內置對象：
1.什麼時候內置對象
可以不加聲明和創建就可以在JSP頁面腳本（Java程序片和Java表達式）中使用的成員變量。
2.九大內置對象
out（JspWriter）：等同與response.getWriter()，用來向客戶端發送文本數據；
config（ServletConfig）：對應“真身”中的ServletConfig；page（當前JSP的真身類型）：當前JSP頁面的“this”，即當前對象；
pageContext（PageContext）：頁面上下文對象；
exception（Throwable）：只有在錯誤頁面中可以使用這個對象；
request（HttpServletRequest）：即HttpServletRequest類的對象；
response（HttpServletResponse）：即HttpServletResponse類的對象；
application（ServletContext）：即ServletContext類的對象；
session（HttpSession）：即HttpSession類的對象，不是每個JSP頁面中都可以使用，如果在某個JSP頁面中設置<%@page session=”false”%>，說明這個頁面不能使用session。
附上jsp轉爲Servlet之後的java代碼，有助於理解Jsp的內置對象的工作原理
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 {
int count = 0;
int getCount(){
int num = 100;
return num = 100*count;
}
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=ISO-8859-1");
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(" </head>\r\n");
out.write(" \r\n");
out.write(" <body>\r\n");
out.write(" ");
out.write("\r\n");
out.write(" \r\n");
out.write(" ");
out.write("\r\n");
out.write(" \r\n");
out.write(" ");
out.print(++count );
out.write("\r\n");
out.write(" ");
out.print( getCount() );
out.write("\r\n");
out.write(" <form action=\"test.do\" method=\"post\">\r\n");
out.write(" <select name =\"color\">\r\n");
out.write(" <option>White</option>\r\n");
out.write(" <option>Red</option>\r\n");
out.write(" <option>Yellow</option>\r\n");
out.write(" <option>Blue</option>\r\n");
out.write(" </select>\r\n");
out.write(" \r\n");
out.write(" <input type=\"checkbox\" name=\"hobby\" value=\"football\">football<br>\r\n");
out.write(" <input type=\"checkbox\" name=\"hobby\" value=\"football\">basketball<br>\r\n");
out.write(" <input type=\"checkbox\" name=\"hobby\" value=\"football\">baseball<br>\r\n");
out.write(" \r\n");
out.write(" <input type=\"submit\">\r\n");
out.write(" </form>\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);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}