单例的三种实现方式

1双重加锁模式

public class Singleton {
    private volatile static Singleton singleton;
    public static Singleton getSingleton(){
        if(singleton==null){
            synchronized (Singleton.class) {
                if(singleton==null){
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }

    private Singleton(){

    }
}

2静态内部类

public class Singleton {
    private Singleton(){

    }

    public Singleton getSingleton(){
        return SingletonInner.singleton;
    }
    private static class SingletonInner{
        protected  static Singleton singleton = new Singleton();

    }
}

3枚举

public enum Singleton {
    Singlton;
    private Singleton(){

    }
}

由于java提供了反射机制，第一种和第二种方式通过反射机制仍然有可能生成多个实例