摘要: JSP的九大内置对象： page,application,session,request,response,out,exception,config,pageContext. JSP的四个作用域：application,session,request,page.
//附上jsp转换成servlet后的java代码
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('
');
out.write('
');
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
out.write("
");
out.write("
");
out.write("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
");
out.write("<html>
");
out.write(" <head>
");
out.write(" <base href="");
out.print(basePath);
out.write("">
");
out.write("
");
out.write(" <title>My JSP 'index.jsp' starting page</title>
");
out.write(" <meta http-equiv="pragma" content="no-cache">
");
out.write(" <meta http-equiv="cache-control" content="no-cache">
");
out.write(" <meta http-equiv="expires" content="0">
");
out.write(" <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
");
out.write(" <meta http-equiv="description" content="This is my page">
");
out.write(" </head>
");
out.write("
");
out.write(" <body>
");
out.write(" ");
out.write("
");
out.write("
");
out.write(" ");
out.write("
");
out.write("
");
out.write(" ");
out.print(++count );
out.write("
");
out.write(" ");
out.print( getCount() );
out.write("
");
out.write(" <form action="test.do" method="post">
");
out.write(" <select name ="color">
");
out.write(" <option>White</option>
");
out.write(" <option>Red</option>
");
out.write(" <option>Yellow</option>
");
out.write(" <option>Blue</option>
");
out.write(" </select>
");
out.write("
");
out.write(" <input type="checkbox" name="hobby" value="football">football<br>
");
out.write(" <input type="checkbox" name="hobby" value="football">basketball<br>
");
out.write(" <input type="checkbox" name="hobby" value="football">baseball<br>
");
out.write("
");
out.write(" <input type="submit">
");
out.write(" </form>
");
out.write(" </body>
");
out.write("</html>
");
} 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);
}
}
} 
application作用域就是服务器启动到关闭的整段时间，在这个作用域内设置的信息可以被所有应用程序使用。application作用域上的信息传递是通过ServletContext实现的.
session作用域比较容易理解，同一浏览器对服务器进行多次访问，在这多次访问之间传递信息，就是session作用域的体现,
一个HTTP请求的处理可能需要多个Servlet合作，而这几个Servlet之间可以通过某种方式传递信息，但这个信息在请求结束后就无效了。Servlet之间的信息共享是通过HttpServletRequest接口的两个方法来实现的。
page对象的作用范围仅限于用户请求的当前页面，对于page对象的引用将在响应返回给客户端之后被释放，或者在请求被转发到其他地方后被释放。对page对象的引用通常存储在pageContext对象中。