NullPointerException when extending Log4j Logger class

Question

I wanted to add some simple methods to the Log4j logger for use in my application.

Tried doing so like this:

public class Logger extends org.apache.log4j.Logger
{
    public Logger(Class type)
    {
        super(type.getName());
    }

    public void info(String format, Object... o)
    {
        info(String.format(format, o));
    }

    public void debug(String format, Object... o)
    {
        debug(String.format(format, o));
    }

    public void warn(String format, Object... o)
    {
        warn(String.format(format, o));
    }

    public void error(String format, Object... o)
    {
        error(String.format(format, o));
    }
}

Figured I should be able to use this like so:

public class TmpTest
{
    public static void main(String[] args) throws Exception
    {
        // Note I'm not even using the special methods here
        new Logger(TmpTest.class).warn("Test");
    }
}

But I get a NullPointerException.

Exception in thread "main" java.lang.NullPointerException
    at org.apache.log4j.Category.warn(Category.java:1004)
    at no.nwn.productconfigurator.TmpTest.main(TmpTest.java:16)

Can I not do it like this? Is there some missing initialization that is done only if I use the getLogger factory methods of the Logger class?

If I can't extend the Logger class, how should I do it?

---

Could it be something about the log4j.xml? Never used this myself before and haven't written it myself.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >
<log4j:configuration debug="true">
    <appender name="stdout" class="org.apache.log4j.ConsoleAppender">
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n"/>
        </layout>
    </appender>
    <appender name="ProductConfigurator" class="org.apache.log4j.DailyRollingFileAppender">
      <!-- 
      <param name="File" value="logs/ProductConfigurator.log" />   
      -->
      <param name="File" value="/var/tomcat/logs/ProductConfigurator.log" />
      <param name="Append" value="true" />
        <layout class="org.apache.log4j.PatternLayout">
          <param name="ConversionPattern" value="%d{yyyy-MMM-dd HH:mm:ss,SSS}. %5p %c{1}:%L - %m%n"/>
        </layout>
    </appender>
    <logger name="org.hibernate" additivity="false">
    <level value="warn" />
        <appender-ref ref="ProductConfigurator"/>
    </logger>
    <logger name="org.hibernate.type" additivity="false">
    <level value="warn" />
        <appender-ref ref="ProductConfigurator"/>
    </logger>
    <logger name="org.hibernate.SQL" additivity="false">
    <level value="warn" />
        <appender-ref ref="ProductConfigurator"/>
    </logger>
    <root>
        <!-- all|trace|debug|info|warn|error|fatal|no -->
        <priority value="info" />
        <appender-ref ref="ProductConfigurator"/>
    </root>
</log4j:configuration>

Answer 1

The category.repository field is null. Look at the source code :

public void warn(Object message) {
    if(repository.isDisabled( Level.WARN_INT))
       return;

    if(Level.WARN.isGreaterOrEqual(this.getEffectiveLevel()))
        forcedLog(FQCN, Level.WARN, message, null);
}

To properly initialize a Logger, you should use the LogManager :

public static void main(String[] args) throws Exception
{              
    LogManager.getLogger(TmpTest.class.getName(), new LoggerFactory() {

        @Override
        public Logger makeNewLoggerInstance(String name) {
            return new Logger(name);
        }
    }).warn("Test");
}

I use a custom factory implementation to instantiate your logger. Not that the classic method to create a logger (Logger.getLogger(Class clazz)) use the LogManager:

static public Logger getLogger(Class clazz) {
    return LogManager.getLogger(clazz.getName());
}

---

If there isn't specific constraint in your project, i recommend you to use slf4j. It provides the methods that you are expecting :

logger.debug("Temperature set to {}. Old temperature was {}.", t, oldT);

Answer 2

Just add super. in front of all your calls to info, warn, etc.

in the following code :

public void info(String format, Object... o) {
    info(String.format(format, o));
}

When you call info(String.format(format, o)), you call the very same method in a recursive way : the first param is the formatted string, and the second (var-args) param is null, so when String.format is called again, you get a NullPointerException.

Therefore you should write this instead :

public void info(String format, Object... o) {
    super.info(String.format(format, o));
}