Intellij IDEA 中 Servlet should have a mapping

1.第一种原因是web.xml路径有问题,在File->Project Structure->Modules->web 选项设置正确路径

2.第二种原因是servlet的jar包导入错误,正确的Maven依赖为

    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>servlet-api</artifactId>
      <version>3.0-alpha-1</version>
  </dependency>