关于JSP里JavaBean的用法分析

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关于JSP里JavaBean的用法分析:我们可以使用Scriptlet的方式: <%! %> / <% %>

还可以用<jsp:useBean>的方式//bean.HelloBean.java:

<%!bean.HelloBean memBean = new bean.HelloBean();%>
    <%memBean.setStr("memBean!!"); %>
    <%=memBean.getStr()%>
 
    <%bean.HelloBean partBean = new bean.HelloBean();%>
    <%partBean.setStr("partBean!!"); %>
    <%=partBean.getStr()%>
    
    <jsp:useBean id="pageBean" scope="page" class="bean.HelloBean">
    </jsp:useBean>
    
    <%pageBean.setStr("pageBean!!"); %>
    <%=pageBean.getStr()%>
    
 	<jsp:useBean id="requestBean" scope="request" class="bean.HelloBean">
    </jsp:useBean>
    
    <%requestBean.setStr("pageBean!!"); %>
    <%=pageBean.getStr()%>
    
    <jsp:useBean id="sessionBean" scope="session" class="bean.HelloBean">
    </jsp:useBean>
    
    <%sessionBean.setStr("pageBean!!"); %>
    <%=sessionBean.getStr()%>
    
    <jsp:useBean id="appBean" scope="application" class="bean.HelloBean">
    </jsp:useBean>
    
    <%sessionBean.setStr("appBean!!"); %>
    <%=sessionBean.getStr()%>
    
  </body>
</html>
</pre>

jsp:useBean各项参数含义：id:对象实例名称scope:Bean作用的范围，

默认为page，对整个jsp页面有效

Scope各项参数的意义：page:仅涵盖使用JavaBean的页面（PageBean.jsp/CounterBean.java）

request:有效范围仅限于使用JavaBean的请求（RequestBean.jsp/RequestBean2.jsp/CounterBean.java）

session:有效范围在用户整个连接过程中（整个会话阶段均有效）（SessionBean.jsp/Session2.jsp/CounterBean.java）

application:有效范围涵盖整个应用程序。也就是对整个web应用均有效（Application.jsp/Application2.jsp/CounterBean.java）

//然后我们去到Tomcat的work目录下查看jsp被编译生成的java文件:

<pre class="java" name="code">/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.22
 * Generated at: 2011-11-02 01:36:52 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.*;
import bean.HelloBean;

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

bean.HelloBean memBean = new bean.HelloBean();  
//<%!bean.HelloBean memBean = new bean.HelloBean();%> <%! %> 使这个bean变量这个类的成员变量

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");
      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("<html>\r\n");
      out.write("  <body>\r\n");
      
      out.write("\r\n");
      out.write("    ");
      out.write("\r\n");
      out.write("    ");
memBean.setStr("memBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(memBean.getStr());
      out.write("\r\n");
      out.write(" \r\n");
      out.write("    ");
      
bean.HelloBean partBean = new bean.HelloBean();  //<%bean.HelloBean partBean = new bean.HelloBean();%> <% %>使bean成为了类的成员函数的局部变量
      out.write("\r\n");
      out.write("    ");
partBean.setStr("partBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(partBean.getStr());
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
      
      bean.HelloBean pageBean = null;  //<jsp:useBean id="pageBean" scope="page" class="bean.HelloBean">
      pageBean = (bean.HelloBean) _jspx_page_context.getAttribute("pageBean", javax.servlet.jsp.PageContext.PAGE_SCOPE);
      if (pageBean == null){
        pageBean = new bean.HelloBean();
        _jspx_page_context.setAttribute("pageBean", pageBean, javax.servlet.jsp.PageContext.PAGE_SCOPE);
        out.write("\r\n");
        out.write("    ");
      }
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
pageBean.setStr("pageBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(pageBean.getStr());
      out.write("\r\n");
      out.write("    \r\n");
      out.write(" \t");
      
      
      bean.HelloBean requestBean = null; //<jsp:useBean id="requestBean" scope="request" class="bean.HelloBean">
      requestBean = (bean.HelloBean) _jspx_page_context.getAttribute("requestBean", javax.servlet.jsp.PageContext.REQUEST_SCOPE);
      if (requestBean == null){
        requestBean = new bean.HelloBean();
        _jspx_page_context.setAttribute("requestBean", requestBean, javax.servlet.jsp.PageContext.REQUEST_SCOPE);
        out.write("\r\n");
        out.write("    ");
      }
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
requestBean.setStr("pageBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(pageBean.getStr());
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
      
      
      bean.HelloBean sessionBean = null;// <jsp:useBean id="sessionBean" scope="session" class="bean.HelloBean">
      synchronized (session) {
        sessionBean = (bean.HelloBean) _jspx_page_context.getAttribute("sessionBean", javax.servlet.jsp.PageContext.SESSION_SCOPE);
        if (sessionBean == null){
          sessionBean = new bean.HelloBean();
          _jspx_page_context.setAttribute("sessionBean", sessionBean, javax.servlet.jsp.PageContext.SESSION_SCOPE);
          out.write("\r\n");
          out.write("    ");
        }
      }
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
sessionBean.setStr("pageBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(sessionBean.getStr());
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
      
      
      bean.HelloBean appBean = null;  // <jsp:useBean id="appBean" scope="application" class="bean.HelloBean">
      synchronized (application) {
        appBean = (bean.HelloBean) _jspx_page_context.getAttribute("appBean", javax.servlet.jsp.PageContext.APPLICATION_SCOPE);
        if (appBean == null){
          appBean = new bean.HelloBean();
          _jspx_page_context.setAttribute("appBean", appBean, javax.servlet.jsp.PageContext.APPLICATION_SCOPE);
          out.write("\r\n");
          out.write("    ");
        }
      }
      out.write("\r\n");
      out.write("    \r\n");
      out.write("    ");
sessionBean.setStr("appBean!!"); 
      out.write("\r\n");
      out.write("    ");
      out.print(sessionBean.getStr());
      out.write("\r\n");
      out.write("    \r\n");
      
      
      out.write("  </body>\r\n");
      out.write("</html>\r\n");
    } 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);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}</pre>

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