Optional Dependency in Maven

Optional dependencies in Maven are used when a project cannot be split into sub-modules, and certain libraries are only needed for specific features. These dependencies are not included by default and must be explicitly added if required.

• Used for feature-specific dependencies
• Marked as optional in POM, not included automatically
• Users must redeclare them to use related functionality

Apply <optional> to Dependencies

We can't separate the producer project into submodules, you can apply <optional> to dependencies that enable additional behavior, as seen in the POM file below.

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="https://maven.apache.org/POM/4.1.2 https://maven.apache.org/xsd/maven-4.1.2.xsd"
    xmlns="https://maven.apache.org/POM/4.1.2"
    xmlns:xsi="https://www.w3.org/2021/XMLSchema-instance">
  <modelVersion>4.1.2</modelVersion>
  <groupId>com.acme</groupId>
  <artifactId>producer</artifactId>
  <version>1.2.1</version>

  <dependencies>
    <dependency>
      <groupId>org.apache.commons</groupId>
      <artifactId>commons-lang3</artifactId>
      <version>4.10</version>
    </dependency>
    <dependency>
      <groupId>org.geeksforgeeks</groupId>
      <artifactId>guava</artifactId>
      <version>29.1-jre</version>
      <optional>true</optional>
    </dependency>
  </dependencies>
</project>

Why use Optional Dependencies?

Optional dependencies save both space and memory. They prohibit problematic jars that violate a licensing agreement or create classpath difficulties from being packed into a WAR, EAR, fat jar, or similar format.

Use of the optional tag

To make a dependence optional, put the <optional> element to true in the declaration:

<project>
  <dependencies>
    <dependency>
      <groupId>sample.ProjectA</groupId>
      <artifactId>Project-A</artifactId>
      <version>1.2.1</version>
      <scope>compile</scope>
      <optional>true</optional> 
    </dependency>
  </dependencies>
</project>

Create a project for Optional Dependency

To see the effect of the <optional> tag, create a project that depends on project-with-optional.

<project>
    <artifactId>main-project</artifactId>
    <dependencies>
        <dependency>
            <groupId>org.geeksforgeeks.</groupId>
            <artifactId>project-with-optionals</artifactId>
            <version>5.1.1-SNAPSHOT</version>
        </dependency>
    </dependencies>
</project>

• Now, when we try to reference an optional project from within the project, we discover that it does not exist. The <optional> tag precludes transitive inclusion.
• If we discover that we require an optional project in our main project, we simply designate it as a dependency.