HTTP响应报文头详解：构建动态内容

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"本资源主要介绍了HTTP响应报文头的相关知识，包括HTTP响应的格式、设置响应头部、理解响应头部的作用，以及如何生成Excel电子表格、动态生成JPEG图像和向浏览器发送增量更新等内容。" 在HTTP协议中，服务器响应报文头（HTTP Response Headers）是服务器向客户端发送响应时包含的一系列元数据，这些元数据提供了关于响应的额外信息，如响应的状态、内容类型、编码方式、缓存控制策略等。以下是对HTTP响应报文头的详细解释： 1. **HTTP响应的格式**： HTTP响应由三部分组成：状态行、响应头部和响应主体。状态行包含了响应的状态码和状态消息，例如"200 OK"表示请求成功。响应头部是由键值对组成的字段，每个字段之间用回车换行分隔，头部结束则用两个连续的回车换行。响应主体通常包含服务器返回给客户端的具体内容。 2. **设置响应头部**：服务器可以通过编程方式设置各种响应头部。例如，`Content-Type`用来指定响应主体的数据类型，如`text/html`代表HTML文档，`application/json`代表JSON数据。`Content-Length`用来告知客户端响应主体的字节大小。`Cache-Control`用于控制缓存策略，如`no-cache`表示不缓存，`max-age`定义了缓存的有效时间。 3. **理解响应头部的作用**：响应头部对于优化网络通信、控制缓存、提供安全性和增强用户体验至关重要。例如，`Date`头提供响应生成的时间，`Server`头标识服务器类型，`Location`头在重定向时告诉客户端新的URL，`Set-Cookie`用于设置或修改客户端的Cookie。 4. **生成Excel电子表格**：服务器可以动态生成Excel文件并作为响应主体发送，通常通过设置`Content-Type`为`application/vnd.ms-excel`来指示浏览器以Excel格式处理响应。服务器端的程序可以利用库如Apache POI在Java中创建和导出Excel数据。 5. **动态生成JPEG图像**：当需要根据用户请求生成特定图像时，服务器可以动态创建JPEG图片并返回。这通常涉及图像处理库，如Java的ImageIO，通过设置`Content-Type`为`image/jpeg`来指示浏览器显示图像。 6. **发送增量更新到浏览器**：使用`Transfer-Encoding: chunked`，服务器可以发送流式响应，即不预先知道响应大小的情况下，分块发送数据。这对于长运行的请求或实时数据更新非常有用，因为客户端可以即时处理接收到的数据，而无需等待整个响应完成。通过理解并正确使用HTTP响应报文头，开发者可以更好地控制HTTP通信，提升应用性能，确保数据安全，优化用户体验。同时，参加相关的培训课程，如书中作者提供的Java编程、Servlets、JSP、Struts、JSF/MyFaces、Ajax、GWT、Ruby/Rails等，可以帮助深入理解和掌握这些技术。

7.3 Building Excel Spreadsheets

205

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

Listing 7.1 presents a simple servlet that builds an Excel spreadsheet that com-

pares apples and oranges. Note that

=SUM(col:col) sums a range of columns in

Excel. Figure 7–1 shows the results.

Figure 7–1 Result of the ApplesAndOranges servlet in Internet Explorer on a system

that has Microsoft Office installed.

Listing 7.1 ApplesAndOranges.java

package coreservlets;

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

/** Servlet that creates Excel spreadsheet comparing

* apples and oranges.

public class ApplesAndOranges extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException {

response.setContentType("application/vnd.ms-excel");

PrintWriter out = response.getWriter();

out.println("\tQ1\tQ2\tQ3\tQ4\tTotal");

out.println("Apples\t78\t87\t92\t29\t=SUM(B2:E2)");

out.println("Oranges\t77\t86\t93\t30\t=SUM(B3:E3)");

}

Chapter 7 ■ Generating the Server Response: HTTP Response Headers

206

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

7.4 Persistent Servlet State

and Auto-Reloading Pages

Suppose your servlet or JSP page performs a calculation that takes a long time to

complete: say, 20 seconds or more. In such a case, it is not reasonable to complete

the computation and then send the results to the client—by that time the client may

have given up and left the page or, worse, have hit the Reload button and restarted

the process. To deal with requests that take a long time to process (or whose results

periodically change), you need the following capabilities:

