写一个servlet，处理包含学号和姓名以及头像文件（图像）的表单，将头像存储到服务器upload目录下面以学号-姓名命名的文件夹下面，文件名为avatar，保持扩展名不变。

创建一个Servlet来处理这种表单提交通常涉及以下步骤：

首先，确保你已经导入了必要的Java Servlet API包，并且已经配置了一个Servlet容器（如Tomcat）。下面是一个简单的Servlet示例，它实现了`doPost`方法来处理HTTP POST请求：

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.Date;

public class AvatarUploaderServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 获取表单数据
        String studentId = request.getParameter("student_id");
        String name = request.getParameter("name");
        String avatarFile = request.getParameter("avatar");

        // 检查是否收到头像文件
        if (avatarFile == null || avatarFile.isEmpty()) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "No image file received.");
            return;
        }

        // 创建上传目录路径
        String uploadDir = "upload/" + studentId + "-" + name + "/";
        File dir = new File(uploadDir);
        if (!dir.exists() && !dir.mkdirs()) {
            response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, "Failed to create directory.");
            return;
        }

        // 获取图片流并保存到服务器
        InputStream inputStream = request.getInputStream();
        String fileName = studentId + "-" + name + "-avatar." + getFileNameExtension(avatarFile); // 保留原始扩展名
        File outputFile = new File(dir, fileName);

        try (BufferedOutputStream outputStream = new BufferedOutputStream(new FileOutputStream(outputFile))) {
            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }
            outputStream.flush();
        } finally {
            inputStream.close(); // 关闭输入流
        }

        response.setContentType("text/html");
        response.getWriter().println("Image uploaded successfully. Filename: " + fileName);
    }

    private String getFileNameExtension(String filenameWithoutExt) {
        int lastDot = filenameWithoutExt.lastIndexOf('.');
        return lastDot == -1 ? "" : filenameWithoutExt.substring(lastDot);
    }
}