突然想到了我第一次面试单例都不太会写，索性这次写一部分

1

package singlePattern;/** * 饿汉式 */public class Singleton1 {    private static Singleton1 instance = new Singleton1();    private  Singleton1() {    }    public static Singleton1 getInstance() {        return instance;    }    public static void main(String[] args) {        Singleton1 instance = Singleton1.getInstance();    }}复制代码

2

package singlePattern;/** * 饿汉式 */public class Singleton2 {    private static Singleton2 instance = null;    static {        instance = new Singleton2();    }    private Singleton2() {    }    public static Singleton2 getInstance() {        return instance;    }}复制代码

3

package singlePattern;/** * 懒汉式，但是这种线程不安全，不存在可用性 */public class Singleton3 {    public static Singleton3 instance = null;    private Singleton3() {}    public static Singleton3 getInstance() {        if (instance == null) {            instance = new Singleton3();        }        return instance;    }}复制代码

4

package singlePattern;/** * 懒汉式，线程相对安全，效率低，每次getInstance都需要持锁 */public class Singleton4 {    private static Singleton4 instance = null;    private Singleton4() {}    public static synchronized Singleton4 getInstance() {        if (instance == null) {            instance = new Singleton4();        }        return instance;    }}复制代码

5

package singlePattern;/** * 懒汉式，但是线程不安全 */public class Singleton5 {    private static Singleton5 instance = null;    private Singleton5() {}    private static Singleton5 getInstance() {        if (instance == null) {            synchronized (Singleton5.class) {                instance = new Singleton5();            }        }        return instance;    }}复制代码

6

package singlePattern;/** * 懒汉式，双重校验锁 */public class Singleton6 {    private static Singleton6 instance = null;    private Singleton6(){}    public static Singleton6 getInstance() {        if (instance == null) {            synchronized (Singleton6.class) {                if (instance == null) {                    instance = new Singleton6();                }            }        }        return instance;    }}复制代码

7

package singlePattern;/** * 内部类，通过静态类初始化时进行instance的创建，类似饿汉式，但是在初始化时别的线程 * 无法干扰，而且也可以在形式上有懒加载的感觉 */public class Singleton7 {    private Singleton7(){}    private static class SingletonHolder {        private static Singleton7 instance = new Singleton7();    }    public static Singleton7 getInstance() {        return SingletonHolder.instance;    }}复制代码

8

package singlePattern;/** * 枚举 */public enum Singleton8 {    instance;    private Singleton8(){}    public void method(){}}复制代码

现在都很推荐用枚举，建议多了解，明天写个相关的博客