Developing JAX-RPC-Based Web Services Using Axis and SOAP
Web services are software applications that are based on a collection of industry standards and can be shared by and used as components of distributed Web-based applications. They can be identified by an URI and their public interfaces can be described and discovered using XML.
With the rising needs of interoperability and application integration, Web services serve as an optimal business solution because they are based on standard Internet protocols. Web services allow applications to process requests to remote and differential systems by speaking a common, non-proprietary language and using common transport protocols (such as HTTP and SMTP).
In this article, the focus will be on solving the previously mentioned problems by building inexpensive Web services using Axis, an open source SOAP implementation provided by Apache. For justification, the Web services build will be jax-rpc based, and in coming articles, I will deal with other methodologies, too.
The Java API for XML-based remote procedure calls (JAX-RPC) simplifies the process of building Web services that incorporate XML-based RPC. It defines mappings between Java types and XML types that attempt to hide the details of XML and provide a familiar method-call paradigm. This article elaborates how developers can use JAX-RPC to implement and call SOAP-based Web services described by the Web Services Description Language (WSDL) using Apache's open source tools—Apache Tomcat for deployment and Apache Axis for SOAP implementation.
JAX-RPC–Based Web Services
JAX-RPC fully embraces the heterogeneous nature of Web services. It allows a JAX-RPC client to talk to another Web service deployed on a different platform and coded in a different language. JAX-RPC provides the specification for invocation modes, client generation, parameter modes, and type-mappings for Java to WSDL and WSDL to Java and client side APIs for invoking the Web service.
Invocation Modes and Clients
JAX-RPC supports three kinds of Web services invocation modes:
- Synchronous request-response: A client invokes a remote method on a Web service and the thread blocks while it is processed by the Web service and receives a return value or an exception.
- One-way RPC mode: A client invokes a remote method on a Web service in one-way mode and the thread does not block and continues execution. The client does not get any return value.
- Non-blocking RPC mode: A client invokes a remote method on a Web service and continues processing in the same thread. Later, the client processes the remote method by performing a blocking receive or polling for return values.
In this article with above modes, we will see how the Java clients can be written or generated using synchronous request-response mode on a Tomcat-Axis combination.
The Web service invocation based on jax-rpc uses pass-by-copy semantics for parameter passing. It does not support the pass-by-reference way of parameter passing.
The following types of parameters are supported by jax-rpc:
- IN type: An IN parameter is passed as a copy. The value of the IN parameter is copied before a Web service invocation. The return value is created as a copy and returned to the Web service client.
- OUT type: An OUT parameter is passed as a copy without any input value to the Web service method. The Web service method fills out the OUT parameter and then returns it back to the client.
- IN OUT type: An INOUT parameter is passed as a copy with an input value to the Web service method. The Web service method uses the input value, process it, fills in the INOUT parameter with a new value and returns it back to the client.
The parameter passing mode for out and inout parameters uses Holder classes. The use of Holder classes enables the mapping to preserve the intended WSDL signature and parameter passing semantics. The JAX-RPC specification includes "Holder classes" for the mapping of simple XML types to the Java data types. The holder classes for primitive ones (for ex, int, float, and so forth) are available with jax-rpc implementation under the javax.xml.rpc.holders package. For the complex XML data types, the name of the Holder class is constructed by appending Holder to the name of the corresponding Java class. These generated Holder classes are packaged as part of the generated sub package named holders in the wsdl to Java mapping.
Each Holder class provides the following methods and fields:
- A public field named 'value'. The type of value is the mapped Java type.
- A default constructor that initializes the value field to a default value.
- A constructor that sets the value field to the passed parameter.
Web Service Invocation
The Apache Tomcat-Axis combination provides a JAX-RPC 1.0 compliant runtime engine, which has both client-side and service-side libraries and deployment tools. Figure 1 elaborates the normal Web service invocation architecture for the synchronous request-response mode. The Application1 with WS client wrapper uses the JAX-RPC runtime to perform a remote procedure call to invoke a public method of Application 2 with the WS server wrapper. The client uses runtime libraries to serialize Java objects to a SOAP message and sends it to the Web service end point, using HTTP transport. As the Web service side that is deployed on Apache Tomcat receives this request, the service-side JAX-RPC runtime deserializes the SOAP message in to Java types and invokes the method on the Web service and in turn makes a call to Application 2. The Web service, after processing the request, sends response back to the client in a similar fashion.
Figure 1. Web Services Invocation Architecture
As of now, we understood the simple Web service invocation based on JAX-RPC. The next part of this article briefly develops a sample Web service, a dynamic client for it on Apache Tomcat with Axis.
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