Maven exec plugin main class missing

It should be mainClass not mainclass.

-Dexec.mainClass="org.hsqldb.Server"

Read more on this here and here.

And hsqldb should be in your maven dependencies:

<dependency> 
  <groupId>org.hsqldb</groupId> 
  <artifactId>hsqldb</artifactId> 
  <version>2.0.0</version> 
</dependency>

Please be aware, that the "configuration" section must be out of the execution section (see example below). Otherwise you get the "missing or invalid mainClass" error.

            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2</version>
                <dependencies>
                    <dependency>
                        <groupId>org.versly</groupId>
                        <artifactId>versly-wsdoc</artifactId>
                        <version>1.0-SNAPSHOT</version>
                        <scope>compile</scope>
                    </dependency>
                </dependencies>
                <executions>
                    <execution>
                        <phase>prepare-package</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>org.versly.rest.wsdoc.RestDocAssembler</mainClass>
                    <arguments>
                        <argument>${project.build.directory}/classes</argument>
                    </arguments>
                </configuration>
            </plugin>