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JSP全称是Java Server Pages，它和servle技术一样，都是SUN公司定义的一种用于开发动态web资源的技术。
JSP这门技术的最大的特点在于，写jsp就像在写html，但它相比html而言，html只能为用户提供静态数据，而Jsp技术允许在页面中嵌套java代码，为用户提供动态数据。
浏览器向服务器发请求，不管访问的是什么资源，其实都是在访问Servlet，所以当访问一个jsp页面时，其实也是在访问一个Servlet，服务器在执行jsp的时候，首先把jsp翻译成一个Servlet，所以我们访问jsp时，其实不是在访问jsp，而是在访问jsp翻译过后的那个Servlet，例如下面的代码：
index.jsp
1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
2 <%
3 String path = request.getContextPath();
4 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
5 %>
6
7 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
8 <html>
9 <head>
10 <base href="<%=basePath%>">
11
12 <title>First Jsp</title>
13
14 </head>
15
16 <body>
17 <%
18 out.print("Hello Jsp");
19 %>
20 </body>
21 </html>
当我们通过浏览器访问index.jsp时，服务器首先将index.jsp翻译成一个index_jsp.class，在Tomcat服务器的work\Catalina\localhost\项目名\org\apache\jsp目录下可以看到index_jsp.class的源代码文件index_jsp.java，index_jsp.java的代码如下：
1 package org.apache.jsp;
2
3 import javax.servlet.*;
4 import javax.servlet.http.*;
5 import javax.servlet.jsp.*;
6 import java.util.*;
7
8 public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
9 implements org.apache.jasper.runtime.JspSourceDependent {
10
11 private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();
12
13 private static java.util.List _jspx_dependants;
14
15 private javax.el.ExpressionFactory _el_expressionfactory;
16 private org.apache.AnnotationProcessor _jsp_annotationprocessor;
17
18 public Object getDependants() {
19 return _jspx_dependants;
20 }
21
22 public void _jspInit() {
23 _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
24 _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
25 }
26
27 public void _jspDestroy() {
28 }
29
30 public void _jspService(HttpServletRequest request, HttpServletResponse response)
31 throws java.io.IOException, ServletException {
32
33 PageContext pageContext = null;
34 HttpSession session = null;
35 ServletContext application = null;
36 ServletConfig config = null;
37 JspWriter out = null;
38 Object page = this;
39 JspWriter _jspx_out = null;
40 PageContext _jspx_page_context = null;
41
42
43 try {
44 response.setContentType("text/html;charset=UTF-8");
45 pageContext = _jspxFactory.getPageContext(this, request, response,
46 null, true, 8192, true);
47 _jspx_page_context = pageContext;
48 application = pageContext.getServletContext();
49 config = pageContext.getServletConfig();
50 session = pageContext.getSession();
51 out = pageContext.getOut();
52 _jspx_out = out;
53
54 out.write(‘\r‘);
55 out.write(‘\n‘);
56
57 String path = request.getContextPath();
58 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
59
60 out.write("\r\n");
61 out.write("\r\n");
62 out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
63 out.write("<html>\r\n");
64 out.write(" <head>\r\n");
65 out.write(" <base href=\"");
66 out.print(basePath);
67 out.write("\">\r\n");
68 out.write(" \r\n");
69 out.write(" <title>First Jsp</title>\r\n");
70 out.write("\t\r\n");
71 out.write(" </head>\r\n");
72 out.write(" \r\n");
73 out.write(" <body>\r\n");
74 out.write(" ");
75
76 out.print("Hello Jsp");
77
78 out.write("\r\n");
79 out.write(" </body>\r\n");
80 out.write("</html>\r\n");
81 } catch (Throwable t) {
82 if (!(t instanceof SkipPageException)){
83 out = _jspx_out;
84 if (out != null && out.getBufferSize() != 0)
85 try { out.clearBuffer(); } catch (java.io.IOException e) {}
86 if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
87 }
88 } finally {
89 _jspxFactory.releasePageContext(_jspx_page_context);
90 }
91 }
92 }
我们可以看到，index_jsp这个类是继承 org.apache.jasper.runtime.HttpJspBase这个类的，通过查看Tomcat服务器的源代码，可以知道在apache-tomcat-6.0.20-src\java\org\apache\jasper\runtime目录下存HttpJspBase这个类的源代码文件，如下图所示：
我们可以看看HttpJsBase这个类的源代码，如下所示：
1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18 package org.apache.jasper.runtime;
19
20 import java.io.IOException;
21
22 import javax.servlet.ServletConfig;
23 import javax.servlet.ServletException;
24 import javax.servlet.http.HttpServlet;
25 import javax.servlet.http.HttpServletRequest;
26 import javax.servlet.http.HttpServletResponse;
27 import javax.servlet.jsp.HttpJspPage;
28 import javax.servlet.jsp.JspFactory;
29
30 import org.apache.jasper.compiler.Localizer;
31
32 /**
33 * This is the super class of all JSP-generated servlets.
34 *
35 * @author Anil K. Vijendran
36 */
37 public abstract class HttpJspBase
38 extends HttpServlet
39 implements HttpJspPage
40
41
42 {
43
44 protected HttpJspBase() {
45 }
46
47 public final void init(ServletConfig config)
48 throws ServletException
49 {
50 super.init(config);
51 jspInit();
52 _jspInit();
53 }
54
55 public String getServletInfo() {
56 return Localizer.getMessage("jsp.engine.info");
57 }
58
59 public final void destroy() {
60 jspDestroy();
61 _jspDestroy();
62 }
63
64 /**
65 * Entry point into service.
