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This month's article looks at aglets, an innovation developed by IBM Japan.
According to Webster's Ninth New Collegiate Dictionary, an aglet is:
In other words, aglets are those little plastic tubes on the ends of your shoelaces. Now, however, there is a new definition of the word aglet: a Java-based autonomous software agent.
As used here, a software agent is a program that can halt itself, ship itself to another computer on the network, and continue execution at the new computer. An agent doesn't restart execution from the beginning at the new computer; it continues where it left off. For example, imagine an agent that increments a counter starting with zero. If that agent counts from zero to ten, then halts and ships itself to another computer, it will not start counting again at zero. It will continue counting starting with ten, because that was where it left off when it halted at its previous computer.
Agents are autonomous because they decide where they will go and what they will do. They control their lifetimes. They can receive requests from external sources, such as other agents, but each individual agent decides whether or not to comply with external requests. Also, agents can decide to perform actions, such as travel across a network to a new computer, independent of any external request.
The Java aglet extends the model of network-mobile code made famous by Java applets. Like an applet, the class files for an aglet can migrate across a network. But unlike applets, when an aglet migrates it also carries its state. An applet is code that can move across a network from a server to a client. An aglet is a running Java program (code and state) that can move from one host to another on a network. In addition, because an aglet carries its state wherever it goes, it can travel sequentially to many destinations on a network, including eventually returning back to its original host.
A Java aglet is similar to an applet in that it runs as a thread (or multiple threads) inside the context of a host Java application. To run applets, a Web browser fires off a Java application to host any applets it may encounter as the user browses from page to page. That application installs a security manager to enforce restrictions on the activities of any untrusted applets. To download an applet's class files, the application creates class loaders that know how to request class files from an HTTP server.
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