How to resolve \"NoClassDefFoundError\" in Maven dependencies when running a JAR file?

Resolving NoClassDefFoundError in Maven Dependencies

Maven is a popular build automation tool that can automatically download and manage dependencies for Java projects. However, sometimes you may encounter a NoClassDefFoundError when running your Maven-built JAR file on the command line.

This error typically occurs because Maven doesn't bundle dependencies into the JAR it builds by default. Without the dependencies on the classpath, the Java Virtual Machine (JVM) can't find the library class files when executing your code.

To resolve this issue, you can manually specify the libraries on the classpath using the -cp parameter. However, this approach can be tedious.

A more efficient solution is to use the maven-shade-plugin to shade the library code into your output JAR file. This plugin automatically creates an "uber-JAR" that contains both your classes and the library classes.

To add the maven-shade-plugin to your POM file, follow these steps:

1. Open your pom.xml file.
2. Add the plugin dependency:

<code class="xml"><dependency>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-shade-plugin</artifactId>
  <version>3.5.2</version>
</dependency></code>

3. Define a build plugin execution:

<code class="xml"><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></code>

4. Save the pom.xml file.
5. Run the following commands:

<code class="bash">mvn package
java -cp target/jar-filename.jar your.main.class</code>

By shading the dependencies, you create an executable JAR that includes all necessary libraries. This resolves the NoClassDefFoundError and allows you to run your application independently.

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