Wednesday, November 17, 2010

Spring NamespaceHandler debugging

While updating a library that used Spring yesterday, I began suffering the dreaded "Unable to locate Spring NamespaceHandler for XML schema namespace" exception.

The net is littered with threads about this exception, all of which end with "put the spring jars in your WEB-INF/lib directory".

I was fairly determined not to do this, as I frequently start a webapp with an embedded instance of Jetty, and use the Eclipse project's classpath for all of the dependencies. Since all of the jars are from the project's classpath, the webapp is always using exactly the same jars that Eclipse is for compiling your code, so you never have the two drift apart.

Too many times, after copying jars to WEB-INF/lib and forgetting about them, I'll upgrade a library, everything compiles fine, but spend an embarrassing amount of time wondering why it's not working in the webapp, before remembering the stale jar in the WEB-INF/lib directory.

Anyway, the real cause of the NamespaceHandler exception in my case was a buggy ClassLoader.getResources implementation.

The way spring works, when doing it's XML parsing/whatever magic, is when it sees "xmlns:tx=...", it wants a new NamespaceHandler that knows how to handle those tags.

To allow extensible NamespaceHandlers, Spring uses ClassLoader.getResources("META-INF/spring.handers") to get a list of all of the spring.handlers files across all of the jar files in the classloader. So if spring-core, spring-tx, spring-etc. all have spring.handlers with NamespaceHandlers in them, each file gets found and loaded.

Here's the rub: whatever Eclipse project classloader that had spring-tx on it only returned spring.handlers files from jars that already had classes loaded from them. The ClassLoader.getResources implementation would not look into jar files that was on its classpath, but had not yet been opened for loading classes from.

Of all things, adding:


Before spring was initialized fixed the NamespaceHandler error. Everything boots up correctly now.

While it took way too long to figure out, I'm pleased that I can continue using the Eclipse project classloader for the webapp I'm starting and avoid the annoying "copy jars to WEB-INF/lib" solution.

I'd like to know which classloader had the buggy getResources implementation, but I've already spent too much time on this so far.

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