客户端应该将压缩文件上传到 S3 文件夹中。然后下载压缩文件并解压缩以对其包含的文件执行各种操作。最初我们告诉我们的客户将其文件压缩成ZIP文件，但这对我们的客户来说太难了。相反，它提交了一个带有 ZIP 扩展名的RAR文件......多么聪明。由于显而易见的原因，无法使用ZIP解压缩算法解压缩RAR文件。
因此，我正在寻找一种方法来找出 S3 下载文件的文件类型，因为我正在使用 Amazon 的 SDK 在 Linux 操作系统上开发 Java 项目。我将根据获得的文件类型来处理如何解压文件。
我看过很多堆栈溢出问题，比如这个，但仅通过查看它们（及其评论），似乎没有一个是 100% 有效的。
找出压缩文件类型的最佳方法是什么？
当以编程方式将文件上传到 Amazon S3 时，可以指定对象的Content-Type. 如果没有指定，正如@Michael-bot 澄清的那样，默认分配的值将是binary/octet-stream. 或者，如果您决定通过 Amazon S3 的 GUI 上传文件，该文件会从其Content-Type文件扩展名中获取（遗憾的是，不是其内容）。如果您可以信任上传文件的人来Content-Type正确设置，请继续查看ObjectMetadata，但如果您不能（像我一样），您将需要另一个解决方案。
因此，如果您正在寻找适用于最常见文件压缩类型的解决方案，Files.probeContentType、Apache Tika和SimpleMagic似乎是可以接受的解决方案。
最后我选择了Files.probeContentType因为它不需要额外的库并且在 Linux 机器上工作得很好（只要文件没有错误的扩展名，有一个解决方法：删除文件扩展名并让它发挥它的魔力）。
一开始会认为从 Amazon 的 S3 下载文件时的响应对象包含文件类型。它确实包含此信息，但是当文件的扩展名与其内容不匹配时，就会出现问题。
import com.amazonaws.services.s3.model.S3Object;
final S3Object s3Object = ...;
final String contentType = s3Object.getObjectMetadata().getContentType();
application/zip即使文件的内容是 Rar 文件，此代码也会返回。所以这个解决方案对我不起作用。
出于这个原因，我花时间构建了一个示例项目，该项目使用不同的方法和可用的库测试了各种场景。顺便说一句，我正在使用Java 8。
测试的文件类型有：
请注意，此处介绍的实现仅用于测试目的。它们不以任何方式被认可用于生产代码，因为它们不考虑文件锁定问题以及我的想象无法考虑的其他问题。=)
import java.io.File;
import javax.activation.MimetypesFileTypeMap;
final File file = new File(basePath + "/" + fileName);
try {
return MimetypesFileTypeMap.getDefaultFileTypeMap().getContentType(file);
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/octet-stream
Rar with Zip extension is: application/octet-stream
Zip with Zip extension is: application/octet-stream
7z with 7z extension is: application/octet-stream
7z with Zip extension is: application/octet-stream
Tar.xz with Tar.xz extension is: application/octet-stream
Tar.xz with Zip extension is: application/octet-stream
Tar.gz with Tar.gz extension is: application/octet-stream
Tar.gz with Zip extension is: application/octet-stream
Rar without extension is: application/octet-stream
Zip without extension is: application/octet-stream
7z without extension is: application/octet-stream
Tar.xz without extension is: application/octet-stream
Tar.gz without extension is: application/octet-stream
当文件类型未被识别时，此方法返回的值为application/octet-stream. 似乎所有场景都失败了，所以我们应该放弃这种方法。
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.BufferedInputStream;
import java.net.URLConnection;
final File file = new File(basePath + "/" + fileName);
try {
final FileInputStream fileInputStream = new FileInputStream(file);
final InputStream inputStream = new BufferedInputStream(fileInputStream);
return URLConnection.guessContentTypeFromStream(inputStream);
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: null
Rar with Zip extension is: null
Zip with Zip extension is: null
7z with 7z extension is: null
7z with Zip extension is: null
Tar.xz with Tar.xz extension is: null
Tar.xz with Zip extension is: null
Tar.gz with Tar.gz extension is: null
Tar.gz with Zip extension is: null
Rar without extension is: null
Zip without extension is: null
7z without extension is: null
Tar.xz without extension is: null
Tar.gz without extension is: null
同样，此方法在所有情况下都失败。看来它的支持非常有限。
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
try {
final Path path = Paths.get(basePath + "/" + fileName);
return Files.probeContentType(path);
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/vnd.rar
Rar with Zip extension is: application/zip
Zip with Zip extension is: application/zip
7z with 7z extension is: application/x-7z-compressed
7z with Zip extension is: application/zip
Tar.xz with Tar.xz extension is: application/x-xz-compressed-tar
Tar.xz with Zip extension is: application/zip
Tar.gz with Tar.gz extension is: application/x-compressed-tar
Tar.gz with Zip extension is: application/zip
Rar without extension is: application/vnd.rar
Zip without extension is: application/zip
7z without extension is: application/x-7z-compressed
Tar.xz without extension is: application/x-xz
Tar.gz without extension is: application/gzip
这种方法效果出奇的好，但不要被愚弄，有一种情况会一直失败。如果文件的扩展名错误（不匹配的是内容），它将报告文件类型为扩展名。它不应该经常发生，但如果一个人非常挑剔，则不要使用此方法。
解决方法：如果设法从文件名中删除扩展名，这将为所有给定方案返回正确的值。
这个库似乎有很多种风格（应用程序、服务器、评估等），但网络上的许多人抱怨它有点“依赖重”。
import org.apache.tika.Tika;
try {
return new Tika().detect(new File(basePath + "/" + fileName));
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/x-rar-compressed
Rar with Zip extension is: application/x-rar-compressed
Zip with Zip extension is: application/zip
7z with 7z extension is: application/x-7z-compressed
7z with Zip extension is: application/x-7z-compressed
Tar.xz with Tar.xz extension is: application/x-xz
Tar.xz with Zip extension is: application/x-xz
