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When a message comes in to the ESB profile of WSO2 Enterprise Integrator (WSO2 EI), the receiving transport selects a message builder based on the message's content type. It uses that builder to process the message's raw payload data and convert it into SOAP. Conversely, when sending a message out from ESB, a message formatter is used to build the outgoing stream from the message. As with message builders, the message formatter is selected based on the message's content type. In a typical routing scenario of the ESB, here is the flow:

You can use the messageType property to change the message's content type as it flows through ESB. For example, if the incoming message is in JSON format and you want to transform it to XML, you could add the messageType property before your mediators in the configuration:

<property name="messageType" value="application/xml" scope="axis2"/>

Configuring message builders and formatters

Message builders and formatters are specified in <EI_HOME>/conf/axis2/axis2.xml (or <EI_HOME>/conf/axis2/tenant-axis2.xml). If you are working in a multi-tenant environment), under the messageBuilders and messageFormatters configuration sections.

The ESB profile of WSO2 EI has a few default message builders, so even if you do not specify them explicitly in axis2.xml or tenant-axis2.xml, they will take effect when messages of those content types come into the ESB profile. If you want to use different builders, specify them in axis2.xml or tenant-axis2.xml to override the defaults. The ESB profile does not have default message formatters, so it is important to specify all of them in the axis2.xml or tenant-axis2.xml configuration. Following are the default message builders:

Content typeMessage Builder













Using message relay

If you want to enable message relay, so that messages of a specific content type are not built or formatted but simply pass through the ESB, you can specify the message relay builder (org.wso2.carbon.relay.BinaryRelayBuilder) for that content type. For more information, see Configuring Message Relay.

Handling messages with no content type

To ensure that messages with no content type are handled gracefully without causing errors, add the following to axis2.xml:

  • In the parameters section: <parameter name="defaultContentType" locked="false">"empty/content"</parameter>
  • In the message builders section: <messageBuilder contentType="empty/content" class="org.wso2.carbon.relay.BinaryRelayBuilder"/> 
  • In the message formatters section: <messageFormatter contentType="empty/content" class="org.wso2.carbon.relay.ExpandingMessageFormatter"/>

Handling text/csv messages

There is no default builder or formatter for messages with the text/csv content type. If you just want to pass these messages through the ESB, you can configure the message relay builder and formatter. If you want to process these messages, you can access the content inside the request/response payload of CSV by configuring the org.apache.axis2.format.PlainTextBuilder and org.apache.axis2.format.PlainTextFormatter for the text/csv content type in axis2.xml. For example:

<messagebuilder contenttype="text/csv" class="org.apache.axis2.format.PlainTextBuilder"/>
<messageformatter contenttype="text/csv" class="org.apache.axis2.format.PlainTextFormatter"/>

When a text/csv message comes into the ESB, the log will include an entry similar to the following, and you can observe that the CSV data is placed inside the payload:

[2013-05-09 13:59:03,478] INFO - LogMediator To: , WSAction: urn:mediate, SOAPAction: urn:mediate, MessageID: urn:uuid:5B9A211341DCC368241368088143463, Direction: request, Envelope: <?xml version='1.0' encoding='utf-8'?><soapenv:envelope xmlns:soapenv=""><soapenv:body><text xmlns="">charitha,wso2,colombo chamara,wso2G,galle </text></soapenv:body></soapenv:envelope>

Handling illegal XML characters in plain text payloads

Plain text payloads that contain illegal XML characters (such as unicodes) will not be successfully processed by the ESB. Therefore, you must configure the system to replace the illegal characters in the payload with an actual character. To enable this configuration, add the parameter shown below (with a suitable unicode value) to the file (stored in the <EI_HOME>/ directory). If this file does not exist in your product by default, be sure to create a new file. 

When this configuration is enabled, all the illegal characters found in a payload will be replaced with the actual character that is represented by the unicode value that you specify for the parameter. The below example uses whitespaces (represented by by the '\u0020' unicode value) to replace illegal characters in payloads.


JSON message builders and formatters

The ESB Profile provides the following message builders and formatters for JSON. These are configured in the Axis2 configuration files located in the <EI_HOME>/conf/axis2 directory. Both types of JSON builders use StAXON as the underlying JSON processor.

Default message builder and formatter

The default message builder and formatter of the ESB Profile of  WSO2 EI are as follows: 

  • org.wso2.carbon.integrator.core.json.JsonStreamBuilder
  • org.wso2.carbon.integrator.core.json.JsonStreamFormatter

The default message builder and formatter maintain the JSON representation intact without converting it to XML during message mediation. You can access the payload content using JSON Path or XPath and convert the payload to XML at any point in the mediation flow.

