单例模式

单线程下可用

class Singleton {
    private static Singleton singleton;
    private Singleton(){
        System.out.println("***************new Singleton***************");
    }
    public static Singleton getInstance() {
        if (singleton == null) {
            singleton = new Singleton();
        }
        return singleton;
    }

    public void print() {
        System.out.println("Hello Singleton***************");
    }
    @Override
    public String toString() {
        return Singleton.class.getName();
    }
}
class  App {
    public static void main(String[] args) {
        Singleton instance = Singleton.getInstance();
        instance.print();
    }
}

多线程下（推荐）

class Singleton2 {
    private static volatile Singleton2 singleton;
    private Singleton2(){
        System.out.println("***************new Singleton2***************");
    }
    public static Singleton2 getInstance() {
        if (singleton == null) {
            synchronized (Singleton2.class) {
                if (singleton == null) {
                    singleton = new Singleton2();
                }
            }
        }
        return singleton;
    }

    public void print() {
        System.out.println("Hello Singleton2***************");
    }
    @Override
    public String toString() {
        return Singleton2.class.getName();
    }
}

class  App2 {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            new Thread(() -> Singleton2.getInstance().print(), "thread-" + i).start();
        }
    }
}