Web Services - Web Services Tutorials

In this section of the Web Services tutorial you will be familiarized with the Web Services.

Introduction

The next generation of distributed computing has arrived. A Web service is a unit of managed code that can be remotely invoked using HTTP, that is, it can be activated using HTTP requests.

Historically speaking, remote access to binary units required platform-specific and sometimes language-specific protocols. For example, DCOM clients access remote COM types using tightly coupled RPC calls. CORBA requires the use of tightly coupled protocol referred to as Internet Inter-ORB Protocol (IIOP), to activate remote types. Enterprise JavaBeans (EJBs) requires a Remote Method Invocation (RMI) Protocol and by and large a specific language (Java). Thus each of these remote invocation architectures needs proprietary protocols, which typically require a tight connection to the remote source.

One can access Web services using nothing but HTTP. Of all the protocols in existence today, HTTP is the one specific wire protocol that all platforms tend to agree on. Thus , using Web services, a Web service developer can use any language he wish and a Web service consumer can use standard HTTP to invoke methods a Web service provides. The bottom line is that we have true language and platform integration . Simple Object Access Protocol (SOAP) and XML are also two key pieces of the Web services architecture.

What is a Web Service

Web services constitute a distributed computer architecture made up of many different computers trying to communicate over the network to form one system. They consist of a set of standards that allow developers to implement distributed applications - using radically different tools provided by many different vendors - to create applications that use a combination of software modules called from systems in disparate departments or from other companies.

A Web service contains some number of classes, interfaces, enumerations and structures that provide black box functionality to remote clients. Web services typically define business objects that execute a unit of work (e.g., perform a calculation, read a data source, etc.) for the consumer and wait for the next request. Web service consumer does not necessarily need to be a browser-based client. Console-baed and Windows Forms-based clients can consume a Web service. In each case, the client indirectly interacts with the Web service through an intervening proxy. The proxy looks and feels like the real remote type and exposes the same set of methods. Under the hood, the proxy code really forwards the request to the Web service using standard HTTP or optionally SOAP messages.

Web Service Standards

Web services are registered and announced using the following services and protocols. Many of these and other standards are being worked out by the UDDI project, a group of industry leaders that is spearheading the early creation and design efforts.

Universal Description, Discovery, and Integration (UDDI) is a protocol for describing available Web services components. This standard allows businesses to register with an Internet directory that will help them advertise their services, so companies can find one another and conduct transactions over the Web. This registration and lookup task is done using XML and HTTP(S)-based mechanisms.

Simple Object Access Protocol (SOAP) is a protocol for initiating conversations with a UDDI Service. SOAP makes object access simple by allowing applications to invoke object methods or functions, residing on remote servers. A SOAP application creates a request block in XML, supplying the data needed by the remote method as well as the location of the remote object itself.

Web Service Description Language (WSDL), the proposed standard for how a Web service is described, is an XML-based service IDL (Interface Definitition Language) that defines the service interface and its implementation characteristics. WSDL is referenced by UDDI entries and describes the SOAP messages that define a particular Web service.

ebXML (e-business XML) defines core components, business processes, registry and repository, messaging services, trading partner agreements, and security.

Implementing Web Services

Here comes a brief step-by-step on how a Web service is implemented.

• A service provider creates a Web service
• The service provider uses WSDL to describe the service to a UDDI registry
• The service provider registers the service in a UDDI registry and/or ebXML registry/repository.
• Another service or consumer locates and requests the registered service by querying UDDI and/or ebXML registries.
• The requesting service or user writes an application to bind the registered service using SOAP in the case of UDDI and/or ebXML
• Data and messages are exchanged as XML over HTTP

Web Service Infrastructure

Even though Web services are being built using existing infrastructure, there exists a strong necessity for a number of innovative infrastructures. The core architectural foundation of Web services are XML, XML namespaces, and XML schema. UDDI, SOAP, WSDL, ebXML and security standards are being developed in parallel by different vendors

Web Services Technologies and Tools

There are a number of mechanisms for constructing Web services. Microsoft has come out with a new object-oriented language C# as the development language for Web services and .NET framework. Microsoft has an exciting tool called Visual Studio .NET in this regard. The back end database can be Microsoft SQL Server 2000 in Windows 2000 Professional.

Sun Microsystems has its own set of technologies and tools for facilitating Web services development. Java Servlets, Java Server Pages (JSPs), Enterprise JavaBeans (EJB) architecture and other Java 2 Enterprise Edition (J2EE) technologies play a very critical role in developing Web services.

There are a number of tools for developing Web services. They are Forte Java IDE, Oracle JDeveloper, and WebGain Studio.

Sun Microsystems has taken an initiative called Sun ONE (Open Network Environment) and is planning to push Java forward as a platform for Web services. It is developing Java APIs for XML-based remote procedure calls and for looking up services in XML registries - two more JAX family APIs: JAX/RPC (Java API for XML Remote Procedure Calls) and JAXR (Java API for XML Registries). These will wrap up implementations of Web services standards, such as SOAP and UDDI.

IBM also for its part has already developed a suite of early-access tools for Web services development. They are Web Services Toolkit (WSTK), WSDL Toolkit, and Web Services Development Environment (WSDE).

Apache Axis is an implementation of the SOAP ("Simple Object Access Protocol") submission to W3C.

From the draft W3C specification:

SOAP is a lightweight protocol for exchanging structured information in a decentralized, distributed environment. It is an XML based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses.

Apache Axis is an Open Source SOAP server and client. SOAP is a mechanism for inter-application communication between systems written in arbitrary languages, across the Internet. SOAP usually exchanges messages over HTTP: the client POSTs a SOAP request, and receives either an HTTP success code and a SOAP response or an HTTP error code. Open Source means that you get the source, but that there is no formal support organization to help you when things go wrong.

