Java EE Servlet下载Zip文件损坏问题排查求助

开发Web服务器时，Servlet调用sendFile方法后，客户端下载的Zip文件出现损坏，服务器端生成的Zip文件正常，但客户端无法打开，使用unzip命令报错。

相关代码 #

Servlet调用代码 #

sendFile(directory, "s" + this.optimizerId + "-" + this.optimizerName + "-" + start.toDate(Defs.DB.DATE_PATTERN) + "-" + end.toDate(Defs.DB.DATE_PATTERN), FileExtension.NONE, ContentType.ZIP, response);

第一个sendFile重载方法 #

public default void sendFile(File file, String fileName, FileExtension extension, ContentType type, HttpServletResponse response) {
    if(type == ContentType.ZIP) {
        String[] files = file.list();
        if(files != null && files.length > 0) {
            byte[] bytes = FileUtils.getZipBytes(file, files);
            sendFile(new String(bytes), fileName, extension, type, response);
        }
    }
}

第二个sendFile重载方法 #

public default void sendFile(String content, String fileName, FileExtension extension, ContentType type, HttpServletResponse response) {
    response.setContentType(type.getExpression());
    response.setHeader("Content-Disposition", "attachment; filename=\"" + FileUtils.encodeForFileName(fileName) + "." + extension.getExtension() + "\"");
    OutputStream out = response.getOutputStream();
    out.write(content.getBytes());
    out.flush();
    out.close();
}

FileUtils.getZipBytes方法 #

public static byte[] getZipBytes(File directory, String[] files) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ZipOutputStream zos = new ZipOutputStream(baos);
    byte bytes[] = new byte[2048];
    for (String fileName : files) {
        FileInputStream fis = new FileInputStream(directory.getPath() + 
                System.getProperty("file.separator") + fileName);
        BufferedInputStream bis = new BufferedInputStream(fis);
        zos.putNextEntry(new ZipEntry(fileName));
        int bytesRead;
        while ((bytesRead = bis.read(bytes)) != -1) {
            zos.write(bytes, 0, bytesRead);
        }
        zos.closeEntry();
        bis.close();
        fis.close();
    }
    zos.flush();
    baos.flush();
    zos.close();
    baos.close();
    return baos.toByteArray();
}

客户端报错信息 #

Archive:  s192-Système U carquefou-2023-01-26 000000-2023-01-26 235959(1).zip

caution:  zipfile comment truncated
error [s192-Système U carquefou-2023-01-26 000000-2023-01-26 235959(1).zip]:  missing 3249308257 bytes in zipfile
  (attempting to process anyway)
error [s192-Système U carquefou-2023-01-26 000000-2023-01-26 235959(1).zip]:  attempt to seek before beginning of zipfile
  (please check that you have transferred or created the zipfile in the
  appropriate BINARY mode and that you have compiled UnZip properly)

问题排查与修复 #

1. 核心问题：二进制数据转字符串导致损坏 #

Zip是二进制格式文件，绝对不能将二进制字节数组转成字符串再转回字节数组。第一个sendFile方法中，new String(bytes)会用JVM默认字符编码（如UTF-8）解析二进制字节，很多不可打印的二进制值会被替换成占位符（如�），后续调用content.getBytes()时，生成的字节数组已经和原Zip数据完全不一致，直接导致文件损坏。

2. 次要问题：流操作不规范与异常缺失 #

• 第一个sendFile方法未处理非ZIP类型、空目录的情况，客户端会收到空响应
• 流操作未使用try-with-resources，可能导致资源泄漏
• 第二个sendFile方法直接关闭OutputStream，可能导致响应未完整发送，且未处理IOException

修复后的代码示例 #

新增字节数组处理的sendFile重载方法，替换字符串中转逻辑

public default void sendFile(File file, String fileName, FileExtension extension, ContentType type, HttpServletResponse response) throws IOException {
    if(type == ContentType.ZIP) {
        String[] files = file.list();
        if(files != null && files.length > 0) {
            byte[] bytes = FileUtils.getZipBytes(file, files);
            sendFile(bytes, fileName, extension, type, response);
        } else {
            // 处理空目录，返回404提示
            response.sendError(HttpServletResponse.SC_NOT_FOUND, "No files available to zip");
        }
    } else {
        // 补充其他文件类型的处理逻辑
    }
}

// 新增处理二进制字节数组的重载方法
public default void sendFile(byte[] content, String fileName, FileExtension extension, ContentType type, HttpServletResponse response) throws IOException {
    response.setContentType(type.getExpression());
    response.setHeader("Content-Disposition", "attachment; filename=\"" + FileUtils.encodeForFileName(fileName) + "." + extension.getExtension() + "\"");
    // 设置Content-Length，让客户端知晓文件大小
    response.setContentLength(content.length);
    
    // 使用try-with-resources自动关闭流
    try(OutputStream out = response.getOutputStream()) {
        out.write(content);
        out.flush();
    }
}

优化FileUtils.getZipBytes方法的流操作

public static byte[] getZipBytes(File directory, String[] files) throws IOException {
    // try-with-resources自动关闭所有流
    try(ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ZipOutputStream zos = new ZipOutputStream(baos)) {
        byte bytes[] = new byte[2048];
        for (String fileName : files) {
            File fileToZip = new File(directory, fileName);
            try(FileInputStream fis = new FileInputStream(fileToZip);
                BufferedInputStream bis = new BufferedInputStream(fis)) {
                zos.putNextEntry(new ZipEntry(fileName));
                int bytesRead;
                while ((bytesRead = bis.read(bytes)) != -1) {
                    zos.write(bytes, 0, bytesRead);
                }
                zos.closeEntry();
            }
        }
        zos.finish(); // 完成Zip输出，确保所有数据写入ByteArrayOutputStream
        return baos.toByteArray();
    }
}

内容的提问来源于stack exchange，提问作者loupTerawatt