1. 懒汉式

    public class Singleton {
          private static Singleton instance = null;
          // 不允许外部实例话
          private Singleton();
          
          public static Singleton getInstance(){
                if (null == instance){
                    instence = new Singleton()
                }
                return instance;
          }
    }
   

为什么说它线程不安全？
同时运行线程A和线程B,它们都要调用Singleton.getInstance(),线程A判断instance为空，线程B也会判断instance为空，实际上，线程A与线程B中的获取的Singleton不是同一对象

如何保证单例的线程安全？

    public class Singleton {
          //添加volatile 保护变量安全
          private volatile static Singleton instance = null;
          // 不允许外部实例话
          private Singleton();
          //双重检查锁定
          public static Singleton getInstance(){
                if (null == instance){
                    synchronized (Singleton.class){
                            if(null == instance){
                                  instence = new Singleton()
                            }
                    }
                }
                return instance;
          }
    }

2. 饿汉式

public class Singleton {
        private static Singleton instance = new Singleton();
        //不允许外部实例话
        private Singleton(){}
        public static Singletion getInstance(){
            return instance;
        }
}

基于ClassLoader机制避免了多线程的同步问题

3. 静态内部类

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

基于ClassLoader机制,Singleton类被装载了，instance不一定被初始化

4. 枚举单例

class B {

}

enum Singleton {
    INSTANCE;

    private B b;
    private Singleton(){
        b = new B();
    }
    public B getInstance(){
        return b;
    }
}

public static void main(String[] args) {
  System.out.println(Singleton.INSTANCE.getInstance().hashCode());
  System.out.println(Singleton.INSTANCE.getInstance().hashCode()); 
}

hashcode是一致的

参考：
http://www.blogjava.net/kenzhh/archive/2013/03/15/357824.html
https://www.cnblogs.com/happy4java/p/11206105.html
https://www.jianshu.com/p/d35f244f3770