How to Remove BOM from an XML file in Java

Question

I need suggestions on the way to remove BOM from an UTF-8 file and create a copy of the rest of the xml file.

Answer 1

Having a tool breaking because of a BOM in an UTF-8 file is a very common thing in my experience. I don't know why there where so many downvotes (but then it gives me the chance to try to get enough vote to win a special SO badge ; )

More seriously: an UTF-8 BOM doesn't typically make that much sense but it is fully valid (although discouraged) by the specs. Now the problem is that a lot of people aren't aware that a BOM is valid in UTF-8 and hence wrote broken tools / APIs that do not process correctly these files.

Now you may have two different issues: you may want to process the file from Java or you need to use Java to programmatically create/fix files that other (broken) tools need.

I've had the case in one consulting gig where the helpdesk would keep getting messages from users that had problems with some text editor that would mess up perfectly valid UTF-8 files produced by Java. So I had to work around that issue by making sure to remove the BOM from every single UTF-8 file we were dealing with.

I you want to delete a BOM from a file, you could create a new file and skip the first three bytes. For example:

... $  file  /tmp/src.txt 
/tmp/src.txt: UTF-8 Unicode (with BOM) English text

... $  ls -l  /tmp/src.txt 
-rw-rw-r-- 1 tact tact 1733 2012-03-16 14:29 /tmp/src.txt

... $  hexdump  -C  /tmp/src.txt | head -n 1
00000000  ef bb bf 50 6f 6b 65 ...

As you can see, the file starts with "ef bb bf", this is the (fully valid) UTF-8 BOM.

Here's a method that takes a file and makes a copy of it by skipping the first three bytes:

 public static void workAroundbrokenToolsAndAPIs(File sourceFile, File destFile) throws IOException {
    if(!destFile.exists()) {
        destFile.createNewFile();
    }

    FileChannel source = null;
    FileChannel destination = null;

    try {
        source = new FileInputStream(sourceFile).getChannel();
        source.position(3);
        destination = new FileOutputStream(destFile).getChannel();
        destination.transferFrom( source, 0, source.size() - 3 );
    }
    finally {
        if(source != null) {
            source.close();
        }
        if(destination != null) {
            destination.close();
        }
    }
}

Note that it's "raw": you'd typically want to first make sure you have a BOM before calling this or "Bad Thinks May Happen" [TM].

You can look at your file afterwards:

... $  file  /tmp/dst.txt 
/tmp/dst.txt: UTF-8 Unicode English text

... $  ls -l  /tmp/dst.txt 
-rw-rw-r-- 1 tact tact 1730 2012-03-16 14:41 /tmp/dst.txt

... $  hexdump -C /tmp/dst.txt
00000000  50 6f 6b 65 ...

And the BOM is gone...

Now if you simply want to transparently remove the BOM for one your broken Java API, then you could use the pushbackInputStream described here: why org.apache.xerces.parsers.SAXParser does not skip BOM in utf8 encoded xml?

private static InputStream checkForUtf8BOMAndDiscardIfAny(InputStream inputStream) throws IOException {
    PushbackInputStream pushbackInputStream = new PushbackInputStream(new BufferedInputStream(inputStream), 3);
    byte[] bom = new byte[3];
    if (pushbackInputStream.read(bom) != -1) {
        if (!(bom[0] == (byte) 0xEF && bom[1] == (byte) 0xBB && bom[2] == (byte) 0xBF)) {
            pushbackInputStream.unread(bom);
        }
    }
    return pushbackInputStream; }

Note that this works, but shall definitely NOT fix the more serious issue where you can have other tools in the work chain not working correctly with UTF-8 files having a BOM.

And here's a link to a question with a more complete answer, covering other encodings as well:

Byte order mark screws up file reading in Java