使用java写出图片的上传下载

图片上传：

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

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;

@WebServlet("/uploadImage")
@MultipartConfig
public class UploadImageServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private static final String UPLOAD_DIR = "uploads";

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String applicationPath = request.getServletContext().getRealPath("");
        String uploadPath = applicationPath + File.separator + UPLOAD_DIR;

        File fileUploadDirectory = new File(uploadPath);
        if (!fileUploadDirectory.exists()) {
            fileUploadDirectory.mkdirs();
        }

        final Part filePart = request.getPart("image");
        final String fileName = getFileName(filePart);

        Path path = Paths.get(uploadPath + File.separator + fileName);
        try (InputStream inputStream = filePart.getInputStream()) {
            Files.copy(inputStream, path);
        }

        response.getWriter().print("Image uploaded successfully!");
    }

    private String getFileName(final Part part) {
        final String partHeader = part.getHeader("content-disposition");
        for (String content : partHeader.split(";")) {
            if (content.trim().startsWith("filename")) {
                return content.substring(content.indexOf("=") + 2, content.length() - 1);
            }
        }
        return null;
    }
}

图片下载：

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/downloadImage")
public class DownloadImageServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private static final String UPLOAD_DIR = "uploads";

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String fileName = request.getParameter("name");
        String applicationPath = request.getServletContext().getRealPath("");
        String downloadPath = applicationPath + File.separator + UPLOAD_DIR + File.separator + fileName;

        File file = new File(downloadPath);
        FileInputStream inputStream = new FileInputStream(file);

        response.setContentType("image/jpeg");
        response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");

        OutputStream outputStream = response.getOutputStream();
        byte[] buffer = new byte[1024];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }

        inputStream.close();
        outputStream.close();
    }
}

注意事项：

1. 这里使用的是 Servlet 3.0 的注解，所以需要在 web.xml 中添加 `<web-app version="3.0">`。

2. form 表单中的 `enctype` 必须是 `multipart/form-data`。

3. `getFileName` 方法可以获取上传文件的文件名。

4. `response.setContentType` 要设置成图片的 MIME 类型，如 `image/jpeg`。

5. `response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");` 中的 `filename` 参数可以指定下载时的文件名。