How to Merge Multiple JAR Files into a Single Executable JAR?

Merging JAR Files into a Single Executable JAR

Problem: How to seamlessly merge multiple JAR files into a single executable JAR, including customization of the main-file manifest?

Solution:

To address the need for consolidating multiple JAR files, a simple and effective solution is to utilize Apache Ant's zipfileset element. This element enables the creation of a new JAR file by combining content from existing JARs.

<code class="xml"><jar id="files" jarfile="all.jar">
    <zipfileset src="first.jar" includes="**/*.java **/*.class"></zipfileset>
    <zipfileset src="second.jar" includes="**/*.java **/*.class"></zipfileset>
</jar></code>

This Ant task will read in the contents of first.jar and second.jar and merge them into a new JAR file named all.jar. The includes attribute specifies which files to include from the source JARs.

Additional Features:

This approach also offers:

• Main-class Specification: To set the main application class, use the zipfielsest element's mainEntry attribute.
• Wrap in an Executable Script: By placing the JAR in a shell script and making it executable, you can retain the convenience of using the JAR file as an executable.

Alternative Methods:

While Ant's zipfileset is a straightforward solution, there are alternative methods:

• Java: Use the JarInputStream and JarOutputStream classes to manually merge JAR files.
• Build Tools: Maven and Gradle both provide plugins that can package multiple JARs into a single executable.
• JAR Packaging Tools: Dedicated tools like JarSplice can create fat JARs with custom configurations.

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