如何在Java中正确地创建一个线程安全的Singleton Factory？ [英] How to correctly make a thread safe Singleton Factory in Java?

问题描述

这是我第一次编写 Factory 类。下面是我的Factory类，我不确定这是否是制作线程安全的Singleton Factory类的正确方法。我将使用此工厂返回客户端的实例？

This is the first time I am writing a Factory class. Below is my Factory class, I am not sure whether this is the correct way of making thread safe Singleton Factory class or not. I will be returning instance of my Client using this factory?

public class ClientFactory {

    private static ClientFactory instance = null;   

    private ClientFactory() {

    }

    public static ClientFactory getInstance() {

        if (instance == null)
        {
            instance =  new ClientFactory();
        }

        return instance;
    }

    public IClient getClient() {

        return new TestClient();
    }
}

这是我的TestClient类 -

And here is my TestClient class -

public class TestClient implements IClient {


}

这就是我将如何使用我的工厂 -

And this is how I am going to use my factory -

IClient client = ClientFactory.getInstance().getClient();

推荐答案

实际上你的工厂不是线程安全的，因为在竞争条件下，您可以在应用程序中拥有多个ClientFactory。让我们假设两个线程：

In fact your factory isn't thread safe, because in race condition you can have more than one ClientFactory living in application. Lets assume two threads:

1. ThreadA正在评估条件'if（instance == null）'并且实例为null，因此它输入语句


2. ThreadB正在评估条件'if（instance == null）'并且实例为null（因为A没有实例化它），所以它输入语句


3. ThreadA创建新的ClientFactory（）并返回它


4. ThreadB创建新的ClientFactory（）并返回它


5. 现在我们有多个应用程序中的ClientFactory。当然，其他线程试图在一段时间后检索实例将始终返回单个实例。

1. ThreadA is evaluating condition 'if (instance == null)' and instance is null, so it enters statement
2. ThreadB is evaluating condition 'if (instance == null)' and instance is null (because A didn't make to instantiate it), so it enters statement
3. ThreadA creates new ClientFactory() and returns it
4. ThreadB creates new ClientFactory() and returns it
5. Now we have more than one ClientFactory in application. Of course other threads trying to retrieve instance some time later will always return single instance.

在我看来，编写单例的最简单方法Java是使用枚举。在你的情况下，它将看起来：

In my opinion the easiest way to write singleton in Java is to use enum. In your case it will looks:

public enum ClientFactory {
  INSTANCE;

  public Company getClient() {
    return new Company();
  }
}

用法：

ClientFactory.INSTANCE.getClient()

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