Putting Maven build directories out-of-tree

So you want to have Maven put its build output somewhere out-of-tree.

There are several reasons to do this:

• You have lots of RAM, and want to speed up builds by putting class files in RAM.
• You have an SSD, and want to reduce needless wear cycles by putting class files in RAM (or on a spinning disk).
• Your source tree is on a network file system, and you want compilation output to be local.

Helpfully, the Maven Way is to have all Maven-generated output go to a dedicated directory, target/, where it can be easily separated. Theoretically, you can probably set project.build.directory to point to wherever you want, and get Maven to build somewhere else. However, if parts of the build system assume that output goes into target/ (a questionable assumption, but I make it myself in pieces of the LensKit site generation workflow).

So, I have put together a Maven snippet you can put in your project’s POM to enable support for out-of-tree builds, automatically creating a work directory in some external directory (specified by the external.build.root Maven property) and making target/ a symbolic link to this work directory. Here it is, just put it in <profiles> in your pom.xml:

If you set external.build.root to e.g. /tmp/maven-builds (at the command line with -D, or in your ~/.m2/settings.xml), then all your Maven output will go into /tmp/maven-builds/<group>:<artifact>. mvn clean will delete this directory tree and remove the symbolic link. If you don’t have a external.build.root, then the profile will not activate and your code will be built as normal.

And if the directory you point external.build.root to is a RAM-based file system (tmpfs on Linux), Maven compiler output will all be stored in RAM and not hit disk at all. On many modern Linux distros, /run is available as a tmpfs; some (including Fedora 19) also make /tmp a tmpfs by default.

A few caveats and explanations:

• This won’t work on Windows, I don’t think, as there are no symbolic links.
• Only works on Java 7, I use NIO.2 to make the symbolic links.
• Why not use maven-antrun-plugin? Because it creates an Ant file in target, then runs it; this results in target/ being created before the code to make the symbolic link is run.
• Why use script-maven-plugin instead of gmaven-plugin? Only because one of the builds I’m using this in uses gmaven-plugin configured with Groovy 2, which breaks the execute goal. script-maven-plugin uses generic BSF and avoids this problem.

Have fun!