Singleton类有几个不同的类加载器 [英] Singleton class with several different classloaders

问题描述

例如，我有类 Singleton ，静态字段 instance ：

E.g I have class Singleton with static field instance:

public class Singleton {

    private static Singleton instance;

    // other code, construct, getters, no matter    
}

我可以使用两个不同的类加载器加载此类两次。我怎么能避免它？这是不安全和危险的。

I can load this class twice with two different classloaders. How could I avoid it? It is unsafe and dangerous.

另外，如果我将实例设置为null，它是否会为这两个类设置为null？

Also, if I set instance to null, would it set to null for both classes?

Singleton singleton = Singleton.getInstance();
singleton = null;

推荐答案

如果你想要一个真正的在类加载器中使用Singleton ，然后你需要一个共同的父来加载有问题的类，或者你需要自己指定类加载器。

If you want a true Singleton across classloaders, then you need a common parent to load the class in question, or you need to specify the classloader yourself.

更新：从@Pshemo的评论下面，博客中的一些内容可能直接来自 JavaWorld文章。我已离开博客条目，因为它可能仍然可以帮助某人，但它值得知道内容最初来自哪里。

Update: From the comment from @Pshemo below a fair bit of the content in the blog below might come directly from a JavaWorld Article. I've left the blog entry in as it may still help someone, but its worth knowing where the content originally came from.

原文：
有一个博客条目为您提供了一种方式要做到这一点（虽然我还没试过！），看起来相当合理

Original: There is a blog entry that gives you a way to do this" (although I havent tried it!), and it looks fairly reasonable

如下所示，我上面链接的代码片段 - 我建议您访问完整上下文的博客：

As requested below here a code snippet from my link above - I do suggest you visit the blog though for the full context:

private static Class getClass(String classname) throws ClassNotFoundException {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    if(classLoader == null) 
        classLoader = Singleton.class.getClassLoader();
      return (classLoader.loadClass(classname));
}

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