Chapter 1. Yet Another Language?

CONTENTS

•  1.1 Enter Java
•  1.2 A Virtual Machine
•  1.3 Java Compared with Other Languages
•  1.4 Safety of Design
•  1.5 Safety of Implementation
•  1.6 Application and User-Level Security
•  1.7 Java and the Web
•  1.8 Java as a General Application Language
•  1.9 A Java Road Map

The greatest challenges and most exciting opportunities for software developers today lie in harnessing the power of networks. Applications created today, whatever their intended scope or audience, will almost certainly be run on machines linked by a global network of computing resources. The increasing importance of networks is placing new demands on existing tools and fueling the demand for a rapidly growing list of completely new kinds of applications.

We want software that works consistently, anywhere, on any platform and that plays well with other applications. We want dynamic applications that take advantage of a connected world, capable of accessing disparate and distributed information sources. We want truly distributed software that can be extended and upgraded seamlessly. We want intelligent applications such as autonomous agents that can roam the Net for us, ferreting out information and serving as electronic emissaries. We know, to some extent, what we want. So why don't we have it?

The problem, historically, has been that the tools for building these applications have fallen short. The requirements of speed and portability have been, for the most part, mutually exclusive, and security has been largely ignored or misunderstood. There are truly portable languages, but they are mostly bulky, interpreted, and slow. These languages are popular as much for their high-level functionality as for their portability. And there are fast languages, but they usually provide speed by binding themselves to particular platforms, so they can meet the portability issue only halfway. There are even a few recent safe languages, but they are primarily offshoots of the portable languages and suffer from the same problems.

1.1 Enter Java

The Java programming language, developed at Sun Microsystems under the guidance of Net luminaries James Gosling and Bill Joy, is designed to be a machine-independent programming language that is both safe enough to traverse networks and powerful enough to replace native executable code. Java addresses the issues raised here and may help us start building the kinds of applications we want.

Initially, most of the enthusiasm for Java centered around its capabilities for building embedded applications for the Web called applets. Applets can be independent programs in and of themselves, or serve as sophisticated frontends to programs running on a server. More recently, interest has shifted to other areas. In Version 1.2, Java introduced Swing, one of the most sophisticated toolkits for building graphical user interfaces in any language. This development allowed Java to become a popular platform for developing traditional client-side application software. Java has also become the premier platform for web applications, using the servlet interface, and for enterprise applications using technologies such as Enterprise JavaBeans . Java network portability makes it the platform of choice for modern distributed applications. This book shows you how to use Java to accomplish real programming tasks, such as building networked applications and creating functional user interfaces.

1.1.1 Java's Origins

The seeds of Java were planted in 1990 by Sun Microsystems patriarch and chief researcher, Bill Joy. Since Sun's inception in the early `80s, it has steadily pushed one idea: "The network is the computer." At the time though, Sun was competing in a relatively small workstation market, while Microsoft was beginning its domination of the more mainstream, Intel-based PC world. When Sun missed the boat on the PC revolution, Joy retreated to Aspen, Colorado, to work on advanced research. He was committed to the idea of accomplishing complex tasks with simple software and founded the aptly named Sun Aspen Smallworks.

Of the original members of the small team of programmers assembled in Aspen, James Gosling is the one who will be remembered as the father of Java. Gosling first made a name for himself in the early `80s as the author of Gosling Emacs, the first version of the popular Emacs editor that was written in C and ran under Unix. Gosling Emacs became popular but was soon eclipsed by a free version, GNU Emacs, written by Emacs' original designer. By that time, Gosling had moved on to design Sun's NeWS window system, which briefly contended with the X Window System for control of the Unix graphical user interface (GUI) desktop in 1987. While some people would argue that NeWS was superior to X, NeWS lost out because Sun kept it proprietary and didn't publish source code, while the primary developers of X formed the X Consortium and took the opposite approach.

Designing NeWS taught Gosling the power of integrating an expressive language with a network-aware windowing GUI. It also taught Sun that the Internet programming community will ultimately refuse to accept proprietary standards, no matter how good they may be. The seeds of Java's remarkably permissive licensing scheme were sown by NeWS's failure. Gosling brought what he had learned to Bill Joy's nascent Aspen project, and in 1992, work on the project led to the founding of the Sun subsidiary, FirstPerson, Inc. Its mission was to lead Sun into the world of consumer electronics.

The FirstPerson team worked on developing software for information appliances, such as cellular phones and personal digital assistants (PDAs). The goal was to enable the transfer of information and real-time applications over cheap infrared and packet-based networks. Memory and bandwidth limitations dictated small and efficient code. The nature of the applications also demanded they be safe and robust. Gosling and his teammates began programming in C++, but they soon found themselves confounded by a language that was too complex, unwieldy, and insecure for the task. They decided to start from scratch, and Gosling began working on something he dubbed "C++ minus minus."

With the foundering of the Apple Newton, it became apparent that the PDA's ship had not yet come in, so Sun shifted FirstPerson's efforts to interactive TV (ITV). The programming language of choice for ITV set-top boxes was to be the near ancestor of Java, a language called Oak. Even with its elegance and ability to provide safe interactivity, Oak could not salvage the lost cause of ITV at that time. Customers didn't want it, and Sun soon abandoned the concept.

At that time, Joy and Gosling got together to decide on a new strategy for their language. It was 1993, and the explosion of interest in the Internet, and the Web in particular, presented a new opportunity. Oak was small, robust, architecture-independent, and object-oriented. As it happens, these are also the requirements for a universal, network-savvy programming language. Sun quickly changed focus, and, with a little retooling, Oak became Java.

1.1.2 Future Buzz?

It would not be overdoing it to say that Java has caught on like wildfire. Even before its first official release, while Java was still a nonproduct, nearly every major industry player had jumped on the Java bandwagon. Java licensees included Microsoft, Intel, IBM, and virtually all major hardware and software vendors. That's not to say that everything has come up roses. Even with all this support, Java has taken a lot of knocks and had some growing pains during its first few years.

An ongoing lawsuit between Sun and Microsoft over the distribution of Java with Internet Explorer has hampered its deployment on the world's most common operating system Windows. Microsoft's involvement with Java has also become one focus of a larger federal lawsuit over serious anticompetitive practices at the company, with court testimony revealing concerted efforts by the software giant to undermine Java's future by introducing incompatibilities in its version of Java. Meanwhile, Microsoft has introduced their own Java-like language called C# (C-sharp) as part of their .NET initiative. This can only be interpreted as an indication of the success of the Java architecture.

