How can I register log4j in an existing J2EE Struts web application?

Question

I recently inherited the J2EE Struts web application, which was written back in 2002. There is no logging in the application, except for the odd System.out.println ().
I added log4j logging so that I can write some information to the console, but I'm worried about how best to approach this. Any suggestions, tips and tricks will be welcome, as I don’t want to spend too much time adding logging to each method or class (or trying to find in which places it is better to keep a log, i.e. Block / code defects).
My current approach is to simply add a few protocols to the several classes that I was looking to understand the code, but are there a few key places where I can add logging to maximize my use of adding log4j?

Edit:
Update progress:
I expanded Struts (v.1) ExceptionHandlerand configured struts-config.xmlto use mine CustomExceptionHandlerinstead of the Struts version. Then, overriding the method execute(), I added some logic to register the exception with log4j. See below:

public class CustomExceptionHandler extends ExceptionHandler {

 static Logger logger = Logger.getLogger (CustomExceptionHandler.class);

 public ActionForward execute (Exception ex, ExceptionConfig ae, ActionMapping mapping, ActionForm formInstance, HttpServletRequest request, HttpServletResponse response) throws ServletException {
  logException (ex);
  return super.execute (ex, ae, mapping, formInstance, request, response);
 }

 private void logException (Throwable thr) {
   // Add code here to log the exception appropiately
   // ie logger.error ("Exception is:" + ex.getMessage ());
}

You must also update the struts-config.xml file:

<global-exceptions>
<exception key="" type="java.lang.Throwable" handler="com.mycompany.CustomExceptionHandler" />
</global-exceptions>

Now I can be sure that I will always correctly record any exceptions.
With the exception of the exceptions that were eaten, I wrap them in unchecked exceptions so that they get to the top:

} catch (Exception e) {
// nothing here
}

Changed to:

} catch (Exception e) {
 throw new RuntimeException (e);
}

Answer 1

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Answer 2

, System.out(System.err), , , System.out :

public class LoggerStream extends PrintStream {
  Logger legacyLogger = Logger.getLogger("app.legacyLogger");

  ...

  public void println(String s){
    legacyLogger.log(s);
  }
}

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