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From "David Gao (JIRA)" <j...@apache.org>
Subject [jira] [Created] (IO-331) BOMInputStream wrongly detects UTF-32LE_BOM files as UTF-16LE_BOM files in method getBOM()
Date Fri, 01 Jun 2012 04:20:22 GMT
David Gao created IO-331:
----------------------------

             Summary: BOMInputStream wrongly detects UTF-32LE_BOM files as UTF-16LE_BOM files
in method getBOM()
                 Key: IO-331
                 URL: https://issues.apache.org/jira/browse/IO-331
             Project: Commons IO
          Issue Type: Bug
          Components: Streams/Writers
    Affects Versions: 2.3
         Environment: OS: Win 7 x64
JDK: 1.7.03
            Reporter: David Gao


Hi,

The BOMInputStream works great for most UTF encoded files when detecting Byte Order Marks.
However, if a file is UTF-32LE encoded with BOM the class takes it as UTF-16LE instead. This
is not expected behavior.

The problem comes from method getBOM(). And the first two bytes for UTF-16LE and UTF-32LE
are the same, which might be the root cause of the problem.

The following lists the bytes for UTF encodings for reference. The content is a BOM followed
by letter 't'.
||Encoding||Byte 1||Byte 2||Byte 3||Byte 4|| || || || 
|UTF8|EF|BB|BF|74| | | |
|UTF16-LE|FF|FE|74|00| | | |
|UTF16-BE|FE|FF|00|74| | | |
|UTF32-LE|FF|FE|00|00|74|00|00|00
|UTF32-BE|00|00|FE|FF|00|00|00|74


I personally used the following code to work around this problem at the moment. Hope it helps.

{code}
	private void detectBOM(InputStream in) throws IOException{
		List<ByteOrderMark> all=availableBOMs();
		int max=0;
        for (ByteOrderMark bom : all) {
            max = Math.max(max, bom.length());
        }
		byte[] firstBytes=new byte[max];
		for (int i = 0; i < max; i++) {
			firstBytes[i]=(byte) in.read();
			System.out.print(Integer.toHexString(firstBytes[i] & 0xff).toUpperCase()+" ");
		}
		
		boolean found=false;
		for (int j = max; j >1; j--) {
			byte[] _copy=Arrays.copyOf(firstBytes, j);
			for (ByteOrderMark mark : all) {
				found=Arrays.equals(_copy, mark.getBytes());
				if (found) {
					System.out.println("\nBOM is: "+mark.getCharsetName());
					break;
				}
			}
			if (found) break;
		}
	}
	
	private static List<ByteOrderMark> availableBOMs(){
		List<ByteOrderMark> all=new ArrayList<ByteOrderMark>();
		all.add(ByteOrderMark.UTF_8);
		all.add(ByteOrderMark.UTF_16BE);
		all.add(ByteOrderMark.UTF_16LE);
		all.add(ByteOrderMark.UTF_32BE);
		all.add(ByteOrderMark.UTF_32LE);
		return all;
	}
{code}

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