CODEWITHSUNDEEP
javafile-io
idiotgenius
idiotgenius

Reputation: 499

Java IO - read a big file while other application writing to it

I want to use java to read a weblogic log file while weblogic is writing log into it (buffered),but I only want read content that is present when I start to read it.

how can I do this ?

public class DemoReader implements Runnable{

    public void run() {
        File f = new File ("c:\\test.txt");
        long length = f.length();
        long readedBytes = 0; 
        System.out.println(length);
        try {
            BufferedReader fr = new BufferedReader(new FileReader(f));
            String line = "";
            while((line = fr.readLine()) != null && readedBytes < length){
                readedBytes += line.getBytes().length;
                if(readedBytes > length){
                    break;
                }else{
                    System.out.println(line);
                }
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

Upvotes: 5

Views: 2842

Answers (2)

Gary
Gary

Reputation: 7257

So long as the log file is only locked for write access you should be able to copy it as @karim79 has suggested. After that the copy belongs to you so you can do whatever you like with it.

Here is some code that should achieve what you're after - it just copies the file byte by byte to the System.out stream:

public class Main {

  public static void main(String[] args) throws IOException {

    // Identify your log file
    File file = new File("path/to/your/logs/example.log");

    // Work out the length at the start (before Weblogic starts writing again)
    long size = file.length();

    // Read in the data using a buffer
    InputStream is = new FileInputStream(file);
    BufferedInputStream bis = new BufferedInputStream(is);

    long byteCount=0;

    int result;
    do {
      // Read a single byte
      result = bis.read();
      if (result != -1)
      {
        // Do something with your log
        System.out.write(result);
      } else {
        // Reached EOF
        break;
      }
      byteCount++;
    } while (byteCount<size);

    // Printing this causes a final flush of the System.out buffer
    System.out.printf("%nBytes read=%d",byteCount);

    bis.close();
    is.close();
  }

}

And there you go.

Notes on log files

If the log file is enormous (say >1Gb) then you should really consider changing your logging configuration to incorporate a rolling log file which will automatically break the logs down into chunks (say 1Mb) which are more suitable for viewing in shell editors (like vim).

Upvotes: 1

khachik
khachik

Reputation: 28703

You can get the size of the file at the moment you are starting to read, and then read N number of bytes (assuming that the file isn't locked by the writer and its content from 0 to N is not going to be changed).

Upvotes: 3

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