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

The last chapter showed how Java's architecture makes it a useful tool for developing software in a networked environment. The next three chapters take a closer look at how Java's architecture accomplishes its suitability for networks. This chapter examines platform independence in detail. It shows how Java's architecture enables programs to run on any platform, discusses the factors that determine the true portability of Java programs, and looks at the relevant tradeoffs.

Why Platform Independence?

One of the key reasons Java technology is useful in a networked environment is that Java makes it possible to create binary executables that will run unchanged on multiple platforms. This is important in a networked environment because networks usually interconnect many different kinds of computers and devices. In a typical enterprise environment, for example, a network might connect Macintoshes in the art department, UNIX workstations in engineering, and PCs running Windows everywhere else. Although this arrangement enables various kinds of computers and devices within the company to share data, it requires a great deal of administration. Such a network presents a system administrator with the task of keeping different platform-specific editions of programs up to date on many different kinds of computers. Programs that can run without change on any networked computer, regardless of the computer's type, make the system administrator's job simpler, especially if those programs can actually be delivered across the network.

In addition, the emerging proliferation of network-enabled embedded devices represents another environment in which Java's platform independence is useful. In the workplace, for example, various kinds of embedded devices, such as printers, scanners, and fax machines, are typically connected to the internal network. Network-connected embedded devices have also appeared in consumer domains, such as the home and car. In the embedded world, Java's platform independence can also help simplify system administration. Jini technology, which aims to bring plug and play to the network, simplifies the task of administering a dynamic environment of network-connected embedded devices for both consumers at home and systems administrators at work. Once a device is plugged into the network, it can access other devices attached to the network, and other devices can access it. To achieve this ease of connectivity, Jini-enabled devices exchange objects across the network, a technique that would be impossible without Java's support for platform independence.

From the developer's perspective, Java can reduce the cost and time required to develop and deploy applications on multiple platforms. Even though historically, many (or most) applications have been supported on only one platform, often the reason was that the cost involved in supporting multiple platforms wasn't worth the added return. Java can help make multi-platform support affordable for more types of programs.

On the other hand, Java's platform independence can act as a disadvantage as well as an advantage for software developers. If you are developing and selling a software product, Java's support for platform independence can help you to compete in more markets. Instead of developing a product that runs only on Windows, for example, you can write one that runs on Windows, OS/2, Solaris, and Linux. With Java, you can have more potential customers. The trouble is, so can everyone else. Imagine, for example, that you have focused your efforts on writing great software for Solaris. Java makes it easier for others to write software that competes in your chosen market niche. With Java, therefore, you may not only end up with more potential customers, but also with more potential competitors.

But perhaps most significantly for developers, the fact that Java code can run unchanged on multiple platforms gives the network a homogeneous execution environment that enables new kinds of distributed systems built around network-mobile objects. APIs such as object serialization, RMI (Remote Method Invocation), and Jini take advantage of this underlying capability to bring object-oriented programming out of the virtual machine and onto the network. (More information on Jini is given in Chapter 4, Network Mobility.)

Java's Architectural Support for Platform Independence

Support for platform independence, like support for security and network-mobility, is spread throughout Java's architecture. All the components of the architecture--the language, the class file, the API, and the virtual machine--play a role in enabling platform independence.

The Java Platform

Java's architecture supports the platform independence of Java programs in several ways, but primarily through the Java Platform itself. The Java Platform acts as a buffer between a running Java program and the underlying hardware and operating system. Java programs are compiled to run on a Java virtual machine, with the assumption that the class files of the Java API will be available at run-time. The virtual machine runs the program; the API gives the program access the underlying computer's resources. No matter where a Java program goes, it need only interact with the Java Platform. It needn't worry about the underlying hardware and operating system. As a result, it can run on any computer that hosts a Java Platform.