As we begin looking at the Java architecture, you'll see that much of what is exciting about Java comes from the self-contained, virtual machine environment in which Java applications run. Java has been carefully designed so that this supporting architecture can be implemented either in software, for existing computer platforms, or in customized hardware, for new kinds of devices. Sun and other industry giants are producing fast Java chips and microprocessors tailored to run media-rich Java applications. Hardware implementations of Java are currently used in smart cards and other embedded systems. They are also planned for larger devices such as network terminals and web pads. Software implementations of Java are available for all modern computer platforms down to portable computing devices such as the popular Palm PDA. Java is also shipping now with many new cell phones.

Many people see Java as part of a trend toward cheap, Internet-based, "operating system-less" appliances that will weave the Net into more and more consumer-related areas. The first attempts at marketing "network computers" as alternatives to the standard PC have not gone very well. (The combination of Windows and cheap PC hardware create a formidable barrier.) But the desktop is only one corner of the network. Only time will tell what people will do with Java, but it's probably worth at least a passing thought that the applet you write today might well be running on someone's wristwatch tomorrow. If that seems too futuristic, remember that you can already get "smart cards" and "wearable" devices such as rings and dog tags that have Java interpreters embedded in them. These devices are capable of doing everything from financial transactions (paying a hotel bill) to unlocking a door (the door to your hotel room) to rerouting phone calls (so your hotel room receives your business calls). The hardware is already here; it can't be long before the rest of the software infrastructure begins to take advantage of it. A Java wristwatch is not a silly notion.

1.2 A Virtual Machine

Java is both a compiled and an interpreted language. Java source code is turned into simple binary instructions, much like ordinary microprocessor machine code. However, whereas C or C++ source is refined to native instructions for a particular model of processor, Java source is compiled into a universal format instructions for a virtual machine.

Compiled Java byte code is executed by a Java runtime interpreter. The runtime system performs all the normal activities of a real processor, but it does so in a safe, virtual environment. It executes a stack-based instruction set and manages a storage heap. It creates and manipulates primitive data types, and loads and invokes newly referenced blocks of code. Most importantly, it does all this in accordance with a strictly defined open specification that can be implemented by anyone who wants to produce a Java-compliant virtual machine. Together, the virtual machine and language definition provide a complete specification. There are no features of the base Java language left undefined or implementation-dependent. For example, Java specifies the sizes of all its primitive data types, rather than leaving it up to the platform implementation.

The Java interpreter is relatively lightweight and small; it can be implemented in whatever form is desirable for a particular platform. On most systems, the interpreter is written in a fast, natively compiled language such as C or C++. The interpreter can be run as a separate application, or it can be embedded in another piece of software, such as a web browser.

Put together this means that Java code is implicitly portable. The same Java application byte code can run on any platform that provides a Java runtime environment, as shown in Figure 1-1. You don't have to produce alternative versions of your application for different platforms, and you don't have to distribute source code to end users.

Figure 1-1. The Java runtime environment

The fundamental unit of Java code is the class. As in other object-oriented languages, classes are application components that hold executable code and data. Compiled Java classes are distributed in a universal binary format that contains Java byte code and other class information. Classes can be maintained discretely and stored in files or archives locally or on a network server. Classes are located and loaded dynamically at runtime as they are needed by an application.

In addition to the platform-specific runtime system, Java has a number of fundamental classes that contain architecture-dependent methods. These native methods serve as the gateway between the Java virtual machine and the real world. They are implemented in a natively compiled language on the host platform and provide low-level access to resources such as the network, the windowing system, and the host filesystem. The rest of Java is written entirely in Java and is therefore portable. This includes fundamental Java tools such as the Java compiler and web browser components, which are also written in Java and are therefore available on all Java platforms.

Historically, interpreters have been considered slow, but because the Java interpreter runs compiled byte code, Java is a relatively fast interpreted language. More importantly, Java has also been designed so that software implementations of the runtime system can optimize their performance by compiling byte code to native machine code on the fly. This is called just-in-time (JIT) or dynamic compilation. Sun claims that with just-in-time compilation, Java code can execute nearly as fast as native compiled code and maintain its transportability and security.

This is an often misunderstood point among those who want to compare language performance. There is only one intrinsic performance hit that compiled Java code suffers at runtime for the sake of security and virtual machine design array bounds checking. Everything else can be optimized down to native code just as it can with a statically compiled language. Going beyond that, the Java language includes more structural information than many other languages, providing more room for optimizations. Also remember that these optimization can be made at runtime, taking into account the actual application characteristics. What can be done at compile time that can't be done better at runtime? Well, there is a trade-off: time.

The problem with a traditional JIT compilation is that optimizing code takes time and is extremely important for good performance on modern computer hardware. So a JIT compiler can produce decent results but may not be able to take the time necessary to do a good job of optimization up front. Sun's compiler technology, called HotSpot, uses a trick called adaptive compilation to solve this problem. If you look at what programs actually spend their time doing, it turns out that they spend almost all their time executing a relatively small part of the code again and again. The chunk of code that is executed repeatedly may only be a small fraction of the total program, but its behavior determines the program's overall performance.

To take advantage of this fact, HotSpot starts out as a normal Java byte code interpreter, but with a difference: it measures (profiles) the code as it is executing, to see what parts are being executed repeatedly. Once it knows which parts of the code are crucial to performance, HotSpot compiles those sections and only those sections into true machine code. Since it compiles only a small portion of the program into machine code, it can afford to take the time necessary to optimize those portions. The rest of the program may not need to be compiled at all just interpreted saving memory and time. In fact Sun's default Java VM can run in one of two modes: client and server, which tell it whether the emphasize quick start-up time and memory conservation or flat out performance.

The technology for doing this is complex, but the idea is essentially simple: optimize the parts of the program that need to go fast, and don't worry about the rest. Another advantage of using an adaptive compiler at runtime is that it can make novel kinds of optimizations that a static (compile-time only) compiler cannot dream of.

1.3 Java Compared with Other Languages

Java is a new language, but it draws on many years of programming experience with other languages in its choice of features. Much can be said in comparing and contrasting Java with other languages. There are at least three pillars necessary to support a universal language for network programming today: portability, speed, and security. Figure 1-2 shows how Java compares to other languages.

Figure 1-2. Programming languages compared

You may have heard that Java is a lot like C or C++, but that's really not true, except at a superficial level. When you first look at Java code, you'll see that the basic syntax looks like C or C++. But that's where the similarities end. Java is by no means a direct descendant of C or a next-generation C++. If you compare language features, you'll see that Java actually has more in common with languages such as Smalltalk and Lisp. In fact, Java's implementation is about as far from native C as you can imagine.

