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Safely manage jar dependencies

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Components installed on HDInsight clusters have dependencies on third-party libraries. Usually, a specific version of common modules like Guava is referenced by these built-in components. When you submit an application with its dependencies, it can cause a conflict between different versions of the same module. If the component version that you reference in the classpath first, built-in components may throw exceptions because of version incompatibility. However, if built-in components inject their dependencies to the classpath first, your application may throw errors like NoSuchMethod.

To avoid version conflict, consider shading your application dependencies.

What does package shading mean?

Shading provides a way to include and rename dependencies. It relocates the classes and rewrites affected bytecode and resources to create a private copy of your dependencies.

How to shade a package?

Use uber-jar

Uber-jar is a single jar file that contains both the application jar and its dependencies. The dependencies in Uber-jar are by-default not shaded. In some cases, this may introduce version conflict if other components or applications reference a different version of those libraries. To avoid this, you can build an Uber-Jar file with some (or all) of the dependencies shaded.

Shade package using Maven

Maven can build applications written both in Java and Scala. Maven-shade-plugin can help you create a shaded uber-jar easily.

The example below shows a file pom.xml which has been updated to shade a package using maven-shade-plugin. The XML section <relocation>…</relocation> moves classes from package com.google.guava into package com.google.shaded.guava by moving the corresponding JAR file entries and rewriting the affected bytecode.

After changing pom.xml, you can execute mvn package to build the shaded uber-jar.

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>3.2.1</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
            <configuration>
              <relocations>
                <relocation>
                  <pattern>com.google.guava</pattern>
                  <shadedPattern>com.google.shaded.guava</shadedPattern>
                </relocation>
              </relocations>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>

Shade package using SBT

SBT is also a build tool for Scala and Java. SBT doesn't have a shade plugin like maven-shade-plugin. You can modify build.sbt file to shade packages.

For example, to shade com.google.guava, you can add the below command to the build.sbt file:

assemblyShadeRules in assembly := Seq(
  ShadeRule.rename("com.google.guava" -> "com.google.shaded.guava.@1").inAll
)

Then you can run sbt clean and sbt assembly to build the shaded jar file.

Next steps