Apache POI XSSFWorkbook ClassNotFoundException

Question

我尝试编写一个Servlet，必须在本地光盘上写入从客户端收到的xlsx文件（此部分有效）并使用Apache POI读取此xlsx文件（这是问题所在。）

我提到我尝试发送和读取一个简单的txt文件，它可以正常工作，从客户端来看它没有任何问题。问题来自服务器在调用XSSFWorkbook构造函数时。

我下载了Apache POI 3.17 zip包，我在项目中包含了所有jar。我尝试使用一个简单的Java Project来测试我的代码并且它可以工作，但是当我需要从Eclipse创建一个Dynamic Web Project（Java EE）时，我收到了这个错误：

Sep 28, 2017 11:47:50 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [app.java.request.ExcelDownload] in context with path [/solutions] threw exception [Servlet execution threw an exception] with root cause
java.lang.ClassNotFoundException: org.apache.poi.xssf.usermodel.XSSFWorkbook
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1333)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1167)
    at app.java.request.ExcelDownload.doPost(ExcelDownload.java:52)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1533)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1489)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

servlet：

package app.java.request;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.List;

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

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

@WebServlet("/data/download")
public class ExcelDownload extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ExcelDownload() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        PrintWriter out = response.getWriter();
//      System.out.println("GET");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);

        DiskFileItemFactory factory = new DiskFileItemFactory();
        ServletFileUpload upload = new ServletFileUpload(factory);

        try {
            List<FileItem> items = upload.parseRequest(request);
            Iterator<FileItem> iter = items.iterator();

            while (iter.hasNext()) {
                    FileItem item = iter.next();
                    if (!item.isFormField()) {
                        File file = new File("D:\\EclipseOxygenWks\\files\\" + item.getName());
                        item.write(file);

                        FileInputStream inp = new FileInputStream(file);
                        Workbook workbook = new XSSFWorkbook(inp);
                        //Workbook workbook = WorkbookFactory.create(inp);
                    }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

有没有人知道为什么我收到这个错误，因为我试图从昨天找到答案？