2019独角兽企业重金招聘Python工程师标准>>> 
一、什么是JSP&＃xff1f;
JSP全称是Java Server Pages&＃xff0c;它和servle技术一样&＃xff0c;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&＃xff0c;写jsp就像在写html&＃xff0c;但它相比html而言&＃xff0c;html只能为用户提供静态数据&＃xff0c;而Jsp技术允许在页面中嵌套java代码&＃xff0c;为用户提供动态数据。
二、JSP原理
2.1、Web服务器是如何调用并执行一个jsp页面的&＃xff1f;
浏览器向服务器发请求&＃xff0c;不管访问的是什么资源&＃xff0c;其实都是在访问Servlet&＃xff0c;所以当访问一个jsp页面时&＃xff0c;其实也是在访问一个Servlet&＃xff0c;服务器在执行jsp的时候&＃xff0c;首先把jsp翻译成一个Servlet&＃xff0c;所以我们访问jsp时&＃xff0c;其实不是在访问jsp&＃xff0c;而是在访问jsp翻译过后的那个Servlet&＃xff0c;例如下面的代码&＃xff1a;
index.jsp
1 <%&＃64; page language&＃61;"java" import&＃61;"java.util.*" pageEncoding&＃61;"UTF-8"%>
2 <%
3 String path &＃61; request.getContextPath();
4 String basePath &＃61; request.getScheme()&＃43;"://"&＃43;request.getServerName()&＃43;":"&＃43;request.getServerPort()&＃43;path&＃43;"/";
5 %>
6
7 DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
8 <html>
9 <head>
10 <base href&＃61;"<%&＃61;basePath%>">
11
12 <title>First Jsptitle>
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&＃xff0c;服务器首先将index.jsp翻译成一个index_jsp.class&＃xff0c;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&＃xff0c;index_jsp.java的代码如下&＃xff1a;
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 &＃61; 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 &＃61; _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
24 _jsp_annotationprocessor &＃61; (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 &＃61; null;
34 HttpSession session &＃61; null;
35 ServletContext application &＃61; null;
36 ServletConfig config &＃61; null;
37 JspWriter out &＃61; null;
38 Object page &＃61; this;
39 JspWriter _jspx_out &＃61; null;
40 PageContext _jspx_page_context &＃61; null;
41
42
43 try {
44 response.setContentType("text/html;charset&＃61;UTF-8");
45 pageContext &＃61; _jspxFactory.getPageContext(this, request, response,
46 null, true, 8192, true);
47 _jspx_page_context &＃61; pageContext;
48 application &＃61; pageContext.getServletContext();
49 config &＃61; pageContext.getServletConfig();
50 session &＃61; pageContext.getSession();
51 out &＃61; pageContext.getOut();
52 _jspx_out &＃61; out;
53
54 out.write(&＃39;\r&＃39;);
55 out.write(&＃39;\n&＃39;);
56
57 String path &＃61; request.getContextPath();
58 String basePath &＃61; request.getScheme()&＃43;"://"&＃43;request.getServerName()&＃43;":"&＃43;request.getServerPort()&＃43;path&＃43;"/";
59
60 out.write("\r\n");
61 out.write("\r\n");
62 out.write("\r\n");
63 out.write("\r\n");
64 out.write(" \r\n");
65 out.write("
66 out.print(basePath);
67 out.write("\">\r\n");
68 out.write(" \r\n");
69 out.write(" \r\n");
70 out.write("\t\r\n");
71 out.write(" \r\n");
72 out.write(" \r\n");
73 out.write(" \r\n");
74 out.write(" ");
75
76 out.print("Hello Jsp");
77
78 out.write("\r\n");
79 out.write(" \r\n");
80 out.write("\r\n");
81 } catch (Throwable t) {
82 if (!(t instanceof SkipPageException)){
83 out &＃61; _jspx_out;
84 if (out !&＃61; null && out.getBufferSize() !&＃61; 0)
85 try { out.clearBuffer(); } catch (java.io.IOException e) {}
86 if (_jspx_page_context !&＃61; null) _jspx_page_context.handlePageException(t);
87 }
88 } finally {
89 _jspxFactory.releasePageContext(_jspx_page_context);
90 }
91 }
92 }
我们可以看到&＃xff0c;index_jsp这个类是继承 org.apache.jasper.runtime.HttpJspBase这个类的&＃xff0c;通过查看Tomcat服务器的源代码&＃xff0c;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&＃xff0c;如下图所示&＃xff1a;
我们可以看看HttpJsBase这个类的源代码&＃xff0c;如下所示&＃xff1a;
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 * &＃64;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&＃xff0c;所以HttpJspBase类是一个Servlet&＃xff0c;而index_jsp又是继承HttpJspBase类的&＃xff0c;所以index_jsp类也是一个Servlet&＃xff0c;所以当浏览器访问服务器上的index.jsp页面时&＃xff0c;其实就是在访问index_jsp这个Servlet&＃xff0c;index_jsp这个Servlet使用_jspService这个方法处理请求。
