剑指Offer-【面试题02：实现Singleton 模式——七种实现方式】

题目：设计一个类，我们只能生成该类的一个实例

package com.cxz.demo02;

/**
 * Created by CXZ on 2016/9/13.
 */
public class SingletonTest {

    /**
     * 单例模式，饿汉式，线程安全
     */
    public static class Singleton {
        private final static Singleton INSTANCE = new Singleton();
        private Singleton() {

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

    /**
     * 单例模式，懒汉式，线程不安全
     */
    public static class Singleton2 {
        private static Singleton2 instance = null;
        private Singleton2() {

        }
        public static Singleton2 getInstance() {
            if (instance == null) {
                instance = new Singleton2();
            }
            return instance;
        }
    }

    /**
     * 单例模式，懒汉式，线程安全，多线程环境下效率高
     * synchronized够保证在同一时刻最多只有一个线程执行该段代码
     */
    public static class Singleton3 {
        private static Singleton3 instance = null;
        private Singleton3() {

        }
        public static synchronized Singleton3 getInstance() {
            if (instance == null) {
                instance = new Singleton3();
            }
            return instance;
        }
    }

    /**
     * 单例模式，懒汉式，变种，线程安全
     */
    public static class Singleton4 {
        private static Singleton4 instance = null;
        static {
            instance = new Singleton4();
        }
        private Singleton4() {

        }
        public static Singleton4 getInstance() {
            return instance;
        }
    }

    /**
     * 单例模式，使用静态内部类，线程安全
     */
    public static class Singleton5 {
        private final static class SingletonHolder {
            private static final Singleton5 INSTANCE = new Singleton5();
        }
        private Singleton5() {

        }
        public static Singleton5 getInstance() {
            return SingletonHolder.INSTANCE;
        }
    }

    /**
     * 静态内部类，使用枚举方式，线程安全
     */
    public enum Singleton6 {
        INSTANCE;
        public void whateverMethon() {

        }
    }

    /**
     * 静态内部类，使用双重校验，线程安全
     */
    public static class Singleton7 {
        private volatile static Singleton7 instance = null;
        private Singleton7() {

        }
    }

    /**
     * main
     * @param args
     */
    public static void main(String[] args) {
        System.out.println(Singleton.getInstance() == Singleton.getInstance());
        System.out.println(Singleton2.getInstance() == Singleton2.getInstance());
        System.out.println(Singleton3.getInstance() == Singleton3.getInstance());
        System.out.println(Singleton4.getInstance() == Singleton4.getInstance());
        System.out.println(Singleton5.getInstance() == Singleton5.getInstance());
        System.out.println(Singleton6.INSTANCE == Singleton6.INSTANCE);
    }

}