javaWEB总结(12):JSP页面的九个隐含对象
前言
jsp本质上是一个servlet，而在jsp中有九个不用声明就可以使用的对象，我们叫他隐含对象。本文基于上文所写，如有需要可查看上一篇文章javaWEB总结(11):JSP简介及原理.
打开上次使用的login_jsp.java文件
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.54
* Generated at: 2016-11-26 07:51:43 UTC
* Note: The last modified time of this file was set to
* the last modified time of the source file after
* generation to assist with modification tracking.
*/
package org.apache.jsp; import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*; public final class login_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent { private static final javax.servlet.jsp.JspFactory _jspxFactory =
javax.servlet.jsp.JspFactory.getDefaultFactory(); private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants; private javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.tomcat.InstanceManager _jsp_instancemanager; public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
return _jspx_dependants;
} public void _jspInit() {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
} public void _jspDestroy() {
} public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
throws java.io.IOException, javax.servlet.ServletException { final javax.servlet.jsp.PageContext pageContext;
javax.servlet.http.HttpSession session = null;
final javax.servlet.ServletContext application;
final javax.servlet.ServletConfig config;
javax.servlet.jsp.JspWriter out = null;
final java.lang.Object page = this;
javax.servlet.jsp.JspWriter _jspx_out = null;
javax.servlet.jsp.PageContext _jspx_page_context = null; try {
response.setContentType("text/html; charset=utf-8");
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\n");
out.write("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n");
out.write("<html>\r\n");
out.write("<head>\r\n");
out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=ISO-8859-1\">\r\n");
out.write("<title>my first jsp</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write("\t");
out.write("Hello World") ;
out.write("\r\n");
out.write("</body>\r\n");
out.write("</html>");
} catch (java.lang.Throwable t) {
if (!(t instanceof javax.servlet.jsp.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 throw new ServletException(t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
我们只看下面这段代码
public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
throws java.io.IOException, javax.servlet.ServletException { //参数中定义了两个隐含变量 request和 response //下面定义了六个隐含变量 pageContext,session,application,config,out,page final javax.servlet.jsp.PageContext pageContext;
javax.servlet.http.HttpSession session = null;
final javax.servlet.ServletContext application;
final javax.servlet.ServletConfig config;
javax.servlet.jsp.JspWriter out = null;
final java.lang.Object page = this; //………………………… //<%%>中的代码 //…………………………
}
可以看到共有八个隐含对象
(1) request: HttpServletRequest的一个对象；
(2) response: HttpServletResponse的一个对象；(页面中几乎不用)
(3) pageContext: 页面的上下文，是PageContext的一个对象，可以从中获取其他8个隐含对象，也可以从中获取当前页面的其他信息(可参考如下API中PageContext对象的方法)；(自定义标签时使用)
| Method Summary | |
|---|---|
abstract void |
forward(String relativeUrlPath)This method is used to re-direct, or "forward" the current ServletRequest and ServletResponse to another active component in the application. |
ErrorData |
getErrorData()Provides convenient access to error information. |
abstract Exception |
getException()The current value of the exception object (an Exception). |
abstract Object |
getPage()The current value of the page object (In a Servlet environment, this is an instance of javax.servlet.Servlet). |
abstract ServletRequest |
getRequest()The current value of the request object (a ServletRequest). |
abstract ServletResponse |
getResponse()The current value of the response object (a ServletResponse). |
abstract ServletConfig |
getServletConfig()The ServletConfig instance. |
abstract ServletContext |
getServletContext()The ServletContext instance. |
abstract HttpSession |
getSession()The current value of the session object (an HttpSession). |
abstract void |
handlePageException(Exception e)This method is intended to process an unhandled 'page' level exception by forwarding the exception to the specified error page for this JSP. |
abstract void |
handlePageException(Throwable t)This method is intended to process an unhandled 'page' level exception by forwarding the exception to the specified error page for this JSP. |
abstract void |
include(String relativeUrlPath)Causes the resource specified to be processed as part of the current ServletRequest and ServletResponse being processed by the calling Thread. |
abstract void |
include(String relativeUrlPath, Causes the resource specified to be processed as part of the current ServletRequest and ServletResponse being processed by the calling Thread. |
abstract void |
initialize(Servlet servlet,ServletRequest request,ServletResponse response,The initialize method is called to initialize an uninitialized PageContext so that it may be used by a JSP Implementation class to service an incoming request and response within it's _jspService() method. |
BodyContent |
pushBody()Return a new BodyContent object, save the current "out" JspWriter, and update the value of the "out" attribute in the page scope attribute namespace of the PageContext. |
abstract void |
release()This method shall "reset" the internal state of a PageContext, releasing all internal references, and preparing the PageContext for potential reuse by a later invocation of initialize(). |
(4) session: 代表游览器和服务器的一次会话，是HttpSession的一个对象；
(5) application: 代表当前web应用，是ServletContext的一个对象；
(6) config: 当前jsp对应的servlet的ServletConfig对象，需要访问当前jsp配置的初始化参数(如下)；(几乎不使用)
<servlet>
<servlet-name></servlet-name>
<jsp-file>
<!-- 与java文件不同在于此标签，填写jsp文件的具体路径
，如WebContent下的index.jsp
则填写/index.jsp
-->
</jsp-file>
</servlet> <servlet-mapping>
<servlet-name></servlet-name>
<url-pattern></url-pattern>
</servlet-mapping>
(7) out: JspWriter的一个对象，调用out.println()可以直接将字符串打印到游览器上；
(8) page: 指向当前JSP对应的Servlet对象的引用，但为Object类型，只能调用Object对象的方法;(几乎不使用)
第九个隐含对象
exception:在声明了page指令的isErrorPage="true"时才可以使用(如下);
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%> <!-- 此处声明了page的isErrorPage -->
<%@ page isErrorPage="true" %> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>my first jsp</title>
</head>
<body>
<% out.write("Hello World") ;
exception.getMessage(); %>
</body>
</html>
作用域范围
pageContext--> request--> session--> application 对属性的作用域从小到大。
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