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Gradle build files are written using Apache Groovy.

Gradle Tests Parallelization

The build is configured to run on a given number of workers that are responsible for all the tasks of the build (running tests, compiling code, checkstyle, findbugs, ...)

Gradle is configured with the forkEvery parameter to run test classes in parallel among the gradle workers.

Each gradle worker uses a single JVM. Inside it, tests are run in sequence. Tests inside a test class are not run in parallel unless you specify a JUnit parallelized runner.

Specifying build targets

Gradle targets can be specified with either the project name or directory, for example:

$ ./gradlew :beam-website:serveWebsite

is the same as

$ ./gradlew -p website serveWebsite

The mapping of directory to project name is in settings.gradle

Troubleshooting build issues

  • the --info flag adds verbose logging
  • the --stacktrace flag adds a stacktrace on failures
  • the --scan flag creates a Gradle Build Scan

Common build logic

Most re-usable steps are in BeamModulePlugin.groovy.

A common style in the build files for methods with several arguments is to use the named argument constructor which allows passing a map for names and values, for example:

class BuildTaskConfiguration {
  String name
  boolean useTestConfig = false
  String baseUrl = ''
  String dockerWorkDir = ''

def createBuildTask = {
  BuildTaskConfiguration config = it as BuildTaskConfiguration

createBuildTask(name:'Local', dockerWorkDir: dockerWorkDir)

Customizing local build configuration

Our Gradle uses various configuration properties which can be overridden to customize build behavior. These are generally used to set external resource locations for testing, such as Google Cloud project or Docker container registry. You can find useful properties by searching for usages of the Gradle findProperty('..') API.

You can specify property values for these on the command-line using -P. For example, to run Dataflow runner precommit tests with your own GCP settings:

./gradlew -PgcpProject=myGoogleCloudProject -PgcsBucket=myGoogleCloudStorageBucket \

Setting build customizations in a user config file

You can also set default Gradle property overrides inside of a user file (documentation). Create the file in your GRADLE_USER_HOME directory (defaults to ~/.gradle on Mac/Linux). In the file, you can set defaults for any Gradle property from the build:


Create Build Scan on failed builds

Another useful customization is to automatically upload a build scan when the build fails. This makes it very easy to share your build results for debugging. To do so, create an init.d/buildScan.gradle file in your GRADLE_USER_HOME directory with the following contents (documentation):

initscript {
repositories {
dependencies {
classpath 'com.gradle:build-scan-plugin:1.13.1'
rootProject {
apply plugin: com.gradle.scan.plugin.BuildScanPlugin
buildScan {
termsOfServiceUrl = ''
termsOfServiceAgree = 'yes'

OutOfMemoryException during the Gradle build

You may want to have a customized file in GRADLE_USER_HOME, and then setup your environment vars in the shell startup script. 

export GRADLE_USER_HOME=~/.gradle

source ~/.bashrc

Update gradle config via editing/creating in root source folder or in GRADLE_USER_HOME (~/.gradle).

cat ~/.gradle/ 

# Gradle has a tendency to reuse daemons during build.
# Disable daemon usage to avoid OOM.

# By default Beam project is configured to run gradle tests in parallel. Disabling this can reduce max memory usage.

# Increase jvm memory.
org.gradle.jvmargs=-Xmx2g -XX:MaxPermSize=512m

Publish a module snapshot release to mavenLocal

For local testing, a module can be published to mavenLocal. E.g. for the kafka module:

./gradlew -Ppublishing --no-parallel -PdistMgmtSnapshotsUrl=~/.m2/repository/ -p sdks/java/io/kafka publish

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1 Comment

  1. Wout Scheepers for publishing to mavenLocal, what about using the publishToMavenLocal task?


    Depends on: All publishPubNamePublicationToMavenLocal tasks

    Copies all defined publications to the local Maven cache, including their metadata (POM files, etc.).