Gradle and Maven have fundamentally different views on how to build a project. Gradle provides a flexible and extensible build model that delegates the actual work to the execution of a graph of tasks. Maven uses a model of fixed, linear phases to which you can attach goals (the things that do the work). This may make migrating between the two seem intimidating, but migrations can be surprisingly easy because Gradle follows many of the same conventions as Maven — such as the standard project structure — and its dependency management works in a similar way.

Here we lay out a series of steps for you to follow that will help facilitate the migration of any Maven build to Gradle:

Create a build scan for the Maven build.

A build scan will make it easier to visualize what’s happening in your existing Maven build. For Maven builds, you will be able to see the project structure, what plugins are being used, a timeline of the build steps, and more. Keep this handy so you can compare it to the Gradle build scans while converting the project.

Develop a mechanism to verify that the two builds produce the same artifacts.

This is a vitally important step to ensure that your deployments and tests don’t break. Even small changes, such as the contents of a manifest file in a JAR, can cause problems. If your Gradle build produces the same output as the Maven build, this will give you confidence in switching over and make it easier to implement the changes that will provide the greatest benefits.

This doesn’t mean that you need to verify every artifact at every stage, although doing so can help you quickly identify the source of a problem. You should focus on the critical output such as final reports and the artifacts that are published or deployed.

You will need to factor in some inherent differences in the build output that Gradle produces compared to Maven. Generated POMs will contain only the information needed for consumption and they will use and scopes correctly for that scenario. You might also see differences in the order of files in archives and of files on classpaths. Most differences will be minor, but it’s worth identifying them and verifying that they are acceptable.

Run an automatic conversion.

This will create all the Gradle build files you need, even for multi-module builds. For simpler Maven projects, the Gradle build will be ready to run!

Create a build scan for the Gradle build.

A build scan will make it easier to visualize what’s happening in the build. For Gradle builds, you’ll be able to see the project structure, the dependencies (regular and inter-project ones), what plugins are being used and the console output of the build.

Your build may fail at this point, but that’s ok; the scan will still run. Compare the build scan for the Gradle build to the one for the Maven build and continue down this list to troubleshoot the failures.

We recommend that you regularly generate build scans during the migration to help you identify and troubleshoot problems. If you want, you can also use a Gradle build scan to identify opportunities to improve the performance of the build.

Verify your dependencies and fix any problems.

Configure integration and functional tests.

Many tests can simply be migrated by configuring an extra source set. If you are using a third-party library, such as FitNesse, look to see whether there is a suitable community plugin available on the Gradle Plugin Portal.

Replace Maven plugins with Gradle equivalents.

In the case of popular plugins, Gradle often has an equivalent plugin that you can use. You might also find that you can replace a plugin with built-in Gradle functionality. As a last resort, you may need to reimplement a Maven plugin via your own custom plugins and task types.

The rest of this chapter looks in more detail at specific aspects of migrating a build from Maven to Gradle.