Apart from using the default message builders and formatters that are shipped with WSO2 ESB, you have the flexibility to use your own custom message builder and formatter. 

Custom Message Builder

Let's look at how to create a custom builder using a sample scenario where you need to Base64 encode an XML entry field. In this  sample, you retrieve the text content from the payload and then Base64 encode the text. This is then converted to SOAP and the content is then processed in the ESB mediation flow.

1. You will first need to write a class implementing org.apache.axis2.builder.Builder interface in the Axis2 Kernel module and then override the processDocument method.  Within the processDocument method, you can define your specific logic to process the payload content as required and then convert it to SOAP format.

   package org.test.builder;
   
   import org.apache.axiom.om.OMAbstractFactory;
   import org.apache.axiom.om.OMElement;
   import org.apache.axiom.om.impl.OMNodeEx;
   import org.apache.axiom.om.impl.builder.StAXBuilder;
   import org.apache.axiom.om.impl.builder.StAXOMBuilder;
   import org.apache.axiom.om.util.StAXParserConfiguration;
   import org.apache.axiom.om.util.StAXUtils;
   import org.apache.axiom.soap.SOAPBody;
   import org.apache.axiom.soap.SOAPEnvelope;
   import org.apache.axiom.soap.SOAPFactory;
   import org.apache.axis2.AxisFault;
   import org.apache.axis2.Constants;
   import org.apache.axis2.builder.Builder;
   import org.apache.axis2.context.MessageContext;
   import org.apache.commons.codec.binary.Base64;
   
   import javax.xml.stream.XMLStreamException;
   import java.io.IOException;
   import java.io.InputStream;
   import java.io.PushbackInputStream;
   
   public class CustomBuilderForTextXml implements Builder{
       public OMElement processDocument(InputStream inputStream, String s, MessageContext messageContext) throws AxisFault {
           SOAPFactory soapFactory = OMAbstractFactory.getSOAP11Factory();
           SOAPEnvelope soapEnvelope = soapFactory.getDefaultEnvelope();
   
           PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
   
           try {
               int byteVal = pushbackInputStream.read();
               if (byteVal != -1) {
                   pushbackInputStream.unread(byteVal);
   
                   javax.xml.stream.XMLStreamReader xmlReader = StAXUtils.createXMLStreamReader(StAXParserConfiguration.SOAP,
                           pushbackInputStream, (String) messageContext.getProperty(Constants.Configuration.CHARACTER_SET_ENCODING));
   
                   StAXBuilder builder = new StAXOMBuilder(xmlReader);
                   OMNodeEx documentElement = (OMNodeEx) builder.getDocumentElement();
                   documentElement.setParent(null);
                   String elementVal = ((OMElement) documentElement).getText();
                   byte[]   bytesEncoded = Base64.encodeBase64(elementVal.getBytes());
                   ((OMElement) documentElement).setText(new String(bytesEncoded ));
                   SOAPBody body = soapEnvelope.getBody();
                   body.addChild(documentElement);
               }
           } catch (IOException e) {
               e.printStackTrace();
           } catch (XMLStreamException e) {
               e.printStackTrace();
           }
   
           return soapEnvelope;
       }
   }

2. Create a JAR file of this class and add it into the classpath of the Axis2 installation i.e. <ESB_HOME>/repository/components/lib folder.
3. To enable you custom builder for content type text/xml, add the following line in the Message Buillders section in <ESB_HOME/repository/conf/axis2/axis2.xml> file:

<messageBuilder contentType="text/xml" class="org.apache.axis2.transport.http.CustomBuilderForTextXml"/>

Custom Message Formatter

Similarly you can write your own Message Formatters as well, in order to manipulate the outgoing payload from ESB.

When creating a custom Message Formatter, you will need to create a class implementing org.apache.axis2.transport.MessageFormatter Interface and then override the writeTo method. Similar to the Builder, you can implement your logic within the writeTo method.

Ensure to add the following line in Message Formatters section in <ESB_HOME/repository/conf/axis2/axis2.xml> file:

<messageFormatter contentType= "text/xml" class="org.apache.axis2.transport.http.SOAPMessageFormatter" />