访问java web应用程序时出错

Question

我今天刚开始为Web开发人员学习Java。我正在学习第二章中的一篇教程，我已经逐行检查过了，一切都是一样的。我对这件事还不太了解，无法解决这个问题。以下是我的错误：

exception

javax.servlet.ServletException: Error instantiating servlet class apress.helloworld.HelloWorld
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655)
    org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1566)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1523)
    java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Unknown Source)
root cause

java.lang.ClassNotFoundException: apress.helloworld.HelloWorld
    org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1324)
    org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1177)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655)
    org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1566)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1523)
    java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Unknown Source)

这是我的代码：

package apress.helloworld;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HellowWorld extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        try {
            response.setContentType("text/html");
            PrintWriter printWriter = response.getWriter();
            printWriter.println("<h2>");
            printWriter.println("hello World!");
            printWriter.println("</h2>");

        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
    }
}

和web.xml (不确定是否相关)

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">

    <display-name>helloworld</display-name>
<servlet>
    <servlet-name>HelloWorld</servlet-name>
    <servlet-class>apress.helloworld.HelloWorld</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
</web-app>

有人能帮我学习java吗？？

Answer 1

您在类名中输入了错误(额外的'w')。修复，重新编译和世界将再次美丽。记住要更改文件名以匹配类名。;)

public class HellowWorld extends HttpServlet 

Answer 2

在您得到的错误消息中，第一部分的底线是“未知源”，这可能是因为它找不到您的类。这是因为它在包"apress.helloworld“下，但将类名拼写为"HellowWorld”。修复类名，一切都应该启动并运行。