Servlet与JSP核心编程 ---- 4-优快云博客

本文链接：https://blog.youkuaiyun.com/iteye_12989/article/details/81958040

本文详细介绍了HTTP请求报头中的关键字段，如Accept、Accept-Encoding等，并通过实例展示了如何利用这些报头进行压缩响应数据，提高传输效率。

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chapter 5 客户请求的处理：http请求报头

5.1 请求报头的读取

在使用request.getHeader返回结果之前，一定要确保它不是null。

5.2 制作所有请求报头的表格

5.3 了解Http1.1请求报头

1.Accept 这个报头指定浏览器或其他客户程序能够处理的MIME类型。

request.getHeader("Accept")

2.Accept-Charset

浏览器可以使用的字符集

3.Accept-Encoding

客户端能够处理的编码类型

4.Accept-Language

标准语言代码的一种，比如en,en-us,da等

5.Authorization

6.connection

客户能否处理持续性的连接。

7.Content-Length

这个报头只适用于POST请求，用来给定POST数据的大小，以字节为单位。

8.Cookie

不要直接使用这个报头，而要使用request.getCookies

9.Host

10.If-Modified-Since

仅当页面在指定的日期之后发生更改的情况下，客户程序才希望获取该页面。

11.If-Unmodified-Since

12. Referer

一用web页面的URL

13.User-Agent

这个报头标识生成请求的浏览器或其他客户程序，根据这个报头，可以针对不同类型的浏览器返回不同的内容。

5.4 发送压缩web页面

gzip文本压缩方案可以极大地减少HTML页面的大小。

Implementing compression is straightforward since support for the gzip format is

built in to the Java programming language by classes in java.util.zip. The servlet

first checks the Accept-Encoding header to see if it contains an entry for gzip. If so,

it uses a PrintWriter wrapped around a GZIPOutputStream and specifies gzip

as the value of the Content-Encoding response header. If gzip is not supported, the

servlet uses the normal PrintWriter and omits the Content-Encoding header.

To make it easy to compare regular and compressed performance with the same

browser, we also added a feature whereby we can suppress compression by including

?disableGzip at the end of the URL

package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Servlet with <B>long</B> output. Used to test
* the effect of the gzip compression.
*/
public class LongServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
// Change the definition of "out" depending on whether
// or not gzip is supported.
PrintWriter out;
if (GzipUtilities.isGzipSupported(request) &&
!GzipUtilities.isGzipDisabled(request)) {
out = GzipUtilities.getGzipWriter(response);
response.setHeader("Content-Encoding", "gzip");
} else {
out = response.getWriter();
}
// Once "out" has been assigned appropriately, the
// rest of the page has no dependencies on the type
// of writer being used.
String docType =
"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
"Transitional//EN\">\n";
String title = "Long Page";
out.println
(docType +
"<HTML>\n" +
"<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<H1 ALIGN=\"CENTER\">" + title + "</H1>\n");
String line = "Blah, blah, blah, blah, blah. " +
"Yadda, yadda, yadda, yadda.";
for(int i=0; i<10000; i++) {
out.println(line);
}
out.println("</BODY></HTML>");
out.close(); // Needed for gzip; optional otherwise.
}
}

package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.zip.*;
/** Three small static utilities to assist with gzip encoding.
* <UL>
* <LI>isGzipSupported: does the browser support gzip?
* <LI>isGzipDisabled: has the user passed in a flag
* saying that gzip encoding should be disabled for
* this request? (Useful so that you can measure
* results with and without gzip on the same browser).
* <LI>getGzipWriter: return a gzipping PrintWriter.
* </UL>
*/
public class GzipUtilities {
/** Does the client support gzip? */
public static boolean isGzipSupported
(HttpServletRequest request) {
String encodings = request.getHeader("Accept-Encoding");
return((encodings != null) &&
(encodings.indexOf("gzip") != -1));
}
/** Has user disabled gzip (e.g., for benchmarking)? */
public static boolean isGzipDisabled
(HttpServletRequest request) {
String flag = request.getParameter("disableGzip");
return((flag != null) && (!flag.equalsIgnoreCase("false")));
}
/** Return gzipping PrintWriter for response. */
public static PrintWriter getGzipWriter
(HttpServletResponse response) throws IOException {
return(new PrintWriter
(new GZIPOutputStream
(response.getOutputStream())));
}
}