Discussing the nuts and bolts of software development

Friday, September 18, 2009


Maven Compiler Tips and Tricks

Apache’s Maven is a great tool for managing a build environment: it keeps track of all project dependencies and provides a number of configurable build phases which can add depth to the build process. Building a project from the ground-up with Maven is a sure-fire way to keep it well organized and easy to maintain – but what about adding Maven on to an existing project, or worse, merging a non-Maven project into a project that already relies on Maven?

What follows is a look at some of the lessons I’ve learned from tweaking compiler plug-ins and digging through search results to debug various Maven-related issues. Hopefully it will be useful to the next developer who happens to hit similar issues, and if you have tips of your own be sure to leave a comment.

The maven-compiler-plugin <include> property

When overriding the default maven-compiler-plugin, the <include> tag may be used to force the compiler to include extra files into the build. There are a couple of interesting points to note here:

It is a filter. Many things in Maven expect a path to a directory, but not the <include> tag. If you have some extra java classes in src/main/java and you pass that to <include> it will fail silently – what you actually want is src/main/java/**/*.java.

It is for the compile-phase only. By default, in addition to using the maven-compiler-plugin during the compile phase, Maven will also use it during the test-compile phase. Most properties will apply to both, but <include> is not one of them; the test-compile phase requires a separate property, <testIncludes>, for any includables it requires[1].

The generate-sources and generate-resources phases

These phases are great for adding source (.java) and resource (.class) files to the compiler before the compile phase occurs. The snippet below shows how to use mojo's build-helper-plugin to add some obscure .class files to the classpath:


Now any .class files in ../obscure/classes (or any subdirectories) will be added to the classpath.

The same plug-in is used for adding .java files, simply tweak the generate-resources and add-resource values to generate-sources and add-source, and the <resources> and <resource> properties to <sources> and <source>.

The maven-compiler-plugin <compilerArgument> property

The <complierArgument> tag may be used to pass command-line arguments directly to the java compiler. Two examples:

<classpath> seems like it would let you add resources to the classpath, but near as I can tell, these aren’t actually used. Maven seems to prefer managing its own classpath, though we can still add/remove entries by overriding the default generate-resources phase (as explained above).

<sourcepath> is a great way to specify multiple source directories for compilation. It takes a semicolon-delimited list of top-level directories containing java files to compile. Alternately, an override of the generate-sources phase may be used here as well.

One gotcha with regards to sourcepath: to get this to work, I had to manually set <fork> to true on the maven-compiler-plugin. In fact, this gets even worse: when <fork> is true, Maven will use the %PATH% environment variable to determine which JRE to use, and this will fail with a totally non-descript error if the path to your JRE contains any spaces – very annoying to track down. This is actually a bug in Maven, logged here (http://jira.codehaus.org/browse/MCOMPILER-30).

The <sourceDirectory> property

A minor but important tag when playing around with various sources and resources, the <sourceDirectory> tag may be used to set the base directory for including java source files. It defaults to src/main/java, but I found when playing around with a lot of sources spread around various directories, it was easiest to set it to the current directory as follows:


Other useful debugging hints

When running into problems, it’s always good to have a few debug flags around to get a little more information out of Maven, which is generally not great at telling you what might be wrong.

Specifying the -e flag while running Maven will print out any exceptions Maven encounters, with the corresponding stack trace.

The <verbose> tag may be added inside any plug-in’s <configuration> property and when toggled to true (default is false) it will print some extra information, including the sourcepath and classpath being used by Maven’s compiler.

Specifying the -X flag while running Maven will document all kinds of intermediate steps Maven takes during the build – much more than -e and <verbose>.

[1] This makes perfect sense, of course: it’s unlikely that you’ll want the same includables for both the normal compiler and the testing compiler. It’s just counter-intuitive compared to the rest of the <configuration> properties.

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