java barcode reader Building web services in J2EE in Java

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Building web services in J2EE
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What is a web service
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A web service is some set of functionality made available to remote applications and services via the Internet. A web service is described in XML, using the Web Services Definition Language (WSDL). As shown later in this section, a WSDL description contains details about what the service is, where to find it on the Web, and how to interact with it. Once created, this description is registered in a well-known location. Figure 4.13 depicts a sample web service. There are currently two popular, competing standards in the web services registry space, the ebXML Registry and Repository and Universal Description, Discovery, and Integration (UDDI). Of these, UDDI is more general-purpose and is rapidly gaining the support of a majority of the industry.
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Basic structure of a web service
UDDI defines a system of interoperating service registries that collaborate over the Internet. Its operation is conceptually similar to the Domain Name System (DNS). In UDDI, businesses that provide web services register them with an official UDDI registrar. The registration is then propagated throughout the system, allowing any potential service consumer to search for and locate it via any UDDI engine. Once a web service has been located, the interaction between producer and consumer takes place over standard Internet protocols such as HTTP, FTP, and SMTP. As described earlier, SOAP has an explicit binding to the HTTP protocol. This has made the use of SOAP for web service integration the de facto standard. Where secure interactions are required, HTTPS can be used instead.
Application integration
Web service interfaces A web service can be designed to use a one-way, message-based interface or a two-way, RPC-style interface. This is consistent with our JAXM examples from the previous section. Message-based web services are said to be document-driven, meaning that only the data passed between the parties is important. Issues like timing and coordination of processing are irrelevant. Data-oriented Web services lend themselves to the message-based style. For example, the SOAP message from listing 4.3 is datadriven, and its web service would employ the message-based style. RPC-style services are interface-driven, synchronous interactions. A web service that is process-oriented will use the RPC-style. For example, the SOAP interaction described in listing 4.1 is synchronous (process-driven), and would use the RPC-style. 4.4.2 Providing web services in J2EE
J2EE web services leverage the HTTP capabilities of servlets for sending and receiving messages. For RPC -style web services, you create a servlet that accepts the inbound request, interacts with the component providing the service, and returns an appropriate response. The most obvious choice for implementing a J2EE web service is a stateless session bean, since web services are self-contained RPC calls that do not maintain state across invocations. The suggested architecture for a J2EE RPC-style web service is depicted in figure 4.14.
J2EE EJB Container
Stateless Session EJB
RPC Java Servlet
HTTP(s)
Web Service Clients
J2EE Web Container
Figure 4.14 RPC-style web service in J2EE
Building web services in J2EE
Message-style web services For message-style web services in J2EE, the inbound servlet can place messages on a JMS topic or queue to be processed asynchronously by a Message Driven EJB. If a response is required after the message has been processed, the Message Driven Bean can place the response on an outbound queue for delivery via an outbound servlet. This architecture is depicted in figure 4.15.
J2EE EJB Container
Message Driven EJB
Outbound JMS Destination
Inbound JMS Destination
Outbound Messaging Adapter
Inbound Message Servlet
J2EE Web Container
HTTP(s)
Web Service Clients
Figure 4.15 Message-style web service in J2EE
J2EE web services component model Since J2EE web services are built upon open standards, vendor tools can generate much of the code and supporting XML descriptions of your web service automatically for you. This allows you to concentrate on the specific service being implemented, rather than the details of SOAP messaging and WSDL. For example, BEA s WebLogic product will generate the WSDL for your web
Application integration
service and provide a client JAR file to be downloaded to remote users of the service. It additionally provides generic proxy servlets to receive and send web service messages using SOAP 1.1 with attachments. The only requirements placed upon you are that you implement your web services using the patterns discussed in the previous section (stateless session EJBs for RPC and Message Driven EJBs for message-style) and that you run those EJBs through a web service generation utility. This can make building web services much faster and easier. If you are using another J2EE server, consult your vendor s documentation for similar functionality. As web services registries and the JAXR API mature, we expect to see automated tools for registering your web services in UDDI and similar systems as well. For now, the registration process is outside the scope of vendor implementations.
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