The surface-level similarities to these languages are worth noting, however. Java borrows heavily from C and C++ syntax, so you'll see lots of familiar language constructs, including an abundance of curly braces and semicolons. Java also subscribes to the C philosophy that a good language should be compact; in other words, it should be sufficiently small and regular so a programmer can hold all the language's capabilities in his or her head at once. Just as C is extensible with libraries, packages of Java classes can be added to the core language components.

C has been successful because it provides a reasonably featureful programming environment, with high performance and an acceptable degree of portability. Java also tries to balance functionality, speed, and portability, but it does so in a very different way. C trades functionality for portability; Java trades speed for portability. Java also addresses security issues while C doesn't.

In the early days, before JIT and adaptive compilation, Java was considerably slower than compiled languages. But as we described in the previous section, Java's performance is now comparable to C or C++ for equivalent tasks.

Scripting languages, such as Perl, Python, and Ruby, are becoming very popular, and for good reason. There's no reason a scripting language can't be suitable for safe, networked applications. But most scripting languages are not designed for serious, large-scale programming. The attraction to scripting languages is that they are dynamic; they are powerful tools for rapid prototyping. Some scripting languages such as Perl also provide powerful tools for text-processing tasks that more general-purpose languages find unwieldy. Scripting languages are also highly portable.

One problem with scripting languages, however, is that they are rather casual about program structure and data typing. Most scripting languages (with a hesitant exception for Perl 5.0 and Python) are not object-oriented. They also have vastly simplified type systems and generally don't provide for sophisticated scoping of variables and functions. These characteristics make them unsuitable for building large, modular applications. Speed is another problem with scripting languages; the high-level, fully interpreted nature of these languages often makes them quite slow.

Java offers some of the essential advantages of a scripting language, along with the added benefits of a lower-level language. Java 1.4 adds a complete Regular Expression API that makes it as powerful as Perl for working with text.

However, don't confuse Java with JavaScript! JavaScript is an object-based scripting language being developed by Netscape and others. It serves as a glue and an "in the document" language for dynamic, interactive HTML-based applications. JavaScript takes its name from its intended integration with Java. You can currently interact with Java applets embedded in HTML using JavaScript. There have been a few portable implementations of JavaScript that would promote it to the level of a general scripting language. For more information on JavaScript, check out JavaScript: The Definitive Guide by David Flanagan (O'Reilly & Associates).

Incremental development with object-oriented components, combined with Java's simplicity, make it possible to develop applications rapidly and change them easily. Many studies have found that development in Java is 10 times faster than in C or C++, strictly based on language features. Java also comes with a large base of core classes for common tasks such as building GUIs and doing network communications. But along with these features, Java has the scalability and software-engineering advantages of more static languages. It provides a safe structure on which to build higher-level frameworks (and even other languages).

As we've already said, Java is similar in design to languages such as Smalltalk and Lisp. However, these languages are currently used mostly as research vehicles, rather than for developing large-scale systems. One reason is that they never developed a standard portable binding to operating-system services, such as the C standard library or the Java core classes. Smalltalk is compiled to an interpreted bytecode format, and it can be dynamically compiled to native code on the fly, just like Java. But Java improves on the design by using a bytecode verifier to ensure the correctness of compiled Java code. This verifier gives Java a performance advantage over Smalltalk because Java code requires fewer runtime checks. Java's bytecode verifier also helps with security issues, something that Smalltalk doesn't address. Smalltalk is a mature language, though, and Java's designers took lessons from many of its features.

Throughout the rest of this chapter, we'll present a bird's-eye view of the Java language. We'll explain what's new and what's not-so-new about Java, how it differs from other languages, and why.

1.4 Safety of Design

You have no doubt heard a lot about the fact that Java is designed to be a safe language. But what do we mean by safe? Safe from what or whom? The security features that attract the most attention for Java are those features that make possible new types of dynamically portable software. Java provides several layers of protection from dangerously flawed code, as well as more mischievous things such as viruses and Trojan horses. In the next section, we'll take a look at how the Java virtual machine architecture assesses the safety of code before it's run, and how the Java class loader (the bytecode loading mechanism of the Java interpreter) builds a wall around untrusted classes. These features provide the foundation for high-level security policies that can allow or disallow various kinds of activities on an application-by-application basis.

In this section, though, we'll look at some general features of the Java programming language. Perhaps more important than the specific security features, although often overlooked in the security din, is the safety that Java provides by addressing common design and programming problems. Java is intended to be as safe as possible from the simple mistakes we make ourselves, as well as those we inherit from legacy software. The goal with Java has been to keep the language simple, provide tools that have demonstrated their usefulness, and let users build more complicated facilities on top of the language when needed.

1.4.1 Syntactic Sweet `n' Low

Java is parsimonious in its features; simplicity rules. Unlike C++, Java doesn't allow programmer-defined operator overloading. (The string concatenation operator + is the only system-defined, overloaded operator in Java.). Java doesn't have a preprocessor, so it doesn't have macros, #define statements, or conditional source compilation. These constructs exist in other languages primarily to support platform dependencies, so in that sense they should not be needed in Java. Conditional compilation is also commonly used for debugging purposes. Debugging code can be included directly in your Java source code by making it conditional on a constant (we'll talk about those in Chapter 4). The Java compiler is smart enough to remove this code when it determines that it won't be called.

Java provides a well-defined package structure for organizing class files. The package system allows the compiler to handle some of the functionality of the traditional make utility (a tool for building executables from source code). The compiler also works with compiled Java classes because all type information is preserved; there is no need for "header" files, as in C/C++. All this means that Java code requires less context to read. Indeed, you may sometimes find it faster to look at the Java source code than to refer to class documentation.

Java replaces some features that have been troublesome in other languages. For example, Java supports only a single inheritance class hierarchy but allows multiple inheritance of interfaces. An interface, like an abstract class in C++, specifies some of the behavior of an object without defining its implementation, a powerful mechanism borrowed from Objective C. It allows a class to implement the behavior of the interface, without needing to be a subclass of anything in particular. Interfaces in Java eliminate the need for multiple inheritance of classes, without causing the problems associated with multiple inheritance. As you'll see in Chapter 4, Java is a simple, yet elegant, programming language.

1.4.2 Type Safety and Method Binding

One attribute of a language is the kind of type checking it uses. Generally, when we categorize a language as static or dynamic, we are referring to the amount of information about variable types that is known at compile time versus what is determined while the application is running.

