java – 为什么NoClassDefFoundError由静态字段初始化失败引起？

这是一个有趣的 java问题.

以下简单的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.

但是当我访问Class时,它仍然可用;

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();
        }
    }   
}

最佳答案

这些问题的答案通常都隐藏在规范…… (§12.4.2)中

初始化类时会发生什么：

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：

5. If the Class object is in an erroneous state, then initialization is not possible. Release the lock on the Class object and throw a NoClassDefFoundError.

6-8继续初始化,8执行初始化器,通常发生的是在步骤9：

9. If the execution of the initializers completes normally, then lock this Class object, label it fully initialized, notify all waiting threads, release the lock, and complete this procedure normally.

但是我们在初始化程序中出错了所以：

10. Otherwise, the initializers must have completed abruptly by throwing some exception E. If the class of E is not Error or one of its subclasses, then create a new instance of the class ExceptionInInitializerError, with E as the argument, and use this object in place of E in the following step. But if a new instance of ExceptionInInitializerError cannot be created because an OutOfMemoryError occurs, then instead use an OutOfMemoryError object in place of E in the following step.

是的,我们看到空指针异常的ExceptionInInitializerError b / c.

11. Lock the Class object, label it erroneous, notify all waiting threads, release the lock, and complete this procedure abruptly with reason E or its replacement as determined in the previous step. (Due to a flaw in some early implementations, a exception during class initialization was ignored, rather than causing an ExceptionInInitializerError as described here.)

然后该类被标记为错误,这就是我们第二次从第5步获得异常的原因.