title: Java - multiple threads writing to same file
body: <p>I am trying to write some content to a file, through multiple threads in Java. Each thread reads a different input file, does some computation and writes some (different) content to the common output file. The problem is that in the end, the output file only contains the content written by the last terminating thread and not the content from the other threads. Relevant code for the threads -  </p>
  
  <pre><code>public void run()
  {
      try
      {
          File file = new File("/home/output.txt");
          if (!file.exists()) 
          {
               file.createNewFile();
          }
          FileWriter fw = new FileWriter(file.getAbsoluteFile());
          BufferedWriter bw = new BufferedWriter(fw);
  
          BufferedReader br = new BufferedReader(new FileReader(inputfile)); // each thread reads a different input file
          String line="";
  
          while((line=br.readLine())!=null)
          {
              String id = line.trim();               // fetch id
  
              StringBuffer sb = processId(userId);   // process id
  
              synchronized(this){
              bw.write(sb.toString() + "\n");        // write to file
              }
          }
          bw.close();
      }
      catch (IOException e)
      {
          e.printStackTrace();
      }
  }
  </code></pre>
  
  <p>How do i make all threads write their content to the common file ?</p>