Java Web Services Basics
A web service normally resides on a server. The application (i.e., the client) that accesses the web service sends a method call over a network to the remote machine, which processes the call and returns a response over the network to the application. This kind of distributed computing is beneficial in many applications. For example, a client application without direct access to a database on a remote server might be able to retrieve the data via a web service. Similarly, an application lacking the processing power to perform specific compu-tations could use a web service to take advantage of another system’s superior resources.
In Java, a web service is implemented as a class. In previous chapters, all the pieces of an application resided on one machine. The class that represents the web service resides on a server—it’s not part of the client application.
Making a web service available to receive client requests is known as publishing a web service; using a web service from a client application is known as consuming a web service. An application that consumes a web service consists of two parts—an object of a proxy class for interacting with the web service and a client application that consumes the web service by invoking methods on the object of the proxy class. The client code invokes methods on the proxy object, which handles the details of communicating with the web service (such as passing method arguments to the web service and receiving return values from the web service) on the client’s behalf. This communication can occur over a local network, over the Internet or even with a web service on the same computer. The web ser-vice performs the corresponding task and returns the results to the proxy object, which then returns the results to the client code. Figure 28.1 depicts the interactions among the client code, the proxy class and the web service. As you’ll soon see, Netbeans creates these proxy classes for you in your client applications.
Requests to and responses from web services created with JAX-WS 2.0 (one of many different web service frameworks) are typically transmitted via SOAP. Any client capable of generating and processing SOAP messages can interact with a web service, regardless of the language in which the web service is written. We discuss SOAP in Section 28.5.
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