Tar.gz with Tar.gz extension is: application/gzip
Tar.gz with Zip extension is: application/gzip
Rar without extension is: application/x-rar-compressed
Zip without extension is: application/zip
7z without extension is: application/x-7z-compressed
Tar.xz without extension is: application/x-xz
Tar.gz without extension is: application/gzip
所有文件都被正确识别，但由于它有它的优点，它也有它的缺点。
优点：
缺点：
import java.net.URL;
import java.net.URLConnection;
try {
final URL url = new URL("file://" + basePath + "/" + fileName);
final URLConnection urlConnection = url.openConnection();
return urlConnection.getContentType();
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: content/unknown
Rar with Zip extension is: application/zip
Zip with Zip extension is: application/zip
7z with 7z extension is: content/unknown
7z with Zip extension is: application/zip
Tar.xz with Tar.xz extension is: content/unknown
Tar.xz with Zip extension is: application/zip
Tar.gz with Tar.gz extension is: application/octet-stream
Tar.gz with Zip extension is: application/zip
Rar without extension is: content/unknown
Zip without extension is: content/unknown
7z without extension is: content/unknown
Tar.xz without extension is: content/unknown
Tar.gz without extension is: content/unknown
它几乎不识别任何文件压缩格式，并通过扩展名而不是其内容来引导自己。
这个项目似乎每年至少更新一次。
import com.j256.simplemagic.ContentInfo;
import com.j256.simplemagic.ContentInfoUtil;
try {
final ContentInfoUtil util = new ContentInfoUtil();
final ContentInfo info = util.findMatch(basePath + "/" + fileName);
return info.getMimeType();
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/x-rar
Rar with Zip extension is: application/x-rar
Zip with Zip extension is: application/zip
7z with 7z extension is: application/x-7z-compressed
7z with Zip extension is: application/x-7z-compressed
Tar.xz with Tar.xz extension is: <EXCEPTION: null>
Tar.xz with Zip extension is: <EXCEPTION: null>
Tar.gz with Tar.gz extension is: application/x-gzip
Tar.gz with Zip extension is: application/x-gzip
Rar without extension is: application/x-rar
Zip without extension is: application/zip
7z without extension is: application/x-7z-compressed
Tar.xz without extension is: <EXCEPTION: null>
Tar.gz without extension is: application/x-gzip
它几乎适用于我们所有的场景，但似乎对于最“晦涩”的压缩格式，如 Tar.xz，它无法检测到它们（并在此过程中抛出异常）。
这个项目自 2010 年以来没有被修改过，所以不要指望支持或更新。为了完整起见，仅在此处列出。
import eu.medsea.mimeutil.MimeUtil2;
try {
final MimeUtil2 mimeUtil = new MimeUtil2();
mimeUtil.registerMimeDetector("eu.medsea.mimeutil.detector.MagicMimeMimeDetector");
return MimeUtil2.getMostSpecificMimeType(mimeUtil.getMimeTypes(basePath + "/" + fileName)).toString();
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/x-rar
Rar with Zip extension is: application/x-rar
Zip with Zip extension is: application/zip
7z with 7z extension is: application/octet-stream
7z with Zip extension is: application/octet-stream
Tar.xz with Tar.xz extension is: application/octet-stream
Tar.xz with Zip extension is: application/octet-stream
Tar.gz with Tar.gz extension is: application/x-gzip
Tar.gz with Zip extension is: application/x-gzip
Rar without extension is: application/x-rar
Zip without extension is: application/zip
7z without extension is: application/octet-stream
Tar.xz without extension is: application/octet-stream
Tar.gz without extension is: application/x-gzip
它识别了一些最流行的文件类型，但无法识别 Tar.xz 和 7z。
不是最漂亮的解决方案，但必须尝试：Ubuntu file command。
import java.io.BufferedReader;
import java.io.InputStreamReader;
try {
final Process process = Runtime.getRuntime().exec("file --mime-type " + basePath + "/" + fileName);
final BufferedReader stdInput = new BufferedReader(new InputStreamReader(process.getInputStream()));
String text = "";
String s;
while ((s = stdInput.readLine()) != null) {
text += s;
}
return text.split(": ")[1];
} catch (final Exception exception) {
return "<EXCEPTION: " + exception.getMessage() + ">";
}
Rar with Rar extension is: application/x-rar
Rar with Zip extension is: application/x-rar
Zip with Zip extension is: application/zip
7z with 7z extension is: application/x-7z-compressed
7z with Zip extension is: application/x-7z-compressed
Tar.xz with Tar.xz extension is: application/x-xz
Tar.xz with Zip extension is: application/x-xz
Tar.gz with Tar.gz extension is: application/gzip
Tar.gz with Zip extension is: application/gzip
Rar without extension is: application/x-rar
Zip without extension is: application/zip
7z without extension is: application/x-7z-compressed
Tar.xz without extension is: application/x-xz
Tar.gz without extension is: application/gzip
它适用于我们所有的场景，但同样，这依赖于File运行代码的系统上存在的命令。