java - 单例模式的正确使用

Question

我正在尝试实现单例模式的示例。我们的问题之一是运行两个线程，每个线程调用 getInstance() 并验证只创建了一个 Singleton 对象实例。

这是我的单例代码；

public class OurSingleton {

    static OurSingleton ourSingleton;
    static int instanceCounter;

    private OurSingleton(){
        instanceCounter++;
    }

    public static synchronized OurSingleton GetSingletonInstance(){

        if( ourSingleton == null){

            ourSingleton = new OurSingleton();

        }
        return ourSingleton;    
    }

    public static int getCounter() {

        return instanceCounter;

    }
}

还有我的主要;

public class Main {

    /**
     * @param args
     */
    public static void main(String[] args) {

        OurSingleton mySingleton = null;

        Thread one = new Thread(new GetSingletonInstance(mySingleton));
        Thread two = new Thread(new GetSingletonInstance(mySingleton));

        one.start();
        two.start();


        System.out.println("Main: " + mySingleton.getCounter());
    }   
}

class GetSingletonInstance implements Runnable {

    int count = 0;
    OurSingleton singleton;

    public GetSingletonInstance(OurSingleton ourSingleton){
        singleton = ourSingleton;
    }

    @Override
    public void run() {
        try {
            while (count < 5000000) {
                singleton.getSingletonInstance();
                count++;    
            }

        } catch (Exception e) {

            e.printStackTrace();
        }

        System.out.println("Thread: " + singleton.getCounter());

    }       
}

当我运行这段代码时，我得到以下输出；

Main: 0 Thread: 1 Thread: 1

有人可以解释这个输出的原因吗？我原以为只存在单一的单一实例。这是否意味着正在线程中创建另一个对象？？任何建议表示赞赏!

Answer 1

您应该避免同步 getInstance 方法(因为它只是在初始化实例后获取实例的不必要开销)。以下是惰性初始化单例的推荐方法:

public class OurSingleton {

    private OurSingleton() { }

    public static OurSingleton getInstance() {
        return Holder.instance;
    }

    private static class Holder {
        private static OurSingleton instance = new OurSingleton();
    }        
}