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<h1>How To Write Your Own Axis2 Transport</h1>

<h2>Prologue</h2>

<p>To stop you from reinventing the wheel, before we get started, I will
quickly list the transports that are already supported in Axis2 with a small
description. </p>

<p></p>
<ul>
  <li><b>HTTP</b> - In the HTTP transport, the transport Listener is a
    Servlet or a Simple HTTP server provided by Axis2. The transport Sender
    uses sockets to connect and send the SOAP Message. Currently we have the
    commons-httpclient based HTTP Transport sender as the default
  transport.</li>
  <li><b>TCP</b> - This is the most simple transport, but needs Addressing
    support to be functional.</li>
  <li><b>SMTP</b> - This can work off a single email account or a mail server.
    The Mail Transport Receiver is a thread that checks for emails in fixed
    time intervals.</li>
</ul>

<p>To understand the rest of this document you will need some understanding
of the architecture of Axis2. If you are not familiar with the Axis2
architecture, please go through the <a
href="Axis2ArchitectureGuide.html">Axis2 Architecture Guide</a> before you
read any further.</p>

<h2>Introduction</h2>

<p>Broadly speaking, a transport inside Axis2 can be classified as a way of
getting messages that arrive though some channel into the Axis2 engine. The
core of Axis2 is transport independent. All data that is transport specific
is striped out of the incoming message and inserted into the MessageContext.
On the outgoing message, all transport specific information like headers are
added and sent.</p>

<p>To write your own transport you will need to primarily write two classes,
one is the TransportSender and the other is the TransportReceiver. To
register a transport with Axis2 you will need to put two entries in the
axis2.xml file. One for the transport receiver and the other for the
transport sender. I will take you through the process of adding the entries
in relevent sections.</p>

<h2>Transport Receiver</h2>

<p>Any message that is coming into Axis2 needs to go through a transport
receiver. All information about how the message is received at the Axis2
server from the wire [or by snail mail for that matter :)] is isolated inside
the transport receiver. It extracts the data that is coming on the wire and
transforms it into a state that the Axis2 server understands.</p>

<p>So now that we have some background information about how transports work
inside Axis2, without further delay lets dive into some coding and start
building out own transport.</p>

<p></p>

<p>To get things stared you will first need to extend from the
org.apache.Axis2.transport.TransportListener class and write you own
transport listener. To create an engine to process the MessageContext we need
a configuration context. The following code fragment will do this. This
should ideally be only done once for the lifetime of the Transport
receiver.</p>

<p><source></p>
<pre>try {
        //Create a factory 
        ConfigurationContextFactory factory = new ConfigurationContextFactory();
        //Use the factory and an Axis2 repository to create a new Configuration Context
        configurationContext = ConfigurationContextFactory.createConfigurationContextFromFileSystem(repository_directory, 
axis2xmllocation);
} catch (Exception e) {
        log.info(e.getMessage());
}</pre>
</source>

<p>Now we need some kind of a Listener to listen to the requests that come
in. This you will need to implement according to the transport that you are
trying to build. After a message is received at the Receiver you can use the
following code to process the request and then forward the message context to
the engine using the engine.receive(msgContext) method.{Following code
snippet is extracted from MailListener as an example}</p>
<source><pre>AxisEngine engine = new AxisEngine(configurationContext);
MessageContext msgContext = null;

// create and initialize a message context
try {
        TransportInDescription transportIn =
                reg.getAxisConfiguration().getTransportIn(new QName(Constants.TRANSPORT_NAME));
        TransportOutDescription transportOut =
                reg.getAxisConfiguration().getTransportOut(new QName(Constants.TRANSPORT_NAME));
        if (transportIn != null &amp;&amp; transportOut != null) {
                //create Message Context
                
                msgContext = new MessageContext(configurationContext, transportIn, transportOut);
                msgContext.setServerSide(true);
                msgContext.setProperty(MailSrvConstants.CONTENT_TYPE, message.getContentType());
                msgContext.setProperty(MessageContext.CHARACTER_SET_ENCODING, message.getEncoding());

                String soapAction = message.getSOAPActionHeader();
                msgContext.setWSAAction(soapAction);
                msgContext.setSoapAction(soapAction);

                // Here we are trying to set the reply to if it is present in the transport information.
                msgContext.setReplyTo(new EndpointReference(message.getReplyTo());

                //Create the SOAP Message -- This code in from the mail transport and will change depending
                //on how the data is handled in each transport.
                ByteArrayInputStream bais = new ByteArrayInputStream(message.getContent().toString().getBytes());
                XMLStreamReader reader = XMLInputFactory.newInstance().createXMLStreamReader(bais);

