Jsp源码浅析（1）
JSP全称是Java Server Pages，它和servle技术一样，都是SUN公司定义的一种用于开发动态web资源的技术。
JSP这门技术的最大的特点在于，写jsp就像在写html，但它相比html而言，
html只能为用户提供静态数据，而Jsp技术允许在页面中嵌套java代码，为用户提供动态数据。
首先，我们给出一个结论：Jsp其实也是Servlet。
浏览器向服务器发请求，不管访问的是什么资源，其实都是在访问Servlet，
所以当访问一个jsp页面时，其实也是在访问一个Servlet，服务器在执行jsp的时候，首先把jsp翻译成一个Servlet，所以我们访问jsp时，其实不是在访问jsp，而是在访问jsp翻译过后的那个Servlet。
为了验证Jsp也是Servlet，我们来新建一个Jsp。
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<html><head><base href="<%=basePath%>"><title>My JSP 'Test.jsp' starting pagetitle><meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0"> <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page">head><body>This is my JSP page. <br><h2>JSPEEEh2><form action=""><input type="text" value="朦朦"/>form>body>
html>
我们访问这个jsp:http://localhost:8080/Jsp/Test.jsp
第一次执行Jsp文件时，在Web容器中找到需要执行的.jsp文件之后，.jsp文件通过转换变为.java文件，.java文件经过编译后，形成*.class文件
最终服务器要执行形成的*.class文件。
之后第二次执行因为已经存在了*.class文件，所以不在需要转换和编译的过程，和Servlet一样，只会被Web容器实例化一次。
修改Jsp后执行：源文件已经被修改过了，所以需要重新转换，重新编译。
那此时Jsp第一次被执行后，转换的java文件和class文件在哪呢？
当我们通过MyEclipse编译的时候，会默认的存放位置，放在tomcat目录下，不需要我们去设置。
默认在Tomcat跟目录下的//work//Catalina//localhost//工程名下
但是如果是Eclipse的话，默认的工程名在Eclipse的WorkSpace下的.metadata目录下，不利于我们查找。我们一般要将Jsp的存储路径放在Tomcat目录下，那这时该怎么设置呢。我们找到Servers，双击Servers设置，按下图设置
现在我们找出相应的java文件
/** Generated by the Jasper component of Apache Tomcat* Version: Apache Tomcat/8.0.35* Generated at: 2017-03-28 13:16:35 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.*;public final class Test_jsp extends org.apache.jasper.runtime.HttpJspBaseimplements 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.lang.String,java.lang.Long> _jspx_dependants;private static final java.util.Set.lang.String> _jspx_imports_packages;private static final java.util.Set.lang.String> _jspx_imports_classes;static {_jspx_imports_packages = new java.util.HashSet<>();_jspx_imports_packages.add("javax.servlet");_jspx_imports_packages.add("java.util");_jspx_imports_packages.add("javax.servlet.jsp");_jspx_imports_packages.add("javax.servlet.http");_jspx_imports_classes = null;}private volatile javax.el.ExpressionFactory _el_expressionfactory;private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;public java.util.Map.lang.String,java.lang.Long> getDependants() {return _jspx_dependants;}public java.util.Set.lang.String> getPackageImports() {return _jspx_imports_packages;}public java.util.Set.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('\r');out.write('\n');String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";out.write("\r\n");out.write("\r\n");out.write("\r\n");out.write("\r\n");out.write(" \r\n");out.write(" My JSP 'Test.jsp' starting page \r\n");out.write(" \r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\t \r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\r\n");out.write(" \r\n");out.write(" \r\n");out.write(" \r\n");out.write(" This is my JSP page.
\r\n");out.write(" JSPEEE
\r\n");out.write(" );out.write(" \r\n");out.write(" \r\n");out.write(" \r\n");out.write("\r\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);}}
}
我们看该类实现了HttpJspBase
public final class Test_jsp extends org.apache.jasper.runtime.HttpJspBaseimplements org.apache.jasper.runtime.JspSourceDependent,org.apache.jasper.runtime.JspSourceImports 我们看HttpJspBase这个类
public abstract class HttpJspBase extends HttpServlet implements HttpJspPage {private static final long serialVersionUID = 1L;protected HttpJspBase() {}@Overridepublic final void init(ServletConfig config) throws ServletException {super.init(config);jspInit();_jspInit();}@Overridepublic String getServletInfo() {return Localizer.getMessage("jsp.engine.info");}@Overridepublic final void destroy() {jspDestroy();_jspDestroy();}/*** Entry point into service.*/@Overridepublic final void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {_jspService(request, response);}@Overridepublic void jspInit() {}public void _jspInit() {}@Overridepublic void jspDestroy() {}protected void _jspDestroy() {}@Overridepublic abstract void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException;
}
我们可以很清楚的看到，这个类实现了HttpServlet,所以，Jsp也是Servlet.
我们再来关注下_jspService方法
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('\r');out.write('\n');String path = request.getContextPath();String basePath = request.getScheme() + "://"+ request.getServerName() + ":" + request.getServerPort()+ path + "/";out.write("\r\n");out.write("\r\n");out.write("\r\n");out.write("\r\n");out.write(" \r\n");out.write(" My JSP 'Test.jsp' starting page \r\n");out.write(" \r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\t \r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\t\r\n");out.write("\r\n");out.write(" \r\n");out.write(" \r\n");out.write(" \r\n");out.write(" This is my JSP page.
