java - tag creates a bean? 如何

Question

嗯，我正在准备网络开发者考试，在阅读时我对“can create a bean”这句话感到困惑

我的理解是，如果我们没有假设我的源文件中存在任何类，它将创建一个类

所以我在 jsp 文件中所做的如下 -

  <jsp:useBean id="User" class=com.User>           ///  I don't have any class by the name User
   <jsp:setProperty name="User" property="name" value="John"/>

  </jsp:useBean>

但是它给了我以下错误--

SEVERE: Servlet.service() for servlet jsp threw exception
org.apache.jasper.JasperException: Cannot find any information on property 'name' in a bean of type 'com.User'
    at org.apache.jasper.runtime.JspRuntimeLibrary.internalIntrospecthelper(JspRuntimeLibrary.java:365)
    at org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(JspRuntimeLibrary.java:308)
    at org.apache.jsp.page_jsp._jspService(page_jsp.java:90)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at com.vishal.FirstServlet.doPost(FirstServlet.java:38)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Unknown Source)
Jul 26, 2013 10:00:39 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet FirstServlet threw exception
org.apache.jasper.JasperException: Cannot find any information on property 'name' in a bean of type 'com.User'
    at org.apache.jasper.runtime.JspRuntimeLibrary.internalIntrospecthelper(JspRuntimeLibrary.java:365)
    at org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(JspRuntimeLibrary.java:308)
    at org.apache.jsp.page_jsp._jspService(page_jsp.java:90)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at com.vishal.FirstServlet.doPost(FirstServlet.java:38)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Unknown Source)

根据错误，我应该有一个名为 User 的类，然后应该有一个名为“name”的字段所以我的看法是错误的

但现在我更渴望理解这句话的真正含义

可以创建bean

谢谢

Answer 1

Jsp:Use bean 标签用于初始化类，而 bean 只不过是属性，所以你应该需要在这里定义，因为它应该知道从哪个类中获取所需的内容那么为什么我们会称之为使用 bean> 请参阅下面的内容可能会对您有所帮助

<jsp:useBean id="p1" class="com.envious.Person"
scope="session"/>
 First Name:
<jsp:setProperty property="firstname" name="p1" value="abc"/><br>
<jsp:getProperty name="p1" property="firstname"/><br>

Last Name:
<jsp:setProperty property="lastname" name="p1" value="xyz"/>
<jsp:getProperty property="lastname" name="p1"/>