In addition to using the default message builders and formatters in WSO2 ESB, you can create your own custom message builders and formatters. 

Custom message builder

Let's look at how to create a custom message 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 the 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., the <ESB_HOME>/repository/components/lib folder.
3. To enable your custom message builder for content type text/xml, add the following line in the Message Buillders section in the <ESB_HOME/repository/conf/axis2/axis2.xml> file:

<messageBuilder contentType="text/xml" class="org.test.builder.http.CustomBuilderForTextXml"/>

Custom message formatter

Similarly, you can write your own message formatter to manipulate the outgoing payload from the ESB.

When creating a custom message formatter, you will need to create a class implementing the org.apache.axis2.transport.MessageFormatter interface and then override the writeTo method. You can implement your logic within the writeTo method.

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

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