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, boolean flush)` 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, String errorPageURL, boolean needsSession, int bufferSize, boolean autoFlush)` 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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javaWEB总结(12):JSP页面的九个隐含对象

前言