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## Frequently Asked Technical Questions

1. What is log4j?
2. Is log4j a reliable logging system?
3. What are the prerequisites for log4j?
4. What are the features of log4j?
5. Is there example code for using log4j?
6. What documentation should I read to learn more about log4j?
7. Is log4j thread-safe?
8. What does log output look like?
9. Why should I use log4j when JDK 1.4 already ships with a logging API?
10. What are Loggers?
11. How can I change log behavior at runtime?
12. What is the fastest way of (not) logging?
13. Are there any suggested ways for naming loggers?
14. How do I get the fully-qualified name of a class in a static block?
15. Can the log output format be customized?
16. What are the configurable options for `FooBarAppender`?
17. What is the recommended way of migrating from java.util.logging to log4j?
18. Is it possible to direct log output to different appenders by level?
19. What does the Windows NT Event Viewer complain about missing descriptions for my event messages when I use the `NTEventLogAppender`?
20. Why can't I map my logger names to the loggers that appear in the NT Event Log when I use the NTEventLogAppender?
21. Are there suggested approaches for logging in JSP pages?
22. Can the outputs of multiple client request go to different log files?
23. Logger instances seem to be create only. Why isn't there a method to remove logger instances?
24. How do I get multiple process to log to the same file?
25. How about the timesamps of events generated by multiple processes across multiple hosts (possibly across multiple timezones)?
26. Why can't log4j find my properties file in a J2EE or WAR application?
27. Is there a way to get log4j to automatically reload a configuration file if it changes?
28. Why should I donate my extensions to log4j back to the project?
29. What should I keep in mind when contributing code?
30. Why am I getting multiple copies of messages in my log file?
31. How do I add a custom level?
32. Why does log4j throw a NullPointerException or print a message about NOPLoggerRepository when shutting down or restarting under Tomcat or during a shutdown hook?
33. Why isn't my rolling file appender properly rolling files or why does logging events get written to the old log file?
34. Why do I see a warning about "No appenders found for logger" and "Please configure log4j properly"?
35. What system properties are checked by log4j?

What is log4j?

  log4j is a tool to help the programmer output log statements to a variety of output targets.

  In case of problems with an application, it is helpful to enable logging so that the problem can be located. With log4j it is possible to enable logging at runtime without modifying the application binary. The log4j package is designed so that log statements can remain in shipped code without incurring a high performance cost. It follows that the speed of logging (or rather not logging) is capital.

  At the same time, log output can be so voluminous that it quickly becomes overwhelming. One of the distinctive features of log4j is the notion of hierarchical loggers. Using loggers it is possible to selectively control which log statements are output at arbitrary granularity.

  log4j is designed with three goals in mind: reliability, speed and flexibility. There is a tight balance between these requirements. We believe that log4j strikes the right balance.

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Is log4j a reliable logging system?

  No. log4j is not reliable. It is a best-effort fail-stop logging system.

  By fail-stop, we mean that log4j will not throw unexpected exceptions at run-time potentially causing your application to crash. If for any reason, log4j throws an uncaught exception, please send an email to the log4j-user@logging.apache.org mailing list. Uncaught exceptions are handled as serious bugs requiring immediate attention.

  Moreover, log4j will not revert to System.out or System.err when its designated output stream is not opened, is not writable or becomes full. This avoids corrupting an otherwise working program by flooding the user's terminal because logging fails. However, log4j will output a single message to System.err indicating that logging can not be performed.

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What are the prerequisites for log4j?

  Log4j 1.2.8 and earlier are compatible with JDK 1.1.x and later, later versions of log4j 1.2 are compatible with JDK 1.2 and later.

  The DOMConfigurator is based on the DOM Level 1 API. The DOMConfigurator.configure(Element) method will work with any XML parser that will pass it a DOM tree.

  The DOMConfigurator.configure(String filename) method and its variants require a JAXP compatible XML parser, for example Xerces or Sun's parser. Compiling the DOMConfigurator requires the presence of a JAXP parser in the classpath.

