JSP原理初探
什么是JSP
JSP的英文名称是Java Server Page，它是sun公司定义的一种用于开发动态网页的技术。
下面让我们来带着几个问题来学习JSP：
- Web服务器是如何实现调用并执行一个JSP页面的？
- JSP页面中的Html排版标签是如何被发送到客户端的？
- 服务器是如何执行JSP中的Java代码的？
- Web服务器在调用JSP时会给JSP什么Java对象？
问题一解答：
当客户端向Web服务器发送JSP请求的时候，JSP引擎就会解析JSP，然后将JSP转成Servlet，再编译成.class文件交由服务器来执行，最后Web服务器把JSP页面发给客户端（浏览器），浏览器就可以显示请求的JSP页面了。在第一次执行请求SP页面的时候会比较慢，因为Web服务器还没有JSP相应的Servlet以及.class文件。所以，下一次请求相同的JSP页面的时候就会比较快，直接执行Web服务器中缓存有的.class文件。
那么，JSP相应的java和.class文件保存在哪里呢？以Tomcat为例，保存在\apache-tomcat-6.0.44\apache-tomcat-6.0.44\work\Catalina\工程名\org\apche目录下。
那么，JSP里面的内容是怎么样被解析的呢？下面先看一个JSP页面和解析后的Java文件：
JSP 源文件
<%@ page language="java" import="java.util.*" pageEncoding="GB18030"%>
<% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'index.jsp' starting page</title>
<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">
<!-- <link rel="stylesheet" type="text/css" href="styles.css"> -->
</head>
<body>
This is my JSP page. <br>
<% Date date=new Date(); %>
<%=date %>
</body>
</html>
解析后的JSP页面
package org.apache.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.util.*;
public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
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;charset=GB18030");
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("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
out.write("<html>\r\n");
out.write(" <head>\r\n");
out.write(" <base href=\"");
out.print(basePath);
out.write("\">\r\n");
out.write(" \r\n");
out.write(" <title>My JSP 'index.jsp' starting page</title>\r\n");
out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n");
out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n");
out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n");
out.write("\t<!--\r\n");
out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n");
out.write("\t-->\r\n");
out.write(" </head>\r\n");
out.write(" \r\n");
out.write(" <body>\r\n");
out.write(" This is my JSP page. <br>\r\n");
out.write(" \r\n");
out.write(" ");
Date date=new Date();
out.write("\r\n");
out.write(" ");
out.print(date );
out.write("\r\n");
out.write(" \r\n");
out.write(" </body>\r\n");
out.write("</html>\r\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);
else log(t.getMessage(), t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
可以知道，解析后的JSP的Java类继承自：org.apache.jasper.runtime.HttpJspBase，并且实现了org.apache.jasper.runtime.JspSourceDependent接口，翻看Tomcat源码看看这个类和接口的实现：
HttpJspBase类
package org.apache.jasper.runtime;
import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.HttpJspPage;
import javax.servlet.jsp.JspFactory;
import org.apache.jasper.compiler.Localizer;
/** * This is the super class of all JSP-generated servlets. * * @author Anil K. Vijendran */
public abstract class HttpJspBase extends HttpServlet implements HttpJspPage {
protected HttpJspBase() {
}
public final void init(ServletConfig config)
throws ServletException
{
super.init(config);
jspInit();
_jspInit();
}
public String getServletInfo() {
return Localizer.getMessage("jsp.engine.info");
}
public final void destroy() {
jspDestroy();
_jspDestroy();
}
/** * Entry point into service. */
public final void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
_jspService(request, response);
}
public void jspInit() {
}
public void _jspInit() {
}
public void jspDestroy() {
}
protected void _jspDestroy() {
}
public abstract void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException;
}
package org.apache.jasper.runtime;
/** * Interface for tracking the source files dependencies, for the purpose * of compiling out of date pages. This is used for * 1) files that are included by page directives * 2) files that are included by include-prelude and include-coda in jsp:config * 3) files that are tag files and referenced * 4) TLDs referenced */
JspSourceDependent 接口//返回当前页面包含的所有的资源文件名列表
package org.apache.jasper.runtime;
/** * Interface for tracking the source files dependencies, for the purpose * of compiling out of date pages. This is used for * 1) files that are included by page directives * 2) files that are included by include-prelude and include-coda in jsp:config * 3) files that are tag files and referenced * 4) TLDs referenced */
public interface JspSourceDependent {
/** * Returns a list of files names that the current page has a source * dependency on. */
// FIXME: Type used is Object due to very weird behavior
// with Eclipse JDT 3.1 in Java 5 mode
public Object getDependants();
}
看完这个类和接口，返回来分析一下JSP页面如何被解析的。
- JSP的脚本片段直接翻译成普通Java代码
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
解析成--> String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
<%
Date date=new Date();
%>
-->解析成Date date=new Date();- JSP脚本表达是直接使用out输出
<%=date %>--> out.print(date );- Html排本标签使用out输出
out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
out.write("<html>\r\n");
out.write(" <head>\r\n");
out.write(" <base href=\"");
out.print(basePath);
out.write("\">\r\n");
out.write(" \r\n");
out.write(" <title>My JSP 'index.jsp' starting page</title>\r\n");
out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n");
out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n");
out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n");
out.write("\t<!--\r\n");
out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n");
out.write("\t-->\r\n");
out.write(" </head>\r\n");
out.write(" \r\n");
out.write(" <body>\r\n");
out.write(" This is my JSP page. <br>\r\n");
out.write(" \r\n");
out.write(" ");
Date date=new Date();
out.write("\r\n");
out.write(" ");
out.print(date );
out.write("\r\n");
out.write(" \r\n");
out.write(" </body>\r\n");
out.write("</html>\r\n");- Web服务器传入八大java对象
1 PageContext pageContext ：代表页面上下文，该对象用于范文JSP之间的共享数据；
2 PageContext _jspx_page_context：代表当前页面上下文
3 HttpSession session
4 ServletContext application：Web应用程序上下文，所有用户公用一个对象；
5 ServletConfig config ：代表Servlet初始化信息
6JspWriter out
7 JspWriter _jspx_out
8 Object page
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;charset=GB18030");
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;
……………………
……………………
……………………
} 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);
else log(t.getMessage(), t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}到这里，我们就对JSP有了初步的了解。