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<h1 align="center">Apache Axis2 Architecture Guide</h1>
<p>This document will give an introduction to Axis2's modular architecture
with explanations on every module.</p>
<p><i>Send your feedback to: <a
href="mailto:axis-dev@ws.apache.org">axis-dev@ws.apache.org</a></i>. Prefix
subject with [Axis2]. To subscribe to mailing list see <a
href="http://ws.apache.org/axis2/mail-lists.html">here.</a></p>
<h2>Contents</h2>
<ul>
<li><a href="#bmBP">The Big Picture</a></li>
<li><p><a href="#requirements">Requirement of Axis2</a></p>
</li>
<li><a href="#thearchi">Axis2 Architecture</a>
<ul>
<li><p><a href="#bmcore">Core Modules</a></p>
</li>
<li><a href="#bmother">Other Modules</a></li>
<li><p><a href="#bmInfoMod">Information Model</a></p>
</li>
<li><a href="#bmXML">XML Processing Model</a></li>
<li><p><a href="#bmSOAPPM">SOAP Processing Model</a></p>
<ul>
<li><a href="#default">Axis2 Default Processing Model</a></li>
<li><p><a href="#incomingsoap">Processing an Incoming SOAP
Message</a></p>
</li>
<li><a></a><a href="#outgoing">Processing of the Outgoing
Message</a></li>
<li><p><a href="#extending">Extending SOAP Processing Model</a></p>
<ul>
<li><a href="#extendingwithhandlers">Extending the SOAP
Processing Model with Handlers</a></li>
<li><p><a href="#extendingwithmodules">Extending the SOAP
Processing Model with Modules</a></p>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="#bmDeployment">Deployment</a>
<ul>
<li><a href="#xmlfile">The <em>axis2.xml</em> file</a></li>
<li><p><a href="#servicearchive">Service Archive</a></p>
</li>
<li><a href="#modulearchive">Module Archive</a></li>
</ul>
</li>
<li><p><a href="#bmClientAPI">Client API</a></p>
<ul>
<li><a href="#oneway">One Way Messaging Support</a></li>
<li><p><a href="#requestresponse">Request Response Messaging
Support</a></p>
</li>
</ul>
</li>
<li><a href="#bmTransports">Transports</a></li>
<li><p><a href="#bmWSDL">Code generation</a></p>
</li>
<li><a href="#bmDB">Data Binding</a>
<ul>
<li><a href="#integration">Integration with Code Generation
Engine</a></li>
<li><p><a href="#serial">Serialization and De-Serialization</a></p>
</li>
</ul>
</li>
</ul>
</li>
</ul>
<h2><a name="bmBP">The Big Picture</a></h2>
<p>A new architecture for Axis was introduced during the August 2004 Summit
in Colombo, Sri Lanka. This new architecture Axis2 is based on is more
flexible, efficient and configurable in comparison to <a
href="http://ws.apache.org/axis/java/architecture-guide.html">Axis1.x
architecture</a>. Some well established concepts from Axis 1.x, like handlers
etc., have been preserved in this new architecture.</p>
<p>Any architecture is a result of what that architecture should yield. The
success of an architecture should be evaluated based on the requirements
expected to be met by that architecture. Let us start our journey into Axis2
by looking at the requirements.</p>
<a name="requirements"></a>
<h2>Requirement of Axis2</h2>
<p>In the SOAP terminology, a participant who is taking part in a Web service
interaction is known as a SOAP Node. Delivery of a single SOAP Message is
defined based on two participants, SOAP Sender and SOAP Receiver. Each SOAP
Message is sent by SOAP Sender and received by SOAP Receiver. A single SOAP
delivery is the most basic unit that builds the Web service interaction.</p>
<p>Each SOAP Node may be written in specific programming language, may it be
Java, C++, .NET or Perl, the Web services allow them to inter operate. This
is possible because on the wire each Web service interaction is done via
SOAP, which is common to every SOAP Node.</p>
<p><img alt="" src="images/archi-guide/soap.gif" name="Graphic1"
align="bottom" width="691" height="319" border="0"></p>
<p>Web service middleware handles the complexity in SOAP messaging and lets
the users work with the programming language they are accustomed to. Axis2
allows java users to invoke Web services using java representations, and
handles the SOAP messaging behind the curtain.</p>
<p>Axis2 handles SOAP processing along with numerous other tasks. This makes
the life of the Web service developer a whole lot easier. Following are the
identified requirements:</p>
<ol>
<li>Provide a framework to process the SOAP messages. The framework should
be extensible and the users should be able to extend the SOAP processing
per service or per operation basis. Furthermore it should be able to
model different Message Exchange Patterns (MEPs) using the processing
framework.</li>
<li>Ability to deploy a Web service (with or without WSDL)</li>
<li>Provide a Client API that can be used to invoke Web services. This API
