jsp基础知识一

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jsp技术是在html文件内容中嵌入java代码,jsp页面发布后,由web容器的jsp引擎将jsp页面转化成java类(实际是servlet类的子类),并编译生成class文件(页面第一次访问时)。

1.jsp注释

格式: 

<%--  注释内容  --%>

jsp注释与html注释内容不同,jsp注释不会输出到客户端(浏览器),而html注释会输出到客户端。

html注释格式: <!-- 注释内容 -->

 

2.jsp声明

格式:

<%!

  表达式1;

  表达式2;

%>

如:

1 <%!
2     public int count;
3     public String info()
4     {
5         return "Hello world!";
6     }
7 %>

 

3.jsp输出表达式

格式:

<%=表达式 %>

如:

1 <%=info() %>
2 <%=count++ %>

 

4.jsp脚本

格式:

<%

  表达式1;

  表达式2;

%>

如:

 1 <table border="1px">
 2     <tr>
 3             <td>数据内容</td>
 4             <td>循环次数</td>
 5     </tr>
 6     <%
 7         for(int i=0;i<10;i++)
 8         {
 9     %>
10         <tr>
11             <td>循环内容</td>
12             <td align="center"><%=i %></td>
13         </tr>
14     <%} %>
15 </table>

 

实例:

技术分享
 1 <%@ page language="java" contentType="text/html; charset=utf-8"
 2     pageEncoding="utf-8"%>
 3 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
 4 <html>
 5 <head>
 6 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
 7 <title>JSP知识内容</title>
 8 </head>
 9 <body>
10 <!-- JSP注释 -->
11 <%-- JSP注释内容 --%>
12 <!-- HTML注释 -->
13 <!-- HTML注释内容 -->
14 
15 <%!
16     public int count;
17     public String info()
18     {
19         return "Hello world!";
20     }
21 %>
22 
23 <br/>
24 输出表达式:<%=info() %>
25 <br/>
26 count=<%=count++ %>
27 
28 <br/>
29 <hr>
30 jsp脚本输出内容:
31 <table border="1px">
32     <tr>
33             <td>数据内容</td>
34             <td>循环次数</td>
35     </tr>
36     <%
37         for(int i=0;i<10;i++)
38         {
39     %>
40         <tr>
41             <td>循环内容</td>
42             <td align="center"><%=i %></td>
43         </tr>
44     <%} %>
45 </table>
46 </body>
47 </html>
index.jsp

eclipse集成tomcat并启动后,在%workspace%\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\work\Catalina\localhost\web项目名\org\apache\jsp目录下会发现index.jsp被转化成index_jsp.java文件。

