Solving a GWT deferred binding mystery

This morning at work, I was running our GWT application with some deferred binding logic in it, and all of a sudden I got this ridiculous message:

[ERROR] Class 'mycompany.rebind.HistoryResourceGenerator' must derive from 'com.google.gwt.core.ext.Generator'
[ERROR] Failure while parsing XML
com.google.gwt.core.ext.UnableToCompleteException: (see previous log entries)
at com.google.gwt.dev.cfg.ModuleDefSchema$ObjAttrCvt.convertToArg(ModuleDefSchema.java:729)
at com.google.gwt.dev.util.xml.HandlerArgs.convertToArg(HandlerArgs.java:64)
at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:214)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:257)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(Abstract...)

It was running fine yesterday when I left work, and now it tells me that my generator isn’t a subclass of the GWT Generator??? Quickly pulled the source, and it’s as clear as day that my generator is properly written. Then what gives?

Searching for an explanation, I pulled the GWT’s source code, and opened com.google.gwt.dev.cfg.ModuleDefSchema and here’s the method in question:

  public Object convertToArg(Schema schema, int lineNumber, String elemName,
      String attrName, String attrValue) throws UnableToCompleteException {

    Object found = singletonsByName.get(attrValue);
    if (found != null) {
      // Found in the cache.
      //
      return found;
    }

    Class<?> foundClass = null;
    try {
      // Load the class.
      //
      ClassLoader cl = Thread.currentThread().getContextClassLoader();
      foundClass = cl.loadClass(attrValue);
      Class<? extends T> clazz = foundClass.asSubclass(fReqdSuperclass);

      T object = clazz.newInstance();
      singletonsByName.put(attrValue, object);
      return object;
    } catch (ClassCastException e) {
      Messages.INVALID_CLASS_DERIVATION.log(logger, foundClass,
          fReqdSuperclass, null);
      throw new UnableToCompleteException();
    } catch (ClassNotFoundException e) {
      Messages.UNABLE_TO_LOAD_CLASS.log(logger, attrValue, e);
      throw new UnableToCompleteException();
    } catch (InstantiationException e) {
      Messages.UNABLE_TO_CREATE_OBJECT.log(logger, attrValue, e);
      throw new UnableToCompleteException();
    } catch (IllegalAccessException e) {
      Messages.UNABLE_TO_CREATE_OBJECT.log(logger, attrValue, e);
      throw new UnableToCompleteException();
    }
  }
}

Because the error message was from Messages.INVALID\_CLASS\_DERIVATION, I put a breakpoint on line 22:

Messages.INVALID_CLASS_DERIVATION.log(logger, foundClass, fReqdSuperclass, null);

and started my app in debug mode. The breakpoint hits, and I noticed that the exception object is reporting ClassCastException from JSONNull to JSONObject. I quickly clicked: The error is from the class that fetches some resource over the net and plugged in to the generator. Following this lead, I found that the server resource the class was fetching wasn’t available any more, which caused the fetcher class to fail, and consequently, the ClassCastException is propagated to the generator and eventually ModuleDefSchema.

The mysterious error message, however, has nothing to do with the real problem. I spent a lot of time thinking that my Generator is wrong, because that’s what the compiler says, but in fact, it’s not. I think the confusion can be avoided if the GWT compiler can log the actual exception object, instead of interpreting the exception for the user. That’s what they’re doing in the try/catch clause in the method shown above: the exception object e is not used in the case of ClassCastException…

Well, the take-home message is: don’t always trust the GWT compiler message. Also, adding log more logging to your generator classes that can be indicative to where the actual problem is.