为什么会出现Jsp？
其实对于服务器来说它只认识Servlet，我们完全可以在Servlet用resp.getWriter().write("");画出网页的界面，但是仅仅一个很简单的界面就要重复的书写resp.getWriter().write("")，并且这还没有加上js、css以及jquery。
但是我们在Servlet写的是java代码可以有逻辑的判断，但是在html、css中无法加入逻辑导致页面是静态的。为了解决这一问题出现了jsp，在jsp中可以书写Java代码、js代码、html代码等。
我们都知道浏览器发送请求到服务器，服务器通过解析其中Servlet的别名找到对应的Servlet并执行其中的service方法做出处理和响应，那么对于服务器来说它只认识Servlet，那么我们编辑的Jsp到底是如何被服务器识别的？
实际上服务器还是将.jsp当作Servlet的别名在你局部配置下的web.xml找有没有对应的Servlet，如果没有找到对应的Servlet就去Tomcat的公共配置下的web.xml（Tomcat安装目录conf下的web.xml）找对应的Servlet。
会找到如下内容：
org.apache.jasper.servlet.JspServlet将对应的jsp文件转为Servlet。
那么转的Servlet在什么地方呢？打开Tomcat下的work文件夹一直点到：
找你正在运行的项目的jsp
测试的jsp：
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<!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=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
666
</body>
</html>
被编译好的Java文件：
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.5.46 * Generated at: 2019-10-16 09:05:10 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 index_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent, org.apache.jasper.runtime.JspSourceImports { 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 = null; } 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 { final java.lang.String _jspx_method = request.getMethod(); if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) { response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD"); 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=ISO-8859-1"); 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("<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> "); out.write("<html> "); out.write("<head> "); out.write("<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> "); out.write("<title>Insert title here</title> "); out.write("</head> "); out.write("<body> "); out.write("666 "); 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); } } }
但是我们发现这个类并没有继承HttpServlet而是继承了org.apache.jasper.runtime.HttpJspBase并且里面也没有service方法而是_jspservice方法。那么Tomcat就会到它的父类中找service方法并进行调用。
实际上它的父类org.apache.jasper.runtime.HttpJspBase继承了HttpServlet。
父类中Service方法就是调用了子类的_jspservice方法（所以说实际上Tomcat执行的就是_jspservice方法）
总结：实际上jsp的本质还是servlet，只不过是为了方面花界面所单独拉出来的一部分而已，到最终的实现还是会回到Servlet当中。
展望：jsp已经是一门比较落后的技术来，因为html、css、js是前端写的，而后端要在其中加入逻辑判断，那么首先你一定得认识前端语言的代码，否则你如何嵌套？这样很不利于前后端的分离。之所以学习是因为很多框架和项目中依然会用到它，但是最终还是会被时代所淘汰。