單例模式的適用場(chǎng)景：各種管理類、各種工廠類

實(shí)現(xiàn)方式：

餓漢式：
優(yōu)點(diǎn)：簡(jiǎn)潔，易懂，個(gè)人更傾向?qū)嶋H中使用這種
缺點(diǎn)：每次類加載時(shí)就會(huì)實(shí)例化 ，不能防止反序列化

package com.example.demo;

public class Singleton1 {

    private static final Singleton1 INSTANCE = new Singleton1();

    private Singleton1(){

    }

    public static Singleton1 getInstance(){
        return INSTANCE;
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Singleton1.getInstance().hashCode());
            }).start();
        }
    }
}

懶漢式
缺點(diǎn)：加鎖效率低

package com.example.demo;

public class Singleton2 {
    private static volatile Singleton2 INSTANCE;
    private Singleton2(){

    }

    public static Singleton2 getInstance(){
        //雙重檢查，第一層是為了如果已經(jīng)實(shí)例化其他線程沒必要走到加鎖那一步
        if(INSTANCE == null) {
            synchronized (Singleton2.class) {
                if(INSTANCE == null) {
                    INSTANCE = new Singleton2();
                }
            }
        }
        return INSTANCE;
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Singleton2.getInstance().hashCode());
            }).start();
        }
    }
}

靜態(tài)內(nèi)部類實(shí)現(xiàn)
缺點(diǎn)：可以被反序列化

package com.example.demo;

public class Singleton3 {
    private Singleton3(){

    }
    private static class Singleton3Holder{
        private final static Singleton3 INSTANCE = new Singleton3();
    }

    public static Singleton3 getInstance(){
        return Singleton3Holder.INSTANCE;
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Singleton3.getInstance().hashCode());
            }).start();
        }
    }
}

枚舉方式實(shí)現(xiàn)
解決了線程問題和反序列化問題，傳說中最完美方式

package com.example.demo;

public enum Singleton4 {
    INSTANCE;

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Singleton4.INSTANCE.hashCode());
            }).start();
        }
    }
}