The parameters 'mainClass' for goal org.codeh aus.mojo:exec-maven-plugin:1.6.0:java are missing or

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.6.0:java (d
efault-cli) on project zaidunduan: The parameters 'mainClass' for goal org.codeh
aus.mojo:exec-maven-plugin:1.6.0:java are missing or invalid -> [Help 1]

<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>1.2.1</version>
<executions>
<execution>
<goals>
<goal>java</goal>
</goals>
</execution>
</executions>
<configuration>
<mainClass>client.EchoClient</mainClass>
</configuration>
</plugin>
</plugins>
</build>