                String soapNamespaceURI = "";
                if(message.getContentType().indexOf(SOAP12Constants.SOAP_12_CONTENT_TYPE) &gt; -1){
                        soapNamespaceURI = SOAP12Constants.SOAP_ENVELOPE_NAMESPACE_URI;
                }else if(message.getContentType().indexOf(SOAP11Constants.SOAP_11_CONTENT_TYPE) &gt; -1){
                        soapNamespaceURI = SOAP11Constants.SOAP_ENVELOPE_NAMESPACE_URI;
                }

                StAXBuilder builder = new StAXSOAPModelBuilder(reader, soapNamespaceURI);

                SOAPEnvelope envelope = (SOAPEnvelope) builder.getDocumentElement();
                msgContext.setEnvelope(envelope);
                if (envelope.getBody().hasFault()) {
                        engine.receiveFault(msgContext);
                } else {
                        engine.receive(msgContext);
                }
        } else {
                throw new AxisFault(Messages.getMessage("unknownTransport",Constants.TRANSPORT_NAME));
        }

} catch (Exception e) {
        try {
                if (msgContext != null) {
                        MessageContext faultContext = engine.createFaultMessageContext(msgContext, e);
                        engine.sendFault(faultContext);
                } else {
                        log.error(e);
                }
        } catch (AxisFault e1) {
                log.error(e);
        }
}</pre>
</source>
<p>Now that we have the coding in place, we need to let Axis2 know about our
new transport receiver. We do this by adding an entry into the axis2.xml
file. If you need to pass any properties for the transport to operate, it can
also be done through the axis2.xml file.</p>
<source><pre>   &lt;transportReceiver name="TRANSPORT_NAME" class="org.apache.Axis2.transport.TRANSPORT_NAME.TRANSPORT_LISTNER_CLASS"&gt;
        &lt;parameter name="PROPERTY_NAME" locked="false"&gt;PROPERTY_VALUE&lt;/parameter&gt;
        &lt;parameter name="PROPERTY_NAME_2" locked="false"&gt;PROPERTY_VALUE_2&lt;/parameter&gt;
  &lt;/transportReceiver&gt;
  </pre>
</source>
<p>Using a code fragment like
<code>Utils.getParameterValue(transportOut.getParameter(MailSrvConstants.SMTP_USER))</code>
we can extract the parameters that we insert into the axis2.xml file.</p>

<p>As you can see getting a new transport receiver up and running is a task that requires very little effort.</p>

<h2>Transport Sender</h2>

<p>Any message that is to be sent out of Axis2, is sent through the Transport
Sender. The Transport Sender needs to be extended from the
org.apache.Axis2.transport.AbstractTransportSender class.</p>

<p>The following bit of code from the abstract transport sender will call the
Transport Sender that you wrote.</p>
<source><pre>// If an EPR is present then the message is going on a diffrent channel.
if (epr != null) {
        out = openTheConnection(epr, msgContext);
        OutputStream newOut = startSendWithToAddress(msgContext, out);
        if (newOut != null) {
                out = newOut;
        }
        writeMessage(msgContext, out);
        finalizeSendWithToAddress(msgContext, out);
        } else {
        out = (OutputStream) msgContext.getProperty(MessageContext.TRANSPORT_OUT);
        if (out != null) {
                startSendWithOutputStreamFromIncomingConnection(msgContext, out);
                writeMessage(msgContext, out);
                finalizeSendWithOutputStreamFromIncomingConnection(msgContext, out);
        } else {
                throw new AxisFault(
                        "Both the TO and Property MessageContext.TRANSPORT_WRITER is Null, No way to send response.");
        }
}
</pre>
</source>
<p>Therefore, depending on if your transport is using the same channel to send
the response or using a different channel, you will need to implement a
sub-set of the methods from the abstract class.</p>

<p>After implementing the necessary methods you can let Axis2 know about your
new transport sender by adding an entry to the axis2.xml file like you did
for the Transport Receiver.</p>

<source><pre>  &lt;transportSender name="TRANSPORT_NAME" class="org.apache.Axis2.transport.TRANSPORT_NAME.TRANSPORT_SENDER_CLASS"&gt;
        &lt;parameter name="PROPERTY_NAME" locked="false"&gt;PROPERTY_VALUE&lt;/parameter&gt;
        &lt;parameter name="PROPERTY_NAME_2" locked="false"&gt;PROPERTY_VALUE_2&lt;/parameter&gt;
  &lt;/transportSender&gt;
  </pre>
</source>

<p>Have a look at org.apache.Axis2.transport.mail.MailTransportSender for a
very simple Transport Sender. Also have a look at
org.apache.Axis2.transport.http.CommonsHTTPTransportSender which is used to
send HTTP responses.</p>

<p>Once we have written our transport receiver and our transport sender, and
inserted the needed entries into the axis2.xml file, we are done. It is as
simple as that! </p>

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