How to use log levels in java

Question

I am developing an application where i need to use the logger functionality  I have read about different levels of logger which are    SEVERE  highest  WARNING INFO CONFIG FINE FINER FINEST    I am not able to understand the usage of each logging level   Can someone give me a good example showing all the logging levels and their usage

User · Answer

Logging has different levels such as   Trace     A fine-grained debug message  typically capturing the flow through the application  Debug- A general debugging event should be logged under this  ALL     All events could be logged  INFO- An informational purpose  information written in plain english  Warn- An event that might possible lead to an error  Error- An error in the application  possibly recoverable   Logging captured with debug level is information helpful to developers as well as other personnel  so it captures in broad range  If your code doesn t have exception or errors then you should be alright to use DEBUG level of logging  otherwise you should carefully choose options

User · Answer

The use of levels is really up tp you   You need to decide what is severe in your application  what is a warning and what is just information   You need to split your logging so that your users can easily set up a level of logging that doesn t kill the system with excessing IO but which will report serious errors so you can fix them

User · Answer

Here is a good introduction to logging in Java  http   www javapractices com topic TopicAction do Id 143  Java comes with a logging API since it s 1 4 2 version  http   download oracle com javase 1 4 2 docs guide util logging overview html  You can also use other logging frameworks like Apache Log4j which is the most popular one  http   logging apache org log4j  I suggest you to use a logging abstraction framework which allows you to change your logging framework without re-factoring you code  So you can starts by using Jul  Java Util Logging  then swith to Log4j without changing you code  The most popular logging facade is slf4j  http   www slf4j org   Regards

User · Answer

This excerpt is from the following awesome post      ERROR     something terribly wrong had happened  that must be   investigated immediately  No system can tolerate items logged on this   level  Example  NPE  database unavailable  mission critical use case   cannot be continued       WARN     the process might be continued  but take extra caution    Actually I always wanted to have two levels here  one for obvious   problems where work-around exists  for example     Current data   unavailable  using cached values     and second  name it  ATTENTION  for   potential problems and suggestions  Example     Application running in   development mode    or    Administration console is not secured with a   password     The application can tolerate warning messages  but they   should always be justified and examined       INFO     Important business process has finished  In ideal world    administrator or advanced user should be able to understand INFO   messages and quickly find out what the application is doing  For   example if an application is all about booking airplane tickets  there   should be only one INFO statement per each ticket saying     Who  booked   ticket from  Where  to  Where      Other definition of INFO message    each action that changes the state of the application significantly    database update  external system request        DEBUG     Developers stuff  I will discuss later what sort of   information deserves to be logged       TRACE     Very detailed information  intended only for development  You   might keep trace messages for a short period of time after deployment   on production environment  but treat these log statements as   temporary  that should or might be turned-off eventually  The   distinction between DEBUG and TRACE is the most difficult  but if you   put logging statement and remove it after the feature has been   developed and tested  it should probably be on TRACE level    PS  Read TRACE as VERBOSE

User · Answer

the different log levels are helpful for tools  whose can anaylse you log files  Normally a logfile contains lots of information  To avoid an information overload  or here an stackoverflow    you can use the log levels for grouping the information

User · Answer

This tip shows how to use Logger in any java application  Logger needs to configure Formatter and Handler  There are many types of handlers and formatters present  In this example FileHandler is used to store all the log messages in a log file  And Simple formatter is used to format the log messages in human readable form   package MyProject   import java io IOException  import java util logging FileHandler  import java util logging Level  import java util logging Logger  import java util logging SimpleFormatter   public class MyLogger      public static void main String   args         Logger logger   Logger getLogger  MyLog        FileHandler fh       try             This block configure the logger with handler and formatter       fh   new FileHandler  c   MyLogFile log   true         logger addHandler fh         logger setLevel Level ALL         SimpleFormatter formatter   new SimpleFormatter          fh setFormatter formatter             the following statement is used to log any messages          logger log Level WARNING  My first log           catch  SecurityException e          e printStackTrace          catch  IOException e          e printStackTrace                    some more examples you can find here  https   docs oracle com javase 7 docs api java util logging Logger html