In a strictly statically typed language such as C or C++, data types are etched in stone when the source code is compiled. The compiler benefits from having enough information to enforce usage rules, so that it can catch many kinds of errors before the code is executed, such as storing a floating-point value in an integer variable. The code then doesn't require runtime type checking, so it can be compiled to be small and fast. But statically typed languages are inflexible. They don't support high-level constructs such as lists and collections as naturally as languages with dynamic type checking, and they make it impossible for an application to safely import new data types while it's running.

In contrast, a dynamic language such as Smalltalk or Lisp has a runtime system that manages the types of objects and performs necessary type checking while an application is executing. These kinds of languages allow for more complex behavior and are in many respects more powerful. However, they are also generally slower, less safe, and harder to debug.

The differences in languages have been likened to the differences among kinds of automobiles.^[1] Statically typed languages such as C++ are analogous to a sports car reasonably safe and fast but useful only if you're driving on a nicely paved road. Highly dynamic languages such as Smalltalk are more like an off-road vehicle: they afford you more freedom but can be somewhat unwieldy. It can be fun (and sometimes faster) to go roaring through the back woods, but you might also get stuck in a ditch or mauled by bears.

Another attribute of a language is the way it binds method calls to their definitions. In an language such as C or C++, the definitions of methods are normally bound at compile time, unless the programmer specifies otherwise. Smalltalk, on the other hand, is called a "late-binding" language because it locates the definitions of methods dynamically at runtime. Early-binding is important for performance reasons; an application can run without the overhead incurred by searching method tables at runtime. But late-binding is more flexible. It's also necessary in an object-oriented language, where a subclass can override methods in its superclass, and only the runtime system can determine which method to run.

Java provides some of the benefits of both C++ and Smalltalk; it's a statically typed, late-binding language. Every object in Java has a well-defined type that is known at compile time. This means the Java compiler can do the same kind of static type checking and usage analysis as C++. As a result, you can't assign an object to the wrong type of variable or call nonexistent methods on an object. The Java compiler goes even further and prevents you from using uninitialized variables (see Chapter 4).

However, Java is fully runtime-typed as well. The Java runtime system keeps track of all objects and makes it possible to determine their types and relationships during execution. This means you can inspect an object at runtime to determine what it is. Unlike C or C++, casts from one type of object to another are checked by the runtime system, and it's possible to use new kinds of dynamically loaded objects with a level of type safety. And since Java is a late-binding language, all methods are like virtual methods in C++. This means that it's possible for a subclass to override methods in its superclass, even a subclass loaded at runtime.

1.4.3 Incremental Development

Java carries all data-type and method-signature information with it from its source code to its compiled bytecode form. This means that Java classes can be developed incrementally. Your own Java classes can also be used safely with classes from other sources your compiler has never seen. In other words, you can write new code that references binary class files, without losing the type safety you gain from having the source code.

A common irritation with C++ is the "fragile base class" problem. In C++, the implementation of a base class can be effectively frozen by the fact that it has many derived classes; changing the base class may require recompilation of the derived classes. This is an especially difficult problem for developers of class libraries. Java avoids this problem by dynamically locating fields within classes. As long as a class maintains a valid form of its original structure, it can evolve without breaking other classes that are derived from it or that make use of it.

1.4.4 Dynamic Memory Management

Some of the most important differences between Java and C or C++ involve how Java manages memory. Java eliminates ad hoc pointers and adds garbage collection and true arrays to the language. These features eliminate many otherwise insurmountable problems with safety, portability, and optimization.

Garbage collection alone should save countless programmers from the single largest source of programming errors in C or C++: explicit memory allocation and deallocation. In addition to maintaining objects in memory, the Java runtime system keeps track of all references to those objects. When an object is no longer in use, Java automatically removes it from memory. You can simply ignore objects you no longer use, with confidence that the interpreter will clean them up at an appropriate time.

Java uses a sophisticated garbage collector that runs intermittently in the background, which means that most garbage collecting takes place during idle times, between I/O pauses, mouse clicks, or keyboard hits. Next-generation runtime systems such as HotSpot have even more advanced garbage collection that can even differentiate the usage patterns of objects (such as short-lived versus long-lived) and optimize their collection. Once you get used to garbage collection, you won't go back. Being able to write air-tight C code that juggles memory without dropping any on the floor is an important skill, but once you become addicted to Java, you can reallocate some of those brain cells to new tasks.

You may hear people say that Java doesn't have pointers. Strictly speaking, this statement is true, but it's also misleading. What Java provides are references a safe kind of pointer and Java is rife with them. A reference is a strongly typed handle for an object. All objects in Java, with the exception of primitive numeric types, are accessed through references. If necessary, you can use references to build all the normal kinds of data structures you're accustomed to building with pointers, such as linked lists, trees, and so forth. The only difference is that with references you have to do so in a type-safe way.

Another important difference between a reference and a pointer is that you can't do pointer arithmetic with references (they can point only to specific objects or elements of an array). A reference is an atomic thing; you can't manipulate the value of a reference except by assigning it to an object. References are passed by value, and you can't reference an object through more than a single level of indirection. The protection of references is one of the most fundamental aspects of Java security. It means that Java code has to play by the rules; it can't peek into places it shouldn't.

Unlike C or C++ pointers, Java references can point only to class types. There are no pointers to methods. People sometimes complain about this missing feature, but you will find that tasks that call for pointers to methods can be accomplished more cleanly using interfaces and adapter classes instead. We should also mention that Java has a sophisticated "reflection" API that actually allows you to reference and invoke individual methods. However this is not the normal way of doing things. We discuss reflection in Chapter 6.

Finally, arrays in Java are true, first-class objects. They can be dynamically allocated and assigned like other objects. Arrays know their own size and type, and although you can't directly define or subclass array classes, they do have a well-defined inheritance relationship based on the relationship of their base types. Having true arrays in the language alleviates much of the need for pointer arithmetic like that in C or C++.

1.4.5 Error Handling

Java's roots are in networked devices and embedded systems. For these applications, it's important to have robust and intelligent error management. Java has a powerful exception-handling mechanism, somewhat like that in newer implementations of C++. Exceptions provide a more natural and elegant way to handle errors. Exceptions allow you to separate error-handling code from normal code, which makes for cleaner, more readable applications.

When an exception occurs, it causes the flow of program execution to be transferred to a predesignated "catcher" block of code. The exception carries with it an object that contains information about the situation that caused the exception. The Java compiler requires that a method either declare the exceptions it can generate or catch and deal with them itself. This promotes error information to the same level of importance as argument and return typing. As a Java programmer, you know precisely what exceptional conditions you must deal with, and you have help from the compiler in writing correct software that doesn't leave them unhandled.

