java不依赖后缀名判断上传文件的类型的两种方法

import java.io.FileInputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class FileTypeDetector {

    // 常见文件类型的魔数字典（十六进制）
    private static final Map<String, String> MAGIC_NUMBERS = new HashMap<>();
    static {
        MAGIC_NUMBERS.put("FFD8FF", "image/jpeg");      // JPEG
        MAGIC_NUMBERS.put("89504E47", "image/png");     // PNG
        MAGIC_NUMBERS.put("47494638", "image/gif");     // GIF
        MAGIC_NUMBERS.put("25504446", "application/pdf"); // PDF
        MAGIC_NUMBERS.put("504B0304", "application/zip"); // ZIP或DOCX/XLSX等
        MAGIC_NUMBERS.put("52617221", "application/x-rar-compressed"); // RAR
    }

    public static String detectFileType(byte[] fileBytes) {
        // 将文件前8字节转为十六进制字符串
        StringBuilder hexBuilder = new StringBuilder();
        for (int i = 0; i < Math.min(8, fileBytes.length); i++) {
            hexBuilder.append(String.format("%02X", fileBytes[i] & 0xFF));
        }
        String fileHeader = hexBuilder.toString();

        // 匹配魔数
        for (Map.Entry<String, String> entry : MAGIC_NUMBERS.entrySet()) {
            if (fileHeader.startsWith(entry.getKey())) {
                return entry.getValue();
            }
        }
        return "application/octet-stream"; // 未知类型
    }

    // 示例：从上传的文件读取字节并检测
    public static void main(String[] args) throws IOException {
        String filePath = "uploaded_file.dat"; // 替换为实际文件路径
        try (FileInputStream fis = new FileInputStream(filePath)) {
            byte[] headerBytes = new byte[8];
            int read = fis.read(headerBytes);
            if (read > 0) {
                String mimeType = detectFileType(headerBytes);
                System.out.println("Detected MIME Type: " + mimeType);
            }
        }
    }
}

<dependency>
    <groupId>org.apache.tika</groupId>
    <artifactId>tika-core</artifactId>
    <version>2.9.1</version> <!-- 使用最新版本 -->
</dependency>

import org.apache.tika.Tika;
import java.io.File;
import java.io.IOException;

public class TikaFileDetector {
    public static void main(String[] args) throws IOException {
        Tika tika = new Tika();
        File uploadedFile = new File("uploaded_file.dat");

        // 检测MIME类型（基于内容）
        String mimeType = tika.detect(uploadedFile);
        System.out.println("Detected MIME Type: " + mimeType);
    }
}

@WebServlet("/upload")
public class FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) {
        Part filePart = request.getPart("file");
        try (InputStream fileStream = filePart.getInputStream()) {
            Tika tika = new Tika();
            
            // 只读取部分内容进行检测（Tika自动优化）
            String mimeType = tika.detect(fileStream);
            
            if (!mimeType.startsWith("image/")) {
                response.sendError(400, "只允许上传图片");
                return;
            }
            // 处理合法文件...
        }
    }
}