Clean way to combine multiple jars?

Clean way to combine multiple jars?

Combining multiple JAR files into a single JAR file can be useful in various scenarios, such as creating a self-contained executable JAR or consolidating libraries. Here's a clean way to combine multiple JAR files into one:

Using Gradle or Maven (Build Tools): If you are using Gradle or Maven as your build tool, you can use the "fat JAR" or "uber JAR" approach to combine multiple JAR files into a single JAR. Here's how to do it with both build tools:

Gradle:

  1. In your build.gradle file, add a plugin to handle fat JAR creation. For example, you can use the "Shadow" plugin:

    plugins {
    id 'com.github.johnrengelman.shadow' version '7.0.0'
}

  2. Configure the Shadow plugin to build your fat JAR:

    shadowJar {
    mergeServiceFiles()
}

  3. Run the ./gradlew shadowJar command to build the fat JAR.

Maven:

  1. In your pom.xml file, add the maven-assembly-plugin to handle creating an uber JAR:

    <build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-assembly-plugin</artifactId>
            <version>3.3.0</version>
            <configuration>
                <descriptorRefs>
                    <descriptorRef>jar-with-dependencies</descriptorRef>
                </descriptorRefs>
            </configuration>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>single</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

  2. Run the mvn package command to build the uber JAR.

Using Command Line (Jar Command): You can also combine JAR files using the jar command-line tool that comes with the Java Development Kit (JDK). This approach is more manual but provides fine-grained control.

  1. Create a directory where you want to combine the JAR files.

  2. Copy all the JAR files you want to combine into this directory.

  3. Open a terminal or command prompt and navigate to the directory containing the JAR files.

  4. Use the jar command to create a new JAR file and add the contents of the other JAR files to it. For example:

    jar cvf combined.jar *

    This command creates a new JAR file named combined.jar and adds all the files from the current directory to it.

    If you need to specify a specific main class or other manifest attributes, you can create a manifest file (manifest.mf) and include it when creating the JAR:

    jar cvfm combined.jar manifest.mf *

The Gradle or Maven approach is recommended for most projects because it automates the process and handles dependencies. However, if you have a specific use case or need fine-grained control, you can use the command-line jar tool.

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