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Object mobility in the Jini environment

Reduce the complexity of installation and configuration of complex Jini applications with mobile code

By Frank Sommers, JavaWorld.com, 01/05/01

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Sir Thomas More, the English philosopher, published a book in 1516 about an imaginary island. Inhabitants of this distant land lived the most ideal life imaginable: the citizens were educated, work was not overbearing and could still provide for a good living, society and government were just, and equality and peace ruled. He named this island Utopia.

If Sir Thomas were to write in the present day, he surely would consider an ideal computing landscape for his island as well. We can also surmise that he would seldom mention software installation, version upgrades, or system configuration. Fortunately, we do not have to imagine a utopian world without these chores, because we can take advantage of technologies like Java, Jini, and code mobility to create a more ideal computing environment here and now.

Alan Cooper, inventor of Visual Basic, mentioned in an online ITworld.com interview (see Resources) that the complexity of software installation and the capabilities of software are typically inversely proportional: the more capable a software package, the more difficult its installation. Anyone who has ever set up an operating system or database-management system would instantly agree with Cooper.

An important Jini promise is to disrupt this trend. As Jini architect Jim Waldo put it: "I dream of a world in which my children will never have to use InstallShield." In a recent JavaWorldinterview (see Resources), Waldo referred to Java's ability to download code and objects on an as-needed basis from anywhere on the network.

Jini turns this capability into a practical benefit, providing a simple mechanism to locate objects on the network based on their functionality and download them to a client. Once a client obtains a reference to an object, it can use that reference to benefit from the object's functionality -- by calling methods on it, for instance. Anything that can be represented by a Java object (which includes practically any device or legacy code as well as Java programs) on the network can participate in a federation of such objects -- or services, as they are called in the Jini world.

To effectively use mobile objects, you must gain an understanding of how objects are located on the network, how they are loaded into a running application and participate in the application's execution, and the techniques used to make the object's class files available for downloading.

A class loader refresher

The Java Virtual Machine's (JVM) job is to execute Java bytecode. Bytecode is stored in Java classfiles, which are loaded into the JVM via a class loader. Loading a class means locating a classfile that contains the desired type, based on the type's name, and then creating the class from that file (see the Java Virtual Machine Specification, Chapter 5). Once a class is loaded into a VM, it is linked into the VM's execution state, which means that it becomes part of the program's execution. Finally, the VM initializes the class by calling a special initialization method, which essentially corresponds to static initialization of the class.

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