should support both the Synchronous and Asynchronous programming
models.</li>
<li>Ability to configure Axis2 and it's components via deployment.</li>
<li>Ability to send and receive SOAP messages with different
transports.</li>
</ol>
<p>Apart from the above functionalities, performance in terms of memory and
speed is a major consideration for Axis2. Axis2 Core Architecture is built on
three specifications- WSDL, SOAP and WS-Addressing. Other specifications like
JAX-RP, SAAJ &amp; WS-Policy are layered on top of the Core Architecture.</p>
<h2><a name="thearchi">Axis2 Architecture</a></h2>
Axis2 architecture lays out some principals to preserve the uniformity. They
are as follows:
<ul>
<li><p>Axis2 architecture separates the logic and the states. Code that
does the processing is stateless inside Axis2. This allows code to be
executed freely by parallel threads.</p>
</li>
<li>All the information is kept in one information model allowing system to
be suspended and resumed.</li>
</ul>
<p>Axis2 architecture is modular. Therefore Axis2 Framework is built up of
core modules which collectively make up the core architecture of Axis2, and
non-core/other modules are layered on top of this core
modules/architecture.</p>
<a name="bmother"></a>
<h3>Core Modules:</h3>
<ul>
<li><a href="#bmInfoMod">Information Model</a>- Axis2 defines a model to
handle information and all states are kept in this model. The model has a
hierarchy for the information. The system manages the life cycle of the
objects in this hierarchy.</li>
<li><p><a href="#bmXML">XML processing Model</a>- Handling the SOAP Message
is the most important and most complex task. The efficiency of this is
the single most important factor that decides the performance. It makes
sense to delegate this task to a separate sub-project itself, under Web
service project, allowing that sub-project (AXIOM) to provide a simple
API for SOAP and XML info-set while hiding the complexities of the
efficient XML processing within the implementation.</p>
</li>
<li><a href="#bmSOAPPM">SOAP Processing Model</a>- This controls the
execution of the processing. The model defines different phases the
execution would walk through, and the user can extend the Processing
Model at some specific places.</li>
<li><p><a href="#bmDeployment">Deployment Model</a>- Axis2 deployment model
allows the user to deploy services, configure the transports, extend the
SOAP Processing model per system, service or operation basis.</p>
</li>
<li><a href="#bmClientAPI">Client API</a>- This provides a convenient API
for users to communicate with Web services using Axis2. There are set of
classes to interact with IN-OUT and IN-Only style Message Exchange
Patterns (MEPs) where those can be used to construct any other MEP.</li>
<li><p><a href="#bmTransports">Transports</a>- Axis2 define a transport
framework that enables the user to use different transports. The
transports fit into specific places in the SOAP processing model. The
implementation provides a few common transports and the user may write
new ones if and when it is needed.</p>
</li>
</ul>
<a name="bmcore"></a>
<h3>Other Modules:</h3>
<ul>
<li><a href="#bmWSDL">Code Generation</a>- Axis2 provides a code generation
tool that will generate server side and client side code along with a
test case. The generated code would simplify the service deployment and
the service invocation. This would increase usability of Axis2.</li>
<li><p><a href="#bmDB">Data Binding</a>- The basic client API of Axis2 lets
the users process SOAP at the infoset level where as data binding extends
it to make it more convenient to the users by encapsulating the infoset
layer and providing a programming language specific interface.</p>
</li>
</ul>
<map name="Graphic2Map" id="g2m">
<area shape="rect" coords="123,31,222,97" href="#bmInfoMod" alt="">
<area shape="rect" coords="239,62,319,134" href="#bmXML" alt="">
<area shape="rect" coords="127,112,218,177" href="#bmSOAPPM" alt="">
<area shape="rect" coords="12,39,89,95" href="#bmDeployment" alt="">
<area shape="rect" coords="0,108,94,156" href="#bmWSDL" alt="">
<area shape="rect" coords="350,31,426,86" href="#bmClientAPI" alt="">
<area shape="rect" coords="350,114,421,164" href="#bmTransports" alt="">
</map>
<p><img src="images/archi-guide/all.png" name="Graphic2" width="426" alt=""
height="189" border="0" align="bottom" usemap="#Graphic2Map"></p>
<h2><a name="bmInfoMod">Information Model</a></h2>
<p>Information Model has two main hierarchies-Contexts and Descriptions. This
model is described in UML notations below.</p>
<p><img src="images/archi-guide/contexts.png" name="Graphic3" align="bottom"