Conclusion

For the last few years, XML has enabled heterogeneous computing environments to share information over the Web. It now offers a simplified means by which to share process as well. From a technical perspective, the advent of Web services is not a revolution in distributed computing. It is instead a natural evolution of XML application from structured representation of information to structured representation of inter-application messaging.

Prior to the advent of Web services, enterprise application integration (EAI) was very difficult due to differences in programming languages and middleware used within organizations. This led to the situation where interoperability was cumbersome and painful. With the arrival of Web services, any application can be integrated as long as it is Internet-enabled.

It is difficult to avoid the popularity and hype that is surrounding Web services. Each software vendor has some initiative concerning Web services and there is always great speculation about the future of the market for them. Whichever way it turns out, Web service architectures provide a very different way of thinking about software development. From client-server to n-tier systems, to distributed computing, Web service applications represent the culmination of each of these architectures in combination with the Internet.

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Difference between Servlet 2.5 and Servlet 2.4

Features of Servlet 2.4

1. Upgraded supports for Http, J2SE, and J2EE: Servlet 2.4 depends on Http1.1 and J2SE 1.3.
   
2. Additional ServletRequest methods : In Servlet 2.4 four new methods are added in the ServletRequest
   
   • getRemotePort(): It returns the IP source port of the client.
   • getLocalName(): It returns the host name on which the request was recieved.
   • getLocalAddr(): It returns the IP address on which the request was recieved.
   • getLocalPort(): It returns the IP port number.
     
3. New Support for internationalization and charset choice: For the support of internationization Servlet 2.4 has added to new methods in the ServletReponse interface.
   
   • setCharacterEncoding(String encoding): The purpose of this method is to set the response's character encoding. This method helps us to pass a charset parameter to setContentType(String) or passing a Locale to setLocale(Locale). We can now avoid setting the charset in the setContentType("text/html;charset=UTF-8") as setCharacterEncoding() methods pairs with the preexisting getCharacterEncoding() method to manipulate and view the response's character encoding.
     
   • getContentType(): It is responsible for returning the response's content type. The content type can be dynamically set with a combination of setContentType(), setLocale(), and setCharacterEncoding() calls, and the method getContentType() provides a way to view the generated type string.
     
4. New features has been added in RequestDispatcher: In Servlet 2.4 five new request attributes has been added to provide the extra information during a RequestDispatcher forward() call. This features has been added is Servlet 2.4 to know the true original request URI. The following request attributes are:
   
   • javax.servlet.forward.request_uri
   • javax.servlet.forward.context_path
   • javax.servlet.forward.servlet_path
   • javax.servlet.forward.path_info
   • javax.servlet.forward.query_string
   
   
5. SingleThreadModel interface has been deprecated: In Servlet 2.4 the SingleThreadModel interface has been deprecated.
   
6. HttpSession details and interaction with logins has been clarified: The new method HttpSession.logout() has been added in the Servlet 2.4. Now session allows zero or negative values in the element to indicate sessions should never time out.
   If the object in the session can't be serialized in a distributed environment then it must throw an IllegalArgumentException.
   
7. Welcome file behavior and Classloading has been clarified: In servlet 2.4 welcome file can be servlets.
   
8. The web.xml file now uses XML Schema: Version 2.4 servers must still accept the 2.2 and 2.3 deployment descriptor formats, but all new elements are solely specified in Schema.

Features of Servlet 2.5

This version has been released on September 26, 2005 by the Sun MicroSystems. It is not necessary that all web servers and application servers support the features of Servlet 2.5. Still most of the popular containers like Tomcat 5.5 and JBoss 4.0 still support Servlet 2.4.

The list of the added features is given below:

1. Dependency on J2SE 5.0: The minimum platform requirement for Servlet2.5 is JDK 1.5. Servet2.5 can't be used in versions below that JDK1.5. All the available features of Jdk1.5 like generics, autoboxing, an improved for loop etc, etc are guaranteed available to Servlet2.5 programmers.
   
2. Support For annotations: Annotations provide a mechanism for decorating java code constructs (classes, methods, fields, etc.) with metadata information. Annotations are mark code in such a way that code processors may alter their behavior based on the metadata information.
   
3. Several web.xml convenience: Servlet 2.5 introduces several small changes to the web.xml file to make it more convenient to use. For example while writing a , we can now use a asterisk in a which will represent all servlets as well as Jsp.
   
   Previously we used to do
   
   <filter-mapping>
   <filter-name>FilterName</filter-name>
   <servlet-name>FilterName</servlet-name>
   </filter-mapping>
   
   Now,
   
   <filter-mapping>
   <filter-name>FilterName</filter-name>
   <servlet-name>*</servlet-name>
   </filter-mapping>
   
   Previously in or there used to be only one , but now we can have multiple , like
   
   <servlet-mapping>
   <servlet-name>abc</servlet-name>
   <url-pattern>/abc/*</url-pattern>
   <url-pattern>/abc/*</url-pattern>
   </servlet-mapping>
   
   Apart from these changes, many more facilities added in web.xml.
   
4. A Handful of removed restrictions: Servlet 2.5 removed a few restrictions around error handling and session tracking. Now it has removed the restriction that the could not call the setStatus() method to alter the error code that triggered them. In session tracking, Servlet 2.5 eased a rule that a servlet called by RequestDispatcher include() couldn't set response headers.
   
5. Some edge case clarifications: : The servlet2.4 specification says that before calling request.getReader() we must call request.setCharacterEncoding(). However there is no such clarification given why it is so. It has been described properly in Servlet 2.5.

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