Java文件上传下载是Web开发中常见的功能，实现代码一般基于Servlet或Spring MVC等框架。下面是实现Java文件上传下载功能的完整攻略，包含示例代码。
1. 文件上传
Java文件上传一般需要使用表单提交，数据由客户端通过HTTP POST请求发送到服务器。客户端可以使用HTML表单或JavaScript+FormData等方式实现。服务端接收到数据后，可以使用Java I/O流等方式将文件保存到指定目录。
1.1 HTML表单方式上传文件
HTML表单中通过元素实现文件选择功能，提交表单时，浏览器会将文件数据包装成multipart/form-data编码格式，发送到服务器。服务端接收到表单提交的数据后，通过request.getParameter()方法无法获取文件数据，需要通过request.getPart()方法获取文件Part对象，进一步操作。
示例代码：
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>文件上传HTML表单</title>
</head>
<body>
<form action="fileUploadServlet" method="post" enctype="multipart/form-data">
<label for="file">选择文件：</label>
<input type="file" name="file" id="file" /><br />
<input type="submit" value="上传" />
</form>
</body>
</html>
@WebServlet("/fileUploadServlet")
@MultipartConfig
public class FileUploadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
Part filePart = request.getPart("file"); // 获取文件Part对象
String fileName = filePart.getSubmittedFileName(); // 获取文件名
InputStream fileContent = filePart.getInputStream(); // 获取文件输入流
String uploadPath = request.getServletContext().getRealPath("/uploads"); // 上传文件保存目录
File uploadDir = new File(uploadPath);
if(!uploadDir.exists()) {
uploadDir.mkdirs();
}
File savedFile = new File(uploadDir, fileName);
try(BufferedInputStream bis = new BufferedInputStream(fileContent);
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(savedFile));) {
byte[] buffer = new byte[1024];
int len;
while((len = bis.read(buffer)) != -1) {
bos.write(buffer, 0, len);
}
}
response.getWriter().println("文件上传成功！");
}
}
1.2 JavaScript+FormData方式上传文件
JavaScript+FormData方式上传文件相对于HTML表单方式具有更好的兼容性和灵活性。通过FormData对象将表单数据提交到服务器，能够同时上传多个文件和其他数据。
示例代码：
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>文件上传JavaScript方式</title>
</head>
<body>
<input type="file" id="file" /><br />
<button onclick="uploadFile()">上传</button>
<script>
function uploadFile() {
var files = document.getElementById('file').files;
var formData = new FormData();
for(var i = 0; i < files.length; i++) {
formData.append('file', files[i]);
}
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
if(xhr.readyState == 4 && xhr.status == 200) {
alert(xhr.responseText);
}
}
xhr.open('POST', 'fileUploadServlet', true);
xhr.send(formData);
}
</script>
</body>
</html>
@WebServlet("/fileUploadServlet")
@MultipartConfig
public class FileUploadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
Collection<Part> fileParts = request.getParts(); // 获取所有上传文件Part对象
String uploadPath = request.getServletContext().getRealPath("/uploads"); // 上传文件保存目录
File uploadDir = new File(uploadPath);
if (!uploadDir.exists()) {
uploadDir.mkdirs();
}
for (Part filePart : fileParts) {
if (filePart.getSubmittedFileName() == null) {
continue; // 忽略非文件Part对象
}
String fileName = filePart.getSubmittedFileName(); // 获取文件名
InputStream fileContent = filePart.getInputStream(); // 获取文件输入流
File savedFile = new File(uploadDir, fileName);
try (BufferedInputStream bis = new BufferedInputStream(fileContent);
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(savedFile));) {
byte[] buffer = new byte[1024];
int len;
while ((len = bis.read(buffer)) != -1) {
bos.write(buffer, 0, len);
}
}
response.getWriter().println("文件" + fileName + "上传成功！<br />");
}
}
}
2. 文件下载
Java文件下载一般需要使用HTTP响应方式实现，客户端通过链接、按钮等方式发送下载请求。服务端将文件数据以application/octet-stream或特定格式的MIME类型形式发送到客户端，客户端会根据响应头中的Content-Disposition属性将文件保存到本地。
2.1 通过Servlet实现文件下载
使用Servlet实现文件下载相对于普通的HTTP服务器具有更好的可定制性和扩展性。可以通过Servlet过滤器或权限控制等方式增加安全性。
示例代码：
@WebServlet("/fileDownloadServlet")
public class FileDownloadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("application/octet-stream");
response.setHeader("Content-Disposition", "attachment;filename=file.txt");
String filePath = getServletContext().getRealPath("/downloads"); // 下载文件路径
File downloadFile = new File(filePath, "file.txt");
try (BufferedInputStream bis = new BufferedInputStream(new FileInputStream(downloadFile));
BufferedOutputStream bos = new BufferedOutputStream(response.getOutputStream());) {
byte[] buffer = new byte[1024];
int len;
while ((len = bis.read(buffer)) != -1) {
bos.write(buffer, 0, len);
}
}
}
}
2.2 通过SpringMVC实现文件下载
使用SpringMVC框架相对于Servlet更加简洁便捷。可以通过ResponseEntity
示例代码：
@GetMapping("/fileDownload")
public ResponseEntity<byte[]> fileDownload() throws IOException {
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
headers.setContentDispositionFormData("attachment", "file.txt");
String filePath = servletContext.getRealPath("/downloads"); // 下载文件路径
File downloadFile = new File(filePath, "file.txt");
byte[] data = Files.readAllBytes(downloadFile.toPath());
return new ResponseEntity<byte[]>(data, headers, HttpStatus.OK);
}
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