyInitialization {
   private static Resource resource;
   public synchronized static Resource getInstance() {
      if (resource == null)
          resource = new Resource();
      return resource;
    }
}

當然，寫法很多，還有其它寫法

另外一種寫法：

@ThreadSafe 
public class EagerInitialization { 
  private static Resource resource = new Resource(); 
  public static Resource getResource() { return resource; } 
}
 

延遲初始化的寫法：

@ThreadSafe 
public class ResourceFactory { 
	private static class ResourceHolder { 
		public static Resource resource = new Resource(); 
	} 
	public static Resource getResource() { 
		return ResourceHolder.resource ; 
	} 
}