How to Merge Multiple JARs into One Executable JAR?

Merging Multiple JARs into One

When aiming to consolidate several JAR files into a single executable JAR, developers often encounter challenges related to incorporating dependencies, setting the main class manifest, and ensuring executability. Fortunately, various tools and techniques simplify this process.

Utilizing Ant's Zipfileset

Ant's zipfileset element allows for the straightforward merging of multiple JAR files. By leveraging the includes attribute, you can selectively include specific files from each source JAR. The jar task handles the creation of a new JAR file with the combined contents.

Customization Using Manifest Attributes

To specify the main class manifest and designate the JAR as executable, utilize the manifest attribute task. This task enables you to set the Main-Class attribute, which identifies the entry point of the application within the JAR. Additionally, setting the Executable attribute to true permits direct execution of the JAR file.

Example Using Ant

To illustrate the process using Ant, consider the below build file:

<code class="xml"><project name="merge-jars" default="merge">

    <taskdef name="jar" classname="org.apache.tools.ant.taskdefs.Jar">
        <classpath>
            <fileset dir="${ant.home}/lib">
                <include name="*.jar"></include>
            </fileset>
        </classpath>
    </taskdef>

    <target name="merge">
        <jar jarfile="merged.jar">
            <manifest>
                <attribute name="Main-Class" value="my.main.Class"></attribute>
            </manifest>
            <zipfileset src="first.jar" includes="**/*.java **/*.class"></zipfileset>
            <zipfileset src="second.jar" includes="**/*.java **/*.class"></zipfileset>
        </jar>
    </target>

</project></code>

Upon executing this build file, a new JAR file named "merged.jar" will be created, incorporating the contents of the "first.jar" and "second.jar" files. The JAR will be configured with the specified main class and designated as executable.

Additional Tools and Techniques

Aside from Ant, other tools and approaches can help merge JAR files:

• JarSplice: A tool that simplifies merging JAR files and specifies the main class.
• Degree: A command-line tool that aids in merging JAR files and customizing the manifest.
• Fat JAR: A technique where all required dependencies are bundled within a single JAR, eliminating the need for additional external JARs.
• Maven Shade Plugin: A plugin that merges JAR files and addresses dependency conflicts.

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