• A way to store data between requests. For data that is not

specific to any one client, store it in a field (instance variable)

of the servlet. For data that is specific to a user, store it in the

HttpSession object (see Chapter 9, “Session Tracking”). For data

that needs to be available to other servlets or JSP pages, store it in

the

ServletContext (see the section on sharing data in Chapter

14, “Using JavaBeans Components in JSP Documents”).

• A way to keep computations running after the response is sent

to the user. This task is simple: just start a

Thread. The thread

started by the system to answer requests automatically finishes when

the response is finished, but other threads can keep running. The only

subtlety: set the thread priority to a low value so that you do not slow

down the server.

• A way to get the updated results to the browser when they are

ready. Unfortunately, because browsers do not maintain an open

connection to the server, there is no easy way for the server to

proactively send the new results to the browser. Instead, the browser

needs to be told to ask for updates. That is the purpose of the

Refresh response header.

Finding Prime Numbers for

Use with Public Key Cryptography

Here is an example that lets you ask for a list of some large, randomly chosen prime

numbers. As you are probably aware, access to large prime numbers is the key to

most public-key cryptography systems, the kind of encryption systems used on the

Web (e.g., for SSL and X509 certificates). Finding prime numbers may take some

time for very large numbers (e.g., 100 digits), so the servlet immediately returns

7.4 Persistent Servlet State and Auto-Reloading Pages

207

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

initial results but then keeps calculating, using a low-priority thread so that it won’t

degrade Web server performance. If the calculations are not complete, the servlet

instructs the browser to ask for a new page in a few seconds by sending it a

Refresh

header.

In addition to illustrating the value of HTTP response headers (

Refresh in this

case), this example shows two other valuable servlet capabilities. First, it shows that

the same servlet can handle multiple simultaneous connections, each with its own

thread. So, while one thread is finishing a calculation for one client, another client

can connect and still see partial results.

Second, this example shows how easy it is for servlets to maintain state between

requests, something that is cumbersome to implement in most competing technolo-

gies (even .NET, which is perhaps the best of the alternatives). Only a single instance

of the servlet is created, and each request simply results in a new thread calling the

servlet’s

service method (which calls doGet or doPost). So, shared data simply

has to be placed in a regular instance variable (field) of the servlet. Thus, the servlet

can access the appropriate ongoing calculation when the browser reloads the page

and can keep a list of the N most recently requested results, returning them immedi-

ately if a new request specifies the same parameters as a recent one. Of course, the

normal rules that require authors to synchronize multithreaded access to shared data

still apply to servlets. Servlets can also store persistent data in the

ServletContext

object that is available through the getServletContext method. ServletContext

has setAttribute and getAttribute methods that let you store arbitrary data

associated with specified keys. The difference between storing data in instance vari-

ables and storing it in the

ServletContext is that the ServletContext is shared

by all servlets and JSP pages in the Web application.

Listing 7.2 shows the main servlet class. First, it receives a request that specifies

two parameters:

numPrimes and numDigits. These values are normally collected

from the user and sent to the servlet by means of a simple HTML form. Listing 7.3

shows the source code and Figure 7–2 shows the result. Next, these parameters are

converted to integers by means of a simple utility that uses

Integer.parseInt

(see Listing 7.6). These values are then matched by the findPrimeList method to

ArrayList of recent or ongoing calculations to see if a previous computation cor-

responds to the same two values. If so, that previous value (of type

PrimeList) is

used; otherwise, a new

PrimeList is created and stored in the ongoing-calculations

Vector, potentially displacing the oldest previous list. Next, that PrimeList is

checked to determine whether it has finished finding all of its primes. If not, the cli-

ent is sent a

Refresh header to tell it to come back in five seconds for updated

results. Either way, a bulleted list of the current values is returned to the client. See

Figures 7–3 through 7–5 for representative results.

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