The Java Language

The Java programming language reflects Java's platform independence in one principal way: the ranges and behavior of its primitive types are defined by the language. In languages such as C or C++, the range of the primitive type int is determined by its size, and its size is determined by the target platform. The size of an int in C or C++ is generally chosen by the compiler to match the word size of the platform for which the program is compiled. This means that a C++ program might have different behavior when compiled for different platforms merely because the ranges of the primitive types are not consistent across the platforms. For example, no matter what underlying platform might be hosting the program, an int in Java behaves as a signed 32-bit two's complement number. A float adheres to the 32-bit IEEE 754 floating point standard. This consistency is also reflected in the internals of the Java virtual machine, which has primitive data types that match those of the language, and in the class file, where the same primitive data types appear. By guaranteeing that primitive types behave the same on all platforms, the Java language itself promotes the platform independence of Java programs.

The Java Class File

As mentioned in the previous chapter, the class file defines a binary format that is specific to the Java virtual machine. Java class files can be generated on any platform. They can be loaded and run by a Java virtual machine that sits on top of any platform. Their format, including the big-endian order of multi-byte values, is strictly defined and independent of any platform that hosts a Java virtual machine.

Scaleability

One aspect of Java's support for platform independence is its scaleability. The Java Platform can be implemented on a wide range of hosts with varying levels of resources, from embedded devices to mainframe computers.

Even though Java first came to prominence by riding on top of a wave that was crashing through the desktop computer industry, the World Wide Web, Java was initially envisioned as a technology for embedded and consumer devices, not desktop computers. Part of the early reasoning behind Java was that although Microsoft and Intel had a dominant clutch on the desktop market, no such dominance existed in the embedded and consumer systems markets. Microprocessors had been appearing in device after device for years--audio-video equipment, cell phones, printers, fax machines, copiers--and the coming trend was that, increasingly, embedded microprocessors would be connected to networks. An original design goal of Java, therefore, was to provide a way for software to be delivered across networks to any kind of embedded device--independent of its microprocessor and operating system.

To accomplish this goal, the Java runtime system (the Java Platform) had to be compact enough to be implemented in software using the resources available to a typical embedded system. Embedded microprocessors often have special constraints, such as small memory footprint, no hard disk, a non- graphical display, or no display. These constraints mean that embedded and consumer systems usually don't have the need, or the memory, to support the full Java API.

To address the special requirements of embedded and consumer systems, Sun created several incarnations of the Java Platform with smaller API requirements for embedded and consumer systems:

These Java Platforms are composed of a Java virtual machine and a smaller shell of runtime libraries than are available in the standard Java Platform. The difference between the standard and the Personal Platform, therefore, is that the Personal Platform guarantees the availability of fewer Java API runtime libraries. The Embedded Platform guarantees fewer APIs than the Personal Platform, and the Card Platform fewer than the Embedded. Yet although each platform addresses a progressively smaller execution environment, with progressively tighter constraints on resources, the APIs are not necessarily subsets of each other. Each API subset is geared towards a particular target, and therefore include just the APIs that make sense for that target.

In addition to guaranteeing the smallest set of APIs, the Card Platform, which is targeted at SmartCards, uses only a subset of the full Java virtual machine instruction set. Only a subset of the features of the Java language are supported by this smaller instruction set. As a result, only Java programs that restrict themselves to features available on the Card Platform can run on a SmartCard.

Although Sun attempted to address the special API needs of the embedded and consumer markets with these three subsets, the special API needs of these markets turned out to be a bit too heterogeneous for the three API subsets to adequately address. Because of the special constraints of embedded systems, especially the small memory footprint and lack of disk storage, vendors of embedded systems are often under tremendous economic pressure to pick and choose APIs. Because of the low price points for embedded devices, vendors often simply can't afford to include APIs that aren't directly needed by their device. Despite the three subsets defined by Sun, vendors still felt the need to define and support their own API subsets.

Eventually, Sun recognized their three subsets wouldn't suffice, and changed their approach to defining API standards for the embedded and consumer worlds. Instead of trying to define one-API-fits-all subsets, such as Personal and Embedded Java, Sun defined a very minimal API set they called the Java 2 Platform, Micro Edition (J2ME). On top of J2ME, Sun planned to facilitate the definition of API subsets by individual industry segments appropriate for their market niche (such as automobile, TV set-top box, screenphone, wireless pagers and cell-phones, personal digital assistents, etc.). Sun called these API subsets "profiles." The old Personal and Embedded platforms become profiles in the new approach.

