Log Levels
Every logger and every message has an associated logging
Level. If a logger receives a message for logging and the message
has a level lower than what the logger is set up to log, then the
logger discards the message. The following levels are pre-defined:
SEVERE, WARNING, INFO, CONFIG, FINE, FINER, FINEST.
Handlers also have levels, and they're used in the same way as for
loggers.
The Logger hierarchy
Loggers are placed in a hierarchy. Given are two loggers: the
root logger (with no name) and the global logger, which
has the root logger as its parent. If you define two
loggers:
Logger logger1 = Logger.getLogger("abc.def");
Logger logger2 = Logger.getLogger("abc.def.Game");
then you'll get this hierarchy:
The consequences of having this hierarchy are:
- If a logger has no defined level, it searches up the tree until
it finds a defined level, and uses it. The root logger has as
default a level of "INFO".
- When a logger has completed its own logging (through its
handlers) it sends the message to its parents' handlers--all the way
up the tree.
There are some rules regarding "sending messages up the tree" that
should be observed:
- First of all you'll only see a message logged if its level is
higher or equal to the level of the logger you use.
- The parent loggers' levels has no significance. It's the loggers'
handlers and their levels that are used.
- A handler is only used if the message has a level that is higher
or equal to the level of the handler.
- If a logger has no handler defined then you simply continue up
the tree.
- All handlers, up to and including the root handler, are
checked.
If, for example, no handlers are explicitly specified for
"abc.def.Game" and "abc.def" then it's only the handler for the
root logger that will produce any output. A logger may however
choose to break this chain by calling its setUseParentHandlers
method with the value false.
Filters
I'll not use time on Filters, but just mention that a
handler also checks it's filter (if one is defined) to see if logging
should be done. A filter is a class that can inspect all the data
that goes into the message and then determine if the message should
be logged. See the resources section if you'd like to read more about
filters.
Configuring the Loggers
The Logging API allows you to define the logging configuration in
detail. The default logging configuration comes from the logging.properties file,
which you find in the JDK's jre/lib directory. In this file
you define the handlers for the root logger:
One handler:
handlers= java.util.logging.ConsoleHandler
Two handlers:
handlers= java.util.logging.FileHandler, java.util.logging.ConsoleHandler
You may also define the default properties for the handlers,
e.g.:
java.util.logging.ConsoleHandler.level = INFO
java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter
These default properties apply to any logger that uses these
handlers.
Finally you may define the level for any named logger like
this:
The root logger:
.level= INFO
Other loggers:
com.xyz.foo.level = SEVERE
If you want to use for example FileHandlers with different
properties, then the logging.properties file can't be used, but the
logging API has plenty of methods that you may use to accomplish the
tasks. A pair of examples:
A logger with a ConsoleHandler with low priority
level:
Logger logger1 = Logger.getLogger("abc.def");
logger1.setLevel(Level.FINEST);
ConsoleHandler c1 = new ConsoleHandler();
c1.setLevel(Level.FINEST);
logger1.addHandler(c1);
A logger with a FileHandler using a home made
formatter:
Logger logger2 = Logger.getLogger("abc.def.ghi");
logger2.setLevel(Level.CONFIG);
FileHandler f2 = new FileHandler("%h/mylog.txt", 10000, 2, false);
f2.setFormatter(new MyFormatter());
f2.setLevel(Level.CONFIG);
logger2.addHandler(f2);
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