  The `org.apache.log4j.net.SMTPAppender` relies on the JavaMail API. It has been tested with JavaMail API version 1.2. The JavaMail API requires the JavaBeans Activation Framework package.

  The `org.apache.log4j.net.JMSAppender` requires the presence of the JMS API as well as JNDI.

  log4j test code relies on the JUnit testing framework.

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What are the features of log4j?

  log4j is optimized for speed.

  log4j is based on a named logger hierarchy.

  log4j is fail-stop. However, altough it certainly strives to ensure delivery, log4j does not guarantee that each log statement will be delivered to its destination.

  log4j is thread-safe.

  log4j is not restricted to a predefined set of facilities.

  Logging behavior can be set at runtime using a configuration file. Configuration files can be property files or in XML format.

  log4j is designed to handle Java Exceptions from the start.

  log4j can direct its output to a file, the console, an `java.io.OutputStream`, `java.io.Writer`, a remote server using TCP, a remote Unix Syslog daemon, to a remote listener using JMS, to the NT EventLog or even send e-mail.

  log4j uses 6 levels, namely TRACE, DEBUG, INFO, WARN, ERROR and FATAL.

  The format of the log output can be easily changed by extending the `Layout` class.

  The target of the log output as well as the writing strategy can be altered by implementations of the `Appender` interface.

  log4j supports multiple output appenders per logger.

  log4j supports internationalization.

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Is there example code for using log4j?

  See the `examples/` directory.

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What documentation should I read to learn more about log4j?

  Make sure to read the short manual. It is also recommended to you read The complete log4j manual which is much more detailed and up to date. Both documents were written by Ceki Gülcü.

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Is log4j thread-safe?

  Yes, log4j is thread-safe. Log4j components are designed to be used in heavily multithreaded systems.

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What does log output look like?

  The log output can be customized in many ways. Moreover, one can completely override the output format by implementing one's own Layout.

  Here is an example output using PatternLayout with the conversion pattern "%r [%t] %-5p %c{2} %x - %m%n"

      176 [main] INFO  examples.Sort - Populating an array of 2 elements in reverse order.
      225 [main] INFO  examples.SortAlgo - Entered the sort method.
      262 [main] DEBUG SortAlgo.OUTER i=1 - Outer loop.
      276 [main] DEBUG SortAlgo.SWAP i=1 j=0 - Swapping intArray[0] = 1 and intArray[1] = 0
      290 [main] DEBUG SortAlgo.OUTER i=0 - Outer loop.
      304 [main] INFO  SortAlgo.DUMP - Dump of interger array:
      317 [main] INFO  SortAlgo.DUMP - Element [0] = 0
      331 [main] INFO  SortAlgo.DUMP - Element [1] = 1
      343 [main] INFO  examples.Sort - The next log statement should be an error message.
      346 [main] ERROR SortAlgo.DUMP - Tried to dump an uninitialized array.
              at org.log4j.examples.SortAlgo.dump(SortAlgo.java:58)
              at org.log4j.examples.Sort.main(Sort.java:64)
      467 [main] INFO  examples.Sort - Exiting main method.

  The first field is the number of milliseconds elapsed since the start of the program. The second field is the thread outputting the log statement. The third field is the level of the log statement. The fourth field is the rightmost two components of the logger making the log request. The fifth field (just before the '-') is the nested diagnostic context (NDC). Note the nested diagnostic context may be empty as in the first two statements. The text after the '-' is the message of the statement.

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Why should I use log4j when JDK 1.4 already ships with a logging API?

  Although both APIs are conceptually similar, the log4j API is significantly more flexible and offers many more features, too numerous to be listed here. You will discover that the additional features and flexibility turn out to be indispensable in the context of a mission-critical application.