Because the Java Platform is compact, it can be implemented on a wide variety of embedded and consumer systems. The potential compactness of the Java Platform, however, does not restrict implementation at the opposite end of the spectrum. The Java Platform also scales up to personal computers, workstations, and mainframes. Although in Java's early years, Java Virtual Machine implementation had scaling difficulties on the server side, virtual machines were tuned for servers and now many implementations yield very good performance on the server side. At this end of the spectrum, Sun has defined an API superset: the Java 2 Enterprise Edition (J2EE). In addition to the standard Java APIs, the J2EE includes other APIs that are useful in enterprise server environments, such as servlets and Enterprise JavaBeans.

In the end, Sun's revised approach to defining APIs yielded three basic API sets, which demonstrate the scaleability of the Java Platform:

At the high end, the existence of the Enterprise Edition signifies the utility of the Java Platform in high-end servers. In the middle, the Standard Edition carries on the tradition, started by Applets in browsers, of the Java Platform on the desktop. At the low end, the Micro Edition, augmented with industry profiles, shows that the Java Platform can scale down and mold itself to meet the requirements of a great variety of consumer and embedded environments.

Factors that Influence Platform Independence

Java's architecture facilitates the creation of platform-independent software, but also allows you to create software that is platform-specific. When you write a Java program, platform independence is an option.

The degree of platform independence of any Java program depends on several factors. As a developer, some of these factors are beyond your control, but most are within your control. Primarily, the degree of platform independence of any Java program you write depends on how you write it.

Java Platform Deployment

The most basic factor determining the a Java program's platform independence is the extent to which the Java Platform has been deployed on multiple platforms. Java programs will only run on computers and devices that host a Java Platform. Thus, before one of your Java programs will run on a particular computer owned by, say, your friend Alicia, two things must happen. First, the Java Platform must be ported to Alicia's particular type of hardware and operating system. Once the port has been done by some Java Platform vendor, that port must in some way get installed on Alicia's computer. So a critical factor determining the true extent of platform independence of Java programs--and one that is beyond the control of the average developer--is the availability of Java Platform implementations and their distribution.

Fortunately for the Java developer, the deployment of the Java Platform has proceeded with great momentum, starting with Web-browsers, then moving on to desktop, workstation, network operating systems, and into many different kinds of consumer and embedded devices. It is increasingly likely, therefore, that your friend Alicia will have a Java Platform implementation on her computer or device.

The Java Platform Version and Edition

The deployment of the Java Platform is a bit more complicated, however, because not all standard runtime libraries are guaranteed to be available at every Java Platform. The basic set of libraries guaranteed to be available at a Java Platform is called the standard API. Sun calls a 1.2 Java virtual machine accompanied by the class files that constitute the standard API the Java 2 Platform, Standard EditionThis edition of the Java Platform has the minimum set of Java API libraries that you can assume will be available at desktop computers and workstations. But as described earlier, Sun also defines API sets for the Micro and Enterprise Editions of the Java 2 Platform, and encourages the development of API profiles to augment the Micro Edition in various consumer and embedded industry segments. In addition, Sun defines some standard runtime libraries that it considers as optional for the Standard Edition, and calls these Standard Extension APIs. These libraries include such services as telephony, commerce, and media such as audio, video, or 3D. If your program uses libraries from the Standard Extension API, it will run anywhere those standard extension API libraries are available, but not on a computer that implements only the basic Standard Edition Platform. Some of the Standard Extension APIs, on the other hand, are guaranteed to be available at any implementation of the Enterprise Edition. Given the variety of API editions and profiles, the Java 2 Platform hardly represents a single homogeneous execution environment that will in all cases enable code that is written once to run anywhere.

