EDIT : ho realizzato una versione corretta su GitHub: https://github.com/gpakosz/UnicodeBOMInputStream
Ecco una classe che ho codificato qualche tempo fa, ho appena modificato il nome del pacchetto prima di incollare. Niente di speciale, è abbastanza simile alle soluzioni pubblicate nel database dei bug di SUN. Incorporalo nel tuo codice e stai bene.
/* ____________________________________________________________________________
*
* File: UnicodeBOMInputStream.java
* Author: Gregory Pakosz.
* Date: 02 - November - 2005
* ____________________________________________________________________________
*/
package com.stackoverflow.answer;
import java.io.IOException;
import java.io.InputStream;
import java.io.PushbackInputStream;
/**
* The <code>UnicodeBOMInputStream</code> class wraps any
* <code>InputStream</code> and detects the presence of any Unicode BOM
* (Byte Order Mark) at its beginning, as defined by
* <a href="http://www.faqs.org/rfcs/rfc3629.html">RFC 3629 - UTF-8, a transformation format of ISO 10646</a>
*
* <p>The
* <a href="http://www.unicode.org/unicode/faq/utf_bom.html">Unicode FAQ</a>
* defines 5 types of BOMs:<ul>
* <li><pre>00 00 FE FF = UTF-32, big-endian</pre></li>
* <li><pre>FF FE 00 00 = UTF-32, little-endian</pre></li>
* <li><pre>FE FF = UTF-16, big-endian</pre></li>
* <li><pre>FF FE = UTF-16, little-endian</pre></li>
* <li><pre>EF BB BF = UTF-8</pre></li>
* </ul></p>
*
* <p>Use the {@link #getBOM()} method to know whether a BOM has been detected
* or not.
* </p>
* <p>Use the {@link #skipBOM()} method to remove the detected BOM from the
* wrapped <code>InputStream</code> object.</p>
*/
public class UnicodeBOMInputStream extends InputStream
{
/**
* Type safe enumeration class that describes the different types of Unicode
* BOMs.
*/
public static final class BOM
{
/**
* NONE.
*/
public static final BOM NONE = new BOM(new byte[]{},"NONE");
/**
* UTF-8 BOM (EF BB BF).
*/
public static final BOM UTF_8 = new BOM(new byte[]{(byte)0xEF,
(byte)0xBB,
(byte)0xBF},
"UTF-8");
/**
* UTF-16, little-endian (FF FE).
*/
public static final BOM UTF_16_LE = new BOM(new byte[]{ (byte)0xFF,
(byte)0xFE},
"UTF-16 little-endian");
/**
* UTF-16, big-endian (FE FF).
*/
public static final BOM UTF_16_BE = new BOM(new byte[]{ (byte)0xFE,
(byte)0xFF},
"UTF-16 big-endian");
/**
* UTF-32, little-endian (FF FE 00 00).
*/
public static final BOM UTF_32_LE = new BOM(new byte[]{ (byte)0xFF,
(byte)0xFE,
(byte)0x00,
(byte)0x00},
"UTF-32 little-endian");
/**
* UTF-32, big-endian (00 00 FE FF).
*/
public static final BOM UTF_32_BE = new BOM(new byte[]{ (byte)0x00,
(byte)0x00,
(byte)0xFE,
(byte)0xFF},
"UTF-32 big-endian");
/**
* Returns a <code>String</code> representation of this <code>BOM</code>
* value.
*/
public final String toString()
{
return description;
}
/**
* Returns the bytes corresponding to this <code>BOM</code> value.
*/
public final byte[] getBytes()
{
final int length = bytes.length;
final byte[] result = new byte[length];
// Make a defensive copy
System.arraycopy(bytes,0,result,0,length);
return result;
}
private BOM(final byte bom[], final String description)
{
assert(bom != null) : "invalid BOM: null is not allowed";
assert(description != null) : "invalid description: null is not allowed";
assert(description.length() != 0) : "invalid description: empty string is not allowed";
this.bytes = bom;
this.description = description;
}
final byte bytes[];
private final String description;
} // BOM
/**
* Constructs a new <code>UnicodeBOMInputStream</code> that wraps the
* specified <code>InputStream</code>.
*
* @param inputStream an <code>InputStream</code>.
*
* @throws NullPointerException when <code>inputStream</code> is
* <code>null</code>.
* @throws IOException on reading from the specified <code>InputStream</code>
* when trying to detect the Unicode BOM.
