觀看文件進行修改並獲取要添加的內容

Question

我知道Java7中的WatchService，但我需要添加到文件中的內容。我怎樣才能做到這一點？

例如：我有一個日誌文件，我想查看並檢查添加到文件的日誌條目。

如果程序退出（例如JVM崩潰），還有什麼方法可以繼續執行？存儲文件指針或類似？

非常感謝您的幫助！
V.

Answer 1

以下代碼應該能夠滿足您的要求。儘管它有點冗長，並且可能需要重構，但它應該可以直接使用。我在意見之間嵌入了我的意見，如有必要可以進一步解釋。

您只需要創建'/tmp/log.txt'文件，執行代碼並向文件添加一些內容。文件保存後，內容將顯示在Java控制檯上。

import java.io.BufferedReader; 
import java.io.File; 
import java.io.FileInputStream; 
import java.io.FileReader; 
import java.io.IOException; 
import java.io.PrintWriter; 

public class FileWatcher { 

    private static final String POINTER_FILE_PATH = "/tmp/pointer.txt"; 

    private static final String LOG_FILE_PATH = "/tmp/log.txt"; 

    public static void main(String[] args) throws IOException, 
      InterruptedException { 
     File logFile = new File(LOG_FILE_PATH); 

     //read the previous position in the log file 
     long pointer = getLastPointer(); 

     while (true) { 
      long fsize = logFile.length(); 

      // calculate the length of new content from the pointer position 
      int length = (int) (fsize - pointer); 

      if (length == 0) { 
       // save CPU cycles if nothing happened in the log file 
       Thread.sleep(100); 
       continue; 
      } 

      if (fsize < pointer) { 
       // throw exception if any file content has been deleted 
       // (doesn't cater for caracter replacement) 
       throw new RuntimeException("Tampered file content"); 
      } 

      // initialise a new buffer for the new content 
      byte[] buf = new byte[length]; 

      int actualLength = readFileContent(pointer, buf, length); 

      // increment the position of the pointer in the file 
      pointer = pointer + actualLength; 

      //save the current pointer 
      storePointer(pointer); 

      // print out the new content 
      System.out.print(new String(buf, 0, actualLength)); 
     } 

    } 

    private static int readFileContent(long pointer, byte[] buf, int length) throws IOException { 
     // open a new input stream to the log file 
     FileInputStream fis = new FileInputStream(LOG_FILE_PATH); 

     try { 
      // skip to the previous position in the log file 
      fis.skip(pointer); 

      // read file content into the buffer and retrieve the actual 
      // number of bytes 
      int actualLength = fis.read(buf, 0, length); 
      return actualLength; 
     } finally { 
      fis.close(); 
     } 
    } 

    private static void storePointer(long pointer) throws IOException { 
     PrintWriter pw = null; 
     try { 
      pw = new PrintWriter(POINTER_FILE_PATH); 
      pw.write(String.valueOf(pointer)); 
      pw.flush(); 
     } finally { 
      if (pw != null) { 
       pw.close(); 
      } 
     } 
    } 

    private static long getLastPointer() throws IOException { 
     long pointer = 0; 
     BufferedReader br = null; 
     try { 
      br = new BufferedReader(new FileReader(POINTER_FILE_PATH)); 
      String line = br.readLine(); 
      if (line != null && line.length() > 0) { 
       pointer = Long.valueOf(line.trim()); 
      } 
     } catch(IOException e) { 
      pointer = 0; 
     } finally { 
      if (br != null) 
       br.close(); 
     } 

     return pointer; 
    } 

}