Monday, January 31, 2011

How to use logger in your Java application

How to log Messages in your application?

Java's logging facility has two parts: a configuration file, and an API for using logging services. It is suitable for simple and moderate logging needs. Log entries can be sent to the following destinations, as either simple text or as XML:
· The console
· A file
· A stream
· Memory
· TCP socket on a remote host

The LEVEL class defines seven levels of logging enlightenment :
FINEST, FINER, FINE, CONFIG, INFO, WARNING, SEVERE .ALL and OFF are defined values as well

The levels in code may be modified as required :
· Upon startup, by using CONFIG to log configuration parameters
· During normal operation, by using INFO to log high-level "heartbeat" information
· When bugs or critical conditions occur, by using SEVERE.
· Debugging information might default to FINE, with FINER and FINEST used occasionally, according to user need.

There is flexibility in how logging levels can be changed at runtime, without the need for a restart:
· By simply changing the configuration file and calling LogManager.readConfiguration.
· By changing the level in the body of code , using the logging API ;
For example, one might automatically increase the logging level in response to unexpected events

The logging levels are in descending order SEVERE, WARNING, INFO, CONFIG, FINE, FINER and FINEST. If we specify log level as

INFO then all the log messages which are equal to INFO and greater (WARNING, SEVERE) levels will be logged.

Levels are attached to the following items:
· An originating logging request (from a single line of code)
· A Logger (usually attached to the package containing the above line of code)
· A Handler (attached to an application)

Here is an example of a logging configuration file :
# Properties file which configures the operation of the JDK
# logging facility. # The system will look for this config file, first using
# a System property specified at startup:
# >java -Djava.util.logging.config.file=myLoggingConfigFilePath #
# If this property is not specified, then the config file is retrieved
# from its default location at:
# JDK_HOME/jre/lib/
# Global logging properties.
# ------------------------------------------
# The set of handlers to be loaded upon startup.
# Comma-separated list of class names.
# (? LogManager docs say no comma here, but JDK example has comma.)
handlers=java.util.logging.FileHandler, java.util.logging.ConsoleHandler

# Default global logging level.
# Loggers and Handlers may override this level

# Loggers
# ------------------------------------------
# Loggers are usually attached to packages.
# Here, the level for each package is specified.
# The global level is used by default, so levels
# specified here simply act as an override.

# Handlers
# -----------------------------------------
# --- ConsoleHandler ---
# Override of global logging level

# --- FileHandler ---
# Override of global logging level
# Naming style for the output file:
# (The output file is placed in the directory
# defined by the "user.home" System property.)

# Limiting size of output file in bytes:

# Number of output files to cycle through, by appending an
# integer to the base file name:

# Style of output (Simple or XML):

Here is an example of using the logging API :

import java.util.logging.*;
* Demonstrate Java's logging facilities, in conjunction
* with a logging config file.
public final class SimpleLogger {

  public static void main(String argv[]) {
    SimpleLogger thing = new SimpleLogger();
   public void doSomething() {
   //Log messages, one for each level
   //The actual logging output depends on the configured
    //level for this package. Calls to "inapplicable"
    //messages are inexpensive.
     fLogger.finest("this is finest");
    fLogger.finer("this is finer");
    fLogger.fine("this is fine");
    fLogger.config("this is config");"this is info");
    fLogger.warning("this is a warning");
    fLogger.severe("this is severe");

    //In the above style, the name of the class and
    //method which has generated a message is placed
    //in the output on a best-efforts basis only.
    //To ensure that this information is always
    //included, use the following "precise log"
    //style instead :
    fLogger.logp(Level.INFO, this.getClass().toString(), "doSomething", "blah");

    //For the very common task of logging exceptions, there is a
    //method which takes a Throwable :
    Throwable ex = new IllegalArgumentException("Some exception text");
    fLogger.log(Level.SEVERE, "Some message", ex);

    //There are convenience methods for exiting and
    //entering a method, which are at Level.FINER :
    fLogger.exiting(this.getClass().toString(), "doSomething");

    //Display user.home directory, if desired.
    //(This is the directory where the log files are generated.)
    //System.out.println("user.home dir: " + System.getProperty("user.home") );

  // PRIVATE //

  //This logger will inherit the config of its parent, and add
  //any further config as an override. A simple style is to use
  //all config of the parent except, perhaps, for logging level.

  //This style uses a hard-coded literal and should likely be avoided:
  //private static final Logger fLogger = Logger.getLogger("");

  //This style has no hard-coded literals, but forces the logger
  //to be non-static.
  //private final Logger fLogger=Logger.getLogger(this.getClass().getPackage().getName());

  //This style uses a static field, but hard-codes a class literal.
  //This is probably acceptable.
  private static final Logger fLogger = Logger.getLogger(SimpleLogger.class.getPackage().getName());


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