Chapter 40. Developing Asynchronous Applications

The asynchronous style of invocation requires extra stub code for the dedicated asynchronous methods defined on the SEI. This special stub code is not generated by default. To switch on the asynchronous feature and generate the requisite stub code, you must use the mapping customization feature from the WSDL 2.0 specification.

Customization enables you to modify the way the Maven code generation plug-in generates stub code. In particular, it enables you to modify the WSDL-to-Java mapping and to switch on certain features. Here, customization is used to switch on the asynchronous invocation feature. Customizations are specified using a binding declaration, which you define using a jaxws:bindings tag (where the jaxws prefix is tied to the http://java.sun.com/xml/ns/jaxws namespace). There are two ways of specifying a binding declaration:

The template for a binding declaration file that switches on asynchronous invocations is shown in Example 40.2, “Template for an Asynchronous Binding Declaration”.

<bindings xmlns:xsd="http://www.w3.org/2001/XMLSchema"
          xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
          wsdlLocation="AffectedWSDL"
          xmlns="http://java.sun.com/xml/ns/jaxws">
  <bindings node="AffectedNode">
    <enableAsyncMapping>true</enableAsyncMapping>
  </bindings>
</bindings>

Where AffectedWSDL specifies the URL of the WSDL contract that is affected by this binding declaration. The AffectedNode is an XPath value that specifies which node (or nodes) from the WSDL contract are affected by this binding declaration. You can set AffectedNode to wsdl:definitions, if you want the entire WSDL contract to be affected. The jaxws:enableAsyncMapping element is set to true to enable the asynchronous invocation feature.

For example, if you want to generate asynchronous methods only for the GreeterAsync interface, you can specify <bindings node="wsdl:definitions/wsdl:portType[@name='GreeterAsync']"> in the preceding binding declaration.

Assuming that the binding declaration is stored in a file, async_binding.xml, you would set up your POM as shown in Example 40.3, “Consumer Code Generation”.

<plugin>
  <groupId>org.apache.cxf</groupId>
  <artifactId>cxf-codegen-plugin</artifactId>
  <version>${cxf.version}</version>
  <executions>
    <execution>
      <id>generate-sources</id>
      <phase>generate-sources</phase>
      <configuration>
        <sourceRoot>outputDir</sourceRoot>
        <wsdlOptions>
          <wsdlOption>
            <wsdl>hello_world.wsdl</wsdl>
            <extraargs>
              <extraarg>-client</extraarg>
              <extraarg>-b async_binding.xml</extraarg>
            </extraargs>
          </wsdlOption>
        </wsdlOptions>
      </configuration>
      <goals>
        <goal>wsdl2java</goal>
      </goals>
    </execution>
  </executions>
</plugin>

The -b option tells the code generator where to locate the external binding file.

For more information on the code generator see Section 44.2, “cxf-codegen-plugin”.

You can also embed the binding customization directly into the WSDL document defining the service by placing the jaxws:bindings element and its associated jaxws:enableAsynchMapping child directly into the WSDL. You also must add a namespace declaration for the jaxws prefix.

Example 40.4, “WSDL with Embedded Binding Declaration for Asynchronous Mapping” shows a WSDL file with an embedded binding declaration that activates the asynchronous mapping for an operation.

<wsdl:definitions xmlns="http://schemas.xmlsoap.org/wsdl/"
                  ...
                  xmlns:jaxws="http://java.sun.com/xml/ns/jaxws"
                  ...>
  ...
  <wsdl:portType name="GreeterAsync">
    <wsdl:operation name="greetMeSometime">
      <jaxws:bindings> <jaxws:enableAsyncMapping>true</jaxws:enableAsyncMapping> </jaxws:bindings>
      <wsdl:input name="greetMeSometimeRequest"
                  message="tns:greetMeSometimeRequest"/>
      <wsdl:output name="greetMeSometimeResponse"
                   message="tns:greetMeSometimeResponse"/>
    </wsdl:operation>
  </wsdl:portType>
  ...
</wsdl:definitions>

When embedding the binding declaration into the WSDL document you can control the scope affected by the declaration by changing where you place the declaration. When the declaration is placed as a child of the wsdl:definitions element the code generator creates asynchronous methods for all of the operations defined in the WSDL document. If it is placed as a child of a wsdl:portType element the code generator creates asynchronous methods for all of the operations defined in the interface. If it is placed as a child of a wsdl:operation element the code generator creates asynchronous methods for only that operation.

It is not necessary to pass any special options to the code generator when using embedded declarations. The code generator will recognize them and act accordingly.

