Question

我正在学习本教程 http://netbeans.org/kb/docs/web/quickstart-webapps.html

并且如上所述我创建了两个jsp页面。我的index.jsp页面看起来像这样

%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
    </head>
    <body>

        Name<form name="Name Input Form" action="response.jsp">
            <input type="text" name="name" value="" />
            <input type="submit" value="OK" />
        </form>

    </body>
</html>

我的response.jsp看起来像这样

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
    </head>
    <body>

        <jsp:useBean id="useBean" scope="session" class="abc.NameHandler" />

        <jsp:setProperty name="myBean" property="name" /><h1>Hello</h1>
        <jsp:getProperty name="myBean" property="name" />

    </body>
</html>

我创建了一个名为abc的包，其中我定义了一个名为NameHandler的类。

package abc;

public class NameHandler 
{

    private String name;

    public NameHandler() 
    {
        name =null;

    }

    /**
     * @return the name
     */
    public String getName() {
        return name;
    }

    /**
     * @param name the name to set
     */
    public void setName(String name) {
        this.name = name;
    }
}

当我运行项目时，index.jsp正确显示并命令输入名称，并且它有一个提交按钮。现在，当我按下提交按钮时，我收到以下错误。

HTTP Status 500 -

type Exception report

message

descriptionThe server encountered an internal error () that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: java.lang.NullPointerException

root cause

java.lang.NullPointerException

note The full stack traces of the exception and its root causes are available in the GlassFish Server Open Source Edition 3.1 logs.
GlassFish Server Open Source Edition 3.1

我不知道为什么会收到此错误。有人可以解释一下吗？

[UPDATE]

这个错误意味着什么？

WARNING: StandardWrapperValve[jsp]: PWC1406: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
    at org.apache.jasper.runtime.JspRuntimeLibrary.internalIntrospecthelper(JspRuntimeLibrary.java:368)
    at org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(JspRuntimeLibrary.java:354)
    at org.apache.jsp.response_jsp._jspService(response_jsp.java from :68)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:403)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:492)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:378)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1534)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
    at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:326)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:227)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:170)
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:822)
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:719)
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1013)
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
    at java.lang.Thread.run(Thread.java:619)

Answer 1

jsp:useBean和jsp:setProperty / jsp:getProperty中的bean的ID和名称应匹配：

   <jsp:useBean id="myBean" scope="session" class="abc.NameHandler" />

   <jsp:setProperty name="myBean" property="name" /><h1>Hello</h1>
   <jsp:getProperty name="myBean" property="name" />

为什么运行java Web应用程序时会出现以下错误？

1 个答案: