如何检查InputStream是否被Gzip化？ [英] How to check if InputStream is Gzipped?

问题描述

有没有办法检查InputStream是否已被gzip压缩？
这是代码：

Is there any way to check if InputStream has been gzipped? Here's the code:

public static InputStream decompressStream(InputStream input) {
    try {
        GZIPInputStream gs = new GZIPInputStream(input);
        return gs;
    } catch (IOException e) {
        logger.info("Input stream not in the GZIP format, using standard format");
        return input;
    }
}

我试过这种方式，但它不起作用expected - 从流中读取的值无效。
编辑：
添加了我用来压缩数据的方法：

I tried this way but it doesn't work as expected - values read from the stream are invalid. Added the method I use to compress data:

public static byte[] compress(byte[] content) {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    try {
        GZIPOutputStream gs = new GZIPOutputStream(baos);
        gs.write(content);
        gs.close();
    } catch (IOException e) {
        logger.error("Fatal error occured while compressing data");
        throw new RuntimeException(e);
    }
    double ratio = (1.0f * content.length / baos.size());
    if (ratio > 1) {
        logger.info("Compression ratio equals " + ratio);
        return baos.toByteArray();
    }
    logger.info("Compression not needed");
    return content;

}

推荐答案

这不是万无一失，但它可能是最简单的，不依赖任何外部数据。像所有体面的格式一样，GZip也以一个神奇的数字开头，可以在不读取整个流的情况下快速检查。

It's not foolproof but it's probably the easiest and doesn't rely on any external data. Like all decent formats, GZip too begins with a magic number which can be quickly checked without reading the entire stream.

public static InputStream decompressStream(InputStream input) {
     PushbackInputStream pb = new PushbackInputStream( input, 2 ); //we need a pushbackstream to look ahead
     byte [] signature = new byte[2];
     int len = pb.read( signature ); //read the signature
     pb.unread( signature, 0, len ); //push back the signature to the stream
     if( signature[ 0 ] == (byte) 0x1f && signature[ 1 ] == (byte) 0x8b ) //check if matches standard gzip magic number
       return new GZIPInputStream( pb );
     else 
       return pb;
}

（幻数的来源： GZip文件格式规范）

更新：我刚刚发现在 GZipInputStream 中还有一个名为 GZIP_MAGIC 的常量，其中包含这个值，所以如果你真的想要，你可以使用它的低两个字节。

Update: I've just dicovered that there is also a constant called GZIP_MAGIC in GZipInputStream which contains this value, so if you really want to, you can use the lower two bytes of it.

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