注:%workspace%为eclipse的workspace路径。

技术分享
  1 /*
  2  * Generated by the Jasper component of Apache Tomcat
  3  * Version: Apache Tomcat/8.0.20
  4  * Generated at: 2016-09-27 14:40:53 UTC
  5  * Note: The last modified time of this file was set to
  6  *       the last modified time of the source file after
  7  *       generation to assist with modification tracking.
  8  */
  9 package org.apache.jsp;
 10 
 11 import javax.servlet.*;
 12 import javax.servlet.http.*;
 13 import javax.servlet.jsp.*;
 14 
 15 public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
 16     implements org.apache.jasper.runtime.JspSourceDependent,
 17                  org.apache.jasper.runtime.JspSourceImports {
 18 
 19 
 20     public int count;
 21     public String info()
 22     {
 23         return "Hello world!";
 24     }
 25 
 26   private static final javax.servlet.jsp.JspFactory _jspxFactory =
 27           javax.servlet.jsp.JspFactory.getDefaultFactory();
 28 
 29   private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;
 30 
 31   private static final java.util.Set<java.lang.String> _jspx_imports_packages;
 32 
 33   private static final java.util.Set<java.lang.String> _jspx_imports_classes;
 34 
 35   static {
 36     _jspx_imports_packages = new java.util.HashSet<>();
 37     _jspx_imports_packages.add("javax.servlet");
 38     _jspx_imports_packages.add("javax.servlet.http");
 39     _jspx_imports_packages.add("javax.servlet.jsp");
 40     _jspx_imports_classes = null;
 41   }
 42 
 43   private javax.el.ExpressionFactory _el_expressionfactory;
 44   private org.apache.tomcat.InstanceManager _jsp_instancemanager;
 45 
 46   public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
 47     return _jspx_dependants;
 48   }
 49 
 50   public java.util.Set<java.lang.String> getPackageImports() {
 51     return _jspx_imports_packages;
 52   }
 53 
 54   public java.util.Set<java.lang.String> getClassImports() {
 55     return _jspx_imports_classes;
 56   }
 57 
 58   public void _jspInit() {
 59     _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
 60     _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
 61   }
 62 
 63   public void _jspDestroy() {
 64   }
 65 
 66   public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
 67         throws java.io.IOException, javax.servlet.ServletException {
 68 
 69 final java.lang.String _jspx_method = request.getMethod();
 70 if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
 71 response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
 72 return;
 73 }
 74 
 75     final javax.servlet.jsp.PageContext pageContext;
 76     javax.servlet.http.HttpSession session = null;
 77     final javax.servlet.ServletContext application;
 78     final javax.servlet.ServletConfig config;
 79     javax.servlet.jsp.JspWriter out = null;
 80     final java.lang.Object page = this;
 81     javax.servlet.jsp.JspWriter _jspx_out = null;
 82     javax.servlet.jsp.PageContext _jspx_page_context = null;
 83 
 84 
 85     try {
 86       response.setContentType("text/html; charset=utf-8");
 87       pageContext = _jspxFactory.getPageContext(this, request, response,
 88                   null, true, 8192, true);
 89       _jspx_page_context = pageContext;
 90       application = pageContext.getServletContext();
 91       config = pageContext.getServletConfig();
 92       session = pageContext.getSession();
 93       out = pageContext.getOut();
 94       _jspx_out = out;
 95 
 96       out.write("\r\n");
 97       out.write("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n");
 98       out.write("<html>\r\n");
 99       out.write("<head>\r\n");
100       out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\">\r\n");
101       out.write("<title>JSP知识内容</title>\r\n");
102       out.write("</head>\r\n");
103       out.write("<body>\r\n");
104       out.write("<!-- JSP注释 -->\r\n");
105       out.write("\r\n");
106       out.write("<!-- HTML注释 -->\r\n");
107       out.write("<!-- HTML注释内容 -->\r\n");
108       out.write("\r\n");
109       out.write("\r\n");
110       out.write("\r\n");
111       out.write("<br/>\r\n");
112       out.write("输出表达式:");
113       out.print(info() );
114       out.write("\r\n");
115       out.write("<br/>\r\n");
116       out.write("count=");
117       out.print(count++ );
118       out.write("\r\n");
119       out.write("\r\n");
120       out.write("<br/>\r\n");
121       out.write("<hr>\r\n");
122       out.write("jsp脚本输出内容:\r\n");
123       out.write("<table border=\"1px\">\r\n");
124       out.write("\t<tr>\r\n");
125       out.write("\t\t\t<td>数据内容</td>\r\n");
126       out.write("\t\t\t<td>循环次数</td>\r\n");
127       out.write("\t</tr>\r\n");
128       out.write("\t");
129 
130         for(int i=0;i<10;i++)
131         {
132     
133       out.write("\r\n");
134       out.write("\t\t<tr>\r\n");
135       out.write("\t\t\t<td>循环内容</td>\r\n");
136       out.write("\t\t\t<td align=\"center\">");
137       out.print(i );
138       out.write("</td>\r\n");
139       out.write("\t\t</tr>\r\n");
140       out.write("\t");
141 } 
142       out.write("\r\n");
143       out.write("</table>\r\n");
144       out.write("</body>\r\n");
145       out.write("</html>\r\n");
146     } catch (java.lang.Throwable t) {
147       if (!(t instanceof javax.servlet.jsp.SkipPageException)){
148         out = _jspx_out;
149         if (out != null && out.getBufferSize() != 0)
150           try {
151             if (response.isCommitted()) {
152               out.flush();
153             } else {
154               out.clearBuffer();
155             }
156           } catch (java.io.IOException e) {}
157         if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
158         else throw new ServletException(t);
159       }
160     } finally {
161       _jspxFactory.releasePageContext(_jspx_page_context);
162     }
163   }
164 }
index_jsp.java

  

  从index_jsp.java内容中可看到,index_jsp类继承org.apache.jasper.runtime.HttpJspBase(该类是HttpServlet类的子类),并包含了_jspInit()、_jspDestory()、_jspService()方法,这三个方法与servlet对应的是init()、destory()、server()方法。

  jsp声明中的变量为index_jsp类的内部变量,jsp声明中的方法为index_jsp类的方法;而jsp脚本中的代码则被生成到index_jsp类的_jspService()方法中,所以,jsp脚本代码块中的变量都是局部变更,不能用private或者是public的修饰。

  浏览器中访问index.jsp页面时,每次刷新页面时,count的值都会加1,因为jsp页面对应的类在web容器中是唯一一个实例(sigleton实例)。

  index.jsp页面输出的源码中,看不到jsp注释内容。

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