How to Solve NoClassDefFoundError When Running Maven Projects from the Command Line?

Dependency Issues in Maven: Understanding NoClassDefFoundError

When integrating Maven into a project, developers may encounter the NoClassDefFoundError while attempting to run their application from the command line. This issue arises due to the default behavior of Maven, which doesn't automatically package dependencies within the generated JAR file.

Root Cause

The NoClassDefFoundError occurs when the Java Virtual Machine (JVM) attempts to execute the application's code but fails to locate the classes for the required library dependencies. These classes are not found on the provided classpath, either manually or automatically determined.

Maven's Default Behavior

By design, Maven assumes that the project's dependencies will be available on the system-wide classpath or within the Maven repository used during the build process. However, when running the application from the command line, these dependencies may not be available by default.

Solution: Dependency Shading

To resolve this issue, developers can shade the library code into the output JAR file. This is an effective approach that allows all required libraries to be bundled together within a single JAR.

Using the Maven Shade Plugin

To accomplish dependency shading, developers can incorporate the maven-shade-plugin into their Maven project. This plugin provides a simple configuration process to generate an "uber-JAR" that packages both the project's classes and the dependencies:

<code class="xml"><project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupid>org.apache.maven.plugins</groupid>
        <artifactid>maven-shade-plugin</artifactid>
        <version>3.5.2</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
  ...
</project></code>

Re-running the Command

Once the shade plugin has been configured and executed, developers can re-run the commands to build and run the application. The resulting JAR will include all necessary dependencies, eliminating the NoClassDefFoundError.

Additional Configuration

The maven-shade-plugin offers additional configuration options to further tailor the shading process. Developers can specify which JARs to include, define a Main-Class for the JAR file, and more. Comprehensive documentation is available on the plugin's website.

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