1.4.6 Threads

Applications today require a high degree of parallelism. Even a very single-minded application can have a complex user interface which requires concurrent activities. As machines get faster, users become more sensitive to waiting for unrelated tasks that seize control of their time. Threads provide efficient multiprocessing and distribution of tasks for both client and server applications. Java makes threads easy to use because support for them is built into the language.

Concurrency is nice, but there's more to programming with threads than just performing multiple tasks simultaneously. In many cases, threads need to be synchronized, which can be tricky without explicit language support. Java supports synchronization based on the monitor and condition model developed by C.A.R. Hoare a sort of lock and key system for accessing resources. The keyword synchronized designates methods for safe, serialized access within an object. Only one synchronized method within the object may run at a given time. There are also simple, primitive methods for explicit waiting and signaling between threads interested in the same object.

Learning to program with threads is an important part of learning to program in Java. See Chapter 8 for a discussion of this topic. For complete coverage of threads, refer to Java Threads by Scott Oaks and Henry Wong (O'Reilly).

1.4.7 Scalability

At the lowest level, Java programs consist of classes. Classes are intended to be small, modular components. They can be separated physically on different systems, retrieved dynamically, stored in a compressed format, and even cached in various distribution schemes. Over classes, Java provides packages, a layer of structure that groups classes into functional units. Packages provide a naming convention for organizing classes and a second level of organizational control over the visibility of variables and methods in Java applications.

Within a package, a class is either publicly visible or protected from outside access. Packages form another type of scope that is closer to the application level. This lends itself to building reusable components that work together in a system. Packages also help in designing a scalable application that can grow without becoming a bird's nest of tightly coupled code.

1.5 Safety of Implementation

It's one thing to create a language that prevents you from shooting yourself in the foot; it's quite another to create one that prevents others from shooting you in the foot.

Encapsulation is a technique for hiding data and behavior within a class; it's an important part of object-oriented design. It helps you write clean, modular software. In most languages, however, the visibility of data items is simply part of the relationship between the programmer and the compiler. It's a matter of semantics, not an assertion about the actual security of the data in the context of the running program's environment.

When Bjarne Stroustrup chose the keyword private to designate hidden members of classes in C++, he was probably thinking about shielding you from the messy details of a class developer's code, not the issues of shielding that developer's classes and objects from attack by someone else's viruses and Trojan horses. Arbitrary casting and pointer arithmetic in C or C++ make it trivial to violate access permissions on classes without breaking the rules of the language. Consider the following code:

// C++ code class Finances {       private:           char creditCardNumber[16];           ...   };      main( ) {       Finances finances;          // Forge a pointer to peek inside the class       char *cardno = (char *)&finances;       printf("Card Number = %s\n", cardno);   }

In this little C++ drama, we have written some code that violates the encapsulation of the Finances class and pulls out some secret information. This sort of shenanigan abusing an untyped pointer is not possible in Java. If this example seems unrealistic, consider how important it is to protect the foundation (system) classes of the runtime environment from similar kinds of attacks. If untrusted code can corrupt the components that provide access to real resources, such as the filesystem, the network, or the windowing system, it certainly has a chance at stealing your credit-card numbers.

In Visual BASIC, it's also possible to compromise the system by peeking, poking, and, under DOS, installing interrupt handlers.

If a Java application is to dynamically download code from an untrusted source on the Internet and run it alongside applications that might contain confidential information, protection has to extend very deep. The Java security model wraps three layers of protection around imported classes, as shown in Figure 1-3.

Figure 1-3. The Java security model

At the outside, application-level security decisions are made by a security manager. A security manager controls access to system resources such as the filesystem, network ports, and the windowing environment. A security manager relies on the ability of a class loader to protect basic system classes. A class loader handles loading classes from the network. At the inner level, all system security ultimately rests on the Java verifier, which guarantees the integrity of incoming classes.

The Java bytecode verifier is a fixed part of the Java runtime system. Class loaders and security managers (or security policies to be more precise), however, are components that may be implemented differently by different applications that load byte code, such as applet viewers and web browsers. All three of these pieces need to be functioning properly to ensure security in the Java environment.^[2]

1.5.1 The Verifier

Java's first line of defense is the bytecode verifier. The verifier reads byte code before it is run and makes sure it is well-behaved and obeys the basic rules of the Java language. A trusted Java compiler won't produce code that does otherwise. However, it's possible for a mischievous person to deliberately assemble bad code. It's the verifier's job to detect this.

Once code has been verified, it's considered safe from certain inadvertent or malicious errors. For example, verified code can't forge references or violate access permissions on objects. It can't perform illegal casts or use objects in unintended ways. It can't even cause certain types of internal errors, such as overflowing or underflowing the operand stack. These fundamental guarantees underlie all of Java's security.

You might be wondering, isn't this kind of safety implicit in lots of interpreted languages? Well, while it's true that you shouldn't be able to corrupt the interpreter with bogus BASIC code, remember that the protection in most interpreted languages happens at a higher level. Those languages are likely to have heavyweight interpreters that do a great deal of runtime work, so they are necessarily slower and more cumbersome.

By comparison, Java byte code is a relatively light, low-level instruction set. The ability to statically verify the Java byte code before execution lets the Java interpreter run at full speed with full safety, without expensive runtime checks.

The verifier is a type of mathematical "theorem prover." It steps through the Java byte code and applies simple, inductive rules to determine certain aspects of how the byte code will behave. This kind of analysis is possible because compiled Java byte code contains a lot more type information than the object code of other languages of this kind. The byte code also has to obey a few extra rules that simplify its behavior. First, most bytecode instructions operate only on individual data types. For example, with stack operations, there are separate instructions for object references and for each of the numeric types in Java. Similarly, there is a different instruction for moving each type of value into and out of a local variable.

Second, the type of object resulting from any operation is always known in advance. There are no bytecode operations that consume values and produce more than one possible type of value as output. As a result, it's always possible to look at the next instruction and its operands and know the type of value that will result.

Because an operation always produces a known type, by looking at the starting state it's possible to determine the types of all items on the stack and in local variables at any point in the future. The collection of all this type information at any given time is called the type state of the stack; this is what Java tries to analyze before it runs an application. Java doesn't know anything about the actual values of stack and variable items at this time, just what kind of items they are. However, this is enough information to enforce the security rules and to ensure that objects are not manipulated illegally.

