Question

Today I spent my afternoon with analysing a NoClassDefFoundError. After verifying the classpath again and again, it turned out that there was a static member of a class that threw an Exception that was ignored the first time. After that every use of the class throw a NoClassDefFoundError without a meaningful stacktrace:

Exception in thread "main" java.lang.NoClassDefFoundError: 
    Could not initialize class InitializationProblem$A
    at InitializationProblem.main(InitializationProblem.java:19)

That’s all. No more lines.

Reduced to the point, this was the problem:

public class InitializationProblem {
    public static class A {
        static int foo = 1 / 0;
        static String getId() {
            return "42";
        }
    }

    public static void main( String[] args ) {
        try {
            new A();
        }
        catch( Error e ) {
            // ignore the initialization error
        }

        // here an Error is being thrown again,
        // without any hint what is going wrong.
        A.getId();
    }
}

To make it not so easy, all but the last call of A.getId() was hidden somewhere in the initialization code of a very big project.

Question:

Now that I’ve found this error after hours of trial and error, I’m wondering if there is a straight forward way to find this bug starting from the thrown exception. Any ideas on how to do this?

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I hope this question will be a hint for anyone else analysing an inexplicable NoClassDefFoundError.

Answer 1

My advice would be to avoid this problem by avoiding static initializers as much as you can. Because these initializers get executed during the classloading process, many frameworks don’t handle them very well, and in fact older VMs don’t handle them very well either.

Most (if not all) static initializers can be refactored into other forms, and in general it makes the problems easier to handle and diagnose. As you’ve discovered, static initializers are forbidden from throwing checked exceptions, so you’ve got to either log-and-ignore, or log-and-rethrow-as-unchecked, none of which make the job of diagnosis any easier.

Also, most classloaders make one-and-only-one attempt to load a given class, and if it fails the first time, and isn’t handled properly, the problem gets effectively squashed, and you end up with generic Errors being thrown, with little or no context.