使用java实现grep功能(FileChannel, Pattern, MappedByteBuffer 直接字节缓冲区，其内容是文件的内存映射区域)

利用正则表达式查找一系列文件,类似于grep功能. 演示了 NIO mapped byte buffers, charsets, and regular expressions

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.CharBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;

/**
 * User: wenzhihong
 * Date: 12-2-23
 * Time: 上午10:06
 */
public class Grep {
    private static Charset charset = Charset.forName("utf-8");
    private static CharsetDecoder decoder = charset.newDecoder();
    
    private static Pattern linePattern = Pattern.compile(".*\r?\n");
    private static Pattern pattern;

    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 e) {
                System.err.println(f + ":" + e);
            }

        }
    }

    private static void grep(File f) throws IOException {
        //取得 FileChannel
        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.MapMode.READ_ONLY, 0, sz);
        //把 字节buffer decode 成 charBuffer
        CharBuffer cb = decoder.decode(bb);
        
        grep(f, cb);

        fc.close();
    }

    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 curLineCs = lm.group();//the current line
            if (pm == null){
                pm = pattern.matcher(curLineCs);
            } else {
                pm.reset(curLineCs);
            }
            
            if (pm.find()){
                System.out.println(f + ":" + lines + ":" + curLineCs);
            }

            if (lm.end() == cb.limit()) { //文件最后
                break;
            }

        }
        
    }

    private static void compile(String pat) {
        try {
            pattern = Pattern.compile(pat);
        } catch (PatternSyntaxException e) {
            System.err.println(e.getMessage());
            System.exit(1);
        }
    }
}