10 tips for migrating from Maven to Gradle

Here’s a quick list of 10 lessons we learned when making the switch to Gradle from Maven for StackFoundation. Coming from a deeply Maven place, these are the things that gave us an “Aha! That’s how you do it in Gradle!” moment. As with any other tool, these are not the only way (nor the best) to do some of these things with Gradle – this is not meant to be a prescriptive list of best practices. Rather, it’s just a few things to help those Mavenistas out there who are thinking of switching to Gradle, or actively switching to Gradle, and figuring out how to get their mind to think Gradle.

1) Forget the GAV, use the directory layout!

Maven folks are used to thinking about a module’s GAV – it’s group, artifact, and version. When you switch to Gradle, you don’t have to think about this so much. Gradle will name projects based on the names of the directories by default. So if you have the following multi-project directory structure:

  • server
    • core
      • src/main/java
    • logging
      • src/main/java

These projects are named server, core, and logging. In Gradle, projects are identified with a fully-qualified path – in this case the paths are going to be  :server,  :server:core and  :server:logging.

Note: You can give projects a group, and version, if you want.

2) Build everything from the root!

More than a GAV, when you start using Gradle regularly, you’ll start thinking of projects by their path.

In Maven, you’re probably used to switching to a particular sub-module directory and then invoking mvn clean install, etc from there. In Gradle, you’re going to kick off all builds from the root of your multi-project setup. And you can simply use a sub-project’s path to kick off a task for that project. For example, you can invoke gradlew :server:logging:buildto build the logging sub-project, within the top-level server project.

3) Use custom tasks!

In Maven, if you need to perform some custom logic for a build or deploy, you go hunting for a particular plug-in, and then see if invoking one of it’s goals at some spot within the fixed Maven build lifecycle accomplishes what you want. If not, you try and look for another plug-in. And another, and then you might try writing one yourself.

Gradle is fundamentally built around tasks. You’re going to end up writing a custom task for a lot of what you want to do. Build a package by combining things in a specific way? Write a task. Deploy a service? Write a task. Setup infrastructure? Write a task. And remember all Gradle scripts are Groovy scripts so you are writing Groovy code when writing your tasks. Most of the time, you won’t write a task from scratch – you’ll start with a plug-in (yes, like in Maven, you’ll start by searching for plug-ins), and one of the tasks it defines, and then customize it!

4) Name your tasks, give them a group and description!

If you have a complex Maven project, you are very likely using a number of profiles, and you will probably have a specific order to build things, and maybe even a specific order to run things with different profiles activated. You’ll end up documenting this on a Wiki or a README file in your Git repo. And then you’ll forget to update that document so that eventually, how exactly something is built is only tribal knowledge.

In Gradle, you create custom tasks. This was already point 3 – but once you create them, you can give these tasks a group and a description. We give our most important custom tasks the group name ‘StackFoundation’. That way when we run a gradlew tasks, we see a list of tasks specific to our project in the list of available tasks to run. A great way to document our tasks.

5) Alias tasks, name them something you will remember!

Picking up from 3 and 4: You can create a task just to alias another task defined by another plugin. For example – the Shadow plugin is the Gradle version of the Maven Shade plugin. You might be happy with the default shadowJar task it provides but if in your project, a more meaningful name for creating that shadow JAR package is createServicePackage, you can create an alias:

task createServicePackage(dependsOn: shadowJar)

Note: It’s not exactly an alias, but close enough.

6) The Shadow plugin is the Gradle version of the Maven Shade plugin

This one is used by enough Maven folks that it’s worth repeating.

7) Use the Gradle wrapper

With Maven, you have to get everyone to setup Maven or use an IDE which comes with a Maven built-in in order to run builds for your project. With Gradle, there’s the Gradle wrapper – and you’re meant to check it in to your team’s repo. Setup your project to use the wrapper, and put it in your source control repo! Your team won’t have to think about getting Gradle.

8) Forget the inheritance parent, use external build scripts to define common tasks

In Maven, you use an inheritance parent to manage dependencies, and plugins.

With Gradle, you can reference other Gradle files from a build.gradle file – you do that using something that looks like: apply from: '../../gradle/services.gradle'. These are called external build scripts and there’s some caveats to using them but they’re a great way to define common tasks. For example, you can create some common tasks for deploying any of the services you use in your projects inside gradle/services.gradle and reference them from your other Gradle files.

Note: You can also put common task code inside buildSrc.

9) Forget the inheritance parent, create custom libraries

In Maven, you use a parent POM to define common dependencies. With Gradle, you can define common dependencies by putting them in an external build script (described in point 6). Here’s an example of a file in gradle/dependencies.gradle which defines some common libraries we use in all of our projects:

repositories {
    mavenLocal()
    mavenCentral()
}

ext {
    libraries = [
            aws            : {
                it.compile('com.amazonaws:aws-java-sdk-s3:1.11.28') {
                    exclude group: 'org.apache.httpcomponents', module: 'httpclient'
                    exclude group: 'com.fasterxml.jackson.core', module: 'jackson-annotations'
                    exclude group: 'com.fasterxml.jackson.core', module: 'jackson-core'
                    exclude group: 'com.fasterxml.jackson.core', module: 'jackson-databind'
                    exclude group: 'com.fasterxml.jackson.dataformat', module: 'jackson-dataformat-cbor'
                }
            },
            awsEcr         : 'com.amazonaws:aws-java-sdk-ecr:1.11.28',
            datamill       : {
                it.compile('foundation.stack.datamill:core:0.1.1-SNAPSHOT') {
                    exclude group: 'org.apache.httpcomponents', module: 'httpclient'
                }
            },
            datamillLambda : 'foundation.stack.datamill:lambda-api:0.1.1-SNAPSHOT',
            junit5 : [
                'org.junit.jupiter:junit-jupiter-api:5.0.0-M4',
                'org.junit.jupiter:junit-jupiter-migration-support:5.0.0-M4'
            ],
    ]
}

Note the use of GAVs to refer to Maven dependencies, and how you can setup exclusions using this approach. With this approach, we get to give our own names to these libraries instead of referring to everything with GAVs. This is especially great for us because colloquially, we refer to our dependencies using these names and this makes looking at project dependency information clear and concise. In addition, we can group multiple Maven dependencies into one custom user library, as with the junit5 example.

Here’s how a particular project defines the libraries as dependencies:

dependencies {
    compile libraries.datamill(it)
    testCompile libraries.junit5
}

10) Doing resource filtering

In Maven, you probably use resource filtering to replace property placeholders in resource files. There’s two equivalents in Gradle – the first is to use ReplaceTokens:

processResources {
    def props = [imageVersion: 'unspecified']
    filesMatching('*.properties') {
        filter(org.apache.tools.ant.filters.ReplaceTokens, tokens: props)
    }
}

This looks for placeholders of the form @imageVersion@, i.e., they’re delimited by @’s. It tolerates missing property names. A second form looks like this:

processResources {
    props = [imageVersion: 'unspecified']
    filesMatching("**/*.yaml") {
        expand props
    }
}

This looks for property placeholders of the form $imageVersion – well, sort of. It’s actually using a template mechanism in Groovy which makes it very powerful but if you use it for simple cases, you’ll probably encounter the following: if a simple placeholder references a missing property, your build will fail in error!

That’s all for now! More lessons from our experience migrating to Gradle at StackFoundation will be for a future post! Hope that helps those of you making the switch from Maven!