java设计模式

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();
  
    // Private constructor suppresses
    // default public constructor
    private Singleton() {};
 
    public static Singleton getInstance() {
        return INSTANCE;
    }
  }

public class Singleton {
    private static volatile Singleton INSTANCE = null;
  
    // Private constructor suppresses 
    // default public constructor
    private Singleton() {};
  
    //Thread safe and performance  promote 
    public static  Singleton getInstance() {
        if(INSTANCE == null){
             synchronized(Singleton.class){
                 // When more than two threads run into the first null check same time, 
                 // to avoid instanced more than one time, it needs to be checked again.
                 if(INSTANCE == null){ 
                     INSTANCE = new Singleton();
                  }
              } 
        }
        return INSTANCE;
    }
  }