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Java's three types of portability

Find out about the types of portability Java supports, and how Microsoft could undermine the technology by subverting the one type that most threatens its hold on the desktop operating system market

By Mark Roulo, JavaWorld.com, 05/01/97

Before jumping into details on each of these three types of portability, though, let's review a few fundamental terms.

Defining some terms

The following terms are used in this article:

Endianism

Endianism refers to the storage order of bytes in a multibyte quantity in a given CPU. For example, the unsigned short 256 (decimal) requires two bytes of storage: a 0x01 and 0x00. These two bytes can be stored in either order: 0x01, 0x00 or 0x00, 0x01. Endianism determines the order in which the two bytes are stored. For practical purposes, endianism usually matters only when CPUs of different endianism must share data.

Java

Java is several different technologies packaged together -- the Java programming language, the Java virtual machine (JVM), and the class libraries associated with the language. This article discusses all of these aspects.

Java virtual machine (JVM)

The JVM is an imaginary CPU for which most Java compilers emit code. Support for this imaginary CPU is what allows Java programs to run without being recompiled on different CPUs. Nothing in the Java programming language requires Java source code to be compiled into code for the JVM instead of into native object code.

In fact, Asymetrix and Microsoft have announced Java compilers that emit native Microsoft Windows applications. (See the Resources section of this article for additional information.)

J-code

J-code is the output emitted by most Java compilers into the class files. J-code can be thought of as object code for the Java virtual machine.

Portability

Portability refers to the ability to run a program on different machines. Running a given program on different machines can require different amounts of work (for example, no work whatsoever, recompiling, or making small changes to the source code). When people refer to Java applications and applets as portable, they usually mean the applications and applets run on different types of machines with no changes (such as recompilation or tweaks to the source code).

Now that we have covered some essential terms, we'll explain each of the three types of Java portability.

Java as a language: source code portability

As a programming language Java provides the simplest and most familiar form of portability -- source code portability. A given Java program should produce identical results regardless of the underlying CPU, operating system, or Java compiler. This idea is not new; languages such as C and C++ have provided the opportunity for this level of portability for many years. However, C and C++ also provide numerous opportunities to create non-portable code as well. Unless programs written in C and C++ are designed to be portable from the beginning, the ability to move to different machines is more theoretical than practical. C and C++ leave undefined details such as the size and endianism of atomic data types, the behavior of floating-point math, the value of uninitialized variables, and the behavior when freed memory is accessed.