Java Servlet编程入门与核心知识点

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《O'Reilly - Java Servlet Programming》是一本深入讲解Java Servlet编程的专业书籍，旨在为读者提供一个全面而实用的学习指南。作者们在撰写时意识到，由于读者背景的多样性，难以满足所有人的需求，因此这本书侧重于原创内容，而非详尽描述已广泛可在线获取或在其他书籍中找到的主题。本书涵盖了丰富的Java技术领域，包括但不限于： 1. **Java基础**：确保读者对Java语言有扎实的理解，包括语言规范、核心类库（FundamentalClassesReference）和Java虚拟机（Java Virtual Machine，JVM）的工作原理。 2. **网络编程**：介绍了Java在网络通信中的应用，这对于理解Servlet如何与客户端和服务器交互至关重要。 3. **线程处理**：Java的多线程支持在Servlet中是不可或缺的，了解这部分内容可以帮助开发人员编写高效的并发代码。 4. **数据库编程**：通过JDBC接口，书中有针对Java与数据库交互的深入介绍，这对于基于Web的应用程序来说非常重要。 5. **分布式计算**：探讨了Java在分布式环境下的应用，如网络编程如何支持分布式系统的设计和实现。 6. **JavaBean开发**：JavaBean是构建可重用组件的基础，书中会介绍如何设计和使用这些轻量级的对象来简化Web应用程序的开发。 7. **安全性和加密**：在Web应用中，安全性不容忽视，书中的内容涵盖了Java的安全模型以及数据加密的相关知识。 8. **图形用户界面（GUI）**：通过Swing库，读者可以学习如何创建现代的、跨平台的用户界面，这对于Web应用可能需要的桌面集成有所准备。 9. **HTTP和HTML**：虽然题目中没有直接提及，但理解这些协议和技术对于Servlet开发者来说至关重要，因为它们是Web应用通信的基础。 10. **Servlet编程**：本书的核心部分，深入剖析了Servlet的工作原理、生命周期、请求处理、响应生成以及与Web容器的交互。除了上述主题，书中的许多概念还引用了外部资料，以补充深入学习和进一步研究。《Java in a Nutshell》系列（包括原版、豪华版和示例版）为读者提供了额外的Java基础知识，而书名本身也体现了其精炼且实用的特点。《O'Reilly - Java Servlet Programming》是一本适合不同层次读者的实用指南，从初学者到经验丰富的开发者都能从中找到所需的技术细节和实践经验。

xvi PREFACE

Java™ Servlet Programming, eMatter Edition

(who looks the spitting image of James Gosling), Jim Driscoll, Rob Clark, and Dave

Brownell.

Thanks also to the members of the jserv-interest mailing list, whose questions and

answers have shaped the content of this book; Will Ramey, an old friend who

didn’t let friendship blind his critical eye; Mike Engber, the man to whom I turned

when I had run out of elegant workarounds and was ready to accept the crazy

things he comes up with; Dave Vandegrift, the first person to read many of the

chapters; Bill Day, author of Java Media Players, who helped intangibly by going

through the book writing process in parallel with me; Michael O’Connell and Jill

Steinberg, editors at JavaWorld, where I did my first professional writing; Doug

Young, who shared with me the tricks he learned writing seven technical books of

his own; and Shoji Kuwabara, Mieko Aono, Song Yung, Matthew Kim, and Alex-

andr Pashintsev for their help translating “Hello World” for Chapter 12.

Finally, thanks to Mom and Dad, for their love and support and for the time they

spent long ago teaching me the basics of writing. And a special thanks to my girl-

friend, Kristi Taylor, who made the small time away from work a pleasure.

And Grandpa, I wish you could have seen this.

Jason Hunter

July 1998

William Crawford

First and foremost, thanks to Shelley Norton, Dr. Isaac Kohane, Dr. James Fackler,

and Dr. Richard Kitz (plus a supporting cast whose contributions were invalu-

able), whose assistance and early support have made everything since possible.

Also, to Martin Streeter of Invantage, Inc., for his support during this project.

Without Rob Leith, Roger Stacey, and Fred Strebeigh, I would probably still be

stuck in the passive voice. Dale Dougherty offered me money in exchange for

words, a twist of events that I still haven’t gotten over. Andy Kwak, Joel Pomerantz,

and Matthew Proto, brave souls all, were willing to read drafts and listen to

complaints at one o’clock in the morning.

And, of course, to Mom and Dad for their years of support, and to my sister Faith

for (usually) letting me get away with being a nerd.

William Crawford

July 1998

Java™ Servlet Programming, eMatter Edition

Chapter 1

In this chapter:

• History of Web

Applications

• Support for Servlets

• The Power of Servlets

1. Introduction 1:

The rise of server-side Java applications is one of the latest and most exciting

trends in Java programming. The Java language was originally intended for use in

small, embedded devices. It was first hyped as a language for developing elaborate

client-side web content in the form of applets. Until recently, Java’s potential as a

server-side development platform had been sadly overlooked. Now, Java is coming

into its own as a language ideally suited for server-side development.

Businesses in particular have been quick to recognize Java’s potential on the

server—Java is inherently suited for large client/server applications. The cross-

platform nature of Java is extremely useful for organizations that have a heteroge-

neous collection of servers running various flavors of the Unix and Windows

operating systems. Java’s modern, object-oriented, memory-protected design

allows developers to cut development cycles and increase reliability. In addition,

Java’s built-in support for networking and enterprise APIs provides access to legacy

data, easing the transition from older client/server systems.

