Servlet与JSP核心编程第二版第一卷英文CoreServletsandJavaServerPages(Volume.1.2nd.Edition)

Servlet

核心编程

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Training courses from the book’s author:

http://courses.coreservlets.com/

• Personally developed and taught by Marty Hall

• Available onsite at your organization (any country)

• Topics and pace can be customized for your developers

• Also available periodically at public venues

• Topics include Java programming, beginning/intermediate servlets

and JSP, advanced servlets and JSP, Struts, JSF/MyFaces, Ajax,

GWT, Ruby/Rails and more. Ask for custom courses!

AN OVERVIEW OF

ERVLET AND JSP

ECHNOLOGY

Topics in This Chapter

• Understanding the role of servlets

• Building Web pages dynamically

• Looking at servlet code

• Evaluating servlets vs. other technologies

• Understanding the role of JSP

Made by Charles Zhu, contact me by visiting http://xifangyuhui.javaeye.com

Chapter 1 ■ An Overview of Servlet and JSP Technology

J2EE training from the author: http://courses.coreservlets.com/

1. Read the explicit data sent by the client.

The end user normally enters this data in an HTML form on a Web

page. However, the data could also come from an applet or a custom

HTTP client program. Chapter 4 discusses how servlets read this data.

2. Read the implicit HTTP request data sent by the browser.

Figure 1–1 shows a single arrow going from the client to the Web

server (the layer where servlets and JSP execute), but there are really

two varieties of data: the explicit data that the end user enters in a

form and the behind-the-scenes HTTP information. Both varieties are

critical. The HTTP information includes cookies, information about

media types and compression schemes the browser understands, and

so forth; it is discussed in Chapter 5.

3. Generate the results.

This process may require talking to a database, executing an RMI or

EJB call, invoking a Web service, or computing the response directly.

Your real data may be in a relational database. Fine. But your database

probably doesn’t speak HTTP or return results in HTML, so the Web

browser can’t talk directly to the database. Even if it could, for security

reasons, you probably would not want it to. The same argument

applies to most other applications. You need the Web middle layer to

extract the incoming data from the HTTP stream, talk to the applica-

tion, and embed the results inside a document.

4. Send the explicit data (i.e., the document) to the client.

This document can be sent in a variety of formats, including text

(HTML or XML), binary (GIF images), or even a compressed format

like gzip that is layered on top of some other underlying format. But,

HTML is by far the most common format, so an important servlet/JSP

task is to wrap the results inside of HTML.

5. Send the implicit HTTP response data.

Figure 1–1 shows a single arrow going from the Web middle layer (the

servlet or JSP page) to the client. But, there are really two varieties of

data sent: the document itself and the behind-the-scenes HTTP infor-

mation. Again, both varieties are critical to effective development.

Sending HTTP response data involves telling the browser or other

client what type of document is being returned (e.g., HTML), setting

cookies and caching parameters, and other such tasks. These tasks are

discussed in Chapters 6 and 7.

1.2 Why Build Web Pages Dynamically?

J2EE training from the author: http://courses.coreservlets.com/

1.2 Why Build Web

Pages Dynamically?

After Marty wrote the first edition of Core Servlets and JavaServer Pages, various of

his non-software-savvy friends and relations would ask him what his book was about.

Marty would launch into a long, technical discussion of Java, object-oriented pro-

gramming, and HTTP, only to see their eyes immediately glaze over. Finally, in exas-

peration, they would ask, “Oh, so your book is about how to make Web pages, right?”

“Well, no,” the answer would be, “They are about how to make programs that

make Web pages.”

“Huh? Why wait until the client requests the page and then have a program build

the result? Why not just build the Web page ahead of time?”

Yes, many client requests can be satisfied by prebuilt documents, and the server

would handle these requests without invoking servlets. In many cases, however, a

static result is not sufficient, and a page needs to be generated for each request.

There are a number of reasons why Web pages need to be built on-the-fly:

• The Web page is based on data sent by the client.

For instance, the results page from search engines and order-

confirmation pages at online stores are specific to particular user

requests. You don’t know what to display until you read the data that

the user submits. Just remember that the user submits two kinds of

data: explicit (i.e., HTML form data) and implicit (i.e., HTTP request

headers). Either kind of input can be used to build the output page. In

particular, it is quite common to build a user-specific page based on a

cookie value.

• The Web page is derived from data that changes frequently.

If the page changes for every request, then you certainly need to build

the response at request time. If it changes only periodically, however,

you could do it two ways: you could periodically build a new Web page

on the server (independently of client requests), or you could wait and

only build the page when the user requests it. The right approach

depends on the situation, but sometimes it is more convenient to do

the latter: wait for the user request. For example, a weather report or

news headlines site might build the pages dynamically, perhaps

returning a previously built page if that page is still up to date.

• The Web page uses information from corporate databases or

other server-side sources.

If the information is in a database, you need server-side processing

even if the client is using dynamic Web content such as an applet.

Imagine using an applet by itself for a search engine site:

Chapter 1 ■ An Overview of Servlet and JSP Technology

J2EE training from the author: http://courses.coreservlets.com/

“Downloading 50 terabyte applet, please wait!” Obviously, that is silly;

you need to talk to the database. Going from the client to the Web tier

to the database (a three-tier approach) instead of from an applet

directly to a database (a two-tier approach) provides increased

flexibility and security with little or no performance penalty. After all,

the database call is usually the rate-limiting step, so going through the

Web server does not slow things down. In fact, a three-tier approach is

often faster because the middle tier can perform caching and

connection pooling.

In principle, servlets are not restricted to Web or application servers that handle

HTTP requests but can be used for other types of servers as well. For example, serv-

lets could be embedded in FTP or mail servers to extend their functionality. And, a

servlet API for SIP (Session Initiation Protocol) servers was recently standardized

(see

http://jcp.org/en/jsr/detail?id=116). In practice, however, this use of servlets

has not caught on, and we’ll only be discussing HTTP servlets.

1.3 A Quick Peek at Servlet Code

Now, this is hardly the time to delve into the depths of servlet syntax. Don’t worry,

you’ll get plenty of that throughout the book. But it is worthwhile to take a quick look

at a simple servlet, just to get a feel for the basic level of complexity.

Listing 1.1 shows a simple servlet that outputs a small HTML page to the client.

Figure 1–2 shows the result.

The code is explained in detail in Chapter 3 (Servlet Basics), but for now, just

notice four points:

• It is regular Java code. There are new APIs, but no new syntax.

• It has unfamiliar import statements. The servlet and JSP APIs are

not part of the Java 2 Platform, Standard Edition (J2SE); they are a

separate specification (and are also part of the Java 2 Platform,

Enterprise Edition—J2EE).

• It extends a standard class (

HttpServlet). Servlets provide a rich

infrastructure for dealing with HTTP.

• It overrides the

doGet method. Servlets have different methods to

respond to different types of HTTP commands.

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