To make it feasible to analyze the type state of the stack, Java places an additional restriction on how Java bytecode instructions are executed: all paths to the same point in the code have to arrive with exactly the same type state.^[3]

1.5.2 Class Loaders

Java adds a second layer of security with a class loader. A class loader is responsible for bringing the byte code for one or more Java classes into the interpreter. Every application that loads classes from the network must use a class loader to handle this task.

After a class has been loaded and passed through the verifier, it remains associated with its class loader. As a result, classes are effectively partitioned into separate namespaces based on their origin. When a loaded class references another class name, the location of the new class is provided by the original class loader. This means that classes retrieved from a specific source can be restricted to interact only with other classes retrieved from that same location. For example, a Java-enabled web browser can use a class loader to build a separate space for all the classes loaded from a given URL.

The search for classes always begins with the built-in Java system classes. These classes are loaded from the locations specified by the Java interpreter's classpath (see Chapter 3). Classes in the classpath are loaded by the system only once and can't be replaced. This means that it's impossible for an applet to replace fundamental system classes with its own versions that change their functionality.

1.5.3 Security Managers

Finally, a security manager is responsible for making application-level security decisions. A security manager is an object that can be installed by an application to restrict access to system resources. The security manager is consulted every time the application tries to access items such as the filesystem, network ports, external processes, and the windowing environment; the security manager can allow or deny the request.

A security manager is most useful for applications that run untrusted code as part of their normal operation. Since a Java-enabled web browser can run applets that may be retrieved from untrusted sources on the Net, such a browser needs to install a security manager as one of its first actions. This security manager then restricts the kinds of access allowed after that point. This lets the application impose an effective level of trust before running an arbitrary piece of code. And once a security manager is installed, it can't be replaced.

In recent versions of Java, the security manager works in conjunction with an access controller that lets you implement security policies by editing a file. Access policies can be as simple or complex as a particular application warrants. Sometimes it's sufficient simply to deny access to all resources or to general categories of services such as the filesystem or network. But it's also possible to make sophisticated decisions based on high-level information. For example, a Java-enabled web browser could use an access policy that lets users specify how much an applet is to be trusted or that allows or denies access to specific resources on a case-by-case basis. Of course, this assumes that the browser can determine which applets it ought to trust. We'll see how this problem is solved shortly.

The integrity of a security manager is based on the protection afforded by the lower levels of the Java security model. Without the guarantees provided by the verifier and the class loader, high-level assertions about the safety of system resources are meaningless. The safety provided by the Java bytecode verifier means that the interpreter can't be corrupted or subverted and that Java code has to use components as they are intended. This, in turn, means that a class loader can guarantee that an application is using the core Java system classes and that these classes are the only way to access basic system resources. With these restrictions in place, it's possible to centralize control over those resources with a security manager.

1.6 Application and User-Level Security

There's a fine line between having enough power to do something useful and having all the power to do anything you want. Java provides the foundation for a secure environment in which untrusted code can be quarantined, managed, and safely executed. However, unless you are content with keeping that code in a little black box and running it just for its own benefit, you will have to grant it access to at least some system resources so that it can be useful. Every kind of access carries with it certain risks and benefits. For example, in the web browser environment, the advantages of granting an untrusted (unknown) applet access to your windowing system are that it can display information and let you interact in a useful way. The associated risks are that the applet may instead display something worthless, annoying, or offensive. Since most people can accept that level of risk, graphical applets and the Web in general are possible.

At one extreme, the simple act of running an application gives it a resource computation time that it may put to good use or burned frivolously. It's difficult to prevent an untrusted application from wasting your time or even attempting a "denial of service" attack. At the other extreme, a powerful, trusted application may justifiably deserve access to all sorts of system resources (e.g., the filesystem, process creation, network interfaces); a malicious application could wreak havoc with these resources. The message here is that important and sometimes complex security issues have to be addressed.

In some situations, it may be acceptable to simply ask the user to "okay" requests. Sun's HotJava web browser can pop up a dialog box and ask the user's permission for an applet to access an otherwise restricted file. However, we can put only so much burden on our users. An experienced person will quickly grow tired of answering questions; an inexperienced user may not be able to answer the questions correctly. Is it okay for me to grant an applet access to something if I don't understand what that is?

Making decisions about what is dangerous and what is not can be difficult. Even ostensibly harmless access, such as displaying a window, can become a threat when paired with the ability of an untrusted application to communicate from your host. The Java Security Manager provides an option to flag windows created by an untrusted application with a special, recognizable border to prevent it from impersonating another application and perhaps tricking you into revealing your password or your secret recipe collection. There is also a grey area, in which an application can do devious things that aren't quite destructive. An applet that can mail a bug report can also mail-bomb your boss. The Java language provides the tools to implement whatever security policies you want. However, what these policies will be ultimately depends on who you are, what you are doing, and where you are doing it.

1.6.1 Signing Classes

Web browsers that run Java applets, such as Sun's HotJava, start by defining a few rules and some coarse levels of security that restrict where applets may come from and what system resources they may access. These rules are sufficient to keep the waving Duke applet from clutching your password file, but they aren't sufficient for applications you'd like to trust with sensitive information. To fully exploit the power of Java, we need to have some nontechnical basis on which to make reasonable decisions about what a program can be allowed to do. This nontechnical basis is trust; basically, you trust certain entities not to do anything that's harmful to you. For a home user, this may mean that you trust the "Bank of Boofa" to distribute applets that let you transfer funds between your accounts, or you may trust L.L. Bean to distribute an applet that debits your Visa account. For a company, this may mean you trust applets originating behind your firewall and perhaps applets from a few high-priority customers, to modify internal databases. In all these cases, you don't need to know in detail what the program is going to do and give it permission for each operation. You only need to know that you trust your local bank.

This doesn't mean that there isn't a technical aspect to the problem of trust. Trusting your local bank when you walk up to the ATM means one thing; trusting some web page that claims to come from your local bank means something else entirely. It would be very difficult to impersonate the ATM two blocks down the street (though it has been known to happen), but, depending on your position on the Net, it's not all that difficult to impersonate a web site or to intercept data coming from a legitimate web site and substitute your own.

That's where cryptography comes in. Digital signatures, together with certificates, are techniques for verifying that data truly comes from the source it claims to have come from and hasn't been modified en route. If the Bank of Boofa signs its checkbook applet, your browser can verify that the applet actually came from the bank, not an imposter, and hasn't been modified. Therefore, you can tell your browser to trust applets that have the Bank of Boofa's signature. Java supports digital signatures; the details are covered in Chapter 3.

