Faster Maven Builds

A while ago, me and a couple of colleagues set about trying to make the CI build faster for a large Maven project. We learned quite a bit along the way, and it’s turned into an unhealthy obsession for all of us. But why did we get so worked up about how much time the build takes anyway?

When someone has pushed the last commit to their pull request, they want to see that successful build so they can merge it and move on. The longer the build takes, the more productivity they’ll lose, context-switching as they try to stay busy - we want a fast feedback loop. Also, a slower build means more time before you can ship to production - including when you need to deploy a fix for a critical issue. If you’re serious about anything like continuous deployment, your build needs to be fast.

In a less direct way, continually pushing to keep your builds fast is good for the health of your codebase. You’ll find and address bugs, inefficiencies and defunct code, and most improvements you make will help developers on the ground as well as the CI pipeline.

Measure

There are a lot of different things we can look at doing, and it’s tempting to just jump in, but first we need to stop and decide how we’re going to measure results. There will be things that you think will save time that actually end up making no difference or even making it worse; if you rush around doing everything all in one go, you won’t be able to tell the good from the bad.

Having created a feature branch in your project, wirh each change you can then run the build a few times and take the average as your measurement. It’s still not exactly scientific, but it should be good enough to sort the wheat from the chaff.

For multi-module builds, you have the reactor summary at the end which should tell you how long each module has taken. This can give you some immediate pointers about where to look first for improvements - but keep in mind that when using the deployAtEnd option, the last module to finish will get the time taken to deploy everything added to its reactor time, which can skew the results. Also, you can use this neat tool to analyse the time taken down to individual goal execution level, which can give you more clarity on why a module is taking so long.

Work around it

Don’t forget about the more indirect things you can do to make your build faster:

  • Less stuff - What can you spin out to be a separate library, or service? Areas of the code that are discreet, well-tested and rarely change are the best candidates.
  • Faster machinery - Hardware and cloud compute power are cheap and plentiful compared to developer time and attention1.
  • Use Gradle? - It’s worth a shot

Configure Maven for better performance

Let’s set these up in our MAVEN_OPTS:

-Xms1024m -Xms2048m -Dmaven.artifact.threads=10

Maven’s default JVM memory args have it maxing out at 512mb - a large-ish project will want more than this. Also, by default, Maven will only try to resolve 5 artifacts at the same time - doubling this should make the dependency resolution faster overall.

Command line arguments

Let’s take a look at the Maven command that’s likely running for your builds2:

mvn clean verify

This seems fine - it’s basically what you would run locally. But we can get things moving faster from here without touching any of our code yet:

mvn verify -T1C

First, we dropped the clean goal. The clean plugin is not very efficient, but more to the point your CI tool should already be giving you a completely clean directory to work in (it will probably do it faster than the clean plugin as well).

The most interesting (for projects with multiple modules at least) is the -T argument, which is an alias for --threads, Maven’s under-publicised option for building reactor modules in parallel. As you’d hope, it’s smart enough to work out the inter-dependencies of your modules and parallelise as much as possible without ever starting a module before other ones it depends on have finished. It will use up to the specified number of threads; the default is 1, meaning modules are built entirely in serial unless you specify otherwise.

With -T1C, we’re telling Maven to use one thread per available CPU core. This seems to be the best balance; every time we tried to use more than this, it performed the same or worse, presumably as too many different threads compete for finite resources.

The vast majority of Maven plugins support running in this multi-threaded mode, including all the first-party ones. If you’re not already doing it, it’s probably the single biggest gain you can make; your mileage may vary, but an instant 80% improvement is not unrealistic for a project with a dozen or more modules.

Maven modules and dependencies

If you have a lot of Maven modules in your project (like, 30+), it’s worth giving them an audit to see if the structure has some inefficiency. Each module in Maven has an inherent cost: initialising tooling, resolving dependencies, building artifacts etc. If you have any modules that are doing comparatively little (i.e. they just contain a handful of classes) then you might have a chance to fold that module into another one and save some time. Bear in mind that, taken too far, this risks negating the benefits of multi-threaded mode.

Another thing worth auditing is the dependencies in your pom.xml files. Dependencies are very easy to add and use, but tend to get left in even when the code has changed and no longer relies on that dependency. This can cause unused dependencies to mount up and waste time in our build. You can use maven-dependency-plugin:analyze to identify unused dependencies and remove them.

Integration tests

“Integration tests” has a bit of a broad definition these days, but for the sake of argument let’s say we mean stuff that runs at the integration-test phase in the Maven lifecycle, using Spring test to bring up an application context against a database and test the app via its entry points.

Naturally, being higher in the pyramid, these tests are going to take a lot longer to run than unit tests. As ever, parallelisation is the key to gaining time, but with the nature of this kind of testing we are more likely to run into problems; for example, any thread safety issues in your application will be brutally exposed.

We can configure the Surefire plugin right to extract the best performance like this:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    ...
    <configuration>
        <parallel>classes</parallel>
        <threadCount>4</threadCount>
        <perCoreThreadCount>true</perCoreThreadCount>
        <trimStackTrace>false</trimStackTrace>
    </configuration>
</plugin>

Notice that in our parallel property, we have “classes” instead of “all” - this means Maven will use multiple threads to run tests, but will only parallelise down to class level, so every test method in a given class will run one at a time on the same thread. This is certainly not ideal, but parallelisation down to method level simply didn’t work with Spring test, as we found to our cost3.

You can do things to work around this limitation; if you have an expensive setup operation (“calculate a quote”, “create a document” etc), the result of which is asserted on by multiple methods that don’t change any state themselves, you may be able to refactor to do that expensive operation fewer times whist maintaining the same coverage.

Ultimately, of course, the tests are only ever going to be as fast as your app lets them be. Running JProfiler or something similar against your tests to find hotspots will help your users just as much as your build times, and is something you should go back to on a regular basis, even if you’re “happy” with how fast your build is.

  1. Seriously, do this. Our builds were pretty well optimised by the time we finished this little sprint, but we recently replaced the infrastructure with the best hardware we could afford, everything is now 3-4x faster and the difference to developer experience has been huge. 

  2. The goal would be verify for a feature branch build, but would be deploy when building a snapshot or release from master - the rest applies either way. Also, what a CI tool like Jenkins or TeamCity actually runs will look very different to this, with the locations of Java, Maven etc heavily parameterised and a bunch of other parameters thrown in, but regardless it should let you configure goals and options, even if it does add some bits and pieces of its own. 

  3. In theory it’s now possible using the latest versions of Spring and JUnit, though we haven’t been able to test it yet.