Running Linux, 5th Edition - Matthias Kalle Dalheimer [420]
The Promise of Java, or Why You Might Want to Use Java
All this may not sound too exciting to you. There are lots of object-oriented programming languages, after all, and with Mozilla plug-ins you can download executable programs from web servers and execute them on your local machine.
But Java is more than just an object-oriented programming language. One of its most exciting aspects is platform independence . That means you can write and compile your Java program and then deploy it on almost every machine, whether it is a lowly '386 running Linux, a powerful Pentium IV running the latest bloatware from Microsoft, or an IBM mainframe. Sun Microsystems calls this "Write Once, Run Anywhere." Unfortunately, real life is not as simple as design goals. There are tiny but frustrating differences that make a program work on one platform and fail on another. With the advent of the GUI library Swing, a large step was made toward remedying this problem.
The neat feature of compiling code once and then being able to run it on another machine is made possible by the Java Virtual Machine (JVM ), the piece of software the interprets the byte code generated by the Java compiler: The Java compiler does not generate object code for a particular CPU and operating system like gcc does — it generates code for the JVM. This "machine" does not exist anywhere in hardware (yet), but is instead a specification. This specification says which so-called opcodes the machine understands and what the machine does when it encounters them in the object file. The program is distributed in binary form containing so-called byte codes that follow the JVM specification.
Now all you need is a program that implements the JVM on your particular computer and operating system. These are available nowadays for just about any platform—no vendor can dare not provide a JVM for its hardware or operating system. Such programs are also called Java interpreters because they interpret the opcodes compiled for the JVM and translate them into code for the native machine.
This distinction, which makes Java both a compiled and an interpreted language, makes it possible for you to write and compile your Java program and distribute it to someone else, and no matter what hardware and operating system she has, she will be able to run your program as long as a Java interpreter is available for it.
Alas, Java 's platform independence comes at a steep price. Because the object code is not object code of any currently existing hardware, it must pass through an extra layer of processing, meaning that programs written in Java typically run 10 to 20 times slower than comparable programs written in, for example, C. Although this does not matter for some cases, in other cases it is simply unacceptable. So-called just-in-time compilers are available that first translate the object code for the JVM into native object code and then run this object code. When the same object code is run a second time, the precompiled native code can be used without any interpretation and thus runs faster. But the speed that can be achieved with this method is still inferior to that of C programs. Newer compilers use a technology called just-in-time compilation, but the promise of an execution speed "comparable to C programs" has not been met yet, and it is doubtful whether it ever will.
Java also distinguishes between applications and applets. Applications are standalone programs that are run from the command line or your local desktop and behave like ordinary programs. Applets, on the other hand, are programs (usually smaller) that run inside your web browser. (To run these programs, the browser needs a Java interpreter inside.) When you browse a web site that contains a Java applet, the web server sends you the object code of the applet, and your browser executes it for you. You can use this for anything from simple animations to complete online banking systems.[*]