After generating the stub code in this way, the GreeterAsync SEI (in the file GreeterAsync.java) is defined as shown in Example 40.5, “Service Endpoint Interface with Methods for Asynchronous Invocations”.

package org.apache.hello_world_async_soap_http;

import org.apache.hello_world_async_soap_http.types.GreetMeSometimeResponse;
...

public interface GreeterAsync
{
  public Future<?> greetMeSometimeAsync(
        java.lang.String requestType,
        AsyncHandler<GreetMeSometimeResponse> asyncHandler
    );

    public Response<GreetMeSometimeResponse> greetMeSometimeAsync(
        java.lang.String requestType
    );

    public java.lang.String greetMeSometime(
        java.lang.String requestType
    );
}

In addition to the usual synchronous method, greetMeSometime(), two asynchronous methods are also generated for the greetMeSometime operation:

The polling approach is the more straightforward of the two approaches to developing an asynchronous application. The client invokes the asynchronous method called OperationNameAsync() and is returned a Response<T> object that it polls for a response. What the client does while it is waiting for a response is depends on the requirements of the application. There are two basic patterns for handling the polling:

Example 40.6, “Non-Blocking Polling Approach for an Asynchronous Operation Call” illustrates using non-blocking polling to make an asynchronous invocation on the greetMeSometime operation defined in Example 40.1, “WSDL Contract for Asynchronous Example”. The client invokes the asynchronous operation and periodically checks to see if the result is returned.

package demo.hw.client;

import java.io.File;
import java.util.concurrent.Future;

import javax.xml.namespace.QName;
import javax.xml.ws.Response;

import org.apache.hello_world_async_soap_http.*;

public final class Client {
  private static final QName SERVICE_NAME
    = new QName("http://apache.org/hello_world_async_soap_http",
                "SOAPService");

  private Client() {}

  public static void main(String args[]) throws Exception {

    // set up the proxy for the client

    Response<GreetMeSometimeResponse> greetMeSomeTimeResp =
      port.greetMeSometimeAsync(System.getProperty("user.name"));

      while (!greetMeSomeTimeResp.isDone()) {
      // client does some work
      }
      GreetMeSometimeResponse reply = greetMeSomeTimeResp.get();
      // process the response

      System.exit(0);
  }
}

The code in Example 40.6, “Non-Blocking Polling Approach for an Asynchronous Operation Call” does the following:

Invokes the greetMeSometimeAsync() on the proxy.

The method call returns the Response<GreetMeSometimeResponse> object to the client immediately. The Apache CXF runtime handles the details of receiving the reply from the remote endpoint and populating the Response<GreetMeSometimeResponse> object.

Checks to see if a response has arrived by checking the isDone() of the returned Response object.

If the response has not arrived, the client continues working before checking again.

When the response arrives, the client retrieves it from the Response object using the get() method.

When using the block polling pattern, the Response object’s isDone() is never called. Instead, the Response object’s get() method is called immediately after invoking the remote operation. The get() blocks until the response is available.

You can also pass a timeout limit to the get() method.

Example 40.7, “Blocking Polling Approach for an Asynchronous Operation Call” shows a client that uses blocking polling.

package demo.hw.client;

import java.io.File;
import java.util.concurrent.Future;

import javax.xml.namespace.QName;
import javax.xml.ws.Response;

import org.apache.hello_world_async_soap_http.*;

public final class Client {
  private static final QName SERVICE_NAME
    = new QName("http://apache.org/hello_world_async_soap_http",
                "SOAPService");

  private Client() {}

  public static void main(String args[]) throws Exception {

    // set up the proxy for the client

    Response<GreetMeSometimeResponse> greetMeSomeTimeResp =
      port.greetMeSometimeAsync(System.getProperty("user.name"));
    GreetMeSometimeResponse reply = greetMeSomeTimeResp.get();
      // process the response
      System.exit(0);
  }
}

An alternative approach to making an asynchronous operation invocation is to implement a callback class. You then call the asynchronous remote method that takes the callback object as a parameter. The runtime returns the response to the callback object.

To implement an application that uses callbacks, do the following:

The callback class must implement the javax.xml.ws.AsyncHandler interface. The interface defines a single method: handleResponseResponse<T>res The Apache CXF runtime calls the handleResponse() method to notify the client that the response has arrived. Example 40.8, “The javax.xml.ws.AsyncHandler Interface” shows an outline of the AsyncHandler interface that you must implement.

public interface javax.xml.ws.AsyncHandler
{
  void handleResponse(Response<T> res)
}

Example 40.9, “Callback Implementation Class” shows a callback class for the greetMeSometime operation defined in Example 40.1, “WSDL Contract for Asynchronous Example”.

package demo.hw.client;

import javax.xml.ws.AsyncHandler;
import javax.xml.ws.Response;

import org.apache.hello_world_async_soap_http.types.*;

public class GreeterAsyncHandler implements AsyncHandler<GreetMeSometimeResponse>
{
  private GreetMeSometimeResponse reply;

  public void handleResponse(Response<GreetMeSometimeResponse>
                             response)
  {
    try
    {
      reply = response.get();
    }
    catch (Exception ex)
    {
      ex.printStackTrace();
    }
  }

  public String getResponse()
  {
    return reply.getResponseType();
  }
}

The callback implementation shown in Example 40.9, “Callback Implementation Class” does the following:

Defines a member variable, response, that holds the response returned from the remote endpoint.

