Java模拟实现grep文件的功能

import java.io.File;
import java.io.FileInputStream;
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;
 
 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.MapMode.READ_ONLY, 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);
             }
         }
     }
 }