懒汉式： 用了之后才初始化

  1. public class Singleton {
  2.     // volatile防止指定重排序， 及内存可见性
  3.     private static volatile Singleton instance = null;
  4.     private Singleton() { }
  5.     public static Singleton getInstance() {
  6.         if (instance == null) {
  7.             synchronized(Singleton.class) {
  8.                 if (instance == null){
  9.                     instance = new Singleton();
  10.                 }
  11.             }
  12.         }
  13.        return instance; 
  14.     }
  15. }

恶汉式

  1. public class Singleton {
  2.     private static Singleton instance = null;
  3.     static {
  4.         instance = new Singleton();
  5.     }
  6.     private Singleton() { }
  7.     public static Singleton getInstance() {
  8.         return this.instance; 
  9.     }
  10. }

破坏懒汉式单例与饿汉式单例