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Chapter 2 of Inside the Java Virtual Machine
Platform Independence
by Bill Venners

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Seven Steps to Platform Independence

Java's architecture allows you to choose between platform independence and other concerns. You make your choice by the way in which you write your program. If your goal is to take advantage of platform- specific features not available through the Java API, to interact with a legacy system, to use an existing library written not written in Java, or to maximize the execution speed of your program, you can use native methods to help you achieve that goal. In such cases, your programs will have reduced platform independence, and that will usually be acceptable. If, on the other hand, your goal is platform independence, then you should follow certain rules when writing your program. The following seven steps outline one path you can take to maximize your program's portability:

  1. Choose a set of host computers and devices that you will claim your program runs on (your "target hosts").
  2. Choose an edition and version of the Java Platform that you feel is well enough distributed among your target hosts. Write your program to run on this version of the Java Platform.
  3. For each target host, choose a set of Java Platform implementations that you will claim your program runs on (your "target runtimes").
  4. Write your program so that it accesses the host computer only through the standard runtime libraries of the Java API. (Don't invoke native methods, or use vendor-specific libraries that invoke native methods.)
  5. Write your program so that it doesn't depend for correctness on timely finalization by the garbage collector or on thread prioritization.
  6. Strive to design a user interface that works well on all of your target hosts.
  7. Test your program on all of your target runtimes and all of your target hosts.

If you follow the seven steps outlined above, your Java program will definitely run on all your target hosts. If your target hosts cover most major Java Platform vendors on most major host computers, there is a very good chance you program will run many other places as well.

If you wish, you can have your program certified as "100% Pure Java." There are several reasons that you may wish to do this if you are writing a program that you intend to be platform independent. For example, if your program is certified 100% Pure, you can brand it with the 100% Pure Java coffee cup icon. You can also potentially participate in co-marketing programs with Sun. You may, however, wish to go through the certification process simply as an added check of the platform independence of your program. In this case, you have the option of just running 100% Pure verification tools you can download for free. These tools will report problems with your program's "purity" without requiring you to go through the full certification process.

The 100% Pure certification is not quite a full measure of platform independence. Part of platform independence is that user's expectations are fulfilled on multiple platforms. The 100% Pure testing process does not attempt to measure user fulfillment. It only checks to make certain your program depends only on the standard APIs. You could write a Java program that passes the 100% Pure tests, but still doesn't work well on all platforms from the perspective of users. Nonetheless, running your code through the 100% Pure testing process can be a worthwhile step on the road to creating a platform independent Java program.

The Politics of Platform Independence

As illustrated in Figure 2-2, Java Platform vendors are allowed to extend the standard components of the Java Platform in non-standard and platform-specific ways, but they must always support the standard components. In the future, Sun Microsystems intends to prevent the standard components of the Java Platform from splitting into several competing, slightly incompatible systems, as happened, for instance, with UNIX. The license that all Java Platform vendors must sign requires compatibility at the level of the Java virtual machine and the Java API, but permits differentiation in the areas of performance and extensions. There is some flexibility, as mentioned above, in the way vendors are allowed to implement threads, garbage-collection, and user interface look and feel. If things go as Sun plans, the core components of the Java Platform will remain a standard to which all vendors faithfully adhere, and the ubiquitous nature of the standard Java Platform will enable you to write programs that really are platform independent.

Figure 2-2. Java Platform implementations from different vendors.

You can rely on the standard components of the Java Platform because every Java Platform vendor must support them. If you write a program that only depends on these components, it should "run anywhere," but may suffer to some extent from the lowest-common-denominator problem. Yet because vendors are allowed to extend the Java Platform, they can give you a way to write platform-specific programs that take full advantage of the features of the underlying host operating system. The presence of both required standard components and permitted vendor extensions at any Java Platform implementation gives developers a choice. This arrangement allows developers to balance platform independence with other concerns.

There is currently a marketing battle raging for the hearts and minds of software developers over how they will write Java programs--in particular, whether or not they will choose to write platform-independent or platform-specific programs. The choice that Java graciously gives to developers also potentially threatens some vested interests in the software industry.

Java's support for platform independence threatens to weaken the "lock" enjoyed by operating system vendors. If all of your software runs on only one operating system, then your next computer will also very likely run that same operating system. You are "locked in" to one operating system vendor because your investment in software depends on an API proprietary to that vendor. You are also likely locked into one hardware architecture because the binary form of your programs requires a particular kind of microprocessor. If instead, much of your software is written to the Java API and stored as bytecodes in class files, it becomes easier for you to migrate to a different operating system vendor the next time you buy a computer. Because the Java Platform can be implemented in software on top of existing operating systems, you can switch operating systems and take all of your old platform-independent Java-based software with you.

Microsoft dominates the desktop operating system market largely because most available software runs only on Microsoft operating systems. It is in Microsoft's strategic interest to continue this status quo, so they are encouraging developers to use Java as a language to write programs that run only on Microsoft platforms. It is in just about every other operating system vendor's strategic interest to weaken Microsoft's lock on the operating system market, so the other players are encouraging developers to write Java programs that are platform independent. For example, Sun, Netscape, IBM, and many others banded together to promote Sun's "100% Pure Java initiative," through which they hoped to educate and persuade developers to go the platform independence route.

Microsoft's approach to Java is to make Windows the best platform on which to develop and run Java programs. They want developers to use Microsoft's tools and libraries whether the developer chooses platform independence or not. Still, in the "spin" Microsoft gives to Java in promotional material to developers, they strongly favor the platform-specific Windows path. They extol the virtues of using Java to write programs that take full advantage of the Windows platform.

Sun and the other operating system vendors behind the 100% Pure Java initiative are attempting to counter Microsoft's spin with some of their own. The promotional material from these companies focuses on the benefits of writing platform-independent Java programs.

On one level, it is a battle between two icons. If you write your Java program Microsoft's way, you get to brand your product with a Windows icon that displays the famous four-paneled Windows logo. If you go the 100% Pure Java route, you get to brand your product with a 100% Pure Java icon that displays the famous steaming coffee cup logo.

As a developer, the politics and propaganda swirling around the software industry need not be a major factor when you decide how to write a particular Java program. For some programs you write, platform independence may be the right approach; for others, a platform-specific program may make more sense. In each individual case, you can make a decision based on what you feel your customers want and how you want to position yourself in the marketplace with respect to your competitors.

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