本文介绍了为什么NoClassDefFoundError由静态字段初始化失败引起?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

这是一个有趣的java问题。

Here is a interesting java question.

以下简单java程序包含静态方法初始化的静态字段。实际上,我强制计算intiailize值的方法引发NullPointException,当我访问这样的静态字段时,会引发NoClassDefFoundError。似乎VM对待Class并不完整。

the following simple java program contains static field initialized by a method statically. Actually, I force the method which calculate the intiailize value to raise a NullPointException, When I access such a static field, a NoClassDefFoundError will raised. it seems the VM treat the Class is not complete.

但是当我访问Class时,它仍然可用;

But when I access the Class, it still available;

有谁知道为什么?

class TestClass {
    public static TestClass instance = init();

    public static TestClass init() {
       String a = null;
       a.charAt(0); //force a null point exception;
       return new TestClass();
    }
}

class MainClass {
    static public void main(String[] args) {
       accessStatic(); // a ExceptionInInitializerError raised cause by NullPointer
       accessStatic(); //now a NoClassDefFoundError occurs;

       // But the class of TestClass is still available; why?
       System.out.println("TestClass.class=" + TestClass.class);
    }

    static void accessStatic() {
        TestClass a;

        try {
            a = TestClass.instance;
        } catch(Throwable e) {
            e.printStackTrace();
        }
    }
}


推荐答案

这些问题的答案通常都隐藏在规范中......

The answer to such questions is usually buried somewhere in the specs... (§12.4.2)

初始化类时会发生什么:

What happens when classes are initialized:

步骤1-4与此问题有些无关。这里的第5步是触发异常的原因:

Steps 1-4 are somewhat unrelated to this question. Step 5 here is what triggers the exception:

6-8继续初始化,8执行初始值设定项,通常在步骤9中执行:

6-8 continue the initialization, 8 executes the initializers, and what usually happens is in step 9:

但我们在初始化程序中出错:

But we got an error in the initializer so:

是的,我们看到 ExceptionInInitializerError b / c的空指针异常。

Yep, we see an ExceptionInInitializerError b/c of the null pointer exception.

然后该类被标记为错误,这就是我们第二次从第5步获得例外的原因。

And then the class is marked erroneous which is why we get the exception from step 5 the second time.

可能是因为 TestClass 仍然存在,它只是标记为错误。它已经加载并验证。

Probably because TestClass still exists, it's just marked erroneous. It has been already loaded and verified.

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07-29 17:06
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