Troubleshooting log4j

# 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.

Logging output is written to a target by using an appender. If no appenders are attached to a category nor to any of its ancestors, you will get the following message when trying to log:

    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

is returned as an option having the key `"fruit"` and the value `"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.

Location information is extracted automatically by the PatternLayout conversion patterns %C, %F, %M and %L. However, some just-in-time (JIT) compilers make it impossible to extract location information. It is also possible that the compiler that generated the byte code may have omitted the LineNumber table as is done by -O option of javac and jikes.

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");

where `MyCategory` is a subclass of `Category`. The problem occurs because the second `getInstance` invocation will retrieve the category created in the fist invocation. This instance is a `Category` object and cannot be cast to `MyCategory`.

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

Naturally you should check the classpath. But you should also be aware of the presence of multiple classloaders in the JVM:

1. the bootstrap classloader
2. the extension classloader
3. the application classloader

If you place log4j.jar in the `jre/lib/ext` directory but place user-defined extensions to log4j in the application classloader classpath, log4j configurators will not find them.

Servlet, JSP and EJB containers inside of application servers usually have their own special classloaders in addition to the three mentioned above. While this provides for a greater degree of separation for different webapps, EJB containers and the application server runtime itself, it can provide headaches to the uninitiated.

Classloaders are usually hierarchically related. The bootstrap loader forms the root with the extension loader as its child. The application loader is the child of the extension loader and it's this "app loader" that we use by default when we write standalone Java programs.

Upon receiving a class load request, the classloader usually delegates it to the parent before attempting to service the request. This allows the bootstrap and extension loaders to deliver any classes that are part of the JDK or its extensions. Only after this delegation fails will the classloader attempt to find the class itself. Note that classloaders do not delegate requests to children.

Application servers often use the application loader for its runtime classes and create separate classloaders for its webapp and EJB containers. These additional classloaders may descend directly from the app server's runtime classloader. If log4j is placed in the classpath of a webapp classloader, another webapp classloader will not necessarily see it. EJBs wouldn't see it either. If log4j is intended to be made available to all objects participating in the app server, it should be included in the classpath of a classloader high enough in the classloader hierarchy to be seen by all classloaders.

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.

If you are trying to log to the Unix syslog under Linux using the SyslogAppender, then the Linux syslog daemon must be configured to accept log input from the network. Otherwise, you will get an `IOException: connection refused`.

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

If during log4j configuration you get a warning about an inexistent property, then you have probably misspelled a property or entered a truly unrecognized property for the component you are trying to configure in the configuration file.

Log4j version 1.0 did not complain about unrecognized properties whereas log4j version 1.1 and later do complain.