XFire, Applets, and 404s

XFire Mapping Files

The application I work on uses XFire as a Java SOAP framework (this project has been replaced by CXF which is why this isn't a bug report with a patch) that is launched via Web Start. At some point we decided to allow it to be run as an applet as well. One of the weird things I ran into when we did this was a ton of 404 errors in the access logs on the server. The applet was repeatedly trying to load these "*.aegis.xml" files. These are mapping files used to configure how XFire maps your types to XML. However, you don't necessarily need these files if you elect to configure the mappings another way. XFire apparently still looks for the mapping file to allow you a mechanism for overriding other mapping methods. Everybody still here? Good.

Repeatedly Loading Non-Existent Resources

Since we don't use the mapping files, they're not in any of the jars that would be loaded by the applet. So what happens is XFire calls XMLClassMetaInfoManager.getDocument whenever it needs to figure out how to map one of your objects to XML. The manager then executes a getResourceAsStream to see if you have a mapping file (either as a primary or backup configuration). This winds up in the classloader which in the case of an applet is the AppletClassLoader. The class loader first tries to find the requested resource locally and if it's not there it tries to load it from the codebase. In our case it's not on the server so you get a 404 in the logs. The fact that the resource isn't available anywhere isn't a problem but the fact that this information isn't cached anywhere means that the next time something looks for the mapping file the whole thing is going to happen all over again.

The code in XMLClassMetaInfoManager.getDocument could easily add and check for a key with a null value in the event that it had previously tried to find the mapping and couldn't. This is what it does instead:

	public Document getDocument(Class clazz) {
		if (clazz == null)
			return null;
		Document doc = (Document) documents.get(clazz.getName());
		if (doc != null) {
			return doc;
		}
		String path = '/' + clazz.getName().replace('.', '/') + ".aegis.xml";
		InputStream is = clazz.getResourceAsStream(path);
		if (is == null) {
			log.debug("Mapping file : " + path + " not found.");
			return null;
		}
		log.debug("Found mapping file : " + path);
		try {
			doc = new StaxBuilder().build(is);
			documents.put(clazz.getName(), doc);
			return doc;
		} catch (XMLStreamException e) {
			log.error("Error loading file " + path, e);
		}
		return null;
	}

(Incidentally, I'm really liking the syntaxhighlighter WordPress plugin.) The AppletClassLoader is off the hook because you can easily say that its behavior is a feature, not a bug.

The Quick Fix

Like I said, XFire is in maintenance mode. The prospect of getting the source code, creating a vendor branch in the SCM, fixing / testing, and finally internally hosting the patched artifact didn't appeal to me on the limited time scale on which I was working. The quick fix (that apparently only works in JRE 1.6) is to set the codebase_lookup parameter to false in the applet tag.

Sadly, the 1.6 restriction means I probably won't get to live with this fix for long and will likely wind up either patching XFire or upgrading to CXF. CXF may or may not still have the problem, but at least I'd be writing a patch for an actively developed library.

To further complicate things, there are other incidents of 404s with resource bundles and XML parsers. The nice thing about the codebase_lookup solution is that it "fixes" those issues as well. If I can't rely on JRE 1.6 as a requirement I'll have a lot more work to do than just patching XFire.

Applets. Yay.

Update

It's probably actually XMLTypeCreator that is causing my problem, but the code is pretty much identical. There are also issues with XMLDocumentationBuilder.loadDocument. I don't blame XFire in particular. There are a lot of projects out there with code that uses getResourceAsStream knowing that it could fail and then not caching the fact that it failed. They then repeat the operation multiple times. This assumes that getResourceAsStream and other such operations are very cheap performance-wise or that it's not worth the effort or memory to cache failed attempts. I'm not sure I buy it in the first place but it certainly goes out the window when you're in an applet and every such call that fails locally then gets executed across the network back to your codebase.

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3 Responses to “XFire, Applets, and 404s”

  1. Running as Root » Blog Archive » More on applets and codebase_lookup Says:

    […] I mentioned in the last post I'm farting around with applets for work. You may remember that the applet was hammering the […]

  2. Running as Root » Blog Archive » Interminably Long Timeouts for META-INF Under IIS Says:

    […] events: I'm working on speeding up a Java applet, sloppy code in applet libraries try to load resources from the server which then 404, you can avoid this by setting the codebase_lookup property to false in the applet […]

  3. Michael Villere Says:

    Good post Robert. I am experiencing similar pains working with Applets on both Windows and Mac. I'm limited to running Java 5 since Java 6 is not well supported yet on the mac side.

    Thanks for the info.
    -Michael

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