本文介绍了JSP中的九大内置对象及其四个作用域，包括page、application、session、request等，并通过示例展示了这些对象如何在实际应用中发挥作用。
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.
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//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(
'\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);
}
}
}
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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。
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