alt="" width="400" height="443" border="0"></p>
<p>( A ----&lt;&gt; B says, B has 1 or more objects of A. A------&gt;B says,
the given relationship holds between A and B.)</p>
<p>The two hierarchies are connected as shown in the above figure. The
Description hierarchy represents the static data. This data may be loaded
from a configuration file that exists throughout the lifetime of Axis2. For
example, deployed Web services, operations, etc. On the other hand, the
context hierarchy holds more dynamic information about the things that have
more than one instances (e.g.Message Context).</p>
<p>These two hierarchies creates a model that provides the ability to search
for key value pairs. When the values are searched at a given level, they are
searched while moving up the hierarchy until a match is found. In the
resulting model the lower levels override the values in the upper levels. For
example, when a value is looked up in the Message Context and is not found,
it would be looked up in the Operation Context etc, up the hierarchy. The
Search is first done up the hierarchy, and if starting point is a Context
then it is search in the Description hierarchy as well.</p>
<p>This allows the user to declare and override values. Result being a very
flexible configuration model. The flexibility could be the <em>Achilles</em>
heel for the system as the search is expensive, specially for something that
does not exist. Yet in the final analysis developers believe that the
flexibility would serve better in this instant.</p>
<table width="955" border="1" cellpadding="2" cellspacing="3">
<col width="112"><col width="371"><col width="103"><col width="336"><tbody>
<tr>
<td><strong>Context</strong></td>
<td><strong>Description</strong></td>
<td><strong>Configuration</strong></td>
<td><strong>Description</strong></td>
</tr>
<tr>
<td width="112"><p>Configuration Context</p>
</td>
<td width="371"><p>Holds the run time status. A deep copy of this would
essentially make a copy of Axis2.</p>
</td>
<td width="103"><p>Axis Configuration</p>
</td>
<td width="336"><p>Holds all global configurations. Transports, global
modules, parameters and services etc.</p>
</td>
</tr>
<tr>
<td width="112"><p>Service Group Context</p>
</td>
<td width="371"><p>Holds information about a particular usage of the
respective service group. The life of a Service Group Context starts
when a user starts interacting with a service that belong to this
service group. This can be used to share information between services
(within the same service group) in a single interaction.</p>
</td>
<td width="103"><p>AxisServiceGroup</p>
</td>
<td width="336"><p>Holds deployment time information about a particular
service group.</p>
</td>
</tr>
<tr>
<td width="112"><p>Service Context</p>
</td>
<td width="371"><p>This context is available throughout the usage of
the respective service. This can be used to share information between
several MEPs of the same service, within a single interaction.</p>
</td>
<td width="103"><p>AxisService</p>
</td>
<td width="336"><p>Hold the Operations and the service level
configurations</p>
</td>
</tr>
<tr>
<td width="112"><p>Operation Context</p>
</td>
<td width="371"><p>Holds the information about the current MEP
instance, maintain the Messages in the current MEP etc.</p>
</td>
<td width="103"><p>AxisOperation</p>
</td>
<td width="336"><p>Holds the operation level configurations</p>
</td>
</tr>
<tr>
<td width="112"><a name="messageContext"></a>
<p>Message Context</p>
</td>
<td width="371"><p>Holds all the information about the Message
currently being executed.</p>
</td>
<td width="103"><p>AxisMessage</p>
</td>
<td width="336"><p>Do not hold any information as yet, but can be used
as a future extension point.</p>
</td>
</tr>
</tbody>
</table>
<a name="bmXML"></a>
<h2>XML Processing Model</h2>
<p>Please refer to the <a href="OMTutorial.html">OM Tutorial</a></p>
<h2><a name="bmSOAPPM">SOAP Processing Model</a></h2>
<p><img src="images/archi-guide/soap-processing.gif" name="Graphic4" alt=""
align="bottom" width="755" height="348" border="0"></p>
<p>The architecture identified two basic actions a SOAP processor should
perform, sending and receiving SOAP messages. The architecture provides two
Pipes ('Flows'), to perform these two basic actions. Axis Engine or the
driver of Axis2 defines two methods send() and receive() to implement these
two Pipes. The two pipes are named <i><b>In</b> Pipe</i> and <i><b>Out</b>
Pipe</i>, and the complex Message Exchange Patterns (MEPs) are constructed by
combining these two pipes.</p>
<p>Extensibility of the SOAP processing model is provided through handlers.
