How to specify frontend for wsdl2java in a pom.xml?

If you are using cxf-codegen-plugin, you can add the arguments in extraargs element:

<executions>
    <execution>
        <configuration>
            <wsdlOptions>
                <wsdlOption>
                    <wsdl>...</wsdl>
                    <extraargs>
                        <extraarg>-fe</extraarg>
                        <extraarg>jaxws21</extraarg>
                    </extraargs>
                </wsdlOption>
            </wsdlOptions>
        </configuration>
        <goals>
            <goal>wsdl2java</goal>
        </goals>
    </execution>
</executions>

Source: http://cxf.apache.org/docs/maven-cxf-codegen-plugin-wsdl-to-java.html

You can use <frontEnd> inside <wsdlOption> or <defaultOption>. The latter is helpful, if you include multiple WSDLs and specified <wsdlRoot>:

<executions>
    <execution>
        <configuration>
            <defaultOptions>
                <frontEnd>jaxws21</frontEnd>
            </defaultOptions>
            <wsdlRoot>${basedir}/src/main/wsdl</wsdlRoot>
            <includes>
                <include>*.wsdl</include>
            </includes>
        </configuration>
        <goals>
            <goal>wsdl2java</goal>
        </goals>
    </execution>
</executions>