\r\n");out.write(" JSPEEE
\r\n");out.write(" );out.write(" \r\n");out.write(" \r\n");out.write(" \r\n");out.write("\r\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);elsethrow new ServletException(t);}} finally {_jspxFactory.releasePageContext(_jspx_page_context);}}我们都知道在jsp中有9大内置对象，其中page对象，request和response已经完成了实例化，这一点在程序中很容易看出来
page就是this，当前对象 request代表HttpServletRequest，response代表HttpResponse，这两个对象在Servlet容器（Tomcat）调用Servlet的Service方法之前
就被实例化
那么其他的对象是怎样被实例化的呢
我们随便百度，很快就可以知道我们可以通过jsp9大内置对象中的pageContext对象获得别的对象引用，那这是怎么做到的呢
我们查看代码一看
application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();
原来这里的实例化application，config，session,out直接调用了PageContext的相应对象呀。也就是说PageContext对象都保存了其他对象的引用
那PageContext对象是怎样得到这些对象的引用的呢（总不可能一开始就存在PageContext对象中把）
我们看代码，发现，得到pageContext是通过 pageContext = _jspxFactory.getPageContext(this, request, response,
null, true, 8192, true);
这个方法得到的
那么一目了然，肯定是通过这个方法将其他内置对象的引用赋给了pageContext
我们就来看下这个方法的具体实现。
JspFactory抽象类
public abstract PageContext getPageContext(Servlet servlet,ServletRequest request, ServletResponse response,String errorPageURL, boolean needsSession, int buffer,boolean autoflush);JspFactory的子类JspFactory，实现了getPageContext方法
public PageContext getPageContext(Servlet servlet, ServletRequest request,ServletResponse response, String errorPageURL, boolean needsSession,int bufferSize, boolean autoflush) {if( Constants.IS_SECURITY_ENABLED ) {PrivilegedGetPageContext dp = new PrivilegedGetPageContext(this, servlet, request, response, errorPageURL,needsSession, bufferSize, autoflush);return AccessController.doPrivileged(dp);} else {return internalGetPageContext(servlet, request, response,errorPageURL, needsSession,bufferSize, autoflush);}}@Overridepublic void initialize(Servlet servlet, ServletRequest request,ServletResponse response, String errorPageURL,boolean needsSession, int bufferSize, boolean autoFlush)throws IOException {_initialize(servlet, request, response, errorPageURL, needsSession,bufferSize, autoFlush);}private void _initialize(Servlet servlet, ServletRequest request,ServletResponse response, String errorPageURL,boolean needsSession, int bufferSize, boolean autoFlush) {// initialize statethis.servlet = servlet;this.config = servlet.getServletConfig();this.context = config.getServletContext();this.errorPageURL = errorPageURL;this.request = request;this.response = response;// initialize application contextthis.applicationContext = JspApplicationContextImpl.getInstance(context);// Setup session (if required)if (request instanceof HttpServletRequest && needsSession)this.session = ((HttpServletRequest) request).getSession();if (needsSession && session == null)throw new IllegalStateException("Page needs a session and none is available");// initialize the initial out ...depth = -1;if (this.baseOut == null) {this.baseOut = new JspWriterImpl(response, bufferSize, autoFlush);} else {this.baseOut.init(response, bufferSize, autoFlush);}this.out = baseOut;// register names/values as per specsetAttribute(OUT, this.out);setAttribute(REQUEST, request);setAttribute(RESPONSE, response);if (session != null)setAttribute(SESSION, session);setAttribute(PAGE, servlet);setAttribute(CONFIG, config);setAttribute(PAGECONTEXT, this);setAttribute(APPLICATION, context);isIncluded = request.getAttribute(RequestDispatcher.INCLUDE_SERVLET_PATH) != null;}
注意最后的setAttribute
其实你发现，就是封装了其他对象而已，并没有做太多的工作
config对象的获取：是通过Servlet容器初始化Servlet对象时，执行构造方法后，调用init(ServletConfig config)，
这时Servlet容器会将ServletConfig对象传递给init(ServletConfig config)
接着才是调用Service。这里调用的是HttpJspBase中的该方法，这个类实现了基类GenericServlet的init(ServletConfig config)
在执行init(ServletConfig config)后，我们便得到了config的引用，所以在Service方法中调用getServletConfig当然可得到
config：即代码中的：this.config = servlet.getServletConfig();接着调用：this.context = config.getServletContext();（ServletConfig保存了ServletContext的引用，可查看源码）session通过request.getSession();得到总之，全都是封装了。一切都是封装，抽象。
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
再说一句：注意这里的setAttribute();!!!!!!!!以后会分析到**
本文来自互联网用户投稿，文章观点仅代表作者本人，不代表本站立场，不承担相关法律责任。如若转载，请注明出处。 如若内容造成侵权/违法违规/事实不符，请点击【内容举报】进行投诉反馈！