Question

I have tried to write the console output to a txt file using this code suggestion (http://www.daniweb.com/forums/thread23883.html#) however I was not successful. What’s wrong?

try {
      //create a buffered reader that connects to the console, we use it so we can read lines
      BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

      //read a line from the console
      String lineFromInput = in.readLine();

      //create an print writer for writing to a file
      PrintWriter out = new PrintWriter(new FileWriter("output.txt"));

      //output to the file a line
      out.println(lineFromInput);

      //close the file (VERY IMPORTANT!)
      out.close();
   }
      catch(IOException e1) {
        System.out.println("Error during reading/writing");
   }

Answer 1

You need to do something like this:

PrintStream out = new PrintStream(new FileOutputStream("output.txt"));
System.setOut(out);

The second statement is the key. It changes the value of the supposedly “final” System.out attribute to be the supplied PrintStream value.

There are analogous methods (setIn and setErr) for changing the standard input and error streams; refer to the java.lang.System javadocs for details.

A more general version of the above is this:

PrintStream out = new PrintStream(
        new FileOutputStream("output.txt", append), autoFlush);
System.setOut(out);

If append is true, the stream will append to an existing file instead of truncating it. If autoflush is true, the output buffer will be flushed whenever a byte array is written, one of the println methods is called, or a \n is written.

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I’d just like to add that it is usually a better idea to use a logging subsystem like Log4j, Logback or the standard Java java.util.logging subsystem. These offer fine-grained logging control via runtime configuration files, support for rolling log files, feeds to system logging, and so on.

Alternatively, if you are not “logging” then consider the following:

• With typical shells, you can redirecting standard output (or standard error) to a file on the command line; e.g.

  $ java MyApp > output.txt   
  

  For more information, refer to a shell tutorial or manual entry.

• You could change your application to use an out stream passed as a method parameter or via a singleton or dependency injection rather than writing to System.out.

Changing System.out may cause nasty surprises for other code in your JVM that is not expecting this to happen. (A properly designed Java library will avoid depending on System.out and System.err, but you could be unlucky.)