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The ability to invoke methods on one Java object from objects residing in another JVM has been a standard Java feature since the JDK 1.1 release. The Remote Method Invocation (RMI) framework makes a remote method invocation follow a local method call's syntax, while also explicitly acknowledging the semantic differences between local and remote object invocation. The combination of that simplicity and power has made RMI a popular distributed programming model.
Its popularity notwithstanding, RMI thus far has been limited to distributed systems that operate within the confines of a single administrative domain. The main reason for that limit: RMI lacked the security features required of wide-area distributed systems—especially systems that perform method invocations across the public Internet. Access to RMI calls' customization features was not part of the standard RMI API, and existing RMI implementations did not provide programmatic access to securing RMI method invocations.
In addition, growing experience with RMI provided a wealth of new feature requests from RMI users. One common developer gripe was the need to explicitly generate stub classes for an RMI server with the rmic compiler instead of dynamically creating proxy classes.
The Sun Microsystems RMI team answered the need for a customization API and many RMI feature requests in the latest Java RMI release, Jini Extensible Remote Invocation (JERI). JERI provides programmatic access to each layer of an RMI call via an API and allows an RMI service deployer to choose the RMI implementation most suitable in a deployment scenario. JERI also defines a uniform mechanism to make remote objects available to answer remote method calls (object exporting), which was not standardized in prior RMI releases. The result of the new features is that RMI calls can now adhere to any security requirement. While distributed system security is not a novel subject, securing RMI calls poses a few special challenges due to RMI's mobile code capability.
When RMI debuted, Web-scale remote method invocation was hardly a feature required of distributed systems. At that time, Web-based distributed computing amounted to simple interaction between Web browsers and Web servers in the form of CGI programs and HTTP servlets. As Web-aware distributed systems' requirements grew over the last several years, the confines of HTTP POST and GET commands for distributed computing became apparent. In response, Simple Object Access Protocol (SOAP) emerged as an XML-based messaging layer on top of HTTP and other Internet protocols. XML-based Web services rely on the exchange of SOAP messages to facilitate distributed computing.
From a Java programmer's viewpoint, SOAP and Java couldn't be more different. SOAP defines a way to encode textual and binary data in XML data structures; Java code, on the other hand, can express not only data, but also algorithms that operate on data. While SOAP-aware APIs, such as SOAP with Attachments API for Java (SAAJ) and Java API for XML-based RPC (JAX-RPC), can make a SOAP-based method invocation syntactically appear as if that invocation was a simple Java method call, no SOAP-aware API can mask the semantic difference between Java and SOAP.
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