Order of loading jar files from lib directory

It's all described in Tomcat's ClassLoading HOW-TO. It's not necessarily in alphabetic order. If you observed that behaviour, it should absolutely not be relied upon if you intend to keep your webapp portable across servers. For example, Tomcat 6 "coincidentally" orders it, but Tomcat 8 does not.

Summarized, the loading order is as follows:

1. bootstrap/system (JRE/lib, then server.loader)
2. webapp libraries (WEB-INF/classes, then WEB-INF/lib)
3. common libraries (common.loader, then Tomcat/lib)
4. webapp-shared libraries (shared.loader)

If you would like to guarantee that JAR X is loaded after JAR Y, then you'd need to put JAR X in one of the places which appears later in the listing above.

There are exceptions however, which are mentioned in the tomcat docs

Lastly, the web application class loader will always delegate first for JavaEE API classes for the specifications implemented by Tomcat (Servlet, JSP, EL, WebSocket). All other class loaders in Tomcat follow the usual delegation pattern.

That means if a webapp contains any JavaEE classes (javax.*), then it will be ignored by tomcat.

For each loader, the classes are just loaded by the JVM in the order whenever they needs to be imported/executed and are not loaded yet.

Actually, it is alphabetical order! (Within a specific directory, e.g. the 'lib' directory that the original poster mentions.)

More specifically, if you look at the source for Tomcat 6, in class FileDirContext, the method list() calls Arrays.sort() on the array of file names of jars that are found.

I have tested this manually as well. I create a war with a JSP that calls HelloWorld.getGreeting(), and put two nearly identical jars containing slightly different versions of HelloWorld into the WEB-INF/lib directory. One says "Hello, world", the other "Goodbye, cruel world".

If I name the "Hello, world" version a.jar, and the "goodbye" version b.jar, and restart Tomcat, I get the "Hello" text. If I name the jars the other 'way round and restart Tomcat, I get the "Goodbye" text.

As far as I have been able to determine, this behaviour is NOT documented, NOT specified, and should NOT be relied on. But it is alphabetical -- for now.