I have an IDEA! Wait... Dude, where's my jar?

The most funny thing in programming for me is how programmers can create problems from nothing and then heroically trying to solve them.

On StackOverflow.com, there are the tales about kinky developers who want to use two Maven artifacts of different versions in their projects. You might as well want to do it for whatever reason.

But what about using two (or more) dependencies with the same artifactId and version?
"Gotta catch 'em all!", you know 😄 Well, you'll be fine unless you're using some convenient tools provided by Intellij IDEA.

The tools are the "Build Artifacts..." button and the configurable tasks (aka configurations, i.e. deployment on application server, which do not directly build artifacts but commence the builds and use the artifacts produced):

Let's look at an example.

Somewhere in pom.xml of our WAR project:

	<!-- same artifactId and version as the artifact below! -->
	<dependency>
	<groupId>a.b.c</groupId>
	<artifactId>foo</artifactId>
	<version>1.0</version>
	</dependency>
	<!-- same artifactId and version as the artifact above! -->
	<dependency>
	<groupId>x.y.z</groupId>
	<artifactId>foo</artifactId>
	<version>1.0</version>
	</dependency>
	<!-- not related in any way to the dependencies above -->
	<dependency>
	<groupId>com.acme</groupId>
	<artifactId>bar</artifactId>
	<version>1.2.3</version>
	</dependency>

view raw pom.xml hosted with ❤ by GitHub

What kind of dragon we'll have to slay here? Well, how about infamous NoClassDefFoundError?

Why would it happen? See, if you build the artifact with Maven alone (via CLI or the Maven toolbar in IDEA) by executing the war:war/war:exploded goals of the Maven WAR plugin, there will be no name clashes as the conflicting dependencies will have their names prefixed with the corresponding groupIds:

dependencies of the WAR artifact built by mvn war:war

dependencies of the exploded WAR artifact built by mvn war:exploded

But what happens when you build one of the artifacts (war/exploded war) either via "Build Artifacts" or a configured task? A name clash. It turns out that IDEA doesn't apply any prefixes, so the content of the lib directory (or whatever the name of the directory in which you've ordered to put the dependencies in is) looks like this:

dependencies of the WAR artifact built by IDEA

dependencies of the exploded WAR artifact built by IDEA

So, after the build you won't really have both of the libraries in the classpath, because in the output directory there will be just one file named foo-1.0.jar which corresponds to one of the "conflicting" dependencies. And when your program tries to load classes from the absent library, that sneaky bastard NoClassDefFoundError is going in for the kill.

But don't worry. There's a way to get the library back and bring the pesky error down. I don't know whether IDEA uses Maven for building Maven artifacts or does it by some other means, but it respects the content of pom.xml and configuration of the Maven WAR plugin in particular. And for this plugin there's an option for setting the pattern for the names of the dependencies, and the option is called <outputFileNameMapping>:

	<build>
	<plugins>
	<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-war-plugin</artifactId>
	<version>3.0.0</version>
	<configuration>
	<outputFileNameMapping>@{groupId}@-@{artifactId}@-@{version}@@{dashClassifier?}@.@{extension}@</outputFileNameMapping>
	</configuration>
	</plugin>
	</plugins>
	</build>

view raw pom.xml hosted with ❤ by GitHub

You just add the option, set the pattern and that's it. When building the artifact, IDEA will now be using the pattern for all the dependencies that it has to put in the artifact's library directory:

dependencies of the WAR artifact built by IDEA with the outputFileNameMapping property configured

dependencies of the WAR artifact built by IDEA with the outputFileNameMapping property configured

A related link explaining the Maven WAR plugin's <outputFileNameMapping> option: https://maven.apache.org/plugins/maven-war-plugin/examples/file-name-mapping.html