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other technologies (such as RMI/IIOP), JAX RPC gives clients greater flexibility

and supports more types of clients. 

J2EE application clients may also use RMI/IIOP to make remote calls over

the network on application business logic. RMI/IIOP is often used for clients

operating in intranet environments, where there is a greater degree of control over

the client's deployment and the J2EE server. While these controlled environments

provide a client container that handles the communication security, passing

through firewalls can be problematic. RMI/IIOP provides clients with secure

access to the application business logic while at the same time taking care of the

details of the client and server communication and marshalling and demarshalling

parameters. 

Java Message Service (JMS) is another means for J2EE clients to communi 

cate with server applications. JMS provides a means for asynchronous communi 

cation. Applications using JMS are better suited to a setting that is behind a

firewall, since messaging systems generally do not work well on the Internet.

(Often, messaging systems are not even exposed on the Internet.) Not only must

developers have some knowledge of how to work with messaging systems, such

as how to set up and use topics or queues, but the messaging system mechanisms

must already be in place. 

Although Web services provide a standard way to exchange XML documents

over HTTP, you can use nonstandard approaches as well. Communication using

the HTTP protocol requires only a simple infrastructure to send and receive mes 

sages. However, the client application must be able to parse the XML documents

representing the messages. Parsing involves mapping the XML data to the client

application's object model. When using this means of communication, the devel 

oper at a minimum needs to write code to send and receive the documents over

HTTP as well as to parse the document data. If such communication must also be

secure, developers would have to include code that uses Secure Socket Layer

(SSL), making the development task more difficult. However, this means of com 

munication may be sufficient, particularly in a closed environment or when clients

are applets. Care should be taken if using this approach, since it is not standard. 

5.2

Scenarios for Web Services Based Client Applications

Developers typically write clients to access preexisting Web services that is,

public or private Web services. At times, the same developers may simultaneously

develop both the service and its clients. Regardless of this, client developers should