java实现文件上传下载和图片压缩代码示例

Java实现文件上传

1. 准备工作

在进行文件上传前，需要在服务器上建立一个存储上传文件的目录，并且需要在前端用HTML5的file标签来设置文件选择框。

2. 前端代码

前端代码使用HTML5的form表单和一个file选择框，具体如下：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>文件上传示例</title>
</head>
<body>
<form action="/file/uploadFile" method="post" enctype="multipart/form-data">
    <label for="file">选择要上传的文件：</label>
    <input type="file" name="file" id="file"><br>
    <input type="submit" name="submit" value="上传">
</form>
</body>
</html>

3. 后端代码

后端代码中使用Spring MVC框架来处理文件上传，controller代码实现如下：

@Controller
@RequestMapping("/file")
public class FileUploadController {

    @RequestMapping(value = "/uploadFile", method = RequestMethod.POST)
    public String uploadFileHandler(@RequestParam("file") MultipartFile file, ModelMap model) {
        if (!file.isEmpty()) {
            try {
                String fileName = file.getOriginalFilename();
                byte[] bytes = file.getBytes();
                // TODO 存储上传文件并且返回下载链接
                model.addAttribute("message", "上传文件成功：" + fileName);
                return "success";
            } catch (Exception e) {
                model.addAttribute("message", "上传文件失败：" + e.getMessage());
                return "error";
            }
        } else {
            model.addAttribute("message", "请选择要上传的文件！");
            return "error";
        }
    }

}

Java实现文件下载

1. 前端代码

文件下载的前端代码可以通过超链接或者一个表单的submit方式来实现，直接下载不需要前端ajax方式。

2. 后端代码

文件下载的后端代码中，需要设置Response的头文件，告诉浏览器要下载的文件的信息，具体实现如下：

@Controller
@RequestMapping("/file")
public class FileDownloadController {

    @RequestMapping(value = "/downloadFile/{file_name:.+}", method = RequestMethod.GET)
    public void downloadFile(@PathVariable("file_name") String fileName, HttpServletResponse response) {
        try {
            File file = new File("存储文件的路径" + fileName);
            response.setContentType("application/octet-stream");
            response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");
            InputStream inputStream = new FileInputStream(file);
            OutputStream outputStream = response.getOutputStream();
            byte[] buffer = new byte[4096];
            int length;
            while ((length = inputStream.read(buffer)) > 0) {
                outputStream.write(buffer, 0, length);
            }
            outputStream.flush();
            outputStream.close();
            inputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

Java实现图片压缩

1. 使用Java自带类库压缩图片

Java自带了图片处理类库，可以使用Java提供的类库来实现图片压缩，具体实现如下：

public static void compressImageByJava(File sourceFile, File targetFile, int targetWidth, int targetHeight, float quality) {
        FileInputStream inputStream;
        try {
            inputStream = new FileInputStream(sourceFile);

            Image img = ImageIO.read(inputStream);
            int width = img.getWidth(null);
            int height = img.getHeight(null);
            if (width > targetWidth) {
                height = height * targetWidth / width;
                width = targetWidth;
            }
            if (height > targetHeight) {
                width = width * targetHeight / height;
                height = targetHeight;
            }

            BufferedImage targetImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
            Graphics2D graphics = targetImage.createGraphics();
            graphics.drawImage(img, 0, 0, width, height, null);
            graphics.dispose();

            FileOutputStream out = new FileOutputStream(targetFile);
            JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
            JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(targetImage);
            param.setQuality(quality, true);
            encoder.encode(targetImage, param);
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

2. 使用第三方类库压缩图片

如果想要更灵活的图片压缩方式，可以使用第三方类库ImageIO和Thumbnails来实现，具体实现如下：

public static void compressImageByThirdParty(File sourceFile, File targetFile, int targetWidth, int targetHeight, float quality) {
        try {
            Thumbnails.of(sourceFile)
                    .size(targetWidth, targetHeight)
                    .outputQuality(quality) // 设置图片质量
                    .toFile(targetFile);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

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