2.2、Jsp页面中的html排版标签是如何被发送到客户端的&＃xff1f;
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&＃61;"http://localhost:8080/JavaWeb_Jsp_Study_20140603/">
5
6 <title>First Jsptitle>
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&＃xff1a;
1 out.write(&＃39;\r&＃39;);
2 out.write(&＃39;\n&＃39;);
3
4 String path &＃61; request.getContextPath();
5 String basePath &＃61; request.getScheme()&＃43;"://"&＃43;request.getServerName()&＃43;":"&＃43;request.getServerPort()&＃43;path&＃43;"/";
6
7 out.write("\r\n");
8 out.write("\r\n");
9 out.write("\r\n");
10 out.write("\r\n");
11 out.write(" \r\n");
12 out.write("
13 out.print(basePath);
14 out.write("\">\r\n");
15 out.write(" \r\n");
16 out.write(" \r\n");
17 out.write("\t\r\n");
18 out.write(" \r\n");
19 out.write(" \r\n");
20 out.write(" \r\n");
21 out.write(" ");
22
23 out.print("Hello Jsp");
24
25 out.write("\r\n");
26 out.write(" \r\n");
27 out.write("\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&＃xff0c;在jsp中编写的java代码会原封不动地翻译成java代码&＃xff0c;如<%out.print("Hello Jsp");%>直接翻译成out.print("Hello Jsp");&＃xff0c;而HTML代码则会翻译成使用out.write("\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("\r\n");的形式输出到浏览器&＃xff0c;浏览器拿到html代码后才能够解析执行html代码。
2.3、Jsp页面中的java代码服务器是如何执行的&＃xff1f;
在jsp中编写的java代码会被翻译到_jspService方法中去&＃xff0c;当执行_jspService方法处理请求时&＃xff0c;就会执行在jsp编写的java代码了&＃xff0c;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。
2.4、Web服务器在调用jsp时&＃xff0c;会给jsp提供一些什么java对象&＃xff1f;
查看_jspService方法可以看到&＃xff0c;Web服务器在调用jsp时&＃xff0c;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 &＃61; this;
7 HttpServletRequest request,
8 HttpServletResponse response
其中page对象&＃xff0c;request和response已经完成了实例化&＃xff0c;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 &＃61; _jspxFactory.getPageContext(this, request, response,null, true, 8192, true);
2 application &＃61; pageContext.getServletContext();
3 config &＃61; pageContext.getServletConfig();
4 session &＃61; pageContext.getSession();
5 out &＃61; pageContext.getOut();
这8个java对象在Jsp页面中是可以直接使用的&＃xff0c;如下所示&＃xff1a;
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")&＃43;"
");//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")&＃43;"
");//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")&＃43;"
");//获取application对象的属性
8 out.print("Hello Jsp"&＃43;"
");//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&＃xff1a;"&＃43;page.getClass()&＃43;"
");//使用page对象
10 out.print("处理请求的Servlet的名字是&＃xff1a;"&＃43;config.getServletName()&＃43;"
");//使用config对象
11 out.print(response.getContentType()&＃43;"
");//使用response对象
12 out.print(request.getContextPath()&＃43;"
");//使用request对象
13 %>
运行结果如下&＃xff1a;
2.5、Jsp最佳实践
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&＃xff0c;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&＃xff0c;在长期的软件实践中&＃xff0c;人们逐渐把servlet作为web应用中的控制器组件来使用&＃xff0c;而把JSP技术作为数据显示模板来使用。其原因为&＃xff0c;程序的数据通常要美化后再输出&＃xff1a;让jsp既用java代码产生动态数据&＃xff0c;又做美化会导致页面难以维护。让servlet既产生数据&＃xff0c;又在里面嵌套html代码美化数据&＃xff0c;同样也会导致程序可读性差&＃xff0c;难以维护。因此最好的办法就是根据这两门技术的特点&＃xff0c;让它们各自负责各的&＃xff0c;servlet只负责响应请求产生数据&＃xff0c;并把数据通过转发技术带给jsp&＃xff0c;数据的显示jsp来做。
2.6、Tomcat服务器的执行流程
第一次执行&＃xff1a;
- 客户端通过电脑连接服务器&＃xff0c;因为是请求是动态的&＃xff0c;所以所有的请求交给WEB容器来处理
- 在容器中找到需要执行的*.jsp文件
- 之后*.jsp文件通过转换变为*.java文件
- *.java文件经过编译后&＃xff0c;形成*.class文件
- 最终服务器要执行形成的*.class文件
第二次执行&＃xff1a;
- 因为已经存在了*.class文件&＃xff0c;所以不在需要转换和编译的过程
修改后执行&＃xff1a;
1.源文件已经被修改过了&＃xff0c;所以需要重新转换&＃xff0c;重新编译。
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