When a SOAP message is being processed the handlers that are registered would
be executed. The handlers can be registered in global, service, or operation
scopes and the final handler chain is calculated combining the handlers from
all the scopes.</p>
<p>The handlers act as interceptors and they process parts of the SOAP
message and provide add-on services. Usually handlers work on the SOAP
headers, yet they may access or change the SOAP Body as well.</p>
<p>When a SOAP message is being sent through the Client API, an <i>Out
Pipe</i> would begin, the <i>Out Pipe</i> invokes the handlers and end with a
Transport Sender that sends the SOAP message to the target endpoint. The SOAP
message is received by a Transport Receiver at the target endpoint, which
reads the SOAP message and starts the <i>In Pipe</i>. The <em>In Pipe</em>
consists of handlers and ends with the <a href="#mr">Message Receiver</a>,
which consumes the SOAP message.</p>
<p>Above explained processing happens for each and every SOAP message
exchanged. After processing one message Axis2 may decide to create other SOAP
messages, in which case more complex message patterns emerge. However Axis2
always view the SOAP message in terms of processing a single message. The
combination of the messages are layered on top of that basic framework.</p>
<p>The two pipes does not differentiate between the Server and the Client.
The SOAP Processing Model handles the complexity and provides two abstract
pipes to the user. The different areas or the stages of the pipes are given
names, and according to the Axis2 slang those are named 'phases'. A Handler
always runs inside a phase, and the phase provides a mechanism to specify the
ordering of handlers. Both Pipes have built in phases, and both define the
areas for 'User Phases' which can be defined by the user.</p>
<h3><a name="default">Axis2 Default Processing Model</a></h3>
<p>Axis2 has some inbuilt handlers that run in inbuilt phases and they create
the default configuration for the Axis2. We will be looking more in to how to
extend the default processing Model in the next section.</p>
There are four special handlers defined in Axis2.
<ol>
<li>Dispatchers - Finds the service and the operation the SOAP message is
directed to. Dispatchers always run on the <em>In-Pipe</em> and inside
the Dispatch phase. The in-built dispatchers dispatch to a particular
operation depending on various conditions like WS-Addressing information,
URI information, SOAP action information, etc.,</li>
</ol>
<ul>
<li><a name="mr">Message Receiver - Consume the SOAP Message and hands that
over to application , Message receiver is the last handler of the
in-pipe</a></li>
<li><p>Transport Sender - Send the SOAP message to the SOAP endpoint the
message is destined to. Always runs as last handler in the out-pipe</p>
</li>
</ul>
<h3><a name="incomingsoap">Processing an Incoming SOAP Message</a></h3>
<p>Incoming SOAP Message is always received by a Transport Receiver waiting
for the SOAP Messages. Once the SOAP Message arrives the transport Headers
are parsed and a</p>
<a href="#messageContext">Message Context</a> is created from the incoming
SOAP Message. Then the <i>In Pipe</i> is executed with the Message Context.