1.7 Java and the Web

The application-level safety features of Java make it possible to develop new kinds of applications that were not feasible before. A web browser that uses the Java runtime system can incorporate Java applets as executable content inside documents. This means that web pages can contain not only static hypertext information but also full-fledged interactive applications. The added potential for use of the Web is enormous. A user can retrieve and use software simply by navigating with a web browser. Formerly static information can be paired with portable software for interpreting and using the information. Instead of just providing some data for a spreadsheet, for example, a web document might contain a fully functional spreadsheet application embedded within it that allows users to view and manipulate the information.

In addition to applets, a more recent model for Internet downloadable application content is Java Web Start. The Web Start API allows your web browser to install applications locally, with stringent security still enforced by the Java runtime system. This system can also automatically update the software when it is used. We'll discuss this more in Chapter 22.

1.7.1 Applets

The term "applet" is used to mean a small, subordinate, or embeddable application. By "embeddable," we mean it's designed to be run and used within the context of a larger system. In that sense, most programs are embedded within a computer's operating system. An operating system manages its native applications in a variety of ways: it starts, stops, suspends, and synchronizes applications; it provides them with certain standard resources; and it protects them from one another by partitioning their environments.

As far as the web browser model is concerned, an applet is just another type of object to display; it's embedded into an HTML page with a special tag. Java-enabled web browsers can execute applets directly, in the context of a particular document, as shown in Figure 1-4. Browsers can also implement this feature using Sun's Java Plug-in, which runs Java just like other browser plug-ins display other kinds of content.

Figure 1-4. Applets in a web document

A Java applet is a compiled Java program, composed of classes just like any Java program. While a simple applet may consist of only a single class, most large applets should be broken into many classes. Classes may be stored in separate files on the server, allowing them to be retrieved as needed, but more generally are packaged together into archives. Java defines a standard archive format the JAR file which is built on the common ZIP archive format.

An applet has a four-part life cycle. When an applet is initially loaded by a web browser, it's asked to initialize itself. The applet is then informed each time it's displayed and each time it's no longer visible to the user. Finally, the applet is told when it's no longer needed, so that it can clean up after itself. During its lifetime, an applet may start and suspend itself, do work, communicate with other applications, and interact with the web browser.

Applets are autonomous programs, but they are confined within the walls of a web browser or applet viewer, and have to play by its rules. We'll be discussing the details of what applets can and can't do as we explore features of the Java language. However, under the most conservative security policies, an applet can interact only with the user and can communicate over the network only with the host from which it originated. Other types of activities, such as accessing files or interacting directly with outside applications, are typically prevented by the security manager that is part of the web browser or applet viewer. But aside from these restrictions, there is no fundamental difference between a Java applet and a standalone Java application.

1.7.2 New Kinds of Media

When it was first released, Java quickly achieved a reputation for multimedia capabilities. Frankly, this wasn't really deserved. At that point, Java provided facilities for doing simple animations and playing audio (which was leaps and bounds beyond static web pages). You could animate and play audio simultaneously, though you couldn't synchronize the two. Still, this was a significant advance for the Web, and people thought it was pretty impressive.

Java's multimedia capabilities have now taken shape. Java now has CD-quality sound, 3D animation, media players that synchronize audio and video, speech synthesis and recognition, and more. The Java Media Framework now supports most common audio and video file formats; the Java Sound API (part of the core classes) can record sound from a computer's microphone.

1.7.3 New Software Development Models

For many years, people have been using integrated development environments (IDEs) to create user interfaces. These environments let you generate applications by moving components around on the screen, connecting components to each other, and so on. In short, designing a user interface becomes a lot more like drawing a picture than like writing code. (Usually these tools also help you write code in the more traditional sense as well.)

For visual development environments to work well, you need to be able to create reusable software components. That's what the JavaBeans architecture is all about: it defines a way to package software as reusable building blocks. A graphical development tool can figure out a component's capabilities, customize the component, and connect it to other components to build applications. JavaBeans takes the idea of graphical development a step further. JavaBeans components, called Beans, aren't limited to visible, user interface components: you can have Beans that are entirely invisible and whose job is purely computational. For example, you can have a Bean that does database access; you can connect this to a Bean that lets the user request information from the database; and you can use another Bean to display the result. You can also have a set of Beans that implement the functions in a mathematical library; you can then do numerical analysis by connecting different functions to each other. In either case, you can create programs without writing a single line of code using Beans from a variety of sources. Granted, someone would have to write the Beans in the first place, but that's a different kind of task.

The JavaBeans APIs are a set of naming and design patterns that work with other Java capabilities reflection and serialization to allow tools to discover the capabilities of components and hook them together. The JavaBeans standard also specifies ways for individual beans to provide explicit information for these builder tools, including user friendly names and appearance information.

Visual development tools that support JavaBeans include Sun's Forte for Java a commercial product that is also available in an open source version called NetBeans (http://www.netbeans.org/), IBM's VisualAge, Inprise's JBuilder (http://www.inprise.com), and WebGain's Visual Cafe (http://www.webgain.com). By using a "bridge," JavaBeans can also function inside ActiveX components, which are used by many IDEs.

1.8 Java as a General Application Language

Java was introduced to the world through the web browser and the Java applet API. However, Java is more than just a tool for building multimedia applications. Java is a powerful, general-purpose programming language that just happens to be safe and architecture-independent. Standalone Java applications are not subject to the restrictions placed on applets; they can perform the same jobs as do programs written in languages such as C and C++.

Any software that implements the Java runtime system can run Java applications. Applications written in Java can be large or small, standalone or component-like, as in other languages. Java applets are different from other Java applications only in that they expect to be managed by a larger application. They are also normally considered untrusted code. In this book, we will build examples of both applets and standalone Java applications. With the exception of the few things untrusted applets can't do, such as access files, all the tools we examine in this book apply to both applets and standalone Java applications.

1.9 A Java Road Map

With everything that's going on, it's hard to keep track of what's available now, what's promised, and what has been around for some time. The following sections comprise a road map that imposes some order on Java's past, present, and future.

1.9.1 The Past: Java 1.0-Java 1.3

Java 1.0 provided the basic framework for Java development: the language itself plus packages that let you write applets and simple applications. Although 1.0 is officially obsolete, there are still a lot of applets in existence that conform to its API.

Java 1.1 superseded 1.0, incorporating major improvements in the AWT package (Java's original GUI facility), a new event pattern, new language facilities such as reflection and inner classes, and many other critical features. Java 1.1 remains important, because it is supported natively by most versions of Netscape Navigator and Microsoft Internet Explorer browsers. For various political reasons, the future of the browser world has been frozen in this condition for many years; to execute applets using any features of Java after Version 1.1, you need to use the Java Plug-in, which allows Netscape and IE to use an up-to-date Java implementation. The latest version of Netscape (6.x) also supports Java 1.3.

