The Builder Mediator can be used to build the actual SOAP message from a message coming in to ESB through the Message Relay. One usage is to use this before trying to log the actual message in case of an error. Also with the Builder Mediator ESB can be configured to build some of the messages while passing the others along.
Syntax of Builder Mediator.
<syn:builder xmlns:syn="http://ws.apache.org/ns/synapse">
<syn:messageBuilder contentType="" class="" [formatterClass=""]/>
</syn:builder>
By default, Builder Mediator uses the axis2 default Message builders for the content types. User can override those by using the optional messageBuilder configuration. See sample configurations of Message builder and Message formatter in Configuring Message Relay.
Like in axis2.xml user has to specify the content type and the implementation class of the messageBuilder. Also user can specify the message formatter for this content type. This is used by the ExpandingMessageFormatter to format the message before sending to the destination.