Maven Reactor Tricks

Not many people know this, but you can use Maven to resume a failed build from the middle, or to build just a subset of projects in a multi-module (a.k.a. “reactor”) build.

Example Reactor Project

Consider this complex multi-module reactor build:

|-- pom.xml
|-- barBusinessLogic
|   `-- pom.xml
|-- bazDataAccess
|   `-- pom.xml
|-- quz
|   |-- pom.xml
|   |-- quzAdditionalLogic
|   |   `-- pom.xml
|   `-- quzUI
|       `-- pom.xml
`-- fooUI
    `-- pom.xml

Suppose project fooUI depends on project barBusinessLogic, which depends on project bazDataAccess.

fooUI --> barBusinessLogic --> bazDataAccess

Furthermore, quzUI depends on quzAdditionalLogic, which depends on barBusinessLogic.

quzUI --> quzAdditionalLogic --> barBusinessLogic --> bazDataAccess

Ordinarily, when you run mvn install from my-root-project, you’ll build the projects in this order:

  1. my-root-project (parent project)
  2. bazDataAccess
  3. barBusinessLogic
  4. fooUI
  5. quz (parent project)
  6. quzAdditionalLogic
  7. quzUI

Resuming the Build with --resume-from

Suppose you’re working on your code and you attempt to run mvn install from my-root-project, but you encounter a test failure in fooUI. You make additional changes to barBusinessLogic without changing bazDataAccess; you know that bazDataAccess is fine, so there’s no need to rebuild/test it. You can then use the --resume-from argument, like this:

mvn install --resume-from=fooUI

That will skip over bazDataAccess and barBusinessLogic and pick up the build where you left off in fooUI. If fooUI succeeds, it will go on to build quzAdditionalLogic and quzUI.

Specify a Subset of Projects with –projects

Suppose you’ve made some changes to fooUI and bazDataAccess and would like to rebuild just those two projects. You can use the --projects argument, like this:

mvn install --projects fooUI,bazDataAccess

That will automatically build just those two projects, saving you the trouble of running Maven in each directory separately.

Making fooUI Without Building quz Using --also-make

Suppose you’re a developer working on fooUI; you don’t want to work on quz right now, but just want to get a working build of fooUI. You can use --also-make, like this:

mvn install --projects fooUI --also-make

--also-make will examine fooUI and walk down its dependency tree, finding all of the projects that it needs to build. In this case, it will automatically build bazDataAccess, barBusinessLogic and fooUI without building quz.

Changing barBusinessLogic and Verifying You Didn’t Break Anything Using --also-make-dependents

Suppose you’ve made a change to barBusinessLogic; you want to make sure you didn’t break any of the projects that depend on you. You also want to avoid rebuilding/testing projects that you know you haven’t changed. In this case, you want to avoid building bazDataAccess. You can use --also-make-dependents, like this:

mvn install --projects barBusinessLogic --also-make-dependents

--also-make-dependents will examine all of the projects in your reactor to find projects that depend on barBusinessLogic; it will automatically build those and anything that depends on them.

Resuming a make Build

When you use --also-make or --also-make-dependents, you run a subset of projects, but that doesn’t mean stuff won’t fail halfway through the build. You can resume an --also-make build from the project that stopped the build by using --resume-from together with --also-make, like this:

mvn install --projects quz/quzUI --also-make --resume-from barBusinessLogic

At Redfin, our Maven reactor has 86 projects; avoiding unnecessary rebuilds is essential to our productivity!