GZIP decompress string and byte conversion

Question

I have a problem in code:

private static String compress(String str)
{
    String str1 = null;
    ByteArrayOutputStream bos = null;
    try
    {
        bos = new ByteArrayOutputStream();
        BufferedOutputStream dest = null;

        byte b[] = str.getBytes();
        GZIPOutputStream gz = new GZIPOutputStream(bos,b.length);
        gz.write(b,0,b.length);
        bos.close();
        gz.close();

    }
    catch(Exception e) {
        System.out.println(e);
        e.printStackTrace();
    }
    byte b1[] = bos.toByteArray();
    return new String(b1);
}

private static String deCompress(String str)
{
    String s1 = null;

    try
    {
        byte b[] = str.getBytes();
        InputStream bais = new ByteArrayInputStream(b);
        GZIPInputStream gs = new GZIPInputStream(bais);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        int numBytesRead = 0;
        byte [] tempBytes = new byte[6000];
        try
        {
            while ((numBytesRead = gs.read(tempBytes, 0, tempBytes.length)) != -1)
            {
                baos.write(tempBytes, 0, numBytesRead);
            }

            s1 = new String(baos.toByteArray());
            s1= baos.toString();
        }
        catch(ZipException e)
        {
            e.printStackTrace();
        }
    }
    catch(Exception e) {
        e.printStackTrace();
    }
    return s1;
}

public String test() throws Exception
    {
        String str = "teststring";
        String cmpr = compress(str);
        String dcmpr = deCompress(cmpr);
}

This code throw java.io.IOException: unknown format (magic number ef1f)

GZIPInputStream gs = new GZIPInputStream(bais);

It turns out that when converting byte new String (b1) and the byte b [] = str.getBytes () bytes are "spoiled." At the output of the line we have already more bytes. If you avoid the conversion to a string and work on the line with bytes - everything works. Sorry for my English.

---

public String unZip(String zipped) throws DataFormatException, IOException {
    byte[] bytes = zipped.getBytes("WINDOWS-1251");
    Inflater decompressed = new Inflater();
    decompressed.setInput(bytes);

    byte[] result = new byte[100];
    ByteArrayOutputStream buffer = new ByteArrayOutputStream();

    while (decompressed.inflate(result) != 0)
        buffer.write(result);

    decompressed.end();

    return new String(buffer.toByteArray(), charset);
}

I'm use this function to decompress server responce. Thanks for help.

Answer 1

You have two problems:

• You're using the default character encoding to convert the original string into bytes. That will vary by platform. It's better to specify an encoding - UTF-8 is usually a good idea.
• You're trying to represent the opaque binary data of the result of the compression as a string by just calling the String(byte[]) constructor. That constructor is only meant for data which is encoded text... which this isn't. You should use base64 for this. There's a public domain base64 library which makes this easy. (Alternatively, don't convert the compressed data to text at all - just return a byte array.)

Fundamentally, you need to understand how different text and binary data are - when you want to convert between the two, you should do so carefully. If you want to represent "non text" binary data (i.e. bytes which aren't the direct result of encoding text) in a string you should use something like base64 or hex. When you want to encode a string as binary data (e.g. to write some text to disk) you should carefully consider which encoding to use. If another program is going to read your data, you need to work out what encoding it expects - if you have full control over it yourself, I'd usually go for UTF-8.

Additionally, the exception handling in your code is poor:

• You should almost never catch Exception; catch more specific exceptions
• You shouldn't just catch an exception and continue as if it had never happened. If you can't really handle the exception and still complete your method successfully, you should let the exception bubble up the stack (or possibly catch it and wrap it in a more appropriate exception type for your abstraction)

Answer 2

When you GZIP compress data, you always get binary data. This data cannot be converted into string as it is no valid character data (in any encoding).

So your compress method should return a byte array and your decompress method should take a byte array as its parameter.

Futhermore, I recommend you use an explicit encoding when you convert the string into a byte array before compression and when you turn the decompressed data into a string again.

Answer 3

When you GZIP compress data, you always get binary data. This data cannot be converted into string as it is no valid character data (in any encoding).

Codo is right, thanks a lot for enlightening me. I was trying to decompress a string (converted from the binary data). What I amended was using InflaterInputStream directly on the input stream returned by my http connection. (My app was retrieving a large JSON of strings)