  The open and collaborative way in which log4j is developped ensures that it continues to preserve and even widen its competitive edge. At some point, input from bright developers from all over the world is bound to make a difference.

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What are Loggers?

  Lggers lie at the heart of log4j. Loggers define a hierarchy and give the programmer run-time control on which statements are printed or not.

  Loggers are assigned levels. A log statement is printed depending on its level and its logger.

  Make sure to read the log4j manual for more information.

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How can I change log behavior at runtime?

  Log behavior can be set using configuration files which are parsed at runtime. Using configuration files the programmer can define loggers and set their levels.

  The `PropertyConfigurator` defines a particular format of a configuration file. See also the `examples/Sort.java` example and associated configuration files.

  Configuration files can be specified in XML. See `log4j.dtd` and `org.log4j.xml.DOMConfigurator` for more details.

  See the various Layout and Appender components for specific configuration options.

  In addition to configuration files, the user may disable all messages belonging to a set of levels. See next item.

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What is the fastest way of (not) logging?

  For some logger `l`, writing,

       l.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));

  incurs the cost of constructing the message parameter, that is converting both integer `i` and `entry[i]` to a String, and concatenating intermediate strings. This, regardless of whether the message will be logged or not.

  If you are worried about speed, then write

         if(l.isDebugEnabled()) {
           l.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));
         }

  or using LogMF from the extras companion write

          LogMF.debug(logger, "Entry number: {0} is {1}", i, entry[i]);

  This way you will not incur the cost of parameter construction if debugging is disabled for logger `l`. On the other hand, if the logger is debug enabled, you will incur the cost of evaluating whether the logger is enabled or not, twice: once in `debugEnabled` and once in `debug`. This is an insignificant overhead since evaluating a logger takes less than 1% of the time it takes to actually log a statement.

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Are there any suggested ways for naming loggers?

  Yes, there are.

  You can name loggers by locality. It turns out that instantiating a logger in each class, with the logger name equal to the fully-qualified name of the class, is a useful and straightforward approach of defining loggers. This approach has many benefits:

  - It is very simple to implement.
  - It is very simple to explain to new developers.
  - It automatically mirrors your application's own modular design.
  - It can be further refined at will.
  - Printing the logger automatically gives information on the locality of the log statement.

  However, this is not the only way for naming loggers. A common alternative is to name loggers by functional areas. For example, the "database" logger, "RMI" logger, "security" logger, or the "XML" logger.

  You may choose to name loggers by functionality and subcategorize by locality, as in "DATABASE.com.foo.some.package.someClass" or "DATABASE.com.foo.some.other.package.someOtherClass".

  You are totally free in choosing the names of your loggers. The log4j package merely allows you to manage your names in a hierarchy. However, it is your responsibility to define this hierarchy.

  Note by naming loggers by locality one tends to name things by functionality, since in most cases the locality relates closely to functionality.

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How do I get the fully-qualified name of a class in a static block?

  You can easily retrieve the fully-qualified name of a class in a static block for class X, with the statement `X.class.getName()`. Note that `X` is the class name and not an instance. The `X.class` statement does not create a new instance of class `X`.

  Here is the suggested usage template:

      package a.b.c;

      public class Foo {
        final static Logger logger = Logger.getLogger(Foo.class);
        ... other code

      }

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Can the log output format be customized?

  Yes, you can extend the `Layout` class to create you own customized log format. Appenders can be parameterized to use the layout of your choice.

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What are the configurable options for `FooBarAppender`?

  Log4j uses JavaBeans style configuration.

  Thus, any setter method in `FooBarAppender` corresponds to a configurable option. For example, in `RollingFileAppender` the `setMaxBackupIndex(int maxBackups)` method corresponds to the `maxBackupIndex` option. The first letter of the option can be upper case, i.e. `MaxBackupIndex` and `maxBackupIndex` are equivalent but not `MAXBACKUPIndex` nor `mAXBackupIndex`.

