That First Gulp of Java
Java's virtual-machine concept improves security as well as portability, and at several levels.
Because a traditional fully compiled program is in native code, it is in an uncomfortably good position to exploit weaknesses in the operating system or hardware, and do serious damage. By contrast, Java bytecode is architecture-neutral, and what it does not know about the platform it cannot exploit. This “passive protection” is only the beginning, however.
Strong typing, including the addition of a boolean type, the replacement of pointers with type-safe references, and the elimination of other troublesome features makes it possible to perform run-time checks that validate a program's correctness.
In addition, the run-time system's Bytecode Verifier validates each program at load time, in several ways: It simply rejects any file that does not adhere to the distinctive bytecode-file format, thus avoiding execution of what might appear to be valid Java instructions but are not. When satisfied that a file is in the proper form, the verifier examines the bytecode itself for ill-formed constructs. It then goes on to search for errors usually not detected before run time, such as stack overflow.
Another part of the JRTS, the Class Loader, further enhances security by isolating classes from each other in separate security domains. To guard against malicious code, it separates classes that are built into the run-time system itself from classes local to the user's account, and separates both of these from classes that come from other users and other systems. An ill-intentioned “foreign class” thus cannot disguise itself as a more trusted class.
Users are understandably concerned that a virus or a Trojan Horse will enter their systems by way of an applet downloaded from the Internet. To guard users' systems, run-time systems employ combinations of security features Java makes possible, above and beyond bytecode verification and class partitioning. A Web browser or other package typically enables users to select from among multiple security levels, so that they may deny or limit “untrusted” applets' access to network connections and local file stores. Clearly visible marks distinguish windows created by trusted and untrusted applets so that the latter cannot masquerade as the former.
Much has been made of the risks inherent in downloading executable code over the notoriously insecure Internet. Experience with “plug-ins” has created some justified worry, but it is important to learn from Mark Twain's proverbial cat, and not shy away from a cool stovetop just because we once jumped onto a hot one. Java is too new for us to dismiss all such concerns blithely, but its many security features make it much safer than comparable technologies.
Some will not be satisfied with any risk level above zero; for them the only counsel can be complete abstinence from the pleasures of the Internet. Others realize that some risk is an inevitable feature of life in this world, and they can protect themselves by obtaining a Java Run-time System from a reliable vendor, through means as secure as those they use to acquire other software. Doing so should bring risks down to a level acceptable by most.
The first uses of any new technology are often relentlessly trivial. If our only exposure to Java has been cutesy animations and downloaded calculators, it all too easy to underrate its potential. We hope this brief overview has shown that Java offers much more than bouncing heads—even though we didn't have the space to describe the neat way Java separates inheritance of implementation from inheritance of interface, and its built-in support for multi-threading, and....
Brian Christeson with John Mitchell, co-authored of Making Sense of Java. They are working on professional courses, other books, a compiler, and consulting/development projects related to Java, Tcl/Tk, and other languages. Brian lectures on OO analysis, design, and programming at major companies in the U.S. and abroad.
John Mitchell with Brian Christeson, Making Sense of Java. They are working on professional courses, other books, a compiler, and consulting/development projects related to Java, Tcl/Tk, and other languages. John developed PDA software in OO assembly language, and writes two columns for JavaWorld magazine.
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