Last updated May 15, 2023

When deploying with Git, it’s common for very large Java applications to exceed the slug size limits of the platform. If this happens, you’ll see an error like this when you deploy:

remote: -----> Compressing...
remote:  !   Compiled slug size 583.3MB is too large, max is 500 MB.

There are a number of ways to mitigate this problem, including switching to non-Git deployment. In this article, you’ll learn how to do this and how to configure your application to reduce it’s slug size.

Using the Maven Clean plugin to remove build artifacts

The Maven Clean plugin can be configured to remove non-essential artifacts after the build is complete. For example, if your application is packaged into a WAR file, and you would like remove all JAR files (because they are duplicated inside the WAR file), then you could configure the plugin like this in your pom.xml:

<plugin>
  <artifactId>maven-clean-plugin</artifactId>
  <version>3.0.0</version>
  <executions>
    <execution>
      <id>clean-jar-artifacts</id>
      <phase>install</phase>
      <goals><goal>clean</goal></goals>
      <configuration>
        <excludeDefaultDirectories>true</excludeDefaultDirectories>
        <filesets>
          <fileset>
            <directory>target/dependency/*.jar</directory>
          </fileset>
        </filesets>
      </configuration>
    </execution>
  </executions>
</plugin>

This removes all target/dependency/*.jar files from the slug after the install phase.

Other common patterns for the <configuration> element include removing everything in the target/ directory except the WAR file:

<fileset>
  <directory>target</directory>
  <excludes>
    <exclude>*.war</exclude>
  </excludes>
</fileset>

Or removing Node.js dependencies that were only needed to precompile JavaScript assets:

<fileset>
  <directory>node_modules</directory>
</fileset>
<fileset>
  <directory>.heroku/node</directory>
</fileset>

This configuration is particularly common when the Java buildpack is combined with the Node.js buildpack to perform JavaScript optimization during the build.

Using doLast in Gradle

For Gradle deployments, you can create a stage task with a doLast directive similar to this:

task stage {
    dependsOn build
    doLast {
        delete fileTree(dir: "build", exclude: "libs")
        delete fileTree(dir: "build/libs", exclude: "*.jar")
    }
}

The files you remove will depend on your application.

Excluding test dependencies

If you are using the maven-dependency-plugin to copy JAR files into the slug at build time, you may want to exclude your test dependencies. You can do this by setting the <includeScope> value in the plugin’s configuration:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-dependency-plugin</artifactId>
  <version>2.4</version>
  <executions>
    <execution>
      <id>copy-dependencies</id>
      <phase>package</phase>
      <goals><goal>copy-dependencies</goal></goals>
      <configuration>
        <includeScope>compile</includeScope>
      </configuration>
    </execution>
  </executions>
</plugin>

The included scopes defaults to test, which includes your test dependencies, so compile will actually exclude them.

Adding a .slugignore file

If your Git repository includes binaries or other large files that are not needed at runtime, you can exclude them from the slug by creating a .slugignore file in the root of your Git repo.

It is very common for the Git repository to include large files that are only needed when running tests. In this case, the .slugignore file might look like this:

*.psd
*.pdf
/test

You can inspect the extracted contents of your slug by running heroku run bash and using commands such as ls and du. This may help you identify large files that need to be added to the .slugignore file. Exactly what should be excluded depends on the needs of each application.

Using the Heroku Maven plugin

The heroku-maven-plugin avoids Git deployment altogether and builds the slug locally before pushing it to Heroku. This allows it to package only what is absolutely necessary for the application to run.

If you need to include additional directories in the slug, you can do so with the <includes> element. For example:

<includes>
  <include>etc/readme.txt</include>
</includes>

If you implement these suggestions, and still have a slug that is too large, please contact Heroku support.