设计模式之-单例模式

1.单利 sigleton

• 1.各种mgr
• 2.各种Factory

使用：

1.私有化对象
2.私有化构造器
3.公有化获取对象方法

第一种：推荐使用，线程安全：

• 类加载到内存之后，就实例化一个单利，JVM保证线程安全；
• 缺点：类装载时候就会实例化

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr01 {

    private static Mgr01 mgr01 = new Mgr01();

    private Mgr01(){}

    public static Mgr01 getInstance(){
        return mgr01;
    }


    public static void main(String[] args) {
        System.out.println(Mgr01.getInstance());
        System.out.println(Mgr01.getInstance());
    }
}

2.lazy loading 懒汉式

• 多线程情况下会产生多个实例

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr02 {

    private static Mgr02 mgr02 ;

    private Mgr02(){}

    public static Mgr02 getInstance(){
        if(mgr02 == null){
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            mgr02 = new Mgr02();
        }
        return mgr02;
    }


    public static void main(String[] args) {
        for(int i = 0; i< 100;i++){
            new Thread(()->{
                System.out.println(Mgr02.getInstance().hashCode());
            }).start();
        }
    }
}

3.lazy loading :加锁，但是性能换安全

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr03 {

    private static Mgr03 mgr02 ;

    private Mgr03(){}

    public static synchronized Mgr03 getInstance(){
        if(mgr02 == null){
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            mgr02 = new Mgr03();
        }
        return mgr02;
    }


    public static void main(String[] args) {
        for(int i = 0; i< 100;i++){
            new Thread(()->{
                System.out.println(Mgr03.getInstance().hashCode());
            }).start();
        }
    }
}

4.lazy loading：将锁更加的细微化，加锁到需要加锁的地方

• 这种方案不可行，if判断对象多线程都会进来执行

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr04 {

    private static Mgr04 mgr02;

    private Mgr04() {
    }

    public static Mgr04 getInstance() {
        if (mgr02 == null) {
            synchronized (Mgr04.class) {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                mgr02 = new Mgr04();
            }
        }
        return mgr02;
    }


    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(Mgr04.getInstance().hashCode());
            }).start();
        }
    }
}

5.双重检查+sync = 安全

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr05 {

    //防止指令重排
    private static volatile Mgr05 mgr02;

    private Mgr05() {
    }

    public static Mgr05 getInstance() {
        if (mgr02 == null) {
            synchronized (Mgr05.class) {
                if (mgr02 == null) {
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    mgr02 = new Mgr05();
                }
            }
        }
        return mgr02;
    }


    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(Mgr05.getInstance().hashCode());
            }).start();
        }
    }
}

6.静态内部类：推荐使用，比第一种更好一点

• 静态内部类在外边类被加载的时候，是不会加载内部类

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public class Mgr06 {

    private Mgr06() {
    }

    private static class Mgr06Holder{
        private static final Mgr06 mgr06 = new Mgr06();
    }

    public static Mgr06 getInstance() {

        return Mgr06Holder.mgr06;
    }


    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(Mgr06.getInstance().hashCode());
            }).start();
        }
    }
}

7.枚举类 ： 完美中的完美

• 不仅可以解决线程同步，还可以防止反序列化

package com.fengzi.design;

/**
 * @author:
 * @Date:2021/12/15/14:00
 * @Description
 */
public enum Mgr07 {

    INSTANCE;

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(Mgr07.INSTANCE.hashCode());
            }).start();
        }
    }
}