Another complicating factor is that in a sense the Java Platform is a moving target--it evolves over time. Although the Java virtual machine is likely to evolve very gradually, the Java API will probably change more frequently. Over time, features will be added to and removed from both the Standard Edition and Standard Extension APIs, and parts of the Standard Extension API may migrate into the Standard Edition. The changes made to the Java Platform should for the most part be upwards compatible, meaning they won't break existing Java programs, but some changes may not be. As obsolete features are removed in a new version of the Java Platform, existing Java programs that depend upon those features won't run on the new version. Also, changes may not be downwards compatible, meaning programs that are compiled for a new version of the Java Platform won't necessarily work on an old version. The dynamic nature of the Java Platform complicates things somewhat for the developer wishing to write a Java program that will run on any computer.

In theory, your program should run on all computers that host a Java 2 Platform, Standard Edition, so long as you depend only upon the runtime libraries in the standard API. In practice, however, new versions of the standard API will take time to percolate everywhere. When your program depends on newly added features of the latest version of the standard API, there may be some hosts that can't run it because they have an older version. This is not a new problem to software developers--programs written for Windows 95, for example, didn't work on the previous version of the operating system, Windows 3.1--but because Java enables the network delivery of software, it becomes a more acute problem. The promise of Java is not only that it is easy to port programs from one platform to another, but that the same piece of binary Java code can be sent across the network and run on any computer or device.

As a developer, you can't control the release cycles or deployment schedules of the Java Platform, but you can choose the Java Platform edition and version that your programs depend upon. In practice, therefore, you will have to decide when a new version of the Java Platform has been distributed to a great enough extent to justify writing programs for that version.

Native Methods

Besides the Java Platform version and edition your program depends on, the other major factor determining the extent of platform independence of your Java program is whether or not you call native methods. The most important rule to follow when you are writing a platform independent Java program is: don't directly or indirectly invoke any native methods that aren't part of the Java API. As you can see in Figure 2-1, calling native methods outside the Java API renders your program platform-specific.



Figure 2-1. A platform-specific Java program.

Calling native methods directly is appropriate in situations where you don't desire platform independence. In general, native methods are useful in three cases: o for accessing features of an underlying host platform that are not accessible through the Java API o for accessing a legacy system or using an already existing library that isn't written in Java o for speeding up the performance of a program by implementing time-critical code as native methods

If you need to use native methods and also need your program to run on several platforms, you'll have to port the native methods to all the required platforms. This porting must be done the old fashioned way, and once you've done it, you'll have to figure out how to deliver the platform-specific native method libraries to the appropriate hosts. Because Java's architecture was designed to simplify multi-platform support, your initial goal in writing a platform-independent Java program should be to avoid native methods altogether and interact with the host only through the Java API.

Non-Standard Runtime Libraries

Native methods aren't inherently incompatible with platform independence. What's important is whether or not the methods you invoke are implemented "everywhere." Implementations of the Java API on operating systems such as Windows or Solaris use native methods to access the host. When you call a method in the Java API, you are certain it will be available everywhere. It doesn't matter if in some places the method is implemented as a native method.

Java Platform implementations can come from a variety of vendors, and although every vendor must supply the standard runtime libraries of the Java API, individual vendors may also supply extra libraries. If you are interested in platform independence, you must remain aware of whether any non-standard runtime libraries you use call native methods. Non-standard libraries that don't call native methods don't degrade your program's platform independence. Using non-standard libraries that do call native methods, however, yields the same result as calling native methods directly--it renders your program platform-specific.

Virtual Machine Dependencies

Two other rules to follow when writing a platform independent Java program involve portions of the Java virtual machine that can be implemented differently by different vendors. The rules are: 1. don't depend upon timely finalization for program correctness, and 2. don't depend upon thread prioritization for program correctness. These two rules address the variations allowed in the Java virtual machine specification for garbage collection and threads.

All Java virtual machines must have a garbage-collected heap, but different implementations can use different garbage collection techniques. This flexibility in the Java virtual machine specification means that the objects of a particular Java program can be garbage collected at completely different times on different virtual machines. This in turn means that finalizers, which are run by the garbage collector before an object is freed, can run at different times on different virtual machines. If you use a finalizer to free finite memory resources, such as file handles, your program may run on some virtual machine implementations but not others. On some implementations, your program could run out of the finite resource before the garbage collector gets around to invoking the finalizers that free the resource.

