How to Build and Package a Java Application?

Understand how to compile and package your Java project into a deployable JAR file using Maven or Gradle.

📌 Why Do I Need to Build My Java Application?

Discloud does not compile Java projects automatically.

You must: ✔ Compile your code into .class files. ✔ Package everything into a single .jar file (including dependencies). ✔ Ensure the META-INF/MANIFEST.MF file correctly defines the Main-Class.

🔧 Choosing a Build Tool

Build Tool

Best For

Required Files

Maven

Large projects, dependency-heavy apps

pom.xml

Gradle

Modern projects with flexible build scripts

build.gradle or build.gradle.kts

📦 Building Your Java Application

Apache Maven is a widely used build automation tool that manages project builds, dependencies, and documentation using a Project Object Model (POM).

Add the Maven Compiler Plugin.

Ensure your pom.xml includes the Maven Compiler Plugin to specify the Java version for compilation. This prevents compatibility issues when running the project.

pom.xml

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.9.9</version>
  <configuration>
    <source>17</source>
    <target>17</target>
  </configuration>
</plugin>

Set <source> and <target> to the Java version required for your project.

Specify the Main Class.

To make your application executable, you need to specify the entry point (main method). This is done using the Maven Shade Plugin, which also ensures that all dependencies are bundled inside a single JAR file.

pom.xml

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-shade-plugin</artifactId>
      <version>3.2.4</version>
      <executions>
        <execution>
          <phase>package</phase>
          <goals>
            <goal>shade</goal>
          </goals>
          <configuration>
            <transformers>
              <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                <!-- NOTE: Replace this with the correct path to your project's main class -->
                <mainClass>com.project.example.Main</mainClass>
              </transformer>
            </transformers>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

Replace com.project.example.Main with the actual fully qualified name of your Main class.
Ensure the Main class contains a valid public static void main(String[] args) method.

Build the Project.

Run the following command to clean old builds and generate the new JAR files:

mvn clean package

After running mvn package, Maven will create two different JAR files inside the target/ directory:

Standard JAR (Original Project JAR).

📌 Filename: original-<artifactId>-<version>.jar 📌 Example: original-com.maven.discordbot-0.0.1-SNAPSHOT.jar 📌 Contents:

Only your project's compiled code.
Does NOT include dependencies.

Do NOT use this file for execution unless dependencies are handled separately.

Fat JAR (Shaded JAR with Dependencies).

📌 Filename: <artifactId>-<version>.jar 📌 Example: com.maven.discordbot-0.0.1-SNAPSHOT.jar 📌 Contents:

Your compiled application code.
All dependencies included (JDA, Apache Commons, etc.).
Manifest file (MANIFEST.MF) with the Main-Class entry.

Use this file for execution and deployment.

Gradle is a flexible build automation tool that uses a Groovy or Kotlin-based DSL to define the build process.

Apply the Java Plugin.

Ensure your build.gradle (Groovy) or build.gradle.kts (Kotlin) applies the Java plugin:

build.gradle

plugins {
    id 'java'
    id 'com.gradleup.shadow' version '8.3.6' // Shadow Plugin for Fat JAR
}

Specify the Main Class.

To make the application executable, define the entry point (main method) inside the JAR manifest.

build.gradle

shadowJar {
    archiveClassifier.set('all')
    manifest {
        attributes(
            'Main-Class': 'com.project.Main' // Replace with your actual main class
        )
    }
}

tasks.build.dependsOn shadowJar // Ensure shadowJar runs when building

build.gradle.kts

import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar

tasks.withType<ShadowJar> {
    archiveClassifier.set("all")
    manifest {
        attributes(mapOf("Main-Class" to "com.project.Main")) // Replace with your actual main class
    }
}

tasks.named("build") {
    dependsOn("shadowJar")
}

Replace "com.project.Main" with the fully qualified name of your Main class.

Build the Project.

To clean old builds and generate new JAR files, run:

gradle clean build

Standard JAR (Regular Project JAR).

📌 Filename: <project-name>.jar📌 Example: discordbot.jar 📌 Contents:

Only your project's compiled code.
Does NOT include dependencies.

Do NOT use this file for execution unless dependencies are handled separately.

Fat JAR (Uber JAR with Dependencies).

📌 Filename: <project-name>-all.jar📌 Example: discordbot-all.jar 📌 Contents:

Your compiled application code.
All dependencies included (JDA, Apache Commons, etc.).
Manifest file (MANIFEST.MF) with the Main-Class entry.

Use this file for execution and deployment.

Recommendation

Rename your JAR file to a simple name like app.jar to avoid issues with special characters.

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Last updated 7 months ago