多线程下单例模式示例（synchronized）

class SingleDemo {

    public static SingleDemo instance = null;

    private SingleDemo() {
        System.out.println(Thread.currentThread().getName() + "\t 我是SingleDemo构造方法");
    }

    public static synchronized SingleDemo getInstance() {
        if (instance == null) {
            instance = new SingleDemo();
        }
        return instance;
    }
}


public class Main {

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            new Thread(() -> {
                SingleDemo.getInstance();
            }, String.valueOf(i)).start();
        }
    }
}