  Layouts options are also defined by their setter methods. The same goes for most other log4j components.

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What is the recommended way of migrating from java.util.logging to log4j?

  We suggest to just use global file search/replace. You should be able to replace all the "java.util.Logger" references with "org.apache.log4j.Logger", and you should be on your way.

  If you're on a Win32 platform, we recommend Textpad. You can use the CTRL+SHIFT+O to open all *.java files from a directory including all its sub-directories, and then use the search/replace function to replace in all files, and then CTRL+SHIFT+S to save all. Should take about 60 seconds! :)

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Is it possible to direct log output to different appenders by level?

  Yes it is. Setting the Threshold option of any appender extending AppenderSkeleton, (most log4j appenders extend AppenderSkeleton) to filter out all log events with lower level than the value of the threshold option.

  For example, setting the threshold of an appender to DEBUG also allow INFO, WARN, ERROR and FATAL messages to log along with DEBUG messages. This is usually acceptable as there is little use for DEBUG messages without the surrounding INFO, WARN, ERROR and FATAL messages. Similarly, setting the threshold of an appender to ERROR will filter out DEBUG, INFO and WARN messages but not ERROR or FATAL messages.

  This policy usually best encapsulates what the user actually wants to do, as opposed to her mind-projected solution.

  See examples/sort4.lcf for an example threshold configuration.

  If you must filter events by exact level match, then you can attach a LevelMatchFilter to any appender to filter out logging events by exact level match.

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What does the Windows NT Event Viewer complain about missing descriptions for my event messages when I use the `NTEventLogAppender`?

  The NT Event Viewer relies on message resource DLLs to properly view an event message. The NTEventLogAppender.dll contains these message resources, but that DLL must be copied to %SYSTEMROOT%\SYSTEM32 for it to work properly.

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Why can't I map my logger names to the loggers that appear in the NT Event Log when I use the NTEventLogAppender?

  Unfotunately, the logger names are hardcoded within the message resource DLL (see previous question about NTEventLogAppender), so there isn't any easy way to override those dynamically... in fact, I don't think it's possible to do it, as you'd have to modify the DLL resources for every application. Since most native applications don't use the Logger column anyway...

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Are there suggested approaches for logging in JSP pages?

  The suggested approach depends on your design requirements. If you or your organization has no constraints on the use of Java in JSP pages, simply use log4j normally in `<% ... %>` statements as indicated in the Short Manual and the rest of the documentation.

  However, if your design calls for a minimum amount of Java in your JSP pages, consider using the Log Taglib from the Jakarta Taglibs project. It provides logging JSP tags that invoke log4j.

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Can the outputs of multiple client request go to different log files?

  Many developers are confronted with the problem of distinguishing the log output originating from the same class but different client requests. They come up with ingenious mechanisms to fan out the log output to different files. In most cases, this is not the right approach.

  It is simpler to use a nested diagnostic context (NDC). Typically, one would NDC.push() client specific information, such as the client's hostname, ID or any other distinguishing information when starting to handle the client's request. Thereafter, log output will automatically include the nested diagnostic context so that you can distinguish logs from different client requests even if they are output to the same file.

  See the `NDC` and the `PatternLayout` classes for more information. The `NumberCruncher` example shows how the NDC can be used to distinguish the log output from multiple clients even if they share the same log file.

  For select applications, such as virtual hosting web-servers, the NDC solution is not sufficient. As of version 0.9.0, log4j supports multiple hierarchy trees. Thus, it is possible to log to different targets from the same logger depending on the current context.

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Logger instances seem to be create only. Why isn't there a method to remove logger instances?

  It is quite nontrivial to define the semantics of a "removed" logger escecially if it is still referenced by the user. Future releases may include a remove method in the Logger class.

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How do I get multiple process to log to the same file?

  You may have each process log to a `SocketAppender`. The receiving `SocketServer` (or `SimpleSocketServer`) can receive all the events and send them to a single log file.

