Applying Regular Expressions on the Contents of a File
The matching routines in java.util.regex require that the input be a
CharSequence object. This example implements a method that
efficiently returns the contents of a file in a CharSequence object.
Here is sample code that uses the method:
// Converts the contents of a file into a CharSequence
// suitable for use by the regex package.
public CharSequence fromFile(String filename) throws IOException {
FileInputStream fis = new FileInputStream(filename);
FileChannel fc = fis.getChannel();
// Create a read-only CharBuffer on the file
ByteBuffer bbuf = fc.map(FileChannel.MapMode.READ_ONLY, 0, (int)fc.size());
CharBuffer cbuf = Charset.forName("8859_1").newDecoder().decode(bbuf);
return cbuf;
}
try {
// Create matcher on file
Pattern pattern = Pattern.compile("pattern");
Matcher matcher = pattern.matcher(fromFile("infile.txt"));
// Find all matches
while (matcher.find()) {
// Get the matching string
String match = matcher.group();
}
} catch (IOException e) {
}
Not quite efficienlty. Charset.forName("8859_1").newDecoder().decode(bbuf) copies (with decoding) the memory-mapped byte buffer causing out-of-memory exception in situations that just accessing the byte buffer never would, thus negating all advantages of file memory-mapping