User · Answer

The java util logging Level documentation does a good job of defining when to use a log level and the target audience of that log level  Most of the confusion with java util logging is in the tracing methods   It should be in the class level documentation but instead the Level FINE field provides a good overview   FINE is a message level providing tracing information  All of FINE  FINER  and FINEST are intended for relatively detailed tracing  The exact meaning of the three levels will vary between subsystems  but in general  FINEST should be used for the most voluminous detailed output  FINER for somewhat less detailed output  and FINE for the lowest volume  and most important  messages  In general the FINE level should be used for information that will be broadly interesting to developers who do not have a specialized interest in the specific subsystem  FINE messages might include things like minor  recoverable  failures  Issues indicating potential performance problems are also worth logging as FINE   One important thing to understand which is not mentioned in the level documentation is that call-site tracing information is logged at FINER   Logger entering A LogRecord with message  quot ENTRY quot   log level FINER      Logger throwing The logging is done using the FINER level   The LogRecord s message is set to  quot THROW quot  Logger exiting A LogRecord with message  quot RETURN quot   log level FINER     If you log a message as FINE you will be able to configure logging system to see the log output with or without tracing log records surrounding the log message   So use FINE only when tracing log records are not required as context to understand the log message   FINER indicates a fairly detailed tracing message  By default logging calls for entering  returning  or throwing an exception are traced at this level   In general  most use of FINER should be left to call of entering  exiting  and throwing  That will for the most part reserve FINER for call-site tracing when verbose logging is turned on  When swallowing an expected exception it makes sense to use FINER in some cases as the alternative to calling trace throwing method since the exception is not actually thrown   This makes it look like a trace when it isn t a throw or an actual error that would be logged at a higher level   FINEST indicates a highly detailed tracing message   Use FINEST when the tracing log message you are about to write requires context information about program control flow   You should also use FINEST for tracing messages that produce large amounts of output data   CONFIG messages are intended to provide a variety of static configuration information  to assist in debugging problems that may be associated with particular configurations  For example  CONFIG message might include the CPU type  the graphics depth  the GUI look-and-feel  etc   The CONFIG works well for assisting system admins with the items listed above   Typically INFO messages will be written to the console or its equivalent  So the INFO level should only be used for reasonably significant messages that will make sense to end users and system administrators   Examples of this are tracing program startup and shutdown   In general WARNING messages should describe events that will be of interest to end users or system managers  or which indicate potential problems   An example use case could be exceptions thrown from AutoCloseable close implementations   In general SEVERE messages should describe events that are of considerable importance and which will prevent normal program execution  They should be reasonably intelligible to end users and to system administrators   For example  if you have transaction in your program where if any one of the steps fail then all of the steps voided then SEVERE would be appropriate to use as the log level

User · Answer

Those are the levels  You d consider the severity of the message you re logging  and use the appropriate levels   It s basically a watermark  the higher the level  the more likely you want to retain the information in the log entry  FINEST would be for messages that are of very little importance  so you d use it for things you usually don t care about but might want to see in some rare circumstance

User · Answer

Generally  you don t need all those levels  SEVERE  WARNING  INFO  FINE might be enough  We re using Log4J  not java util logging directly  and the following levels  which might differ in name from other logging frameworks     ERROR  Any error exception that is or might be critical  Our Logger automatically sends an email for each such message on our servers  usage   logger error  message      WARN  Any message that might warn us of potential problems  e g  when a user tried to log in with wrong credentials - which might indicate an attack if that happens often or in short periods of time  usage  logger warn  message      INFO  Anything that we want to know when looking at the log files  e g  when a scheduled job started ended  usage  logger info  message      DEBUG  As the name says  debug messages that we only rarely turn on   usage  logger debug  message        The beauty of this is that if you set the log level to WARN  info and debug messages have next to no performance impact  If you need to get additional information from a production system you just can lower the level to INFO or DEBUG for a short period of time  since you d get much more log entries which make your log files bigger and harder to read   Adjusting log levels etc  can normally be done at runtime  our JBoss instance checks for changes in that config every minute or so

[java] How to use log levels in java

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