Implements handleResponse().

This implementation simply extracts the response and assigns it to the member variable reply.

Implements an added method called getResponse().

This method is a convenience method that extracts the data from reply and returns it.

Example 40.10, “Callback Approach for an Asynchronous Operation Call” illustrates a client that uses the callback approach to make an asynchronous call to the GreetMeSometime operation defined in Example 40.1, “WSDL Contract for Asynchronous Example”.

package demo.hw.client;

import java.io.File;
import java.util.concurrent.Future;

import javax.xml.namespace.QName;
import javax.xml.ws.Response;

import org.apache.hello_world_async_soap_http.*;

public final class Client {
  ...

  public static void main(String args[]) throws Exception
  {
    ...
    // Callback approach
    GreeterAsyncHandler callback = new GreeterAsyncHandler();

    Future<?> response =
      port.greetMeSometimeAsync(System.getProperty("user.name"),
                                callback);
    while (!response.isDone())
    {
      // Do some work
    }
    resp = callback.getResponse();
    ...
    System.exit(0);
  }
}

The code in Example 40.10, “Callback Approach for an Asynchronous Operation Call” does the following:

Instantiates a callback object.

Invokes the greetMeSometimeAsync() that takes the callback object on the proxy.

The method call returns the Future<?> object to the client immediately. The Apache CXF runtime handles the details of receiving the reply from the remote endpoint, invoking the callback object’s handleResponse() method, and populating the Response<GreetMeSometimeResponse> object.

Uses the returned Future<?> object’s isDone() method to check if the response has arrived from the remote endpoint.

Invokes the callback object’s getResponse() method to get the response data.

Consumers making asynchronous requests will not receive the same exceptions returned when they make synchronous requests. Any exceptions returned to the consumer asynchronously are wrapped in an ExecutionException exception. The actual exception thrown by the service is stored in the ExecutionException exception’s cause field.

Exceptions generated by a remote service are thrown locally by the method that passes the response to the consumer’s business logic. When the consumer makes a synchronous request, the method making the remote invocation throws the exception. When the consumer makes an asynchronous request, the Response<T> object’s get() method throws the exception. The consumer will not discover that an error was encountered in processing the request until it attempts to retrieve the response message.

Unlike the methods generated by the JAX-WS framework, the Response<T> object’s get() method throws neither user modeled exceptions nor generic JAX-WS exceptions. Instead, it throws a java.util.concurrent.ExecutionException exception.

The framework stores the exception returned from the remote service in the ExecutionException exception’s cause field. The details about the remote exception are extracted by getting the value of the cause field and examining the stored exception. The stored exception can be any user defined exception or one of the generic JAX-WS exceptions.

Example 40.11, “Catching an Exception using the Polling Approach” shows an example of catching an exception using the polling approach.

package demo.hw.client;

import java.io.File;
import java.util.concurrent.Future;

import javax.xml.namespace.QName;
import javax.xml.ws.Response;

import org.apache.hello_world_async_soap_http.*;

public final class Client
{
  private static final QName SERVICE_NAME
    = new QName("http://apache.org/hello_world_async_soap_http",
                "SOAPService");

  private Client() {}

  public static void main(String args[]) throws Exception
  {
    ...
    // port is a previously established proxy object.
    Response<GreetMeSometimeResponse> resp =
       port.greetMeSometimeAsync(System.getProperty("user.name"));

    while (!resp.isDone())
    {
      // client does some work
    }

    try
    {
      GreetMeSometimeResponse reply = greetMeSomeTimeResp.get();
      // process the response
    }
    catch (ExecutionException ee)
    {
       Throwable cause = ee.getCause();
       System.out.println("Exception "+cause.getClass().getName()+" thrown by the remote service.");
    }
  }
}

The code in Example 40.11, “Catching an Exception using the Polling Approach” does the following:

Wraps the call to the Response<T> object’s get() method in a try/catch block.

Catches a ExecutionException exception.

Extracts the cause field from the exception.

If the consumer was using the callback approach the code used to catch the exception would be placed in the callback object where the service’s response is extracted.