java servlet 文件上传 req.getPart null 返回空值

import javax.servlet.ServletException;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.Part;
import java.io.IOException;
import java.io.InputStream;
@WebServlet("/testServlet")
@MultipartConfig(maxFileSize = 5*1024*1024)
public class fileUpload extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        其他数据还可以跟之前一样正常读，当然，也可以用req.getPart读
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username);
        System.out.println(password);
//        读取文件
        Part file = req.getPart("file");
        System.out.println(file.getName());
//        输出文件内容
        InputStream inputStream = file.getInputStream();
        byte[] buf = new byte[1024];
        int n = inputStream.read(buf);
        String s = new String(buf,0,n,"utf-8");
        System.out.println(s);
    }
}

<servlet>
    <servlet-name>FileServlet</servlet-name>
    <servlet-class>com.momo.servlet.xml.controller.FileServlet</servlet-class>
    <multipart-config>
        <max-file-size>5242880</max-file-size>
    </multipart-config>
</servlet>
<servlet-mapping>
    <servlet-name>FileServlet</servlet-name>
    <url-pattern>/file/*</url-pattern>
</servlet-mapping>