<p>Let us see what happens at each phase of the execution. This process may
happen either in the server or in the Client.</p>
<ol>
<li><strong>Transport Phase</strong> - The handlers are in the phase meant
to process transport specific information such as validating incoming
message by looking at various transport headers, add data into message
context etc.</li>
<li><strong>Pre-Dispatch Phase</strong>- The main functionality of the
handlers in this phase is to populate message context in order to do the
dispatching. As an example, processing of addressing headers of the SOAP
message happen in this phase.Addressing handlers extract information and
put them in to the message context.</li>
<li><strong>Dispatch Phase</strong> - The Dispatchers run in this phase and
tries to find the correct service and operation this particular message
is destined to.<br>
The post condition of the dispatch phase (any phase can contain a post
condition) checks whether a service and an operation was found by the
dispatchers. If not the execution will halt and throws out a "service not
found error".</li>
<li><strong>User Defined Phases</strong> - Users are allowed to engage
their custom handlers here.</li>
<li>Message Validation Phase - Once the user level execution has taken
place this phase validates whether SOAP Message Processing has taken
place correctly.</li>
<li><strong>Message Processing Phase</strong> - The Business logic of the
SOAP message is executed here. A <a href="#mr">Message Receiver</a> is
registered with each Operation. This Message receiver (associated to the
particular operation) will executed as the last Handler of this
phase.</li>
</ol>
<p>There may be other handlers in any of these phases. Users may use custom
handlers to override the mechanics in each of these phases.</p>
<h3><a name="outgoing">Processing of the Outgoing Message</a></h3>
<p><em>Out Pipe</em> is simpler because the service and operation to dispatch
is known by the time the pipe is executed. The <em>Out Pipe</em> may be
initiated by the</p>
<a href="#mr">Message Receiver</a> or the Client API implementation.Phases of
the <em>Out Pipe</em> are described below:
<ol>
<li>Message Initialize Phase - First phase of the <em>Out Pipe</em>. Serves
as the placeholder for the custom handlers</li>
<li>User Phases - This executes handlers in user defined phases</li>
<li>Transports Phase - Execute any transport handlers taken from the
associated transport configuration. The last handler would be a transport
sender which would send the SOAP message to the target endpoint.</li>
</ol>
<h3><a name="extending">Extending SOAP Processing Model</a></h3>
<p>Above we discussed the default processing model of Axis2. Now let us
discuss the extension mechanism for the SOAP processing model. After all, the
whole effort of making this SOAP engine/processing model was focused much on
making it extendable.</p>
<p>Idea behind introducing step wise processing of the SOAP message in terms
of handlers &amp; phases is to allow easier modification of the processing
order. The notion of phases makes it easier to place handlers in between
other handlers. This enables modification on the default processing behavior.
SOAP Processing Model can be extended with <a
href="#extendingwithhandlers">handler</a> or <a
href="#extendingwithmodules">modules</a>.</p>
<a name="extendingwithhandlers"></a>
<h4>Extending the SOAP Processing Model with Handlers</h4>
The handlers in a module can specify the phase they need to be placed in.
Furthermore they can specify their location inside a phase by providing phase
rules. Phase rules will place a handler
<ol>
<li>as the first handler in a phase.</li>
<li>or as the last handler in a phase.</li>
<li>or before a given handler</li>
<li>or after a given handler</li>
</ol>
<h4><a name="extendingwithmodules">Extending the SOAP Processing Model with
Modules</a></h4>
<p>Axis2 defines an entity called a 'module' that can introduce handlers and
Web service operations. A Module in terms of Axis2 usually acts as a
convenient packaging that includes</p>
<ul>
<li>a set of handlers and</li>
<li>an associated descriptor which includes the phase rules</li>
</ul>
. Modules have the concept of being 'available' and 'engaged'. 'Availability'
means the module is present in the system, but has not been activated, i.e.,
the handlers included inside the module have not been used in the processing
mechanism. When a module is 'engaged' it becomes active and the handlers get
placed in the proper phases. The handlers will act in the same way as
explained in the previous section. Usually a module will be used to implement
a WS-* functionality such as WS-Addressing.
<p>Apart from the extension mechanism based on the handlers, the WS-*
specifications may suggest a requirement for adding new operations. For
example, once a user add a Reliable Messaging capability to a service, the
"Create Sequence" operation needs to be available to the service end point.
This can be implemented by letting the modules define the operations. Once
the module is engaged to a service, the necessary operations will be added to
that service.</p>
<p>A service, operations or the system may engage a module. Once the module
is engaged the handlers and the operations defined in the module are added to
the entity that engaged them.</p>
<p>Modules can not be added (no hot deployment) while the Axis2 engine is
running, but they will be available once the system is restarted.</p>
<a name="bmDeployment"></a>
<h2>Deployment</h2>
<p>The Deployment Model provides a concrete mechanism to configure Axis2.
