深入分析JSP
首先,面试的时候经常会被问到JSP和Servlet有什么关系?
从表面上看,JSP处理页面展示,Servlet处理页面跳转;更进一步就是有些面试宝典上说得:JSP编译后是类Servlet文件
那么到底两者是什么关系?见下面分析
一个简单的JSP文件HelloWorld.jsp
<%@ page buffer="16kb" %> <%! int a = 0; %> <% String message = "Hello World!"; out.print(a); %> <%=message%>
将它拷贝到Tomcat下\webapps\ROOT下
经过Tomcat编译后看一下文件夹在我本地是\apache-tomcat-6.0.14\work\Catalina\localhost\_\org\apache\jsp,
你会发现有个HelloWorld_jsp.java,内容如下:
package org.apache.jsp; import javax.servlet.*; import javax.servlet.http.*; import javax.servlet.jsp.*; public final class HelloWorld_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent { int a = 0; private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory(); private static java.util.List _jspx_dependants; private javax.el.ExpressionFactory _el_expressionfactory; private org.apache.AnnotationProcessor _jsp_annotationprocessor; public Object getDependants() { return _jspx_dependants; } public void _jspInit() { _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory(); _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName()); } public void _jspDestroy() { } public void _jspService(HttpServletRequest request, HttpServletResponse response) throws java.io.IOException, ServletException { PageContext pageContext = null; HttpSession session = null; ServletContext application = null; ServletConfig config = null; JspWriter out = null; Object page = this; JspWriter _jspx_out = null; PageContext _jspx_page_context = null; try { response.setContentType("text/html"); pageContext = _jspxFactory.getPageContext(this, request, response, null, true, 16384, 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'); out.write('\r'); out.write('\n'); String message = "Hello World!"; out.print(a); out.write('\r'); out.write('\n'); out.print(message); out.write('\r'); out.write('\n'); } catch (Throwable t) { if (!(t instanceof SkipPageException)){ out = _jspx_out; if (out != null && out.getBufferSize() != 0) try { out.clearBuffer(); } catch (java.io.IOException e) {} if (_jspx_page_context != null) _jspx_page_context.handlePageException(t); } } finally { _jspxFactory.releasePageContext(_jspx_page_context); } } }
发现这个类的方法_jspInit()、_ jspDestroy()、_jspService()和Servlet中的init()、destory()、service()非常像,其实这个文件就是一个servlet。
下面说明几点:
1)这个类extends org.apache.jasper.runtime.HttpJspBase,而HttpJspBase extends javax.servlet.http.HttpServlet
2)我们知道JSP的内置对象可以在页面上直接用,原理何在?看一下上面的_jspService()方法中的变量声明 包含6个对象除了request、reponse和exception,并进行了初始化
3)可以看到_jspService()处理主要的页面输出,对比JSP的代码
4)在JSP中我们设定了缓存<%@ page buffer="16kb" %> 在HelloWorld_jsp.java中体现在pageContext = _jspxFactory.getPageContext(this, request, response, null, true, 16384, true);