JSP的本质是什么？
【摘要】 概述
JSP页面在本质上就是Servlet程序，当JSP页面在首次被访问时，Ｗeb容器就会将JSP页面转化为Servlet，只需要做一次。如index.jsp在首次被访问时，Ｗeb容器会将其翻译成一个index_jsp.java文件，即Servlet代码；Servlet程序要被WEB容器调用执行，必须在web.xml中注册映射；Servlet类继承了类org.apac...
概述
- JSP页面在本质上就是Servlet程序，当JSP页面在首次被访问时，Ｗeb容器就会将JSP页面转化为Servlet，只需要做一次。如index.jsp在首次被访问时，Ｗeb容器会将其翻译成一个index_jsp.java文件，即Servlet代码；
- Servlet程序要被WEB容器调用执行，必须在web.xml中注册映射；
- Servlet类继承了类org.apache.jasper.runtime.HttpJspBase(Tomcat API,Tomcat容器实现的),这个类实现了HttpJspPage, JspPage, Servlet, ServletConfig等这些接口，因此WEB容器必须实现这些接口。
WEB容器： Tomcat、Jetty、Apache、Nginx、Jboss、Undertow、Websphere、Kangle、Resin等。
二、重要API
Servlet API：https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html
Tomcat API：https://tomcat.apache.org/tomcat-8.5-doc/api/index.html
JSP API：http://tomcat.apache.org/tomcat-8.5-doc/jspapi/index.html
三、jsp页面转化为servlet程序
（１）index.jsp：
<html>
<body>
<h2>
<% out.println("Hello Tomcat");
%>
</h2>
</body>
</html>
（２）index.jsp在首次被访问时，WEB容器将jsp转化为Servlet类，即index.jsp转化为index_jsp.java：
index_jsp.java:
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.47
* Generated at: 2019-11-07 08:52:49 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"); 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("<html>\n"); out.write("<body>\n"); out.write("<h2>\n"); out.println("Hello Tomcat"); out.write("\n"); out.write("</h2>\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); }
}
}
从转化后的代码，可知jsp页面里的HTML、CSS、JS，最终都被嵌套在Servlet程序的java代码里。我们可以推断jsp页面只是方便我们编写Servlet程序，就像在拼接一个长长的字符串。但是在前后端分离的开发中（可以参考《前后端分离的趋势》），jsp显然是不适应的。
四、分析index_jsp.java类
- 注释部分
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.47
* ...
*/
注释里说*.jsp文件都会被Apache Tomcat容器的Jasper组件转化为Servlet的子类。再次证明，jsp程序的本质就是Servlet程序。
- Servlet类的继承关系
public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent, org.apache.jasper.runtime.JspSourceImports {
HttpJspBase实现了Serializable, HttpJspPage, JspPage, Servlet, ServletConfig这些接口。而HttpJspBase是Tomcat API，所以Tomcat容器实现了这些。
- 内置对象out、request、response、session、application
我们分析index_jsp.java的_jspService方法来学习这些内置对象。
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"); 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;
…
（１）request
request是_jspService方法的参数，类型为HttpServletRequest，专门用来获得客户端向服务端请求的信息。
（２）reponse
reponse是_jspService方法的参数，类型为HttpServletResponse，专门用来填充要发送给客户端的数据。
（３）out、session、application
_jspxFactory对象调用 getPageContext()方法返回 PageContext对象。再通过调用PageContext对象的相应方法来初始out、session、application这三个内置对象。
private static final javax.servlet.jsp.JspFactory _jspxFactory = javax.servlet.jsp.JspFactory.getDefaultFactory();
从上面的代码可知：
out是一个 JspWriter对象。JspWriter对象与ServletResponse的PrintWriter关联着，所以你会发现不用直接使用reponse对象，也可以往response对象写数据。
JspWriter的方法请参考：http://tomcat.apache.org/tomcat-8.5-doc/jspapi/index.html
session是一个 HttpSession对象。用来维护会话期间用户的信息。关闭浏览器此会话就会被关闭。
HttpSession的方法请参考：https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html
application是一个 ServletContext对象。它的生命周期与程序一样长，只有程序被关闭，它才会消亡。
ServletContext的方法请参考：https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html
小知识：HTML、CSS、JS的处理是由客户端的浏览器来完成的，WEB服务器只需要将这些资源返回给浏览器即可，动态部分都是组装好再发送的。
谢谢阅读！
文章来源: blog.csdn.net，作者：WongKyunban，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/weixin_40763897/article/details/103041960
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