Java 1.2, dubbed "Java 2" by Sun, was a major release in December 1998. It provided many improvements and additions, mainly in terms of the set of APIs that were bundled into the standard distributions. The most notable additions were the inclusion of the Swing GUI package as a core API and a new, full-fledged 2D drawing API. Swing is Java's advanced user interface toolkit with capabilities far exceeding the old AWT's. (Swing, AWT, and some other packages have been variously called the JFC, or Java Foundation Classes.) Java 1.2 also added a proper Collections API to Java.

Java 1.3, released in early 2000, added minor features but was primarily focused on performance. With Version 1.3, Java got significantly faster on many platforms and Swing got many bug fixes. In this timeframe, Java enterprise APIs such as Servlets and Enterprise JavaBeans also matured.

1.9.2 The Present: Java 1.4

This book includes all the latest and greatest improvements through the final release of Java 1.4. This release provides many important and long-awaited features including language assertions, regular expressions, preferences and logging APIs, a new I/O system for high-volume applications, standard support for XML, fundamental improvements in AWT and Swing, and a greatly matured Java Servlets API for web applications.

Here's a brief overview of the most important features of the current core Java API:

JDBC (Java Database Connectivity)

A general facility for interacting with databases. (Introduced in Java 1.1.)

RMI (Remote Method Invocation)

Java's distributed objects system. RMI lets you call methods on objects hosted by a server running somewhere else on the network. (Introduced in Java 1.1.)

Java Security

A facility for controlling access to system resources, combined with a uniform interface to cryptography. Java Security is the basis for signed classes, which were discussed earlier.

JFC (Java Foundation Classes)

A catch-all for a number of new features, including the Swing user interface components; "pluggable look-and-feel," which means the ability of the user interface to adapt itself to the "look-and-feel" of the platform you're using; drag and drop; and accessibility, which means the ability to integrate with special software and hardware for people with disabilities.

Java 2D

Part of JFC; enables high-quality graphics, font manipulation, and printing.

Internationalization

The ability to write programs that adapt themselves to the language the user wants to use; the program automatically displays text in the appropriate language. (Introduced in Java 1.1.)

JNDI (Java Naming and Directory Interface)

A general service for looking up resources. JNDI unifies access to directory services such as LDAP, Novell's NDS, and others.

The following "standard extension" APIs aren't part of the core Java distribution; you may have to download them separately.

JavaMail

A uniform API for writing email software.

Java 3D

A facility for developing applications with 3D graphics.

Java Media

Another catch-all that includes Java 2D, Java 3D, the Java Media Framework (a framework for coordinating the display of many different kinds of media), Java Speech (for speech recognition and synthesis), Java Sound (high-quality audio), Java TV (for interactive television and similar applications), and others.

Java Servlets

A facility that lets you write server-side web applications in Java.

Java Cryptography

Actual implementations of cryptographic algorithms. (This package was separated from Java Security for legal reasons.)

JavaHelp

A facility for writing help systems and incorporating them in Java programs.

Enterprise JavaBeans

A component architecture for building distributed server-side applications.

Jini

An extremely interesting catch-all that is designed to enable massively distributed computing, including computing on common household appliances. In a few years, your stereo may be able to execute Java programs.

Java Card

A version of Java for very small (i.e., credit card-sized) devices, which have severe limitations on speed and memory.

In this book, we'll try to give you a taste of as many features as possible; unfortunately for us (but fortunately for Java software developers), the Java environment has become so rich that it's impossible to cover everything in a single book.

1.9.3 The Future

You can think of the first four years of Java development as a "big bang," followed by an "inflationary" phase as Sun added new features and improved old features, at an incredible rate. Things seem to be slowing down now: new APIs aren't being announced as often, and those that are announced tend to be more specialized. At least for the moment, the Java world is stabilizing. This is especially good news for the business world, which is rapidly building a new infrastructure based on Java APIs.

Probably the most exciting front for Java now is in the area of small devices. The Java "Java 2 Micro Edition" or J2ME is a subset of Java designed to fit on devices with limited capabilities. The reference platform for the J2ME architecture is the Palm PDA. Java is also now beginning to ship in cell phones, allowing downloadable applications and media.

1.9.4 Availability

You have several choices for Java development environments and runtime systems. Sun's Java software development kit is available for Solaris, Linux, and Windows. Visit Sun's Java web site at http://java.sun.com for more information about obtaining the latest Java SDK (Version 1.4 is included on the accompanying CD-ROM; view CD content online at http://examples.oreilly.com/learnjava2/CD-ROM/). There are also Java ports for other platforms, including NetWare, HP-UX, OSF/1 (including Digital Unix), Silicon Graphics' IRIX, and various IBM operating systems (including AIX, OS/2, OS/390, and OS/400). For more information, see the web pages maintained by the vendor you're interested in. Sun maintains a web page summarizing porting efforts at http://java.sun.com/cgi-bin/java-ports.cgi. Another good source for current information is the Java FAQ from the comp.lang.java newsgroup.

Most versions of Netscape Navigator and Microsoft Internet Explorer come with their own Java runtime system that runs Java applets and supports Java 1.1. Neither supports later Java releases at present, although Netscape 6 does support Java 1.3. To ameliorate the problem in general, Sun has released a Java Plug-in that allows you to specify and install the latest versions of Java; it is distributed with the Java development kit (SDK) and runtime systems (JRE) for Windows.

[1]  The credit for the car analogy goes to Marshall P. Cline, author of the C++ FAQ.

[2]  You may have seen reports about various security flaws in Java. While these weaknesses are real, it's important to realize that they have been found in the various implementations of components, namely Sun's, Netscape's, and Microsoft's Java virtual machines, not in the basic security model itself. One of the reasons Sun has released the source code for Java is to encourage people to search for weaknesses so that they can be removed.

[3]  The implications of this rule are of interest mainly to compiler writers. The rule means that Java byte code can't perform certain types of iterative actions within a single frame of execution. A common example would be looping and pushing values onto the stack. This is not allowed because the path of execution would return to the top of the loop with a potentially different type state on each pass, and there is no way that a static analysis of the code can determine whether it obeys the security rules. This restriction makes it possible for the verifier to trace each branch of the code just once and still know the type state at all points. Thus, the verifier can ensure that instruction types and stack value types always correspond, without actually following the execution of the code. For a more thorough explanation of all this, see The Java Virtual Machine by Jon Meyer and Troy Downing (O'Reilly).

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