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Chapter 1 of Inside the Java Virtual Machine
Introduction to Java's Architecture
by Bill Venners

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At the heart of Java technology lies the Java virtual machine--the abstract computer on which all Java programs run. Although the name "Java" is generally used to refer to the Java programming language, there is more to Java than the language. The Java virtual machine, Java API, and Java class file work together with the language to make Java programs run.

The first four chapters of this book (Part I. "Java's Architecture") show how the Java virtual machine fits into the big picture. They show how the virtual machine relates to the other components of Java's architecture: the class file, API, and language. They describe the motivation behind--and the implications of- -the overall design of Java technology.

This chapter gives an introduction to Java as a technology. It gives an overview of Java's architecture, discusses why Java is important, and looks at Java's pros and cons.

Why Java?

Over the ages people have used tools to help them accomplish tasks, but lately their tools have been getting smarter and interconnected. Microprocessors have appeared inside many commonly used items, and increasingly, they have been connected to networks. As the heart of personal computers and workstations, for example, microprocessors have been routinely connected to networks. They have also appeared inside devices with more specific functionality than the personal computer or the workstation. Televisions, VCRs, audio components, fax machines, scanners, printers, cell phones, personal digital assistants, pagers, and wrist-watches--all have been enhanced with microprocessors; most have been connected to networks. Given the increasing capabilities and decreasing costs of information processing and data networking technologies, the network is rapidly extending its reach.

The emerging infrastructure of smart devices and computers interconnected by networks represents a new environment for software--an environment that presents new challenges and offers new opportunities to software developers. Java is well suited to help software developers meet challenges and seize opportunities presented by the emerging computing environment, because Java was designed for networks. Its suitability for networked environments is inherent in its architecture, which enables secure, robust, platform- independent programs to be delivered across networks and run on a great variety of computers and devices.

The Challenges and Opportunities of Networks

One challenge presented to software developers by the increasingly network- centric hardware environment is the wide range of devices that networks interconnect. A typical network usually has many different kinds of attached devices, with diverse hardware architectures, operating systems, and purposes. Java addresses this challenge by enabling the creation of platform-independent programs. A single Java program can run unchanged on a wide range of computers and devices. Compared with programs compiled for a specific hardware and operating system, platform-independent programs written in Java can be easier and cheaper to develop, administer, and maintain.

Another challenge the network presents to software developers is security. In addition to their potential for good, networks represent an avenue for malicious programmers to steal or destroy information, steal computing resources, or simply be a nuisance. Virus writers, for example, can place their wares on the network for unsuspecting users to download. Java addresses the security challenge by providing an environment in which programs downloaded across a network can be run with customizable degrees of security.

One aspect of security is simple program robustness. Like devious code written by malicious programmers, buggy code written by well-meaning programmers can potentially destroy information, monopolize compute cycles, or cause systems to crash. Java's architecture guarantees a certain level of program robustness by preventing certain types of pernicious bugs, such as memory corruption, from ever occurring in Java programs. This establishes trust that downloaded code will not inadvertently (or intentionally) crash, but it also has an important benefit unrelated to networks: it makes programmers more productive. Because Java prevents many types of bugs from ever occurring, Java programmers need not spend time trying to find and fix them.

One opportunity created by an omnipresent network is online software distribution. Java takes advantage of this opportunity by enabling the transmission of binary code in small pieces across networks. This capability can make Java programs easier and cheaper to deliver than programs that are not network- mobile. It can also simplify version control. Because the most recent version of a Java program can be delivered on-demand across a network, you needn't worry about what version your end-users are running. They will always get the most recent version each time they use your program.

Mobile code gives rise to another opportunity: mobile objects, the transmission of both code and state across the network. Java realizes the promise of object mobility in its APIs for object serialization and RMI (Remote Method Invocation). Built on top of Java's underlying architecture, object serialization and RMI provide an infrastructure that enables the various components of distributed systems to share objects. The network-mobility of objects makes possible new models for distributed systems programming, effectively bringing the benefits of object-oriented programming to the network.

Platform independence, security, and network-mobility--these three facets of Java's architecture work together to make Java suitable for the emerging networked computing environment. Because Java programs are platform independent, network-mobility of code and objects is more practical. The same code can be sent to all the computers and devices the network interconnects. Objects can be exchanged between various components of a distributed system, which can be running on different kinds of hardware. Java's built-in security framework also helps make network-mobility of software more practical. By reducing risk, the security framework helps to build trust in a new paradigm of network-mobile software.

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