具有多个不同类加载器的单例类 [英] Singleton class with several different classloaders

问题描述

例如我有类 Singleton 和静态字段 instance:

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

public class Singleton {

    private static Singleton instance;

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

我可以用两个不同的类加载器两次加载这个类.我怎么能避免呢?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 instance 设置为 null，它是否会为两个类都设置为 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 Singleton 跨类加载器，那么你需要一个共同的父级来加载有问题的类，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:下面来自@Pshemo 的评论，下面博客中的相当一部分内容可能直接来自 JavaWorld 文章.我已经留下了博客条目，因为它可能仍然对某人有帮助，但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法"(虽然我没试过！)，而且看起来还算合理

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

根据下面的要求，我上面链接中的代码片段 - 我建议您访问博客以获取完整的上下文:

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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