创建型模式--单例模式

饿汉模式

提前创建好了

//饿汉式
public class Singleton {
    private static final Singleton instance = new Singleton();
    private Singleton() {
        //私有构造方法，防止外部实例化
    }
    public static Singleton getInstance() {
        return instance;
    }
}

懒汉式

先用先创建

public class Singleton {
    private static volatile Singleton instance;
    private Singleton(){
        //私有构造方法，防止外部实例化
    }
    public static Singleton getInstance(){
        if(instance == null){
            synchronized (Singleton.class){
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}