单例模式之——饿汉模式、懒汉模式

饿汉模式

优点：线程安全，不存在并发问题

缺点：在类加载之前就创建好了对象

public class Singleton {
    private Singleton(){}
    private static Singleton singleton = new Singleton();
    public static Singleton getInstance(){
        return singleton;
    }

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

懒汉模式（不推荐使用）

缺点：线程不安全，存在并发问题

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

懒汉模式 DCL（double check lock）(推荐)

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

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