Another variation allowed in different implementations of the Java virtual machine involves thread prioritization. The Java virtual machine specification guarantees that all runnable threads at the highest priority in your program will get some CPU time. The specification also guarantees that lower priority threads will run when higher priority threads are blocked. The specification does not, however, prohibit lower priority threads from running when higher priority threads aren't blocked. On some virtual machine implementations, therefore, lower priority threads may get some CPU time even when the higher priority threads aren't blocked. If your program depends for correctness on this behavior, however, it may work on some virtual machine implementations but not others. To keep your multi-threaded Java program platform independent, you must rely on synchronization--not prioritization--to coordinate inter-activity between threads.

User Interface Dependencies

Another major variation between different Java Platform implementations is user interface. User interface is one of the more difficult issues in writing platform independent Java programs. The AWT user interface library gives you a set of basic user-interface components that map to native components on each platform. The Swing library gives you advanced components that don't map directly to native components. From this raw material, you must build an interface that users on many different platforms will feel comfortable with. This is not always an easy task.

Users on different platforms are accustomed to different ways of interacting with their computer. The metaphors are different. The components are different. The interaction between the components is different. Although the AWT and Swing libraries make it fairly easy to create a user interface that runs on multiple platforms, they don't necessarily make it easy to devise an interface that keeps users happy on multiple platforms.

Bugs in Java Platform Implementations

One final source of variation among different implementations of the Java Platform is bugs. Although Sun has developed a comprehensive suite of tests that Java Platform implementations must pass, it is still possible that some implementations will be distributed with bugs in them. The only way you can defend yourself against this possibility is through testing. If there is a bug, you can determine through testing whether the bug affects your program, and if so, attempt to find a work-around.

Testing

Given the allowable differences between Java Platform implementations, the platform dependent ways you can potentially write a Java program, and the simple possibility of bugs in any particular Java Platform implementation, you should if possible test your Java programs on all platforms you plan to claim it runs on. Java programs are not platform independent to a great enough extent that you only need test them on one platform. You still need to test a Java program on multiple platforms, and you should probably test it on the various Java Platform implementations that are likely to be found on each host computer you claim your program runs on. In practice, therefore, testing your Java program on the various host computers and Java Platform implementations that you plan to claim your program works on is a key factor in making your program platform independent.

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.

Platform Independence and Network-Mobile Objects

As mentioned previously in this chapter, the original design target for Java technology was embedded devices. This was done in part because given that the desktop was controlled by Microsoft and Intel, embedded devices represented the most open market. But also, embedded devices were targeted because they were destined to play a role in a coming hardware revolution -- the proliferation of diskless embedded devices connected to high-bandwidth (often, wireless) networks.

Three years after Java was first released by Sun, Sun announced Jini. Jini is an attempt at defining an architecture for the "computer" represented by the emerging environment of embedded and consumer devices connected to a ubiquitous network. The Jini architecture relies heavily on network-mobile objects. In a world of Jini-enabled devices, objects fly across the network between Java Platform implementations in embedded and consumer devices, desktop computers, and servers. The Java Platform implementations that will host these network-mobile objects will reside in a great variety of devices and computer hardware, which will be manufactured by many different vendors. This architecture significantly raises the bar for platform independence.

For Jini to work in the real world, objects written by one device vendor will have to execute properly in Java Platform execution environments provided by other device vendors. Testing your network-mobile code on all platforms it will eventually run on, as recommended by the Seven Steps to Platform Independence presented earlier in this chapter, will be basically impossible. Because so many vendors will be producing so many different kinds of devices, with new devices appearing at an ever increasing rate, it will be generally impossible to predict all the places that network-mobile code embedded in any particular device will execute. Thus, other approaches to testing will have to be developed, such as compatibility test suites for network- mobile code. In addition, for Jini to work in the real world, the homogeneity of execution environments must be realized to the greatest extent possible. And lastly, programmers will likely need to consider the possibility of differences in execution environments when they write network-mobile code, and program defensively.

The Resources Page

For links to more information about Java and platform independence, visit the resources page for this chapter: http://www.artima.com/insidejvm/resources


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