对于Servlet 学习过后,那么接触到JSP,那么可以发现JSP编程则会突然喜爱上,因为JSP不仅仅也是一个servlet,还能帮我们直接从servlet的容器(Tomcat)为我们直接生成对应的servlet代码。至于更加详细的介绍大家可以自行baidu~goole
下面让我们首先写一个JSP,然后大家就明白了其用途:
1. 首先我们编写如下一个jsp文件,这里起名 himi.jsp,其内容如下:
- <html>
- <head>
- </head>
- <body>
- <%
- out.println("HelloWorld!");
- %>
- </body>
- </html>
OK,将其之间放置我们上一篇创建的webapp根目录即可;
然后尝试访问一下这个himi.jsp 观察如下:(这里Himi在tomcat目录下创建的webapp叫“MyWebApp”)
OK,发现Servlet 返回给Client端一个“ Hello World! ”字符串
大家先到tomcat目录下的/work/Catalina/localhost/MyWebApp/org/apache/jsp文件夹下我们可以看到如下两个文件:
a)himi_jsp.class b)himi_jsp.java
不二话打开 himi_jsp.java 如下:
- /*
- * Generated by the Jasper component of Apache Tomcat
- * Version: Apache Tomcat/7.0.27
- * Generated at: 2012-05-24 16:07:57 UTC
- * Note: The last modified time of this file was set to
- * the last modified time of the source file after
- * generation to assist with modification tracking.
- */
- package org.apache.jsp;
- import javax.servlet.*;
- import javax.servlet.http.*;
- import javax.servlet.jsp.*;
- public final class himi_jsp extends org.apache.jasper.runtime.HttpJspBase
- implements org.apache.jasper.runtime.JspSourceDependent {
- private static final javax.servlet.jsp.JspFactory _jspxFactory =
- javax.servlet.jsp.JspFactory.getDefaultFactory();
- private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;
- private javax.el.ExpressionFactory _el_expressionfactory;
- private org.apache.tomcat.InstanceManager _jsp_instancemanager;
- public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
- return _jspx_dependants;
- }
- public void _jspInit() {
- _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
- _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
- }
- public void _jspDestroy() {
- }
- public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
- throws java.io.IOException, javax.servlet.ServletException {
- final javax.servlet.jsp.PageContext pageContext;
- javax.servlet.http.HttpSession session = null;
- final javax.servlet.ServletContext application;
- final javax.servlet.ServletConfig config;
- javax.servlet.jsp.JspWriter out = null;
- final java.lang.Object page = this;
- javax.servlet.jsp.JspWriter _jspx_out = null;
- javax.servlet.jsp.PageContext _jspx_page_context = null;
- try {
- response.setContentType("text/html");
- pageContext = _jspxFactory.getPageContext(this, request, response,
- null, true, 8192, true);
- _jspx_page_context = pageContext;
- application = pageContext.getServletContext();
- config = pageContext.getServletConfig();
- session = pageContext.getSession();
- out = pageContext.getOut();
- _jspx_out = out;
- out.write("<html>\n");
- out.write("\t<head>\n");
- out.write("\t</head>\n");
- out.write("\n");
- out.write("\t<body>\n");
- out.write("\t\t");
- out.println("HelloWorld!");
- out.write("\n");
- out.write("\t</body>\n");
- out.write("</html>");
- } catch (java.lang.Throwable t) {
- if (!(t instanceof javax.servlet.jsp.SkipPageException)){
- out = _jspx_out;
- if (out != null && out.getBufferSize() != 0)
- try { out.clearBuffer(); } catch (java.io.IOException e) {}
- if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
- }
- } finally {
- _jspxFactory.releasePageContext(_jspx_page_context);
- }
- }
- }
看到这里大家应该明白了,当客户端访问jsp后,jsp默认生成一个 servlet 并且编译字节码,(当然是第一次访问生成并编译,后续访问就不会再次编译了);
JSP 语法:
拐回头来看 himi.jsp 代码:
<html> <head> </head> <body> <% out.println("HelloWorld!"); %> </body> </html>
【JSP 自动生成的servlet代码,其实内置了一些对象,如下:】
out 就是PrintWriter;
req 就是HttpServletRequest,
resp 就是HttpServletResponse
1. <% ... %> 两个符号之间是我们的servlet代码;也就是程序代码块,在其中可以任意放置java代码;
也就是说客户端访问jsp后默认将我们himi.jsp中的内容转换成servlet;
2. 其中注视的方式:
1) <%--... ...--%> 2) <%//... ...%> 3) <%/*... ...*/%>
要注意:以上注视都是服务器端的注视方式,如果你在jsp使用客户端注视方式:
<!-- ... --> 那要格外注意!如果你在客户端注视方式加入<%...%>如下:
<!-- <% %> -->中是无法阻止服务器端的代码!只能保证<%..%>不保证在客户端显示出来罢了;
3. <%! ... %> 表示将其中的代码成为自动生成servlet类的成员变量或者成员函数;
4. <%= ... %> 等同于 <% out.println(); %>
5. request.getParameter("xxx"); 可以动态获取到xxx这个属性的参数;
例如传入一个 xxx.jsp?xxx=yyy
6. <%@Directive 属性="属性值"%> 编译期间的命令;
Directive 常用: a) page b) include c) taglib
JSP服务器端和客户端代码互相调用方法:
示例代码段1:
xx.jsp <script> <%out.println("var a =10");%> </script>
示例代码段2:
xx.jsp <script> var a =' <% out.print(23); %> '; </script>
注意 :
1)JSP服务器端代码<%...%>总执行在Client端代码之前
2)代码段2 要注意 “ print(23) ”而不是“println(23)” 因为println是会换行造成客户端语法错误!;