Java library search order

Question

Background / example (but question is probably broader than this):

I'm trying to write a Java application that accesses a Google AppEngine server. To set up the project for this, I followed the steps outlined in the accepted answer here:

Developing a Java Application that uses an AppEngine database

I am now running into a problem where I'm trying to execute an HttpURLConnection-request in the Java client application (i.e. not in the AppEngine server code), but Google's AppEngine library seems to have replaced the Java version of this connection with its own urlFetch()-implementation. This leads to me getting the following error: "The API package 'urlfetch' or call 'Fetch()' was not found.".

Actual question:

What determines the order in which Java looks through libraries to find needed class-implementations? Is there a way to modify this order (specifically in Eclipse), so that the actual JRE-functions take precedence over a third-party-library that is also needed. Or is there maybe something special going on with the implementation of Url in the example given above, that cannot be resolved by specifying a library order?

Update:

Turns out the problem I was seeing had nothing to do with the order in which classes were loaded. The AppEngine server code explicitly calls setContentHandlerFactory(...) to register its own handler during execution rather than at library load time (see here for a fix to this specific issue). So, while my "actual question" might still stand, I haven't actually yet come across a scenario where it matters...

Answer 1

You might have to define a custom ClassLoader.

Also, take a look at this answer.

Answer 2

Inside Eclipse, you can adjust the classpath order. Right click your project, choose Properties, Java Build Path, then click the "Order and Export" tab. However, of course, this won't affect your program when running outside Eclipse.