Java实现文件上传服务器和客户端

下面从服务器和客户端两个角度，分别讲解Java实现文件上传的完整攻略。

一、服务器方案

1.1 前置准备

首先，我们需要引入Java的文件上传相关依赖包：commons-fileupload和commons-io。这两个包的作用是支持多种文件上传方式，并且后者还提供了一些方便的工具类来处理文件操作。

引入依赖参考pom.xml文件配置:

<dependencies>
        <dependency>
            <groupId>commons-fileupload</groupId>
            <artifactId>commons-fileupload</artifactId>
            <version>1.4</version>
        </dependency>
        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.11.0</version>
        </dependency>
</dependencies>

1.2 服务器端代码实现

服务器端代码实现大致分成以下四个步骤：

1. 创建Servlet，配置上传文件的相关参数：

@WebServlet("/upload")
@MultipartConfig(
    location = "/tmp", // 指定上传文件的目录
    maxFileSize = 1024 * 1024 * 10, // 最大文件上传大小
    maxRequestSize = 1024 * 1024 * 50, // 最大请求体大小
    fileSizeThreshold = 1024 * 1024, // 文件大小临界值(当文件大小超过这个值时，会写入磁盘)
)
public class UploadServlet extends HttpServlet {
    // ...
}

1. 调用request.getPart()方法获取上传文件的part对象，然后使用InputStream获取文件流和part.getSubmittedFileName()获取文件名：

Part partFile = request.getPart("file");
String fileName = partFile.getSubmittedFileName();
InputStream inputStream = partFile.getInputStream();

1. 使用Apache commons-io的FileUtils.copyInputStreamToFile()方法将inputStream中的文件流写入到目标文件：

File targetFile = new File(uploadPath, fileName);
FileUtils.copyInputStreamToFile(inputStream, targetFile);

1. 处理上传过程中出现的异常或者错误，并返回上传结果：

response.setContentType("application/json;charset=UTF-8");
String json = "{\"message\":\"ok\"}";
response.getWriter().write(json);

1.3 示例

下面是一个基于SpringBoot的上传文件服务器的示例代码：

@RestController
public class FileController {
    @PostMapping(value="/upload", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
    public ResponseEntity<Object> upload(@RequestParam("file") MultipartFile file) throws IOException {
        File targetFile = new File("/tmp/" + file.getOriginalFilename());
        file.transferTo(targetFile);
        return ResponseEntity.ok().build();
    }
}

二、客户端方案

2.1 前置准备

在客户端实现文件上传的过程中，我们需要使用Java的HttpURLConnection和BufferedReader类。具体来说，我们先构造HttpURLConnection对象，并设置相关的属性，比如请求的方式和请求头参数。然后，将要上传的文件读入内存中，然后构造一个OutputStream对象并将文件写入请求中。最后，处理服务器端的返回结果。

2.2 客户端代码实现

客户端上传文件通常分成以下三个步骤：

1. 配置请求参数：

URL url = new URL("http://localhost:8080/upload");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
connection.setDoOutput(true);

1. 构造表单数据：

OutputStream outputStream = connection.getOutputStream();
String formdataTemplate = "\r\n--" + boundary + "\r\nContent-Disposition: form-data; name=\"%s\"\r\n\r\n%s";
outputStream.write(String.format(formdataTemplate, "name", "testname").getBytes("utf-8"));
outputStream.write(String.format(formdataTemplate, "email", "testemail").getBytes("utf-8"));
outputStream.write(String.format(formdataTemplate, "file", file.getName()).getBytes("utf-8"));

1. 上传文件并处理结果：

FileInputStream inputStream = new FileInputStream(file);
int bytesRead = -1;
byte[] buffer = new byte[4096];
while ((bytesRead = inputStream.read(buffer)) != -1) {
    outputStream.write(buffer, 0, bytesRead);
}
outputStream.write("\r\n--".getBytes("utf-8"));
outputStream.write(boundary.getBytes("utf-8"));
outputStream.write("--\r\n".getBytes("utf-8"));

BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream(), "utf-8"));
String line = "";
while ((line = in.readLine()) != null) {
    System.out.println(line);
}

inputStream.close();
outputStream.flush();
outputStream.close();

2.3 示例

下面是一个基于Java URL的上传文件的客户端示例代码：

public class FileUploadTest {
    public static void main(String[] args) throws Exception {
        String boundary = UUID.randomUUID().toString();
        File file = new File("test.jpg");
        URL url = new URL("http://localhost:8080/upload");

        HttpURLConnection connection = (HttpURLConnection)url.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
        connection.setDoOutput(true);

        OutputStream outputStream = connection.getOutputStream();

        String formdataTemplate = "\r\n--" + boundary + "\r\nContent-Disposition: form-data; name=\"%s\"\r\n\r\n%s";
        outputStream.write(String.format(formdataTemplate, "name", "testname").getBytes("utf-8"));
        outputStream.write(String.format(formdataTemplate, "email", "testemail").getBytes("utf-8"));
        outputStream.write(String.format(formdataTemplate, "file", file.getName()).getBytes("utf-8"));

        FileInputStream inputStream = new FileInputStream(file);
        int bytesRead = -1;
        byte[] buffer = new byte[4096];
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.write("\r\n--".getBytes("utf-8"));
        outputStream.write(boundary.getBytes("utf-8"));
        outputStream.write("--\r\n".getBytes("utf-8"));
        inputStream.close();

        BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream(), "utf-8"));
        String line = "";
        while ((line = in.readLine()) != null) {
            System.out.println(line);
        }

        outputStream.flush();
        outputStream.close();
    }
}

这篇文章介绍了常见的文件上传操作。在实际开发中，我们可以根据业务需求进行修改和扩展。在此基础上，还可以加入安全认证和限流等措施加强上传文件的安全性和稳定性。