JAVA单例模式有哪些？

 单例模式的俩种方式：饿汉式class Singleton {  private static Singleton instance=new Singleton();  private Singleton(){}  static Singleton getInstance() {      return instance;  }}懒汉式class Singleton {  private static Singleton instance=null;  private Singleton(){}  static Singleton getInstance() {      if(instance==null)      instance=new Singleton();      return instance;  }}