HttpServlet类中的init（）函数导致问题[重复]答案

【问题标题】：init() function in HttpServlet class causes problems [duplicate]HttpServlet类中的init（）函数导致问题[重复]
【发布时间】：2020-10-10 13:25:07
【问题描述】：

我正在尝试创建一个 HttpServlet 实现。问题是，init() 方法每次都会触发并抛出异常。

我只想创建自制的servlts而不使用Spring等。顺便说一句，我使用的是tomcat（现在是本地安装的），但是我对servlts完全陌生，所以非常欢迎详细的回答。提前致谢！

    10-Oct-2020 15:07:48.667 SEVERE [http-nio-8080-exec-7] org.apache.catalina.core.ApplicationContext.log Servlet.init() for servlet [SubmissionsController] threw exception
    java.lang.NoClassDefFoundError: javax/persistence/Query
        at Controllers.SubmissionsController.init(SubmissionsController.java:19)
        at javax.servlet.GenericServlet.init(GenericServlet.java:158)
        at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1134)
        at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1089)
        at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:761)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:135)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
        at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
        at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374)
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
        at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
        at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:748)

提交控制器

//@WebServlet(urlPatterns = "/submissions")
public class SubmissionsController extends HttpServlet {
    private SQLService sqlService;

    @Override
    public void init()throws ServletException{
        sqlService = SQLService.getInstance();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        out.println("<h1> hello schmockes </h1>");
    }

   /* @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //SQLService sqlService = SQLService.getInstance();
        System.out.println("Inside doGet");

   //     int id = Integer.parseInt(req.getParameter("id"));

 //       System.out.println(id);
    }
    */
    @Override
    public void destroy(){}
}

和 web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <servlet>
        <servlet-name>SubmissionsController</servlet-name>
        <servlet-class>Controllers.SubmissionsController</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>SubmissionsController</servlet-name>
        <url-pattern>/welcome</url-pattern>
    </servlet-mapping>
</web-app>

【问题讨论】：

SQLService.getInstance(); 长什么样子？

标签： java maven tomcat servlets java-8

【解决方案1】：

将此库添加到您的项目中：

   <dependency>
       <groupId>javax.persistence</groupId>
       <artifactId>persistence-api</artifactId>
       <version>1.0.2</version>
       <scope>provided</scope>
    </dependency>

或：

   <dependency>
       <groupId>javax</groupId>
       <artifactId>javaee-api</artifactId>
       <version>6.0</version>
       <scope>provided</scope>
   </dependency>

【讨论】：

OP 正在使用 Tomcat。 Tomcat 根本不提供 JPA 或 Java EE。 Tomcat 是一个准系统 JSP/Servlet 容器。