This model has three entities that provide the configuration.</p>
<a name="xmlfile"></a>
<h3>The axis2.xml file</h3>
<p>This file holds the global configuration for the client and server, and
provide following information:</p>
<ol>
<li>The global parameters</li>
<li>Registered transports in and transport outs</li>
<li>User defined phase names</li>
<li>Modules that are engaged globally (to all services)</li>
<li>Globally defined <a href="#mr">Message Receivers</a></li>
</ol>
<a name="servicearchive"></a>
<h3>Service Archive</h3>
<p>Service archive must have a <em>META-INF/<a
href="resources/schemas/services.xsd">services.xml</a></em> file and may
contain the dependent classes. The <em>services.xml</em> file has following
information.</p>
<ol>
<li>Service level parameters</li>
<li>Modules that are engaged service level</li>
<li>Service Specific <a href="#mr">Message Receivers</a></li>
<li>Operations inside the service</li>
</ol>
<h3><a name="modulearchive">Module Archive</a></h3>
<p>Module archive must have a META-INF/<a
href="resources/schemas/module.xsd">module.xml</a> file and dependent
classes. The <em>module.xml</em> file has Module parameters and the
Operations defined in the module.</p>
<p>When the system is starting up Axis2 ask the deployment model to create a
Axis Configuration. Deployment Model first finds the axis2.xml file and build
the global configuration. Then it checks for the module archives and then for
the service archives. After that the corresponding services and modules are
added to the Axis Configuration. System will build contexts on top of the
Axis Configurations. After this Axis2 is ready to send or receive the SOAP
messages. Hot deployment is only allowed for services.</p>
<a name="bmClientAPI"></a>
<h2>Client API</h2>
<p>There are three parameters that decide the nature of the Web service
interaction.</p>
<ol>
<li>Message Exchange Pattern (MEP)</li>
<li>The Behavior of the transport, whether it's One-Way or Two-Way</li>
<li>Synchronous/ Asynchronous behavior of the Client API</li>
</ol>
<p>Variations of the three parameters can result in indefinite number of
scenarios, even though Axis2 is built on a core that support any messaging
interaction, the developers were compelled to support only two most widely
used Message Exchange Patterns (MEPs).</p>
<p>Two supported MEPs are One-Way and the In-Out (Request-Response) scenarios
in the Client API. The implementation is based on a class called
<code>ServiceClient</code> and there are extensions for each MEP that Axis2
Client API supports.</p>
<h3><a name="oneway">One Way Messaging Support</a></h3>
<p>The One-Way support is provided by the <code>fireAndForget</code> method
of <code>ServiceClient</code>. For one way invocations one can use HTTP ,
SMTP and TCP transports. In the case of the HTTP transport the return channel
is not used and the HTTP 202 OK is returned in the return Channel.</p>
<a name="requestresponse"></a>
<h3>In-Out (Request Response) Messaging Support</h3>
<p>The In-Out support is provided by the <code>sendReceive()</code> method in
ServiceClient. This provides a much simpler interface for the user. The
Client API has four ways to configure a given Message Exchange</p>
<ol>
<li>Blocking or Non-Blocking nature - this can be decided by using
<code>sendReceive()</code> or <code>sendReceiveNonBlocking()</code>
methods</li>
<li>Sender transport - transport used to send the SOAP Message</li>
<li>Listener transport - transport the Response is received</li>
<li>Use Separate Channel - determines whether the response is send over a
separate transport connection or not. This can be false only when sender
and listener transport is same and is a Two-Way transport.</li>
</ol>
<p>Depending on the values of the above four parameter, Axis2 behave
differently.</p>
<a name="bmTransports"></a>
<h2>Transports</h2>
<p>Axis2 has two basic constructs for transports, namely; Transport Senders
and Transport Receivers . These are accessed via the AxisConfiguration.</p>
<p>The incoming transport is the transport via which the AxisEngine receives
the message. The outgoing transport is decided based on the addressing
information (wsa:ReplyTo and wsa:FaultTo). If addressing information is not
available and if server is trying to respond, then the out going transport
will be the outputstream of the incoming transport (if it is two-way
transport).</p>
<p>At the client side the user is free to specify the transport to be
used.</p>
<p>Transport Senders and Transport Receivers contains following
information.</p>
<ol>
<li>Transport Sender for Out Configuration</li>
<li>Transport Listener for In Configuration</li>
<li>Parameters of the transport</li>
</ol>
<p>Each and every transport out configuration defines a transport sender.
