java - 为什么不能使用 ExceptionInInitializerError 调用 initCause ？

Question

为什么使用 initCause 调用 ExceptionInInitializerError 会产生 IllegalStateException ？

public class Test  {
   public static final void main(String[] ignored)  {
      try  {
         ExceptionInInitializerError err = new ExceptionInInitializerError("1. Fail!");
         err.initCause(new NullPointerException("null!"));
         throw  err;
      }  catch(Exception x)  {
         x.printStackTrace();
      }

      System.out.println();

      try  {
         IllegalStateException isx = new IllegalStateException("2. Fail!");
         isx.initCause(new NullPointerException("null!"));
         throw  isx;
      }  catch(Exception x)  {
         x.printStackTrace();
      }

      System.out.println();

      try  {
         throw  new IllegalStateException("3. Fail!", new NullPointerException("null!"));
      }  catch(Exception x)  {
         x.printStackTrace();
      }
   }
}

输出:

java.lang.IllegalStateException: Can't overwrite cause with java.lang.NullPointerException: null!
        at java.lang.Throwable.initCause(Throwable.java:456)
        at Test.main(Test.java:5)
Caused by: java.lang.ExceptionInInitializerError: 1. Fail!
        at Test.main(Test.java:4)

java.lang.IllegalStateException: 2. Fail!
        at Test.main(Test.java:14)
Caused by: java.lang.NullPointerException: null!
        at Test.main(Test.java:15)

java.lang.IllegalStateException: 3. Fail!
        at Test.main(Test.java:24)
Caused by: java.lang.NullPointerException: null!
    ... 1 more

我在文档中没有看到任何关于构造函数或类本身的内容，要么说(a)不允许，要么(b)为什么不允许。我确实在 JavaDoc 中看到了 "There is no saved throwable object."，但是，对我来说，这意味着可以设置一个。

这是“只是因为”，还是有其他原因导致您无法设置 ExceptionInInitializerError 的原因？

Answer 1

当我研究这个问题时，我正在查看ExceptionInInitializerError的源代码。

首先，class JavaDoc中有这个:

As of release 1.4, this exception has been retrofitted to conform to the general purpose exception-chaining mechanism. The "saved throwable object" that may be provided at construction time and accessed via the getException() method is now known as the cause, and may be accessed via the Throwable.getCause() method, as well as the aforementioned "legacy method."

没有任何关于“不要使用initCause”的内容，但这似乎是您将得到的最接近的结果。

constructors明确地使这成为不可能，并且内部注释比 JavaDoc 中的任何内容都清晰得多(只说“没有保存的可抛出对象”):

/**
   <P>... There is no saved throwable object.</P>
 **/
public ExceptionInInitializerError() {
   initCause(null);  // Disallow subsequent initCause
}
public ExceptionInInitializerError(Throwable thrown) {
   initCause(null);  // Disallow subsequent initCause
   this.exception = thrown;
}
public ExceptionInInitializerError(String s) {
   super(s);
   initCause(null);  // Disallow subsequent initCause
}

不太明白真正的原因，但它似乎是无法改造的遗留代码和“只是因为”的混合体。