The default message builder and formatter can also be used by default for JSON mapping when you expose datasources as data services via the ESB Profile of WSO2 EI.

Other message builders and formatters

Other message builders and formatters can be enabled by adding the required parameters to the axis2.xml file as explained below.

If you want to convert the JSON representation to XML before the mediation flow begins, you need to add the message builder and formatter shown below. Note that some data loss can occur during the JSON to XML to JSON conversion process.

  • <parameter name="passthruJsonBuilder">org.apache.synapse.commons.json.JsonBuilder</parameter>

  • <parameter name="passthruJsonFormatter">org.apache.synapse.commons.json.JsonFormatter</parameter>

You can enable the following builders and formatters for JSON mapping in data services.

  • <parameter name="dsJsonBuilder">org.apache.axis2.json.gson.JsonBuilder</parameter>
  • <parameter name="dsJsonFormatter">org.apache.axis2.json.JSONMessageFormatter</parameter>

You can also enable the following builders and formatters if necessary. Note that this is necessary for compatibility with WSO2 ESB:

  • <parameter name="passthruJSONBuilder">org.apache.axis2.json.JSONBuilder</parameter>
  • <parameter name="passthruJSONFormatter">org.apache.axis2.json.JSONMessageFormatter</parameter> 
  • <parameter name="passthruJSONBuilder">org.apache.axis2.json.JSONStreamBuilder</parameter> 
  • <parameter name="passthruJSONFormatter">org.apache.axis2.json.JSONStreamFormatter</parameter>

  • <parameter name="passthruJSONBuilder">org.apache.axis2.json.JSONBadgerfishOMBuilder</parameter>
  • <parameter name="passthruJSONFormatter">org.apache.axis2.json.JSONBadgerfishMessageFormatter</parameter>

Always use the same type of builder and formatter combination. Mixing different builders and formatters will cause errors at runtime.

If you want the ESB Profile of WSO2 EI to handle JSON payloads that are sent using a media type other than application/json, you must register the JSON builder and formatter for that media type in the following two files at minimum (for best results, register them in all Axis2 configuration files found in the <EI_HOME>/conf/axis2 directory):

  • <EI_HOME>/conf/axis2/axis2.xml
  • <EI_HOME>/conf/axis2/axis2_blocking_client.xml

For example, if the media type is text/javascript, register the message builder and formatter as follows:

<messageBuilder contentType="text/javascript" 

<messageFormatter contentType="text/javascript" 

When you modify the builders/formatters in Axis2 configuration, make sure that you have enabled only one correct message builder/formatter pair for a given media type.

Validating JSON messages

If you want the  JSON builder to validate JSON messages that are received by the ESB, the following property should be added to the file stored in the <EI_HOME>/conf/ directory. This validation ensures that erroneous JSON messages are rejected by the ESB.


Writing a custom Message Builder and Formatter

In addition to using the default message builders and formatters in WSO2 EI, 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 WSO2 EI 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.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;
    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 =;
                if (byteVal != -1) {
           xmlReader = StAXUtils.createXMLStreamReader(StAXParserConfiguration.SOAP,
                            pushbackInputStream, (String) messageContext.getProperty(Constants.Configuration.CHARACTER_SET_ENCODING));
                    StAXBuilder builder = new StAXOMBuilder(xmlReader);
                    OMNodeEx documentElement = (OMNodeEx) builder.getDocumentElement();
                    String elementVal = ((OMElement) documentElement).getText();
                    byte[]   bytesEncoded = Base64.encodeBase64(elementVal.getBytes());
                    ((OMElement) documentElement).setText(new String(bytesEncoded ));
                    SOAPBody body = soapEnvelope.getBody();
            } catch (IOException e) {
            } catch (XMLStreamException e) {
            return soapEnvelope;
  2. Create a JAR file of this class and add it into the classpath of the Axis2 installation, i.e., the <EI_HOME>/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 <EI_HOME>/conf/axis2/axis2.xml file (or <EI_HOME>/conf/axis2/tenant-axis2.xml if you are working in a multi-tenant environment):
<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 WSO2 EI.

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 <EI_HOME>/conf/axis2/axis2.xml file (or <EI_HOME>/conf/axis2/tenant-axis2.xml if you are working in a multi-tenant environment):

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

The class name used in the above line should be the name used for the class when writing the formatter.

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