📄 chap01.html
字号:
<!-- All material contained herein is copyright (c) McGraw-Hill Professional Books
All Rights Reserved. No use of this material may be made without express written
permission of the copyright holder. HTML conversions by Mega Space [barry@megaspace.com] -->
<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1">
<TITLE>Understanding Digital Signatures: Inside the Java Virtual Machine
by Bill Venners - Beta Version</TITLE>
</HEAD>
<BODY BGCOLOR="#FFFFFF">
<TABLE BORDER="0" WIDTH="100%">
<TR><TD><A HREF="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" target="bottom"><IMG SRC="hotkey.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/hotkey.gif" ALIGN="LEFT" BORDER="0" WIDTH="40" HEIGHT="40" ALT="Orders"></A>
<IMG SRC="order_text.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/order_text.gif" WIDTH="103" HEIGHT="41" ALT="Orders"></TD>
<TD ALIGN="RIGHT"><A HREF="intro.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/venners/intro.html"><IMG SRC="backward.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/backward.gif" BORDER="0" ALT="Backward" WIDTH="32" HEIGHT="32"></A> <A HREF="chap02.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/venners/chap02.html"><IMG SRC="forward.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/forward.gif" BORDER="0" ALT="Forward" WIDTH="32" HEIGHT="32"></A></TD></TR>
<TR><TD COLSPAN="2"><A HREF="mailto:computing@mcgraw-hill.com"><IMG SRC="hotkey.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/hotkey.gif" ALIGN="LEFT" BORDER="0" WIDTH="40" HEIGHT="40" ALT="Comments"></A>
<IMG SRC="comment_text.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/comment_text.gif" WIDTH="73" HEIGHT="39" ALT="Comments"></TD></TR>
<TR><TD COLSPAN="2"><FONT FACE="ARIEL,HELVETICA" SIZE="-1"><I>© 1997 The McGraw-Hill Companies, Inc. All rights reserved. <BR>Any use of this Beta Book is subject to the rules stated in the <A HREF="http://www.mcgraw-hill.com/corporate/news_info/copyrttm.htm" tppabs="http://www.mcgraw-hill.com/corporate/news_info/copyrttm.htm" target="_top">Terms of Use</A>.</I></FONT><br>
<script language="javascript">
document.write("<a href='http://banners.linkbuddies.com/click.php?id=237296'><img src='http://banners.linkbuddies.com/image.php?id=237296&ref=" + document.referrer + "' width=468 height=60 alt='Click Here' border=0></a>");
</script></TD></TR>
</TABLE>
<HR>
<P><H1>Chapter One</H1></P>
<P><H2>Introduction to Java韘 Architecture</H2></P>
<P>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 Java language to make the Java phenomenon possible.</P>
<P>The first four chapters of this book (Part I. "Java韘 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韘 architecture: the class file, API, and language. They describe the motivation behind--and the implications of--the overall design of Java technology.</P>
<P>This chapter gives an introduction to Java as a technology. It gives an overview of Java韘 architecture, discusses why Java is important, and looks at Java韘 pros and cons.</P>
<H3><EM><P>Why Java?</P>
</EM></H3><P>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.</P>
<P>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.</P>
<P>Java technology is a tool well suited to help you meet the challenges and seize the opportunities presented by the emerging computing environment. 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.</P>
<H3><EM><P>The Challenges and Opportunities of Networks</P>
</EM></H3><P>One challenge presented to developers by a networked computing 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.</P>
<P>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. A downloaded program can do anything it wants inside the boundaries of the secure environment, but can韙 read or write data outside those boundaries.</P>
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -