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BOM Character: Unknown guest in your file

Many times I have seen special character like “  ” in starting of file and most of the time I ignored it. But recently it started causing more trouble, when my JS validation utility started failing. And log file shows same special character again. After more investigation, I find out it was introduce when I creates a new file in Eclipse.

So, what is this symbol?

It is a BOM (byte order mark) character used to signal the endianness (byte order) of a text file or stream[wiki]. This character differ based on encoding type like

Encoding
Symbol
UTF-8 
UTF-16 (BE) þÿ
UTF-16 (LE) ÿþ

Next question is, how to remove this character from my file. And if you are using Notepad++ editor it will be much easier. You have to select the “Encoding” option from menu bar and choose “Convert to UTF-8 without BOM”.




How to handle BOM character in your code?

I will suggest to use some sophisticated library instead of rolling out your own solution. And “Apache Commons IO” library has dedicated class “BOMInputStream” for this problem.

Some code snippet from API doc

1. Detect and exclude a UTF-8 BOM
 BOMInputStream bomIn = new BOMInputStream(in);
 if (bomIn.hasBOM()) {
     // has a UTF-8 BOM
 }

2. Detect a UTF-8 BOM (but don't exclude it)
 boolean include = true;
 BOMInputStream bomIn = new BOMInputStream(in, include);
 if (bomIn.hasBOM()) {
     // has a UTF-8 BOM
 }

3. Detect Multiple BOMs
 BOMInputStream bomIn = new BOMInputStream(in, 
   ByteOrderMark.UTF_16LE, ByteOrderMark.UTF_16BE,
   ByteOrderMark.UTF_32LE, ByteOrderMark.UTF_32BE
   );
 if (bomIn.hasBOM() == false) {
     // No BOM found
 } else if (bomIn.hasBOM(ByteOrderMark.UTF_16LE)) {
     // has a UTF-16LE BOM
 } else if (bomIn.hasBOM(ByteOrderMark.UTF_16BE)) {
     // has a UTF-16BE BOM
 } else if (bomIn.hasBOM(ByteOrderMark.UTF_32LE)) {
     // has a UTF-32LE BOM
 } else if (bomIn.hasBOM(ByteOrderMark.UTF_32BE)) {
     // has a UTF-32BE BOM
 }

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