（1）私有化构造方法
（2）私有化静态本类对象作为属性
（3）提供公有静态方法获取本类对象

public class Client {
    public static void main(String[] args) {
        Singleton.getInstance();
    }

    static class Singleton {
        private static final Singleton instance = new Singleton();

        private Singleton(){}

        public static Singleton getInstance(){
            return  instance;
        }

    }
}

//饿汉式静态代码块单例模式
public class HungryStaticSingleton {
    private static final HungryStaticSingleton instance;

    static {
        instance = new HungryStaticSingleton();
    }

    private HungryStaticSingleton(){}

    public static HungryStaticSingleton getInstance(){
        return  instance;
    }
}

/**
 * 优点：执行效率高，性能高，没有任何的锁
 * 缺点：某些情况下，可能会造成内存浪费
 */
//饿汉式静态代码块单例模式
public class HungrySingleton {

    private static final HungrySingleton instance = new HungrySingleton();

    private HungrySingleton(){}

    public static HungrySingleton getInstance(){
        return  instance;
    }
}

/**
 * 优点：节省了内存,线程安全
 * 缺点：性能低
 */
//懒汉式单例模式在外部需要使用的时候才进行实例化
public class LazySimpleSingletion {
    //静态块，公共内存区域
    private static LazySimpleSingletion instance;

    private LazySimpleSingletion(){}

    public synchronized static LazySimpleSingletion getInstance(){
        if(instance == null){
            instance = new LazySimpleSingletion();
        }
        return instance;
    }
}

public class Singleton {
    private static Singleton instance = null;
    public  static Singleton getInstance() {
        synchronized (Singleton.class) {
            if(null == instance) {
                instance = new Singleton();
            }
        }
        return instance;
    }
}

/**
 * 优点:性能高了，线程安全了
 * 缺点：可读性难度加大，不够优雅
 */
public class LazyDoubleCheckSingleton {
    private volatile static LazyDoubleCheckSingleton instance;
    private LazyDoubleCheckSingleton(){}

    public static LazyDoubleCheckSingleton getInstance(){
        //检查是否要阻塞
        if (instance == null) {
            synchronized (LazyDoubleCheckSingleton.class) {
                //检查是否要重新创建实例
                if (instance == null) {
                    instance = new LazyDoubleCheckSingleton();
                    //指令重排序的问题
                }
            }
        }
        return instance;
    }
}

/*
  ClassPath : LazyStaticInnerClassSingleton.class
              LazyStaticInnerClassSingleton$LazyHolder.class
   优点：写法优雅，利用了Java本身语法特点，性能高，避免了内存浪费,不能被反射破坏
   缺点：不优雅
 */
//这种形式兼顾饿汉式单例模式的内存浪费问题和synchronized的性能问题
//完美地屏蔽了这两个缺点
//自认为史上最牛的单例模式的实现方式
public class LazyStaticInnerClassSingleton {
    //使用LazyInnerClassGeneral的时候，默认会先初始化内部类
    //如果没使用，则内部类是不加载的
    private LazyStaticInnerClassSingleton(){
        if(LazyHolder.INSTANCE != null){
            throw new RuntimeException("不允许创建多个实例");
        }
    }
    //每一个关键字都不是多余的，static是为了使单例的空间共享，保证这个方法不会被重写、重载
    private static LazyStaticInnerClassSingleton getInstance(){
        //在返回结果以前，一定会先加载内部类
        return LazyHolder.INSTANCE;
    }

    //利用了Java本身语法特点，内部类默认不加载
    private static class LazyHolder{
        private static final LazyStaticInnerClassSingleton INSTANCE = new LazyStaticInnerClassSingleton();
    }

}

public enum EnumSingleton {
    INSTANCE;

    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public static EnumSingleton getInstance(){return INSTANCE;}
}