*/
public UnicodeBOMInputStream(final InputStream inputStream) throws NullPointerException,
IOException
{
if (inputStream == null)
throw new NullPointerException("invalid input stream: null is not allowed");
in = new PushbackInputStream(inputStream,4);
final byte bom[] = new byte[4];
final int read = in.read(bom);
switch(read)
{
case 4:
if ((bom[0] == (byte)0xFF) &&
(bom[1] == (byte)0xFE) &&
(bom[2] == (byte)0x00) &&
(bom[3] == (byte)0x00))
{
this.bom = BOM.UTF_32_LE;
break;
}
else
if ((bom[0] == (byte)0x00) &&
(bom[1] == (byte)0x00) &&
(bom[2] == (byte)0xFE) &&
(bom[3] == (byte)0xFF))
{
this.bom = BOM.UTF_32_BE;
break;
}
case 3:
if ((bom[0] == (byte)0xEF) &&
(bom[1] == (byte)0xBB) &&
(bom[2] == (byte)0xBF))
{
this.bom = BOM.UTF_8;
break;
}
case 2:
if ((bom[0] == (byte)0xFF) &&
(bom[1] == (byte)0xFE))
{
this.bom = BOM.UTF_16_LE;
break;
}
else
if ((bom[0] == (byte)0xFE) &&
(bom[1] == (byte)0xFF))
{
this.bom = BOM.UTF_16_BE;
break;
}
default:
this.bom = BOM.NONE;
break;
}
if (read > 0)
in.unread(bom,0,read);
}
/**
* Returns the <code>BOM</code> that was detected in the wrapped
* <code>InputStream</code> object.
*
* @return a <code>BOM</code> value.
*/
public final BOM getBOM()
{
// BOM type is immutable.
return bom;
}
/**
* Skips the <code>BOM</code> that was found in the wrapped
* <code>InputStream</code> object.
*
* @return this <code>UnicodeBOMInputStream</code>.
*
* @throws IOException when trying to skip the BOM from the wrapped
* <code>InputStream</code> object.
*/
public final synchronized UnicodeBOMInputStream skipBOM() throws IOException
{
if (!skipped)
{
in.skip(bom.bytes.length);
skipped = true;
}
return this;
}
/**
* {@inheritDoc}
*/
public int read() throws IOException
{
return in.read();
}
/**
* {@inheritDoc}
*/
public int read(final byte b[]) throws IOException,
NullPointerException
{
return in.read(b,0,b.length);
}
/**
* {@inheritDoc}
*/
public int read(final byte b[],
final int off,
final int len) throws IOException,
NullPointerException
{
return in.read(b,off,len);
}
/**
* {@inheritDoc}
*/
public long skip(final long n) throws IOException
{
return in.skip(n);
}
/**
* {@inheritDoc}
*/
public int available() throws IOException
{
return in.available();
}
/**
* {@inheritDoc}
*/
public void close() throws IOException
{
in.close();
}
/**
* {@inheritDoc}
*/
public synchronized void mark(final int readlimit)
{
in.mark(readlimit);
}
/**
* {@inheritDoc}
*/
public synchronized void reset() throws IOException
{
in.reset();
}
/**
* {@inheritDoc}
*/
public boolean markSupported()
{
return in.markSupported();
}
private final PushbackInputStream in;
private final BOM bom;
private boolean skipped = false;
} // UnicodeBOMInputStream
E lo stai usando in questo modo:
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;
public final class UnicodeBOMInputStreamUsage
{
public static void main(final String[] args) throws Exception
{
FileInputStream fis = new FileInputStream("test/offending_bom.txt");
UnicodeBOMInputStream ubis = new UnicodeBOMInputStream(fis);
System.out.println("detected BOM: " + ubis.getBOM());
System.out.print("Reading the content of the file without skipping the BOM: ");
InputStreamReader isr = new InputStreamReader(ubis);
BufferedReader br = new BufferedReader(isr);
System.out.println(br.readLine());
br.close();
isr.close();
ubis.close();
fis.close();
fis = new FileInputStream("test/offending_bom.txt");
ubis = new UnicodeBOMInputStream(fis);
isr = new InputStreamReader(ubis);
br = new BufferedReader(isr);
ubis.skipBOM();
System.out.print("Reading the content of the file after skipping the BOM: ");
System.out.println(br.readLine());
br.close();
isr.close();
ubis.close();
fis.close();
}
} // UnicodeBOMInputStreamUsage