JSP在请求的时候，会先转化成Servlet（其实就是个java类），然后生成class文件，再提供服务。 那么生成的java、class文件在哪呢？
首先应该了解的是Tomcat在Eclipse的映射关系：Tomcat的服务器目录配置
可以了解到，Tomcat在Eclipse中提供了三种位置配置选项：
1 Use workspace metadata 2 Use Tomcat installation 3 Use custom location
分别对应三种情况说一下，注意要把Server中发布的目录全部删除，然后clean后才能修改该配置项。（示例如下：1、2、3、4、5）
1，如果Server Locations选择了第一项：Use workspace metadata
服务器目录，即生成的字节码和java文件所在的目录。它在你的Eclipse的工作目录中。（比如我的工作目录是在D:program2，那么在该目录下就可以看到这个.metadata目录了。）
参考上面的配置目录D:program2.metadata.pluginsorg.eclipse.wst.server.core mp0
这就是Tomcat映射的目录，在这个目录中有一个work目录和一个wtpwebapps目录（wtpwebapps目录则存放对应的部署资源文件）
work目录中顺着：workCatalinalocalhost项目名字orgapachejsp 就可以找到你的项目对应的java文件和class文件。
我的：D:program2.metadata.pluginsorg.eclipse.wst.server.core mp0workCatalinalocalhostjsp_demoorgapachejsp
2.如果选择的是第二项，Use Tomcat installation
如果勾选的use tomcat installation，那么你的tomcat目录将被选定为tomcat所安装的目录。
此时，观察上面的：Server path在D:apache-tomcat-9.0.36。很好找了！
注意要先访问jsp后，才会生成.class文件。上面的Server Locations选择Use workspace metadata 也一样，下面是，选择第二个（Use Tomcat installation...）的截图演示）：
另外，选择第二个选项，代表Tomcat的部署目录，这个也能解决访问http://localhost:8080时，出现404。因为，启动一个tomcat，我们自己的worspace是没有相关的内容的（选择第一个），访问的话肯定会报404错误。
3.如果勾选第三项，Use custom location
目录的结构与上面的类似，上面这两个都是不可以修改的，要么是workspace所在的目录，要么是tomcat的目录。
选择该项，可以自定义生成的空间。
JSP代码示例：
<%@ page import="java.util.Map"%> <%@ page import="java.util.HashMap"%> <%@page contentType="text/html;charset=UTF-8" language="java"%> <html> <head> <title>Insert title here</title> </head> <body> <%--练习 --%> <%--1.声明类属性 --%> <%!private Integer id; private String name; private static Map<String, Object> map;%> <%--2.声明static静态代码块 --%> <%!static { map = new HashMap<String, Object>(); map.put("key1", "value1"); map.put("key2", "value2"); map.put("key3", "value3"); }%> <%--3.声明类方法 --%> <%!public int abc() { return 12; }%> <%--4.声明内部类 --%> <%!public static class A { private Integer id = 12; private String abc = "abc"; }%> <%--练习： 1.输出整型 2.输出浮点型 3.输出字符串 4.输出对象 --%> <%=12%><br/> <%=12.12%><br/> <%="我是字符串"%><br/> <%=map%><br/> <%=request.getParameter("username")%><br/> <%--练习 --%> <%--1.代码脚本----if 语句 --%> <% int i=13; if(i==12){ %> <h1>哈哈哈，我来了！</h1> <% }else{ %> <h1>我走了</h1> <% } %> <br/> <%--2.代码脚本----for 循环语句 --%> <table border="1" cellspacing="0"> <% for(int j=0;j<10;j++){ %> <tr> <td>第<%=j+1%>行</td> </tr> <% } %> </table> <%--3.翻译后Java文件中_jspService方法内的代码都可以写 --%> <% String username=request.getParameter("username"); System.out.println("用户名的请求参数值："+ username); %> </body> </html>
JSP页面的本质是一个Servlet程序。
当我们第一次访问jsp页面时，Tomcat服务器会帮我们把jsp页面翻译成一个java源文件。并对它进行编译为.class字节码程序。
我们打开Java源文件不难发现其里面的内容：
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/9.0.36 * Generated at: 2020-07-08 11:04:29 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.*; import java.util.Map; import java.util.HashMap; public final class b_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent, org.apache.jasper.runtime.JspSourceImports { private Integer id; private String name; private static Map<String, Object> map; static { map = new HashMap<String, Object>(); map.put("key1", "value1"); map.put("key2", "value2"); map.put("key3", "value3"); } public int abc() { return 12; } public static class A { private Integer id = 12; private String abc = "abc"; } 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 static final java.util.Set<java.lang.String> _jspx_imports_packages; private static final java.util.Set<java.lang.String> _jspx_imports_classes; static { _jspx_imports_packages = new java.util.HashSet<>(); _jspx_imports_packages.add("javax.servlet"); _jspx_imports_packages.add("javax.servlet.http"); _jspx_imports_packages.add("javax.servlet.jsp"); _jspx_imports_classes = new java.util.HashSet<>(); _jspx_imports_classes.add("java.util.Map"); _jspx_imports_classes.add("java.util.HashMap"); } private volatile javax.el.ExpressionFactory _el_expressionfactory; private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager; public java.util.Map<java.lang.String,java.lang.Long> getDependants() { return _jspx_dependants; } public java.util.Set<java.lang.String> getPackageImports() { return _jspx_imports_packages; } public java.util.Set<java.lang.String> getClassImports() { return _jspx_imports_classes; } public javax.el.ExpressionFactory _jsp_getExpressionFactory() { if (_el_expressionfactory == null) { synchronized (this) { if (_el_expressionfactory == null) { _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory(); } } } return _el_expressionfactory; } public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() { if (_jsp_instancemanager == null) { synchronized (this) { if (_jsp_instancemanager == null) { _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig()); } } } return _jsp_instancemanager; } public void _jspInit() { } 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 { if (!javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) { final java.lang.String _jspx_method = request.getMethod(); if ("OPTIONS".equals(_jspx_method)) { response.setHeader("Allow","GET, HEAD, POST, OPTIONS"); return; } if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method)) { response.setHeader("Allow","GET, HEAD, POST, OPTIONS"); response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS"); return; } } 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=UTF-8"); 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(" "); out.write(" "); out.write(" "); out.write("<html> "); out.write("<head> "); out.write("<title>Insert title here</title> "); out.write("</head> "); out.write("<body> "); out.write(" "); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.print(12); out.write("<br/> "); out.write(" "); out.print(12.12); out.write("<br/> "); out.write(" "); out.write(" "); out.print("我是字符串"); out.write("<br/> "); out.write(" "); out.print(map); out.write("<br/> "); out.write(" "); out.print(request.getParameter("username")); out.write("<br/> "); out.write(" "); out.write(' '); out.write(' '); out.write(' '); out.write(' '); out.write(' '); int i=13; if(i==12){ out.write(" "); out.write(" <h1>哈哈哈，我来了！</h1> "); out.write(" "); }else{ out.write(" "); out.write(" <h1>我走了</h1> "); out.write(" "); } out.write(" "); out.write(" <br/> "); out.write(" "); out.write(" "); out.write(" "); out.write(" <table border="1" cellspacing="0"> "); out.write(" "); for(int j=0;j<10;j++){ out.write(" "); out.write(" <tr> "); out.write(" <td>第"); out.print(j+1); out.write("行</td> "); out.write(" </tr> "); out.write(" "); } out.write(" "); out.write(" </table> "); out.write(' '); out.write(' '); out.write(' '); String username=request.getParameter("username"); System.out.println("用户名的请求参数值："+ username); out.write(" "); out.write(" "); out.write("</body> "); 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 { if (response.isCommitted()) { out.flush(); } else { 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); } } }