Log4j troubleshooting

Ceki Gülcü Paul Glezen
February 2002

Here is a list of commonly encountered problems when using log4j:

• log4j tells me to initialize properly.

• Duplicates in log4j output.

• Options are not parsed correctly.

• Caller location information is printed as a "?" character.

• ClassCastException when instantiating a Category subclasses.

• My EJB-Servlet-JSP says log4j class not found or not defined.

• I cannot connect to syslogd under linux.

• log4j:WARN No such property [xyz] in some.appender.or.layout

log4j tells me to initialize properly.

log4j: No appenders could be found for category (some.category.name).
log4j: Please initialize the log4j system properly.

Log4j does not have a default logging target. It is the user's responsibility to ensure that all categories can inherit an appender. This can be easily achieved by attaching an appender to the root category.

Duplicates in log4j output.

The reason for observing duplicates in log4j output is either due to having added the same appender multiple times to the same category (typically root) or having added the same appender to different categories ignoring the fact that appenders are inherited cumulatively.

log4j does not eliminate appender duplicates. In other words, if you add the same appender to a category n times, that appender will be invoked n times to append to its target.

A slightly different cause is adding different appenders all sharing the same underlying output target to some category. In the most common occurrence of this phenomenon, the BasicConfigurator.configure() method is invoked multiple times. Each time it is invoked, this method adds an appender with a System.out target to the root category.

One other common mistake is to forget that appenders are inherited cumulatively from the hierarchy. For example, if you add an appender, say A, to the root category, all other categories will inherit A as an appender. Thus, if you add A to a category, say C, then an enabled statement of category C, will print to A twice, once because A is in root and once because it is in C.

Options are not parsed correctly.

The PropertyConfigurator relies on java.util.Properties class to read in the configuration file. This class preserves spaces in options. For example,

fruit=orange  

The spaces in the value, i.e. "orange ", are due to invisible spaces at the end of the example shown above. Thus, some of the options might not be interpreted correctly due to trailing spaces.

Caller location information is printed as a "?" character.

You can remedy this problem by disabling the JIT compiler and by compiling the code without the -O option.

In wrappers or subclasses of Category

Wrappers or subclasses of Category need supply their fully qualified class name to the Category.log method or to Category.forcedLog methods so that the caller location information can be extracted correctly.

This approach will work correctly in all cases except if the class invoking the extended category instance has the same prefix as the extended category class. For example, calling an instance of com.foo.BarCategory from the com.foo.BarCategoryTest class will not yield the correct caller information. To circumvent this "bug", either perform the tests from a class with a different name or add a dot to the fully qualified name of the extending class that you supply to Category.log to Category.forcedLog methods. For the com.foo.BarCategory example, supply the string "com.foo.BarCategory.".

ClassCastException when instantiating a Category subclasses.

This exception is thrown because log4j does not support homonyms. For example, the following will systematically throw a ClassCastException

  Category c1 = Category.getInstance("bad");
  MyCategory c2 = (MyCategory) MyCategory.getInstance("bad");

By default, the PropertyConfigurator will create and configure org.apache.log4j.Category objects. Thus, if you try to instantiate a category subclass for an already existing category, and try to cast it to the subclass type, you will systematically get a ClassCastException.

To address this problem, the PropertyConfigurator admits the log4j.categoryFactory key. The value of this key will be used as the factory to invoke when instantiating Category objects.

The DOMConfigurator has a finer grain method for setting the class of the category object to instantiate.

Log4j class not found/defined

A good article on classloaders with examples using IBM's WebSphere application server can be found here in PDF format.

I cannot log to syslogd under linux.

This can be done by adding the -r option when starting the daemon. Or more precisely:

1. Login as the root user
2. Edit file /etc/rc/init.d/syslog

   case "$1" in
     start)
           echo -n "Starting system logger: "
           daemon syslogd -r
   

3. /etc/rc/init.d/syslog restart

log4j:WARN No such property [xyz] in some.appender.or.layout