如何修复 Servlet 的 HTTP 状态 500？

Question

我在 jsp 页面中有一个表单，我正在尝试连接到 servlet MyServlet。我使用的是 Tomcat 10.x。我不断收到以下错误：

HTTP Status 500 – Internal Server Error

Type Exception Report

Message Error instantiating servlet class [Servlets.MyServlet]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

jakarta.servlet.ServletException: Error instantiating servlet class [Servlets.MyServlet]
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:673)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:340)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:391)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744)
    org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
    org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
    org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.base/java.lang.Thread.run(Thread.java:1589)
Root Cause

java.lang.NoSuchMethodException: Servlets.MyServlet.<init>()
    java.base/java.lang.Class.getConstructor0(Class.java:3641)
    java.base/java.lang.Class.getConstructor(Class.java:2324)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:673)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:340)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:391)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744)
    org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
    org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
    org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.base/java.lang.Thread.run(Thread.java:1589)
Note The full stack trace of the root cause is available in the server logs.

Apache Tomcat/10.1.18

我知道它没有在给定位置找到 servlet（？），但我不确定如何解决该问题。任何帮助将不胜感激。

这是我的文件：

index.jsp

<html>
<body>
<h2>Hello World!</h2>
<form action="MyServlet" method="get">
    <input type="submit" value="Enter">
</form>
</body>
</html>

MyServlet.java

package Servlets;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;

@WebServlet("/MyServlet")
public class MyServlet extends HttpServlet {

    MyServlet(){
        super();
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.example</groupId>
  <artifactId>BaseProject</artifactId>
  <packaging>war</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>BaseProject Maven Webapp</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>jakarta.servlet</groupId>
      <artifactId>jakarta.servlet-api</artifactId>
      <version>6.0.0</version>
      <scope>provided</scope>
    </dependency>
  </dependencies>
  <build>
    <finalName>BaseProject</finalName>
  </build>
</project>

我尝试从 MyServlet.java 中删除 '@WebServlet("/MyServlet")' 并手动将 servlet 和 servlet 映射添加到 web.xml，如下所示：

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
  <display-name>Archetype Created Web Application</display-name>
  
  <servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>Servlets.MyServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/MyServlet</url-pattern>
  </servlet-mapping>

</web-app>

但是，这仍然导致了 500 错误。

Answer 1

将 servlet 的构造函数公开，以便 Tomcat 可以运行它。

public MyServlet() {
    super();
}

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