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How about the timesamps of events generated by multiple processes across multiple hosts (possibly across multiple timezones)?

  The timestamp is created when the logging event is created. That is so say, when the `debug`, `info`, `warn`, `error` or `fatal` method is invoked. Thus, the timestamp is unaffected by the time at which event arrive at a remote socket server.

  Timestamps are stored in UTC format inside the event. Consequently, when displayed or written to a log file, timestamps appear in the same timezone as the host displaying or creating the logfile. Note that because the clocks of various machines may not be synchronized, there may be timestamp inconsistencies between events generated on different hosts. The EnhancedPatternLayout in the extras companion supports explicit specification of the timezone using a pattern like "%d{HH:mm}{GMT}".

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Why can't log4j find my properties file in a J2EE or WAR application?

  The short answer: the log4j classes and the properties file are not within the scope of the same classloader.

  The long answer (and what to do about it): J2EE or Servlet containers utilize Java's class loading system. Sun changed the way classloading works with the release of Java 2. In Java 2, classloaders are arranged in a hierarchial parent-child relationship. When a child classloader needs to find a class or a resource, it first delegates the request to the parent.

  Log4j only uses the default `Class.forName()` mechanism for loading classes. Resources are handled similarly. See the documentation for `java.lang.ClassLoader` for more details.

  So, if you're having problems, try loading the class or resource yourself. If you can't find it, neither will log4j. ;)

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Is there a way to get log4j to automatically reload a configuration file if it changes?

  Yes. Both the DOMConfigurator and the PropertyConfigurator support automatic reloading through the `configureAndWatch` method. See the API documentation for more details.

  Because the `configureAndWatch` launches a separate wathdog thread, and because there is no way to stop this thread in log4j 1.2, the `configureAndWatch` method is unsafe for use in J2EE envrironments where applications are recycled.

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Why should I donate my extensions to log4j back to the project?

  Contrary to the GNU Public License (GPL) the Apache Software License does not make any claims over your extensions. By extensions, we mean totally new code that invokes existing log4j classes. You are free to do whatever you wish with your proprietary log4j extensions. In particular, you may choose to never release your extensions to the wider public.

  We are very careful not to change the log4j client API so that newer log4j releases are backward compatible with previous versions. We are a lot less scrupulous with the internal log4j API. Thus, if your extension is designed to work with log4j version `n`, then when log4j release version `n+1` comes out, you will probably need to adapt your proprietary extensions to the new release.

  Thus, you will be forced to spend precious resources in order to keep up with log4j changes. This is commonly referred to as the "stupid-tax." By donating the code and making it part of the standard distribution, you save yourself the unnecessary maintenance work.

  If your extensions are useful then someone will eventually write an extension providing the same or very similar functionality. Your development effort will be wasted. Unless the proprietary log4j extension is business critical, there is little reason for not donating your extensions back to the project.

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What should I keep in mind when contributing code?

  1. Write a test case for your contribution.

     There is nothing more irritating than finding the bugs in debugging (i.e. logging) code. Writing a test case takes some effort but is crucial for a widely used library such as log4j. Writing a test case will go a long way in earning you the respect of fellow developers. See the tests/ directory for exiting test cases.

  2. Stick to the existing indentation style even if you hate it.

     Alternating between indentation styles makes it hard to understand the source code. Make it a little harder on yourself but easier on others.

     Log4j has adopted a rather conservative approach by following the Code Conventions for the JavaTM Programming Language. We use 2 (two) spaces for indentation and no tabs.

  3. Please do not both modify the code and change the indentation in a single commit.

     If you change the code and reformat it at the same time and then commit, the commit notification message will be hard to read. It will contain many diffs associated with the reformatting in addition to logical changes.

     If you must reformat and change the code, then perform each step separately. For example, reformat the code and commit. Following that, you can change the logic and commit. The two steps can be performed in the reverse order just as well. You can first change the logic and commit and only later reformat and commit.

