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From "Silvert, Stan" <S...@mediaocean.com>
Subject Clever Logging?
Date Mon, 14 Jan 2002 20:39:41 GMT
I am responsible for defining standards around how our developers use Log4J.
When our company started using Log4J a couple of months ago, I told
developers to use this technique in their code:

public class MyClass {
    private static final Category LOG =
Category.getInstance(MyClass.class.getName());
    private static boolean DEBUG() { return LOG.isDebugEnabled(); }
}

So, for all debug statements, write something like this: 
  if (DEBUG()) LOG.debug("The value of my vars are: " + v1 + " and " + v2);

If not a debug statement, just do something like this:
    LOG.info("My info message");

That all worked fine, except that developers would make the mistake of
putting the wrong class name inside the getInstance() method.  This would
usually happen when they would cut and paste code.  

So, one of our developers came up with a clever solution to cut down on the
mistakes.  He created a logging utility that could automatically read the
stack frames, discover which class logged the message, and invoke logging
from the proper Category.

Here is how he did it:

public class LogUtil {    
    /**
     * Extends <code>java.lang.SecurityManager</code> to provide the
<code>getClassName()</code>
     * method.
     */
    private static class ClassGetter extends SecurityManager {
        // Inner class necessary because getClassContext() is protected
        
        /**
         * Returns the name of the calling class.
         * @return  the name of the calling class
         */
        public String getClassName() {
            // getClassContext() returns the current execution stack trace
            // as an array of classes -- the element at index 0 is the class
            // of the currently executing method, the element at index 1 is
            // the class of that method's caller, and so on. So, 0 is going
            // to be this class (ClassGetter), 1 will be LogUtil, and 2 will
            // be the magic number we are after.
            return getClassContext()[2].getName();
        }
    }
    private static ClassGetter classGetter = new ClassGetter();

    /**
     * Returns the name of the calling class.
     * Intended to be used from static methods or initializers to
automagically
     * determine the calling class's name.
     * @return  the name of the calling class
     */
    public static String getClassName() {
        return classGetter.getClassName();
    }
    
    ///////////////////////////////////////////////////////////////////

    /**
     * Returns <code>true</code> if debugging is enabled for a given
category
     *
     * @return  <code>true</code> if debugging is enabled,
     *          <code>false</code> if not
     */
    public static boolean DEBUG() {
        Category cat = Category.getInstance(classGetter.getClassName());
        return cat.isDebugEnabled();
    }            
    
    ///////////////////////////////////////////////////////////////////

    /**
     * Logs the given message with <code>DEBUG</code> priority, to the
category
     * automagically derived from the calling class name.
     *
     * @param  msg  the message to log
     */
    public static void debug(String msg) {
        Category cat = Category.getInstance(classGetter.getClassName());
        cat.debug(msg);
    }
}

This looks very nice and seems to solve all the problems quite well - except
for one thing.  I have been reading about how some JIT's can remove stack
frames during optimization.  This would cause messages to once again be
logged under the wrong Category.

My questions to the folks on this list are, 
What is your opinion of both techniques?
Do you know other developers who have tried the "clever" approach and run
into the "disappearing stack frame" problem?
Is there another solution we are overlooking?  (short of disabling cut and
paste from everyone's IDE)

Thanks in advance for your input.

Stan Silvert


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