JSP 文件需要转换成 Java 类。在访问 JSP 文件的时候，并不直接执行 JSP 文件，而是执行 JSP 文件对应的 Java 类(class 文件) 的对象， 这个 Java 文件是服务器根据我们提供的 JSP 文件创建的。在 Tomcat 安装目录下的 work 目录中有 JSP 文件转换成的 Java 文件。<%!%>中定义的变量是类变量，<%%>中定义的变量是局部变量。举例如下：
index.jsp
<%@ page language="java" contentType="text/html; charset=GB18030" pageEncoding="GB18030"%> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=GB18030"> <title>欢迎页面</title> </head> <body> <%!int hitnumber=1; %> <%int jubu=1; %> </body> </html>
对应的Java类：
index_jsp.java
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.0.3 * Generated at: 2015-05-06 06:16:43 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 xixihaha_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent { int hitnumber=1; 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; charset=GB18030"); 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("\r\n"); out.write("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n"); out.write("<html>\r\n"); out.write("<head>\r\n"); out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=GB18030\">\r\n"); out.write("<title>欢迎页面</title>\r\n"); out.write("</head>\r\n"); out.write("<body>\r\n"); out.write('\r'); out.write('\n'); int jubu=1; out.write("\r\n"); out.write("</body>\r\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); else throw new ServletException(t); } } finally { _jspxFactory.releasePageContext(_jspx_page_context); } } }