  4. Make every effort to stick to the JDK 1.1 API.

     One of the important advantages of log4j is its compatibility with JDK 1.1.x.

  5. Always keep it simple, small and fast when possible.

     It's all about the application not about logging.

  6. Identify yourself as a contributor at the top of the relevant file.

  7. Take responsibility for your code.

     Authoring software is very much like running a marathon. It takes time and endurance.

  8. Did we mention sticking with the indentation style?

  9. Did we mention writing test cases?

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Why am I getting multiple copies of messages in my log file?

  There are several reasons this can occur:

  - Repeated configuration of log4j: By default, each call to PropertyConfigurator.configure or DOMConfigurator.configure is culmulative. If your configuration file defines a file appender, calling PropertyConfigurator.configure three times will create three distinct instances and connect each of them to the specified logger.
  - Attaching the same appender to multiple loggers: if an appender is attached to the root logger and child logger, then any message sent to the child logger will go to the appender twice (unless additivity is set to false).

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How do I add a custom level?

  It is possible, but rarely appropriate. The request is commonly for a level named something like "audit" that doesn't obviously fit in the progression "trace", "debug", "info", "warn", "error" and "fatal". In that case, the request for a level is really a request for a mechanism to specify a different audience. The appropriate mechanism is to use a distinct logger name (or tree) for "audit" related messages.

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Why does log4j throw a NullPointerException or print a message about NOPLoggerRepository when shutting down or restarting under Tomcat or during a shutdown hook?

  Tomcat will, by default, clear all static members when unloading classes, however this process can trigger initialization of classes which may then call a class that has been cleared resulting in a NullPointerException or some undesirable behavior. Bug 40212) describes the problem in detail and has a patch which at this writing has not been applied to Tomcat. Glassfish had a similar problem and accepted the patch.

  The following are recommended to avoid this problem:

  1. Set the org.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES system property to false.
  2. Apply the patch from bug 40212 or upgrade to a version that has that patch applied.
  3. Upgrade to log4j 1.2.16 or later to better defend against class loader attack or provide a better diagnostic message when it does occur.

  It is impossible for log4j to defend against all attacks on its internal state by a class loader. There is a limit to the defensive measures that will be incorporated.

  For more background, see bugs 40212, 41939, 43867, 40159, 43181, 41316 and 37956.

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Why isn't my rolling file appender properly rolling files or why does logging events get written to the old log file?
  Most commonly this is due to multiple appenders attempting to use the same file path and most likely by having multiple independent instances of log4j read the same configuration file, however having the log file open by another process (an editor, backup utility) can also interfere with rolling. No provided file appender is reliable when multiple instances are writing to the same file path and java.io provides no mechanism to coordinate writing between JVM's.

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Why do I see a warning about "No appenders found for logger" and "Please configure log4j properly"?
  This occurs when the default configuration files log4j.properties and log4j.xml can not be found and the application performs no explicit configuration. log4j uses Thread.getContextClassLoader().getResource() to locate the default configuration files and does not directly check the file system. Knowing the appropriate location to place log4j.properties or log4j.xml requires understanding the search strategy of the class loader in use. log4j does not provide a default configuration since output to the console or to the file system may be prohibited in some environments. Also see FAQ: Why can't log4j find my properties in a J2EE or WAR application?.

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What system properties are checked by log4j?
  log4j.debug
    If true, log4j will output internal debugging messages to the console.
  log4j.defaultInitOverride
    If true, log4j will not perform default initialization, that is check for log4j.properties or log4j.xml, at the first logging request.
  log4j.configuration
    URL for default initialization configuration file.
  log4j.configurationClass
    Class name for configurator to process default initialization configuration file.
  log4j.ignoreTCL
    If true, the thread class loader will be ignored when loading classes.

  The SMTPAppender may be influenced by mail.smtp and mail.smtps system properties.

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