Logger

public class Logger extends Object

A Logger object is used to log messages for a specific system or application component. Loggers are normally named, using a hierarchical dot-separated namespace. Logger names can be arbitrary strings, but they should normally be based on the package name or class name of the logged component, such as java.net or javax.swing. In addition it is possible to create "anonymous" Loggers that are not stored in the Logger namespace.

Logger objects may be obtained by calls on one of the getLogger factory methods. These will either create a new Logger or return a suitable existing Logger. It is important to note that the Logger returned by one of the getLogger factory methods may be garbage collected at any time if a strong reference to the Logger is not kept.

Logging messages will be forwarded to registered Handler objects, which can forward the messages to a variety of destinations, including consoles, files, OS logs, etc.

Each Logger keeps track of a "parent" Logger, which is its nearest existing ancestor in the Logger namespace.

Each Logger has a "Level" associated with it. This reflects a minimum Level that this logger cares about. If a Logger's level is set to null, then its effective level is inherited from its parent, which may in turn obtain it recursively from its parent, and so on up the tree.

The log level can be configured based on the properties from the logging configuration file, as described in the description of the LogManager class. However it may also be dynamically changed by calls on the Logger.setLevel method. If a logger's level is changed the change may also affect child loggers, since any child logger that has null as its level will inherit its effective level from its parent.

On each logging call the Logger initially performs a cheap check of the request level (e.g., SEVERE or FINE) against the effective log level of the logger. If the request level is lower than the log level, the logging call returns immediately.

After passing this initial (cheap) test, the Logger will allocate a LogRecord to describe the logging message. It will then call a Filter (if present) to do a more detailed check on whether the record should be published. If that passes it will then publish the LogRecord to its output Handlers. By default, loggers also publish to their parent's Handlers, recursively up the tree.

Each Logger may have a ResourceBundle name associated with it. The named bundle will be used for localizing logging messages. If a Logger does not have its own ResourceBundle name, then it will inherit the ResourceBundle name from its parent, recursively up the tree.

Most of the logger output methods take a "msg" argument. This msg argument may be either a raw value or a localization key. During formatting, if the logger has (or inherits) a localization ResourceBundle and if the ResourceBundle has a mapping for the msg string, then the msg string is replaced by the localized value. Otherwise the original msg string is used. Typically, formatters use java.text.MessageFormat style formatting to format parameters, so for example a format string "{0} {1}" would format two parameters as strings.

When mapping ResourceBundle names to ResourceBundles, the Logger will first try to use the Thread's ContextClassLoader. If that is null it will try the SystemClassLoader instead. As a temporary transition feature in the initial implementation, if the Logger is unable to locate a ResourceBundle from the ContextClassLoader or SystemClassLoader the Logger will also search up the class stack and use successive calling ClassLoaders to try to locate a ResourceBundle. (This call stack search is to allow containers to transition to using ContextClassLoaders and is likely to be removed in future versions.)

Formatting (including localization) is the responsibility of the output Handler, which will typically call a Formatter.

Note that formatting need not occur synchronously. It may be delayed until a LogRecord is actually written to an external sink.

The logging methods are grouped in five main categories:

  • There are a set of "log" methods that take a log level, a message string, and optionally some parameters to the message string.

  • There are a set of "logp" methods (for "log precise") that are like the "log" methods, but also take an explicit source class name and method name.

  • There are a set of "logrb" method (for "log with resource bundle") that are like the "logp" method, but also take an explicit resource bundle name for use in localizing the log message.

  • There are convenience methods for tracing method entries (the "entering" methods), method returns (the "exiting" methods) and throwing exceptions (the "throwing" methods).

  • Finally, there are a set of convenience methods for use in the very simplest cases, when a developer simply wants to log a simple string at a given log level. These methods are named after the standard Level names ("severe", "warning", "info", etc.) and take a single argument, a message string.

For the methods that do not take an explicit source name and method name, the Logging framework will make a "best effort" to determine which class and method called into the logging method. However, it is important to realize that this automatically inferred information may only be approximate (or may even be quite wrong!). Virtual machines are allowed to do extensive optimizations when JITing and may entirely remove stack frames, making it impossible to reliably locate the calling class and method.

All methods on Logger are multi-thread safe.

Subclassing Information: Note that a LogManager class may provide its own implementation of named Loggers for any point in the namespace. Therefore, any subclasses of Logger (unless they are implemented in conjunction with a new LogManager class) should take care to obtain a Logger instance from the LogManager class and should delegate operations such as "isLoggable" and "log(LogRecord)" to that instance. Note that in order to intercept all logging output, subclasses need only override the log(LogRecord) method. All the other logging methods are implemented as calls on this log(LogRecord) method.

Constant Summary

String GLOBAL_LOGGER_NAME GLOBAL_LOGGER_NAME is a name for the global logger.

Field Summary

public static final Logger global This field was deprecated. Initialization of this field is prone to deadlocks. The field must be initialized by the Logger class initialization which may cause deadlocks with the LogManager class initialization. In such cases two class initialization wait for each other to complete. The preferred way to get the global logger object is via the call Logger.getGlobal(). For compatibility with old JDK versions where the Logger.getGlobal() is not available use the call Logger.getLogger(Logger.GLOBAL_LOGGER_NAME) or Logger.getLogger("global").

Protected Constructor Summary

Logger(String name, String resourceBundleName)
Protected method to construct a logger for a named subsystem.

Public Method Summary

void
addHandler(Handler handler)
Add a log Handler to receive logging messages.
void
config(String msg)
Log a CONFIG message.
void
entering(String sourceClass, String sourceMethod, Object[] params)
Log a method entry, with an array of parameters.
void
entering(String sourceClass, String sourceMethod, Object param1)
Log a method entry, with one parameter.
void
entering(String sourceClass, String sourceMethod)
Log a method entry.
void
exiting(String sourceClass, String sourceMethod, Object result)
Log a method return, with result object.
void
exiting(String sourceClass, String sourceMethod)
Log a method return.
void
fine(String msg)
Log a FINE message.
void
finer(String msg)
Log a FINER message.
void
finest(String msg)
Log a FINEST message.
static Logger
getAnonymousLogger(String resourceBundleName)
Create an anonymous Logger.
static Logger
getAnonymousLogger()
Create an anonymous Logger.
Filter
getFilter()
Get the current filter for this Logger.
final static Logger
getGlobal()
Return global logger object with the name Logger.GLOBAL_LOGGER_NAME.
Handler[]
getHandlers()
Get the Handlers associated with this logger.
Level
getLevel()
Get the log Level that has been specified for this Logger.
static Logger
getLogger(String name, String resourceBundleName)
Find or create a logger for a named subsystem.
static Logger
getLogger(String name)
Find or create a logger for a named subsystem.
String
getName()
Get the name for this logger.
Logger
getParent()
Return the parent for this Logger.
ResourceBundle
getResourceBundle()
Retrieve the localization resource bundle for this logger for the current default locale.
String
getResourceBundleName()
Retrieve the localization resource bundle name for this logger.
boolean
getUseParentHandlers()
Discover whether or not this logger is sending its output to its parent logger.
void
info(String msg)
Log an INFO message.
boolean
isLoggable(Level level)
Check if a message of the given level would actually be logged by this logger.
void
log(Level level, String msg, Object[] params)
Log a message, with an array of object arguments.
void
log(