写一个java上传文件的代码

以下是一个简单的Java上传文件的代码示例：

import java.io.File;
import java.io.IOException;
import org.apache.commons.io.FileUtils;
import org.apache.commons.io.FilenameUtils;
import org.apache.commons.io.IOUtils;
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 javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class FileUploadServlet extends HttpServlet {
 
    private static final long serialVersionUID = 1L;
    private static final String UPLOAD_DIRECTORY = "uploads";
    private static final int MEMORY_THRESHOLD   = 1024 * 1024 * 3;  
    private static final int MAX_FILE_SIZE      = 1024 * 1024 * 40; 
    private static final int MAX_REQUEST_SIZE   = 1024 * 1024 * 50; 
 
    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
 
        // 检测是否为多媒体上传
        if (!ServletFileUpload.isMultipartContent(request)) {
           
            return;
        }
 
        // 配置上传参数
        DiskFileItemFactory factory = new DiskFileItemFactory();
        factory.setSizeThreshold(MEMORY_THRESHOLD);
        factory.setRepository(new File(System.getProperty("java.io.tmpdir")));
 
        ServletFileUpload upload = new ServletFileUpload(factory);
        upload.setFileSizeMax(MAX_FILE_SIZE);
        upload.setSizeMax(MAX_REQUEST_SIZE);
 
        // 构造临时路径来存储上传的文件
        String uploadPath = getServletContext().getRealPath("")
                + File.separator + UPLOAD_DIRECTORY;
 
        // 如果目录不存在则创建
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) {
            uploadDir.mkdir();
        }
 
        try {
            // 解析请求的内容提取文件数据
            @SuppressWarnings("unchecked")
            List<FileItem> formItems = upload.parseRequest(request);
 
            if (formItems != null && formItems.size() > 0) {
                // 迭代表单数据
                for (FileItem item : formItems) {
                    // 处理不在表单中的字段
                    if (!item.isFormField()) {
                        String fileName = new File(item.getName()).getName();
                        String filePath = uploadPath + File.separator + fileName;
                        File storeFile = new File(filePath);
 
                        // 在控制台输出文件的上传路径
                        System.out.println(filePath);
 
                        // 保存文件到硬盘
                        item.write(storeFile);
                        request.setAttribute("message",
                            "文件上传成功!");
                    }
                }
            }
        } catch (Exception ex) {
            request.setAttribute("message",
                    "错误信息: " + ex.getMessage());
        }
        // 跳转到结果页面
        getServletContext().getRequestDispatcher("/result.jsp").forward(
                request, response);
    }
}

此代码段使用 Apache Commons FileUpload 库来处理文件上传。要使用此代码段，您需要添加以下依赖项：

<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.4</version>
</dependency>
 
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.6</version>
</dependency>

请注意，此代码段仅为演示目的。在生产环境中，您应该添加更多的错误检查和安全性检查，例如限制上传文件的大小和类型等。