Java静态初始化程序是否安全？ [英] Are Java static initializers thread safe?

问题描述

我正在使用静态代码块来初始化我所拥有的注册表中的某些控制器。因此，我的问题是，我可以保证这个静态代码块只在首次加载类时才会被调用一次吗？我知道我无法保证何时会调用此代码块，我猜它是在Classloader首次加载时。我意识到我可以在静态代码块中的类上进行同步，但我的猜测实际上这是怎么回事？

I'm using a static code block to initialize some controllers in a registry I have. My question is therefore, can I guarantee that this static code block will only absolutely be called once when the class is first loaded? I understand I cannot guarantee when this code block will be called, I'm guessing its when the Classloader first loads it. I realize I could synchronize on the class in the static code block, but my guess is this is actually what happens anyway?

简单的代码示例是;

class FooRegistry {

    static {
        //this code must only ever be called once 
        addController(new FooControllerImpl());
    }

    private static void addController(IFooController controller) { 
        // ...
    }
}

或者我应该这样做;

class FooRegistry {

    static {
        synchronized(FooRegistry.class) {
            addController(new FooControllerImpl());
        }
    }

    private static void addController(IFooController controller) {
        // ...
    }
}

推荐答案

是的，Java静态初始化程序是线程安全的（使用您的第一个选项）。

Yes, Java static initializers are thread safe (use your first option).

但是，如果您想确保代码只是在确保该类仅由单个类加载器加载后执行一次。每个类加载器执行一次静态初始化。

However, if you want to ensure that the code is executed exactly once you need to make sure that the class is only loaded by a single class-loader. Static initialization is performed once per class-loader.

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