Java servlets are a key component of server-side Java development. A servlet is a

small, pluggable extension to a server that enhances the server’s functionality.

Servlets allow developers to extend and customize any Java-enabled server—a web

server, a mail server, an application server, or any custom server—with a hitherto

unknown degree of portability, flexibility, and ease. But before we go into any

more detail, let’s put things into perspective.

History of Web Applications

While servlets can be used to extend the functionality of any Java-enabled server,

today they are most often used to extend web servers, providing a powerful, effi-

cient replacement for CGI scripts. When you use a servlet to create dynamic

2CHAPTER 1: INTRODUCTION

Java™ Servlet Programming, eMatter Edition

content for a web page or otherwise extend the functionality of a web server, you

are in effect creating a web application. While a web page merely displays static

content and lets the user navigate through that content, a web application

provides a more interactive experience. A web application can be as simple as a

keyword search on a document archive or as complex as an electronic storefront.

Web applications are being deployed on the Internet and on corporate intranets

and extranets, where they have the potential to increase productivity and change

the way that companies, large and small, do business.

To understand the power of servlets, we need to step back and look at some of the

other approaches that can be used to create web applications.

Common Gateway Interface

The Common Gateway Interface, normally referred to as CGI, was one of the first

practical techniques for creating dynamic content. With CGI, a web server passes

certain requests to an external program. The output of this program is then sent

to the client in place of a static file. The advent of CGI made it possible to imple-

ment all sorts of new functionality in web pages, and CGI quickly became a de

facto standard, implemented on dozens of web servers.

It’s interesting to note that the ability of CGI programs to create dynamic web

pages is a side effect of its intended purpose: to define a standard method for an

information server to talk with external applications. This origin explains why CGI

has perhaps the worst life cycle imaginable. When a server receives a request that

accesses a CGI program, it must create a new process to run the CGI program and

then pass to it, via environment variables and standard input, every bit of informa-

tion that might be necessary to generate a response. Creating a process for every

such request requires time and significant server resources, which limits the

number of requests a server can handle concurrently. Figure 1-1 shows the CGI

life cycle.

Figure 1-1. The CGI life cycle

CGI-based Web Server

Main Process

Request for CGI1

Request for CGI2

Request for CGI1

Child Process for CGI1

Child Process for CGI2

Child Process for CGI1

HISTORY OF WEB APPLICATIONS 3

Java™ Servlet Programming, eMatter Edition

Even though a CGI program can be written in almost any language, the Perl

programming language has become the predominant choice. Its advanced text-

processing capabilities are a big help in managing the details of the CGI interface.

Writing a CGI script in Perl gives it a semblance of platform independence, but it

also requires that each request start a separate Perl interpreter, which takes even

more time and requires extra resources.

Another often-overlooked problem with CGI is that a CGI program cannot

interact with the web server or take advantage of the server’s abilities once it

begins execution because it is running in a separate process. For example, a CGI

script cannot write to the server’s log file.

For more information on CGI programming, see CGI Programming on the World

Wide Web by Shishir Gundavaram (O’Reilly).

FastCGI

A company named Open Market developed an alternative to standard CGI named

FastCGI. In many ways, FastCGI works just like CGI—the important difference is

that FastCGI creates a single persistent process for each FastCGI program, as

shown in Figure 1-2. This eliminates the need to create a new process for each

request.

Although FastCGI is a step in the right direction, it still has a problem with process

proliferation: there is at least one process for each FastCGI program. If a FastCGI

program is to handle concurrent requests, it needs a pool of processes, one per

request. Considering that each process may be executing a Perl interpreter, this

approach does not scale as well as you might hope. (Although, to its credit, FastCGI

can distribute its processes across multiple servers.) Another problem with FastCGI

is that it does nothing to help the FastCGI program more closely interact with the

server. As of this writing, the FastCGI approach has not been implemented by some

Figure 1-2. The FastCGI life cycle

Main Process

Request for CGI1

Request for CGI2

Request for CGI1

Single Child Process for CGI1

Single Child Process for CGI2

FastCGI-based Web Server

4CHAPTER 1: INTRODUCTION

Java™ Servlet Programming, eMatter Edition

of the more popular servers, including Microsoft’s Internet Information Server.

Finally, FastCGI programs are only as portable as the language in which they’re

written.

For more information on FastCGI, see http://www.fastcgi.com/.

mod_perl

If you are using the Apache web server, another option for improving CGI perfor-

mance is using mod_perl. mod_perl is a module for the Apache server that embeds a

copy of the Perl interpreter into the Apache httpd executable, providing complete

access to Perl functionality within Apache. The effect is that your CGI scripts are

precompiled by the server and executed without forking, thus running much

more quickly and efficiently. For more information on mod_perl, see http://perl.

apache.org/.

PerlEx

PerlEx, developed by ActiveState, improves the performance of CGI scripts written

in Perl that run on Windows NT web servers (Microsoft’s Internet Information

Server, O’Reilly’s WebSite Professional, and Netscape’s FastTrack Server and

Enterprise Server). PerlEx uses the web server’s native API to achieve its perfor-

mance gains. For more information, see http://www.activestate.com/plex/.

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