Question

Usually loggers are used to log given message (error, warnings or information) to a log file (simple text file or xml)

Answer 1

Well, the obvious choces are Strategy (one logger that can be configured to use different output options) and Composite (multiplex output over several output outions).

So something like this (in Java):

public class Logger {

  public interface LogOutput {
    void out(final String s);
  }

  // Composite - use to send logging to several destinations
  public LogOutput makeComposite(final LogOutput... loggers) {
    return new LogOutput() {
      void out(final String s) {
        for (final LogOutput l : loggers) {
          l.out(s);
        }
      }
    }
  }


  private static currentLogger = new LogOutput() {
    void out(final String s) {
       // Do nothing as default - no strategy set
    }
  }

  public static log(final String s) {
    currentLogger.out(s);
  }


  // Strategy: Set a new strategy for output
  public static setLogger(final LogOutput l) {
    currentLogger = l;
  }

}