66 */
67 public final void service(HttpServletRequest request, HttpServletResponse response)
68 throws ServletException, IOException
69 {
70 _jspService(request, response);
71 }
72
73 public void jspInit() {
74 }
75
76 public void _jspInit() {
77 }
78
79 public void jspDestroy() {
80 }
81
82 protected void _jspDestroy() {
83 }
84
85 public abstract void _jspService(HttpServletRequest request,
86 HttpServletResponse response)
87 throws ServletException, IOException;
88 }
HttpJspBase类是继承HttpServlet的，所以HttpJspBase类是一个Servlet，而index_jsp又是继承HttpJspBase类的，所以index_jsp类也是一个Servlet，所以当浏览器访问服务器上的index.jsp页面时，其实就是在访问index_jsp这个Servlet，index_jsp这个Servlet使用_jspService这个方法处理请求。
浏览器接收到的这些数据
1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 <html> 3 <head> 4 <base href="http://localhost:8080/JavaWeb_Jsp_Study_20140603/"> 5 6 <title>First Jsp</title> 7 8 </head> 9 10 <body> 11 Hello Jsp 12 </body> 13 </html>
都是在_jspService方法中使用如下的代码输出给浏览器的：
1 out.write(‘\r‘);
2 out.write(‘\n‘);
3
4 String path = request.getContextPath();
5 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
6
7 out.write("\r\n");
8 out.write("\r\n");
9 out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
10 out.write("<html>\r\n");
11 out.write(" <head>\r\n");
12 out.write(" <base href=\"");
13 out.print(basePath);
14 out.write("\">\r\n");
15 out.write(" \r\n");
16 out.write(" <title>First Jsp</title>\r\n");
17 out.write("\t\r\n");
18 out.write(" </head>\r\n");
19 out.write(" \r\n");
20 out.write(" <body>\r\n");
21 out.write(" ");
22
23 out.print("Hello Jsp");
24
25 out.write("\r\n");
26 out.write(" </body>\r\n");
27 out.write("</html>\r\n");
在jsp中编写的java代码和html代码都会被翻译到_jspService方法中去，在jsp中编写的java代码会原封不动地翻译成java代码，如<%out.print("Hello Jsp");%>直接翻译成out.print("Hello Jsp");，而HTML代码则会翻译成使用out.write("<html标签>\r\n");的形式输出到浏览器。在jsp页面中编写的html排版标签都是以out.write("<html标签>\r\n");的形式输出到浏览器，浏览器拿到html代码后才能够解析执行html代码。
在jsp中编写的java代码会被翻译到_jspService方法中去，当执行_jspService方法处理请求时，就会执行在jsp编写的java代码了，所以Jsp页面中的java代码服务器是通过调用_jspService方法处理请求时执行的。
查看_jspService方法可以看到，Web服务器在调用jsp时，会给Jsp提供如下的8个java对象
1 PageContext pageContext; 2 HttpSession session; 3 ServletContext application; 4 ServletConfig config; 5 JspWriter out; 6 Object page = this; 7 HttpServletRequest request, 8 HttpServletResponse response
其中page对象，request和response已经完成了实例化，而其它5个没有实例化的对象通过下面的方式实例化
1 pageContext = _jspxFactory.getPageContext(this, request, response,null, true, 8192, true); 2 application = pageContext.getServletContext(); 3 config = pageContext.getServletConfig(); 4 session = pageContext.getSession(); 5 out = pageContext.getOut();
这8个java对象在Jsp页面中是可以直接使用的，如下所示：
1 <%
2 session.setAttribute("name", "session对象");//使用session对象,设置session对象的属性
3 out.print(session.getAttribute("name")+"<br/>");//获取session对象的属性
4 pageContext.setAttribute("name", "pageContext对象");//使用pageContext对象,设置pageContext对象的属性
5 out.print(pageContext.getAttribute("name")+"<br/>");//获取pageContext对象的属性
6 application.setAttribute("name", "application对象");//使用application对象,设置application对象的属性
7 out.print(application.getAttribute("name")+"<br/>");//获取application对象的属性
8 out.print("Hello Jsp"+"<br/>");//使用out对象
9 out.print("服务器调用index.jsp页面时翻译成的类的名字是："+page.getClass()+"<br/>");//使用page对象
10 out.print("处理请求的Servlet的名字是："+config.getServletName()+"<br/>");//使用config对象
11 out.print(response.getContentType()+"<br/>");//使用response对象
12 out.print(request.getContextPath()+"<br/>");//使用request对象
13 %>
运行结果如下：
Jsp最佳实践就是jsp技术在开发中该怎么去用。
不管是JSP还是Servlet，虽然都可以用于开发动态web资源。但由于这2门技术各自的特点，在长期的软件实践中，人们逐渐把servlet作为web应用中的控制器组件来使用，而把JSP技术作为数据显示模板来使用。其原因为，程序的数据通常要美化后再输出：让jsp既用java代码产生动态数据，又做美化会导致页面难以维护。让servlet既产生数据，又在里面嵌套html代码美化数据，同样也会导致程序可读性差，难以维护。因此最好的办法就是根据这两门技术的特点，让它们各自负责各的，servlet只负责响应请求产生数据，并把数据通过转发技术带给jsp，数据的显示jsp来做。
第一次执行：
第二次执行：
修改后执行：
1.源文件已经被修改过了，所以需要重新转换，重新编译。
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原文地址：http://www.cnblogs.com/wl310538259/p/5164611.html
