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显示文件的倒数XX行
2006/05/12,17:43
读取日志时经常想到这个方法,但是苦于Java的文件操作函数太少.

想到使用随机文件类读取最后一行,然后回退读取,判断回车,直到回车数到达你要求的函数,然后就ok了

使用google搜索一下,竟然找到一个现成了,太爽了

http://crawler.archive.org/xref-test/org/archive/crawler/util/LogReader.html

不过这个东西实在非我能承受,只好copy出来,注明出处了

相关的片断:

  1. /**
  2.      * Implementation of a unix-like 'tail' command
  3.      *
  4.      * @param aFileName a file name String
  5.      * @return An array of two strings is returned. At index 0 the String
  6.      *         representation of at most 10 last lines is located.
  7.      *         At index 1 there is an informational string about how large a
  8.      *         segment of the file is being returned.
  9.      *         Null is returned if errors occur (file not found or io exception)
  10.      */
  11.     public static String[] tail(String aFileName) {
  12.         return tail(aFileName, 10);
  13.     }
  14.  
  15.     /**
  16.      * Implementation of a unix-like 'tail -n' command
  17.      *
  18.      * @param aFileName a file name String
  19.      * @param n int number of lines to be returned
  20.      * @return An array of two strings is returned. At index 0 the String
  21.      *         representation of at most n last lines is located.
  22.      *         At index 1 there is an informational string about how large a
  23.      *         segment of the file is being returned.
  24.      *         Null is returned if errors occur (file not found or io exception)
  25.      */
  26.     public static String[] tail(String aFileName, int n) {
  27.         try {
  28.             return tail(new RandomAccessFile(new File(aFileName),"r"),n);
  29.         } catch (FileNotFoundException e) {
  30.             e.printStackTrace();
  31.             return null;
  32.         }
  33.     }
  34.  
  35.     /**
  36.      * Implementation of a unix-like 'tail -n' command
  37.      *
  38.      * @param raf a RandomAccessFile to tail
  39.      * @param n int number of lines to be returned
  40.      * @return An array of two strings is returned. At index 0 the String
  41.      *         representation of at most n last lines is located.
  42.      *         At index 1 there is an informational string about how large a
  43.      *         segment of the file is being returned.
  44.      *         Null is returned if errors occur (file not found or io exception)
  45.      */
  46.     public static String[] tail(RandomAccessFile raf, int n) {
  47.         int BUFFERSIZE = 1024;
  48.         long pos;
  49.         long endPos;
  50.         long lastPos;
  51.         int numOfLines = 0;
  52.         String info=null;
  53.         byte[] buffer = new byte[BUFFERSIZE];
  54.         StringBuffer sb = new StringBuffer();
  55.         try {
  56.             endPos = raf.length();
  57.             lastPos = endPos;
  58.  
  59.             // Check for non-empty file
  60.             // Check for newline at EOF
  61.             if (endPos > 0) {
  62.                 byte[] oneByte = new byte[1];
  63.                 raf.seek(endPos - 1);
  64.                 raf.read(oneByte);
  65.                 if ((char) oneByte[0] != '
') {
  66.                     numOfLines++;
  67.                 }
  68.             }
  69.  
  70.             do {
  71.                 // seek back BUFFERSIZE bytes
  72.                 // if length of the file if less then BUFFERSIZE start from BOF
  73.                 pos = 0;
  74.                 if ((lastPos - BUFFERSIZE) > 0) {
  75.                     pos = lastPos - BUFFERSIZE;
  76.                 }
  77.                 raf.seek(pos);
  78.                 // If less then BUFFERSIZE avaliable read the remaining bytes
  79.                 if ((lastPos - pos) < BUFFERSIZE) {
  80.                     int remainer = (int) (lastPos - pos);
  81.                     buffer = new byte[remainer];
  82.                 }
  83.                 raf.readFully(buffer);
  84.                 // in the buffer seek back for newlines
  85.                 for (int i = buffer.length - 1; i >= 0; i--) {
  86.                     if ((char) buffer[i] == '
') {
  87.                         numOfLines++;
  88.                         // break if we have last n lines
  89.                         if (numOfLines > n) {
  90.                             pos += (i + 1);
  91.                             break;
  92.                         }
  93.                     }
  94.                 }
  95.                 // reset last postion
  96.                 lastPos = pos;
  97.             } while ((numOfLines <= n) && (pos != 0));
  98.  
  99.             // print last n line starting from last postion
  100.             for (pos = lastPos; pos < endPos; pos += buffer.length) {
  101.                 raf.seek(pos);
  102.                 if ((endPos - pos) < BUFFERSIZE) {
  103.                     int remainer = (int) (endPos - pos);
  104.                     buffer = new byte[remainer];
  105.                 }
  106.                 raf.readFully(buffer);
  107.                 sb.append(new String(buffer));
  108.             }
  109.  
  110.             info = buildDisplayingHeader(sb.length(), raf.length());
  111.         } catch (FileNotFoundException e) {
  112.             sb = null;
  113.         } catch (IOException e) {
  114.             e.printStackTrace();
  115.             sb = null;
  116.         } finally {
  117.             try {
  118.                 if (raf != null) {
  119.                     raf.close();
  120.                 }
  121.             } catch (IOException e) {
  122.                 e.printStackTrace();
  123.             }
  124.         }
  125.         if(sb==null){
  126.             return null;
  127.         }
  128.         String[] tmp = {sb.toString(),info};
  129.         return tmp;
  130.     }

 

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