Including dependencies in a jar with Maven

Is there a way to force Maven (2.0.9) to include all the dependencies in a single jar file?

I have a project the builds into a single jar file. I want the classes from dependencies to be copied into the jar as well.

Update: I know that I can't just include a jar file in a jar file. I'm searching for a way to unpack the jars that are specified as dependencies, and package the class files into my jar.

You can do this using the maven-assembly plugin with the "jar-with-dependencies" descriptor. Here's the relevant chunk from one of our pom.xml's that does this:

  <build>
    <plugins>
      <!-- any other plugins -->
      <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>single</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
        </configuration>
      </plugin>
    </plugins>
  </build>