Servlet：原因：java.lang.IllegalArgumentException：無法轉換參數：null

Question

這是錯誤日志：

  java.lang.IllegalArgumentException: Can't convert argument: null
        at org.apache.tomcat.util.IntrospectionUtils.convert(IntrospectionUtils.java:889)
        at org.apache.tomcat.util.digester.CallMethodRule.end(CallMethodRule.java:476)
        at org.apache.tomcat.util.digester.Digester.endElement(Digester.java:1057)
        at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
        at org.apache.tomcat.util.digester.Digester.parse(Digester.java:1543)
        at org.apache.catalina.startup.ContextConfig.parseWebXml(ContextConfig.java:1694)
        at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1209)
        at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:882)
        at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:317)
        at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
        at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:89)
        at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5081)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1033)
        at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:774)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1033)
        at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:291)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.StandardService.startInternal(StandardService.java:443)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:727)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:620)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:303)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:431)

    Oct 04, 2015 12:53:40 AM org.apache.catalina.startup.ContextConfig parseWebXml
    SEVERE: Parse error in application web.xml file at jndi:/localhost/ConfigImplicitObj/WEB-INF/web.xml
    org.xml.sax.SAXParseException; systemId: jndi:/localhost/ConfigImplicitObj/WEB-INF/web.xml; lineNumber: 9; columnNumber: 18; Error at (9, 18) : Can't convert argument: null
        at org.apache.tomcat.util.digester.Digester.createSAXException(Digester.java:2687)
        at org.apache.tomcat.util.digester.Digester.createSAXException(Digester.java:2713)
        at org.apache.tomcat.util.digester.Digester.endElement(Digester.java:1060)
        at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
        at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
        at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
        at org.apache.tomcat.util.digester.Digester.parse(Digester.java:1543)
        at org.apache.catalina.startup.ContextConfig.parseWebXml(ContextConfig.java:1694)
        at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1209)
        at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:882)
        at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:317)
        at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
        at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:89)
        at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5081)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1033)
        at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:774)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1033)
        at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:291)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.StandardService.startInternal(StandardService.java:443)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:727)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:620)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:303)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:431)
    Caused by: java.lang.IllegalArgumentException: Can't convert argument: null
        at org.apache.tomcat.util.IntrospectionUtils.convert(IntrospectionUtils.java:889)
        at org.apache.tomcat.util.digester.CallMethodRule.end(CallMethodRule.java:476)
        at org.apache.tomcat.util.digester.Digester.endElement(Digester.java:1057)
        ... 36 more


Oct 04, 2015 12:53:40 AM org.apache.catalina.startup.ContextConfig parseWebXml
SEVERE: Occurred at line 9 column 18
Oct 04, 2015 12:53:40 AM org.apache.catalina.startup.ContextConfig configureStart
SEVERE: Marking this application unavailable due to previous error(s)
Oct 04, 2015 12:53:40 AM org.apache.catalina.core.StandardContext startInternal
SEVERE: Error getConfigured
Oct 04, 2015 12:53:40 AM org.apache.catalina.core.StandardContext startInternal
SEVERE: Context [/ConfigImplicitObj] startup failed due to previous errors
Oct 04, 2015 12:53:40 AM org.apache.catalina.loader.WebappClassLoader validateJarFile
INFO: validateJarFile(C:\Users\SouRAV\workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\DemoServlet\WEB-INF\lib\servlet-api.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class
Oct 04, 2015 12:53:40 AM org.apache.coyote.AbstractProtocolHandler start
INFO: Starting ProtocolHandler ["http-bio-8080"]
Oct 04, 2015 12:53:40 AM org.apache.coyote.AbstractProtocolHandler start
INFO: Starting ProtocolHandler ["ajp-bio-8009"]
Oct 04, 2015 12:53:40 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 1162 ms

web.xml中

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>Simple</servlet-name>
    <servlet-class>Simple</servlet-class>
  </servlet>
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>WelcomeServlet</servlet-name>
    <servlet-class>WelcomeServlet</servlet-class>
  </servlet>


  <servlet-mapping>
    <servlet-name>Simple</servlet-name>
    <url-pattern>/go</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>WelcomeServlet</servlet-name>
    <url-pattern>/welcome</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>login.html</welcome-file>
  </welcome-file-list>
</web-app>

我只是在研究servlet，而我正在項目的開始。 但是，它顯示一個錯誤。 我試過了，但是找不到錯誤。

Answer 1

我遇到了同樣的問題，解決方案是刪除web.xml中的內容。 原因是我更改了項目的名稱，因此myeclipse也更改了web.xml。 我也是初學者。 希望這對其他人有幫助。

Answer 2

<servlet-class>WelcomeServlet</servlet-class>.

在此還提供包名稱。 像這樣：

<servlet-class>packages.WelcomeServlet</servlet-class>