自定义异常【将工具类的错误抛给调用者servlet，将原本的控制界面跳转工作交还给servlet】

package com.shan.upload;
import java.io.File;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.UUID;
import javax.servlet.ServletContext;
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.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.apache.commons.io.FilenameUtils;
public class FileUtil {
    // 允许接收的图片类型
    private static final String ALLOWED_IMAGE_TYPE = "png;gif;jpg;jpeg;jiff";
    public static void upload(HttpServletRequest req) throws IOException {
        req.setCharacterEncoding("utf-8");
        // 解析检查请求
        boolean isMultipart = ServletFileUpload.isMultipartContent(req);
        if (!isMultipart) {
            return;
        }
        // 创建FileItemFactory对象---目的是为了创建FileItem对象【FileItem对象是form表单的表单控件的封装】
        DiskFileItemFactory factory = new DiskFileItemFactory();
        // 创建文件上传
        ServletFileUpload upload = new ServletFileUpload(factory);
        // 解析请求
        List<FileItem> items;
        try {
            items = upload.parseRequest(req);
            for (FileItem item : items) {
                String filedName = item.getFieldName();
                if (item.isFormField()) {
                    // 普通表单控件
                    String value = item.getString();
                } else {
                    // ---------------判断文件格式是否允许
                    // 判断方式一：采用拓展名进行判断
                    String[] allowedImageTtype = ALLOWED_IMAGE_TYPE.split(";");
                    String ext = FilenameUtils.getExtension(item.getName());
                    // 将数组转化为list集合，这样就可以直接利用人家的contain方法，不用自己写遍历　　　　　　　　　　　　// ---------------文件格式错误，抛出自定义的异常类型LoginException----------
                    if (!Arrays.asList(allowedImageTtype).contains(ext)) {
                        throw new LoginException("亲，请上传正确的图片格式!");
                    }
                    // -----------------------------------
                    // 避免文件名相同的覆盖【使用唯一的UUID】
                    String fileName = UUID.randomUUID().toString() + "." + FilenameUtils.getExtension(item.getName());
                    // 保存到当前应用下["/" 表示是当前应用-webapp]
                    // 获取servletContext 当前类是servlet类的话，即可通过super获取，否则就通过请求获取
                    String dir = req.getServletContext().getRealPath("/upload");
                    item.write(new File(dir, fileName));// 把二进制数据写到哪个文件中
                }
            }
        } catch (LoginException e) {   //因为除了自定义的错误外还有其他错误的存在，一般要使用最大的错误类型Exception 捕获　　　//需要在最大错误类型Exception之前捕获下咱自定义的错误 LoginException，然后将它往外抛，抛给调用者[UploadServlet]
         throw e;    
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

package com.shan.upload;
public class LoginException extends RuntimeException{
    private static final long serialVersionUID = 1L;
    /**
     * @param message 异常的信息
     * @param cause   异常的根本原因
     */
    public LogicException(String message, Throwable cause) {
        super(message, cause);
    }
    /**
     * @param message 错误信息
     */
    public LogicException(String message) {
        super(message);
    }
}

package com.shan.upload;
import java.io.File;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.List;
import java.util.Scanner;
import java.util.UUID;
import javax.servlet.ServletContext;
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.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.apache.commons.io.FilenameUtils;
@WebServlet("/upload")
public class UploadServlet extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException {
        req.setCharacterEncoding("utf-8");
        try {
            FileUtil.upload(req);
        }catch (LoginException e) {    //捕获自定义的异常，获取错误信息，然后控制界面跳转
            String errorMsg = e.getMessage();
            req.setAttribute("errorMsg", errorMsg);
            req.getRequestDispatcher("/input.jsp").forward(req, resp);
        }
    }
}

.......} catch(FileSizeLimitExceededException e) {    //捕获A错误类型，为了使用B异常处理，在捕获A错误类型后，抛成B异常
    throw new LoginException("亲，单个文件大小不能超过1kb", e);
}catch (LogicException e) {
    throw e;    //将错误进行往外抛，抛给调用者【UploadServlet】 } catch (Exception e) {
    e.printStackTrace();
 }