grep.java

java应用开发详解

// @(#)Grep.java	1.1 01/05/10
//
// Search a list of files for lines that match a given regular-expression
// pattern.  Demonstrates NIO mapped byte buffers, charsets, and regular
// expressions.

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;
import java.util.regex.*;


public class Grep {

    // Charset and decoder for ISO-8859-15
    private static Charset charset = Charset.forName("ISO-8859-15");
    private static CharsetDecoder decoder = charset.newDecoder();

    // Pattern used to parse lines
    private static Pattern linePattern
	= Pattern.compile(".*\r?\n");

    // The input pattern that we're looking for
    private static Pattern pattern;

    // Compile the pattern from the command line
    //
    private static void compile(String pat) {
	try {
	    pattern = Pattern.compile(pat);
	} catch (PatternSyntaxException x) {
	    System.err.println(x.getMessage());
	    System.exit(1);
	}
    }

    // Use the linePattern to break the given CharBuffer into lines, applying
    // the input pattern to each line to see if we have a match
    //
    private static void grep(File f, CharBuffer cb) {
	Matcher lm = linePattern.matcher(cb);	// Line matcher
	Matcher pm = null;			// Pattern matcher
	int lines = 0;
	while (lm.find()) {
	    lines++;
	    CharSequence cs = lm.group(); 	// The current line
	    if (pm == null)
		pm = pattern.matcher(cs);
	    else
		pm.reset(cs);
	    if (pm.find())
		System.out.print(f + ":" + lines + ":" + cs);
	    if (lm.end() == cb.limit())
		break;
	}
    }

    // Search for occurrences of the input pattern in the given file
    //
    private static void grep(File f) throws IOException {

	// Open the file and then get a channel from the stream
	FileInputStream fis = new FileInputStream(f);
	FileChannel fc = fis.getChannel();

	// Get the file's size and then map it into memory
	int sz = (int)fc.size();
	MappedByteBuffer bb = fc.map(FileChannel.MAP_RO, 0, sz);

	// Decode the file into a char buffer
	CharBuffer cb = decoder.decode(bb);

	// Perform the search
	grep(f, cb);

	// Close the channel and the stream
	fc.close();
    }

    public static void main(String[] args) {
	if (args.length < 2) {
	    System.err.println("Usage: java Grep pattern file...");
	    return;
	}
	compile(args[0]);
	for (int i = 1; i < args.length; i++) {
	    File f = new File(args[i]);
	    try {
		grep(f);
	    } catch (IOException x) {
		System.err.println(f + ": " + x);
	    }
	}
    }

}