Transport sender sends the SOAP message, depending on its configuration.</p>
<p>Transport receiver waits for the SOAP Messages and for each SOAP Message
that arrives, it uses the <i>In Pipe</i> to process the SOAP Message.</p>
<p>Axis2 Presently support the following transports:</p>
<ol>
<li>HTTP - In HTTP transport the transport listener is a servlet or
org.apache.axis2.transport.http.SimpleHTTPServer provided by Axis2. The
transport sender uses commons-httpclient to connect and send the SOAP
Message.</li>
<li>TCP - This is the most simplest transport, but needs the WS -
Addressing support to be functional.</li>
<li>SMTP - This works off a single email account. Transport receiver is a
thread that checks for emails in fixed time intervals.</li>
<li>JMS</li>
</ol>
<a name="bmWSDL" id="bmWSDL"></a>
<h2>Code Generation</h2>
<p>Although the basic objective of the code generation tools has not changed,
the code generation module of Axis2 has taken a different approach to
generate code. Primarily the change is in the use of templates, namely XSL
templates which gives the code generator the flexibility to generate code in
multiple languages.</p>
<p>The basic approach is to set the code generator to generate an XML and
parse it with a template to generate the code file. The following figure
describes how this shows up in the architecture of the tool.</p>
<p><img src="images/archi-guide/CodegenArchitecture-new.gif" name="Graphic6"
alt="" align="bottom" border="0"></p>
<p>The fact here is that it is the same information that is extracted from
the WSDL no matter what code is generated. First, an AxisService is populated
from a WSDL. Then the code generator extracts information from the
AxisService and creates an XML which is language independent. This emitted
XML is then parsed with the relevant XSL to generate code for the relevant
language. No matter what the output language, the process is the same except
for the template that is being used.</p>
<h2><a name="bmDB" id="bmDB">Data Binding</a></h2>
<h3>Integration with Code Generation Engine</h3>
<p>Databinding for Axis2 is implemented in an interesting manner. Databinding
has not been included in the core deliberately and hence the code geneation
allows different data binding frameworks to be plugged in. This is done
through an extension mechanism where the codegen engine calls extensions
first and then executes the core emitter. The extensions populate a map of
QNames vs. class names that is passed to the code generator on which the
emitter operates on.</p>
<p><strong>The following diagram shows the structure:</strong></p>
<p><img src="images/codegen.gif" name="Graphic7" align="bottom"
border="0"></p>
<p><strong>The following databinding extensions are available:</strong></p>
<ol>
<li><strong>ADB</strong> - ADB (Axis Data Binding ) is a simple framework
that allows simple schemas to be compiled. It is lightweight and simple,
works off StAX and fairly performant. However, it does not support the
complete set of schema constructs and is likely to complain for certain
schemas!</li>
<li><strong>XMLBeans</strong> - XMLbeans claims that it supports the
complete schema specification and it is the choice, if full schema
support is needed!</li>
<li><strong>JAX-Me</strong> - JaxMe support has been added in a similar
manner to XMLbeans and serves as another option for the user</li>
<li><strong>JibX</strong> - This is the most recent addition to the family
of databinding extensions and it is also another option the users have
for data binding.</li>
</ol>
<h3><a name="serial" id="serial">Serialization and De-Serialization of Data
bound classes</a></h3>
<p>AXIOM is based on a StAX API (Streaming API for XML). Xml-beans supports
StAX API. Data binding in Axis2 is achieved through interfacing the AXIOM
with the Xml-beans using the StAX API which is supported by both parties. At
the time of the code generation there will be utility methods generated
inside the stub (or the message receiver) that can de-serialize from AXIOM to
data bound object and serialize from data bound object to AXIOM. For example,
if the WSDL has an operation called "echoString", once the code is generated
the following methods will be generated inside the relevant classes.</p>
<pre>public static
org.apache.axiom.om.OMElementtoOM(org.soapinterop.xsd.EchoStringParamDocument
param)// This method will handle the serialization.
public static org.apache.xmlbeans.XmlObject
fromOM(org.apache.axis2.om.OMElement param, java.lang.Class type) //This
method will handle the de-serialization.</pre>
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