Generating an executable JAR

The JAR artifact generated by Maven works well when used as a dependency in another project. However, it cannot be run as an executable without manually specifying the main class and explicitly specifying the dependencies that the project uses in the classpath.

What if we want to create an executable JAR for the project? This may be useful when the JAR needs to be tested or the project is a simple tool that should be invoked without additional effort.

How to do it...

  1. Open a simple Maven project (project-with-executable-jar):
  2. Add the following section in the pom file:
    <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>2.6</version>
        <configuration>
          <archive>
            <manifest>
              <addClasspath>true</addClasspath>
              <mainClass>com.packt.cookbook.App</mainClass>
            </manifest>
          </archive>
        </configuration>
      </plugin>
    </plugins>
  </build>
  3. Add the plugin configuration to copy over the dependencies to the target folder:
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <version>2.9</version>
        <executions>
          <execution>
            <id>copy-dependencies</id>
            <phase>package</phase>
            <goals>
              <goal>copy-dependencies</goal>
            </goals>
            <configuration>
            <outputDirectory>${project.build.directory}</outputDirectory>
              <excludeArtifactIds>junit</excludeArtifactIds>
            </configuration>
          </execution>
        </executions>
      </plugin>
  4. Run the following command:
    mvn clean package
  5. Observe the target folder:
    How to do it...
  6. Run the generated JAR file:
              java -jar project-with-executable-jar-1.0-SNAPSHOT.jar
  7. Observe the output:
    C:projectsapache-maven-cookbookproject-with-executable-jar	arget>java -jar project-with-executable-jar-1.0-SNAPSHOT.jar
06:40:18.437 [main] INFO  com.packt.cookbook.App - Hello World

How it works...

We have made the following configurations to the Maven JAR plugin in our pom file:

  • Added classpath: This adds all the dependant JARs to the manifest classpath section
  • Specified the main class: This information is again updated in the manifest

We also added the copy-dependencies goal of the Maven Dependency plugin to copy over the required dependencies to the folder where the executable JAR is generated.

When we then run the executable JAR, it uses the manifest file to determine the main class as well as the dependencies, loads them, and runs.

Let us look at the manifest file generated:

How it works...

Evidently, for this to work, the executable JAR should be accompanied by the dependencies that it uses. In the Creating an assembly recipe in Chapter 11, Advanced Maven Usage, we will learn how to create an assembly with all the dependencies, which can be distributed more easily.

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