Writing Web Service Clients Using Axis2's Primary APIs

This section presents a complex yet powerful XML based client API, which is intended for advanced users. However, if you are a new user, we recommend using code generation given in the Advanced User's Guide.

Web services can be used to provide a wide-range of functionality to the user from simple, quick operations such as "getStockQuote" to time consuming business services. When we utilize (invoke using client applications) these Web services, we cannot always use simple generic invocation paradigms that suite all the timing complexities involved in the service operations. For example, if we use a single transport channel (such as HTTP) to invoke a Web service with an IN-OUT operation that takes a long time to complete, then most often we may end up with "connection time outs". Further, if there are simultaneous service invocations that we need to perform from a single client application, then the use of a "blocking" client API will degrade the performance of the client application. Similarly, there are various other consequences such as One-Way transports that come into play when we need them. Let's try to analyze some common service invocation paradigms.

Many Web service engines provide users with Blocking and Non-Blocking client APIs.

Blocking API - Once the service invocation is called, the client application hangs, regaining control only when the operation completes, after which the client receives a response or a fault. This is the simplest way of invoking Web services, and it also suites many business situations.
Non-Blocking API - This is a callback or polling based API. Here, once a service invocation is called, the client application immediately regains control and the response is retrieved using the callback object provided. This approach allows the client application to invoke several Web services simultaneously without needing to wait for the response of previous operations.

Both these mechanisms work at the API level. Let's name the asynchronous behavior that we can get using the Non-Blocking API as API Level Asynchrony.

Both mechanisms use single transport connections to send the request and to receive the response. They severely lag the capability of using two transport connections for the request and the response (either One-Way or Two-Way). So both these mechanisms fail to address the problem of long running transactions, as the transport connection may still time-out before the operation completes. A possible solution would be to use two separate transport connections for request and response. The asynchronous behavior that we gain using this solution can be called Transport Level Asynchrony.

By combining API Level Asynchrony and Transport Level Asynchrony, we can obtain four different invocation patterns for Web services as shown in the following table.

API (Blocking/Non-Blocking)	Dual Transports (Yes/No)	Description
Blocking	No	The simplest and most familiar invocation pattern
Non-Blocking	No	Using callbacks or polling
Blocking	Yes	This is useful when the service operation is Request-Response in nature but the transport used is One-Way (e.g. SMTP)
Non-Blocking	Yes	This is can be used to gain the maximum asynchronous behavior. Non blocking at the API level and also at the transport level.

Axis2 provides the user with all these possibilities to invoke Web services.

The following section presents clients that use some of the different possibilities presented above to invoke a Web Service using ServiceClients. All the samples mentioned in this guide are located at the "samples\userguide\src" directory of the binary distribution.

This section presents four types of clients.

Request-Response, Blocking Client
One Way Client, Non-Blocking
Request-Response, Non-Blocking that uses one transport connection
Request-Response, Non-Blocking that uses two transport connections

Request-Response, Blocking Client

The client code below will invoke the "echo" operation of "MyService" using a pure blocking single-channel invocation.

   try {
                  
      OMElement payload = ClientUtil.getEchoOMElement();
      Options options = new Options();
      options.setTo(targetEPR); // this sets the location of MyService service
            
      ServiceClient serviceClient = new ServiceClient();
      serviceClient.setOptions(options);

      OMElement result = serviceClient.sendReceive(payload);
      
      System.out.println(result);
   } catch (AxisFault axisFault) {
      axisFault.printStackTrace();
   } 
}

The lines highlighted in green show the set of operations that you need to perform in order to invoke the Web service in this manner.

To test this client, use the provided Ant build file that can be found in the "Axis2_HOME/samples/userguide" directory. Run the "run.client.blocking" target. If you can see the response OMElement printed in your command line, then you have successfully tested the client.

One Way Client, Non-Blocking

In the Web service "MyService", we had an IN-ONLY operation with the name "ping" (see Creating a New Web Service). Let's write a client to invoke this operation. The client code is as follows:

 
   try {
      OMElement payload = ClientUtil.getPingOMElement();
      Options options = new Options();
      options.setTo(targetEPR);
      ServiceClient serviceClient = new ServiceClient();
      serviceClient.setOptions(options);
      serviceClient.fireAndForget(payload);
      /**
       * We need to wait some time for the message to be sent.  Otherwise,
       * if we immediately exit this function using the main thread, 
       * the request won't be sent.
       */
      Thread.sleep(500);
   } catch (AxisFault axisFault) {
      axisFault.printStackTrace();
   }

Since we are calling an IN-ONLY web service, we can directly use the fireAndForget() in the ServiceClient to invoke this operation. This will not block the invocation and will return the control immediately back to the client. You can test this client by running the target "run.client.ping" of the Ant build file at "Axis2Home/samples/userguide".

