Example code using Synapse $url XPath variable to retrieve query param. Details can be found at http://asitha.github.io/cs/programming/wso2/esb/mediators/2017/08/14/get-query-params-with-wso2-esb/

retrieve.query.param.with.$url.xml

<?xml version="1.0" encoding="UTF-8"?>
<api xmlns="http://ws.apache.org/ns/synapse" name="StoreAPI" context="/store">
    <resource methods="GET" uri-template="/view*">
        <inSequence>
            <!--Read and set the parameteres to a property -->
            <property name="name" expression="$url:name" scope="default" type="STRING"/>
            <property name="age" expression="$url:age" scope="default" type="STRING"/>

            <!--Directly use in a payload factory as follows -->
            <payloadFactory media-type="xml">
                <format>
                    <user xmlns="">
                        <name>$1</name>
                        <age>$2</age>
                    </user>
                </format>
                <args>
                    <arg evaluator="xml" expression="$url:name"/>
                    <arg evaluator="xml" expression="$url:age"/>
                </args>
            </payloadFactory>
            <respond/>
        </inSequence>
    </resource>
</api>