outils
Class CharsetToolkit

java.lang.Object
  extended by outils.CharsetToolkit

public class CharsetToolkit
extends java.lang.Object

Utility class to guess the encoding of a given byte array. The guess is unfortunately not 100% sure. Especially for 8-bit charsets. It's not possible to know which 8-bit charset is used. Except through statistical analysis. We will then infer that the charset encountered is the same as the default standard charset.

On the other hand, unicode files encoded in UTF-16 (low or big endian) or UTF-8 files with a Byte Order Marker are easy to find. For UTF-8 files with no BOM, if the buffer is wide enough, it's easy to guess.

Tested against a complicated UTF-8 file, Sun's implementation does not render bad UTF-8 constructs as expected by the specification. But with a buffer wide enough, the method guessEncoding() did behave correctly and recognized the UTF-8 charset.

A byte buffer of 4KB or 8KB is sufficient to be able to guess the encoding.

Usage:

 // guess the encoding
 Charset guessedCharset = com.glaforge.i18n.io.CharsetToolkit.guessEncoding(file, 4096);

 // create a reader with the charset we've just discovered
 FileInputStream fis = new FileInputStream(file);
 InputStreamReader isr = new InputStreamReader(fis, guessedCharset);
 BufferedReader br = new BufferedReader(isr);

 // read the file content
 String line;
 while ((line = br.readLine())!= null)
 {
     System.out.println(line);
 }
 

Date: 18 juil. 2002

Author:
Guillaume LAFORGE

Field Summary
private  byte[] buffer
           
private  java.nio.charset.Charset defaultCharset
           
private  boolean enforce8Bit
           
 
Constructor Summary
CharsetToolkit(byte[] buffer)
          Constructor of the com.glaforge.i18n.io.CharsetToolkit utility class.
CharsetToolkit(byte[] buffer, java.nio.charset.Charset defaultCharset)
          Constructor of the com.glaforge.i18n.io.CharsetToolkit utility class.
 
Method Summary
static java.nio.charset.Charset[] getAvailableCharsets()
          Retrieves all the available Charsets on the platform, among which the default charset.
 java.nio.charset.Charset getDefaultCharset()
          Retrieves the default Charset
static java.nio.charset.Charset getDefaultSystemCharset()
          Retrieve the default charset of the system.
 boolean getEnforce8Bit()
          Gets the enforce8Bit flag, in case we do not want to ever get a US-ASCII encoding.
 java.nio.charset.Charset guessEncoding()
          Guess the encoding of the provided buffer.
static java.nio.charset.Charset guessEncoding(java.io.File f, int bufferLength)
           
static java.nio.charset.Charset guessEncoding(java.io.File f, int bufferLength, java.nio.charset.Charset defaultCharset)
           
private static boolean hasUTF16BEBom(byte[] bom)
          Has a Byte Order Marker for UTF-16 Big Endian (utf-16 and ucs-2).
private static boolean hasUTF16LEBom(byte[] bom)
          Has a Byte Order Marker for UTF-16 Low Endian (ucs-2le, ucs-4le, and ucs-16le).
private static boolean hasUTF8Bom(byte[] bom)
          Has a Byte Order Marker for UTF-8 (Used by Microsoft's Notepad and other editors).
private static boolean isContinuationChar(byte b)
          If the byte has the form 10xxxxx, then it's a continuation byte of a multiple byte character;
private static boolean isFiveBytesSequence(byte b)
          If the byte has the form 11110xx, then it's the first byte of a five-bytes sequence character.
private static boolean isFourBytesSequence(byte b)
          If the byte has the form 11110xx, then it's the first byte of a four-bytes sequence character.
private static boolean isSixBytesSequence(byte b)
          If the byte has the form 1110xxx, then it's the first byte of a six-bytes sequence character.
private static boolean isThreeBytesSequence(byte b)
          If the byte has the form 1110xxx, then it's the first byte of a three-bytes sequence character.
private static boolean isTwoBytesSequence(byte b)
          If the byte has the form 110xxxx, then it's the first byte of a two-bytes sequence character.
static void main(java.lang.String[] args)
           
 void setDefaultCharset(java.nio.charset.Charset defaultCharset)
          Defines the default Charset used in case the buffer represents an 8-bit Charset.
 void setEnforce8Bit(boolean enforce)
          If US-ASCII is recognized, enforce to return the default encoding, rather than US-ASCII.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

buffer

private byte[] buffer

defaultCharset

private java.nio.charset.Charset defaultCharset

enforce8Bit

private boolean enforce8Bit
Constructor Detail

CharsetToolkit

public CharsetToolkit(byte[] buffer)
Constructor of the com.glaforge.i18n.io.CharsetToolkit utility class.

Parameters:
buffer - the byte buffer of which we want to know the encoding.

CharsetToolkit

public CharsetToolkit(byte[] buffer,
                      java.nio.charset.Charset defaultCharset)
Constructor of the com.glaforge.i18n.io.CharsetToolkit utility class.

Parameters:
buffer - the byte buffer of which we want to know the encoding.
defaultCharset - the default Charset to use in case an 8-bit charset is recognized.
Method Detail

setDefaultCharset

public void setDefaultCharset(java.nio.charset.Charset defaultCharset)
Defines the default Charset used in case the buffer represents an 8-bit Charset.

