Java mobile code

Supplementary section for Understanding Networked Applications: A First Course, Morgan Kaufmann, 1999. Copyright notice: Permission is granted to copy and distribute this material for educational purposes only, provided that this copyright notice remains attached. An illustrative example of MC middleware is Java from Sun Microsystems [Arn96][Fla96]. Java is several things. First and foremost, it is a “pure” object-oriented language, one that unlike C++ requires programs to be composed exclusively of objects. Java also defines an entire mobile code system, including a virtual machine and a set of libraries. MC and MO are generally based on objects and components, and Java is an example of this. The Java language is modeled after C++, but with some important differences. First, Java is unabashedly object-oriented, as it dictates that programs are composed of collaborating objects (C++ doesn’t). Second, Java attempts to “clean up” C++ by removing features that make programs more difficult to write and read, and more importantly and make it unsafe as a mobile code language (see the sidebar "Java and Security"). Third, Java adds direct language support for some useful features for distributed computing applications, such as threads. The Java Virtual Machine (VM) defines the instruction set of a virtual microprocessor—one different than most actual microprocessors—and then emulates this virtual processor on the real microprocessor. (A microprocessor that natively implements VM bytecode instructions is also possible.) The primitive instructions for the VM are written in bytecode, composed of instructions for the VM. A Java compiler translates the Java source code into bytecode that can then be exe-