Request-Response, Non-Blocking that uses one transport connection

In the "EchoBlockingClient" once the serviceClient.sendReceive(payload); is called, the client is blocked till the operation is complete. This behavior is not desirable when there are many Web service invocations to be done in a single client application or within a GUI. A solution would be to use a Non-Blocking API to invoke Web services. Axis2 provides a callback based non-blocking API for users.

A sample client for this can be found under "Axis2_HOME/samples/userguide/src/userguide/clients" with the name "EchoNonBlockingClient". If we consider the changes that the developer may have to do with respect to the "EchoBlockingClient" that we have already seen, it will be as follows:

serviceClient.sendReceiveNonblocking(payload, callback);

The invocation accepts a Callback object as a parameter. Axis2 client API provides an abstract Callback with the following methods:

public abstract void onComplete(AsyncResult result);
public abstract void onError(Exception e);
public boolean isComplete() {}

The developer is expected to override the onComplete() and onError() methods of their Callback subclass. The Axis2 engine calls the onComplete() method once the Web service response is received by the Axis2 Client API (ServiceClient). This eliminates the blocking nature of the web service request-response invocation.

To run the sample client ("EchoNonBlockingClient") you can simply use the run.client.nonblocking target of the Ant file found in the "Axis2_HOME/samples/userguide" directory.

Request-Response, Non-Blocking that uses two transport connections

The solution provided above by the Non-Blocking API has one limitation when it comes to Web service invocations that take a long time to complete. The limitation is due to the use of single transport connections to invoke the Web service and retrieve the response. In other words, the client API provides a non-blocking invocation mechanism for developers, but the request and the response still come in a single transport (Two-Way transport) connection like HTTP. Long running Web service invocations or Web service invocations using One-Way transports such as SMTP cannot be utilized by simply using a non-blocking API invocation.

The simplest solution is to use separate transport connections (either One-Way or Two-Way) for the request and response. The next problem that needs to be solved, however, is subsequently correlating each request with its response. WS-Addressing provides a neat solution to this using <wsa:MessageID> and <wsa:RelatesTo> headers. Axis2 provides support for an addressing based correlation mechanism and a complying Client API to invoke Web services with two transport connections. (The core of Axis2 does not depend on WS-Addressing, but contains a set of parameters, like in addressing, that can be populated by any method. WS-Addressing is one of the uses that may populate them. Even the transports can populate them. Hence, Axis2 has the flexibility to use different addressing standards.)

Users can select between Blocking and Non-Blocking APIs for the Web service clients with two transport connections. By simply using a boolean flag, the same API can be used to invoke Web services (IN-OUT operations) using two separate transport connections. Let's see how it's done using an example. The following code fragment shows how to invoke the same "echo" operation using Non-Blocking API with two transport connections. The ultimate asynchrony!!

   try {
      OMElement payload = ClientUtil.getEchoOMElement();

      Options options = new Options();
      options.setTo(targetEPR);
      options.setTransportInProtocol(Constants.TRANSPORT_HTTP);
      options.setUseSeparateListener(true);
      options.setAction("urn:echo");  // this is the action mapping we put within the service.xml

      //Callback to handle the response
      Callback callback = new Callback() {
         public void onComplete(AsyncResult result) {
            System.out.println(result.getResponseEnvelope());
         }

         public void onError(Exception e) {
            e.printStackTrace();
         }
      };

      //Non-Blocking Invocation            
      sender = new ServiceClient();            
      sender.engageModule(new QName(Constants.MODULE_ADDRESSING));
      sender.setOptions(options);            
      sender.sendReceiveNonBlocking(payload, callback);            
   
      //Wait till the callback receives the response.            
      while (!callback.isComplete()) {                
         Thread.sleep(1000);            
      }               
   } catch (AxisFault axisFault) {            
     axisFault.printStackTrace();
   } catch (Exception ex) {
     ex.printStackTrace();
   } finally {
      try {
         //Close the Client Side Listener.
         sender.cleanup();
      } catch (AxisFault axisFault) {
        //have to ignore this
      }
   }

The boolean flag (value True) in the options.setUseSeparateListener(...) method informs the Axis2 engine to use separate transport connections for the request and response. Finally sender.cleanup() informs the Axis2 engine to stop the client side listener, which started to retrieve the response.

To run the sample client ("EchoNonBlockingDualClient") you can simply use the "run.client.nonblockingdual" target of the Ant file found in the "Axis2_HOME/samples/userguide/" directory.