Parameters:
defaultCharset - the default Charset to be returned by guessEncoding() if an 8-bit Charset is encountered.

setEnforce8Bit

public void setEnforce8Bit(boolean enforce)
If US-ASCII is recognized, enforce to return the default encoding, rather than US-ASCII. It might be a file without any special character in the range 128-255, but that may be or become a file encoded with the default charset rather than US-ASCII.

Parameters:
enforce - a boolean specifying the use or not of US-ASCII.

getEnforce8Bit

public boolean getEnforce8Bit()
Gets the enforce8Bit flag, in case we do not want to ever get a US-ASCII encoding.

Returns:
a boolean representing the flag of use of US-ASCII.

getDefaultCharset

public java.nio.charset.Charset getDefaultCharset()
Retrieves the default Charset

Returns:

guessEncoding

public java.nio.charset.Charset guessEncoding()

Guess the encoding of the provided buffer.

If Byte Order Markers are encountered at the beginning of the buffer, we immidiately return the charset implied by this BOM. Otherwise, the file would not be a human readable text file.

If there is no BOM, this method tries to discern whether the file is UTF-8 or not. If it is not UTF-8, we assume the encoding is the default system encoding (of course, it might be any 8-bit charset, but usually, an 8-bit charset is the default one).

It is possible to discern UTF-8 thanks to the pattern of characters with a multi-byte sequence.

 UCS-4 range (hex.)        UTF-8 octet sequence (binary)
 0000 0000-0000 007F       0xxxxxxx
 0000 0080-0000 07FF       110xxxxx 10xxxxxx
 0000 0800-0000 FFFF       1110xxxx 10xxxxxx 10xxxxxx
 0001 0000-001F FFFF       11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
 0020 0000-03FF FFFF       111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
 0400 0000-7FFF FFFF       1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
 

With UTF-8, 0xFE and 0xFF never appear.

Returns:
the Charset recognized.

guessEncoding

public static java.nio.charset.Charset guessEncoding(java.io.File f,
                                                     int bufferLength)
                                              throws java.io.FileNotFoundException,
                                                     java.io.IOException
Throws:
java.io.FileNotFoundException
java.io.IOException

guessEncoding

public static java.nio.charset.Charset guessEncoding(java.io.File f,
                                                     int bufferLength,
                                                     java.nio.charset.Charset defaultCharset)
                                              throws java.io.FileNotFoundException,
                                                     java.io.IOException
Throws:
java.io.FileNotFoundException
java.io.IOException

isContinuationChar

private static boolean isContinuationChar(byte b)
If the byte has the form 10xxxxx, then it's a continuation byte of a multiple byte character;

Parameters:
b - a byte.
Returns:
true if it's a continuation char.

isTwoBytesSequence

private static boolean isTwoBytesSequence(byte b)
If the byte has the form 110xxxx, then it's the first byte of a two-bytes sequence character.

Parameters:
b - a byte.
Returns:
true if it's the first byte of a two-bytes sequence.

isThreeBytesSequence

private static boolean isThreeBytesSequence(byte b)
If the byte has the form 1110xxx, then it's the first byte of a three-bytes sequence character.

Parameters:
b - a byte.
Returns:
true if it's the first byte of a three-bytes sequence.

isFourBytesSequence

private static boolean isFourBytesSequence(byte b)
If the byte has the form 11110xx, then it's the first byte of a four-bytes sequence character.

Parameters:
b - a byte.
Returns:
true if it's the first byte of a four-bytes sequence.

isFiveBytesSequence

private static boolean isFiveBytesSequence(byte b)
If the byte has the form 11110xx, then it's the first byte of a five-bytes sequence character.

Parameters:
b - a byte.
Returns:
true if it's the first byte of a five-bytes sequence.

isSixBytesSequence

private static boolean isSixBytesSequence(byte b)
If the byte has the form 1110xxx, then it's the first byte of a six-bytes sequence character.

Parameters:
b - a byte.
Returns:
true if it's the first byte of a six-bytes sequence.

getDefaultSystemCharset

public static java.nio.charset.Charset getDefaultSystemCharset()
Retrieve the default charset of the system.

Returns:
the default Charset.

hasUTF8Bom

private static boolean hasUTF8Bom(byte[] bom)
Has a Byte Order Marker for UTF-8 (Used by Microsoft's Notepad and other editors).

Parameters:
bom - a buffer.
Returns:
true if the buffer has a BOM for UTF8.

hasUTF16LEBom

private static boolean hasUTF16LEBom(byte[] bom)
Has a Byte Order Marker for UTF-16 Low Endian (ucs-2le, ucs-4le, and ucs-16le).

Parameters:
bom - a buffer.
Returns:
true if the buffer has a BOM for UTF-16 Low Endian.

hasUTF16BEBom

private static boolean hasUTF16BEBom(byte[] bom)
Has a Byte Order Marker for UTF-16 Big Endian (utf-16 and ucs-2).

Parameters:
bom - a buffer.
Returns:
true if the buffer has a BOM for UTF-16 Big Endian.

getAvailableCharsets

public static java.nio.charset.Charset[] getAvailableCharsets()
Retrieves all the available Charsets on the platform, among which the default charset.

Returns:
an array of Charsets.

main

public static void main(java.lang.String[] args)
                 throws java.io.FileNotFoundException,
                        java.io.IOException
Throws:
java.io.FileNotFoundException
java.io.IOException