JSP其实就是Servlet，在运行时，JSP将由JSP编译器进行转换，将JSP转换为Java代码，由JSP所创建的类都实现了Servlet接口，所以JSP就是Servlet，它也有声明声明方法，其实请求该JSP页面实际上也是在调用该JSP转换过了的Servlet的service方法，我们到tomcat目录下查看对应的Servlet代码
<%@ page contentType="text/html;charset=UTF-8" language="java" %><html> ?<head> ???<title>第一个web程序</title> ?</head> ?<body> ???Hello,World! ?</body></html>
该JSP的名字是index.jsp，我们看到它对应的Servlet的名字是index_jsp.java，由tomcat生成的源码如下:
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.0.52 * Generated at: 2018-10-09 11:42:02 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=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("\n"); ?????out.write("\n"); ?????out.write("<html>\n"); ?????out.write(" ?<head>\n"); ?????out.write(" ???<title>第一个web程序</title>\n"); ?????out.write(" ?</head>\n"); ?????out.write(" ?<body>\n"); ?????out.write(" ???Hello,World!\n"); ?????out.write(" ?</body>\n"); ?????out.write("</html>\n"); ???} 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); ???} ?}}这个类继承了org.apache.jasper.runtime.HttpJspBase，而该抽象类继承了HTTPServlet，而且类中还有一个 _jspService方法，参数的类型是HttpRequest和HttpResponse，可以猜测在HttpJspBase类中，将在声明周期方法service中调用这个 _jspService方法，再来看方法的具体实现，方法首先获取了请求方式，如果请求方式不是GET或者POST或者HEAD，那么客户端将收到一个错误状态码，"JSPs only permit GET POST or HEAD"，可见JSP只允许GET、POST和HEAD的请求方式，我们一开始在JSP页面中写的HTML文本，都使用了JspWriter的write方法发送到客户端了，我们使用IDE中在JSP页面的HTML文本部分打一个断点，然后浏览器请求该jsp，我们发现断点被命中了，并且调试器的栈堆将显示代码停留在_jspService方法中，这也证明了JSP其实就是Servlet，
JSP的原理
原文地址：https://www.cnblogs.com/yusiming/p/9762513.html