Introduction
The morale effects are startling. Enthusiasm jumps when there is a running system, even a simple one. Efforts redouble when the first picture from a new graphics software system appears on the screen, even if it is only a rectangle. One always has, at every stage in the process, a working system. I find that teams can grow much more complex entities in four months than they can build.
—FREDERIC P. BROOKS, JR., The Mythical Man-Month
What is logback?
Logback is intended as a successor to the popular log4j project.
It was designed by Ceki Gülcü, log4j's founder. It
builds upon a decade long experience gained in designing
industrial-strength logging systems. The resulting product,
logback is faster with a smaller footprint than all existing
logging systems, sometimes by a wide margin. Logback also offers
unique and rather useful features such as Markers, parameterized
logging statements, conditional stack tracing and powerful event
filtering. These are only few examples of useful features
logback has to offer. For its own error reporting, logback
relies on Status
objects, which greatly facilitate
troubleshooting. You may wish to rely on Status objects in
contexts other than logging. Logback-core bundles Joran, a
powerful and generic configuration system, which can be put to
use in your own projects to great effect.
First Baby Step
Requirements
Logback-classic module requires the presence slf4j-api.jar, logback-core.jar in addition to logback-classic.jar on the classpath.
Let us now begin experimenting with logback.
Example 1.1: Basic template for logging (logback-examples/src/main/java/chapter1/HelloWorld1.java)package chapter1; import org.slf4j.Logger; import org.slf4j.LoggerFactory; public class HelloWorld1 { public static void main(String[] args) { Logger logger = LoggerFactory.getLogger("chapter1.HelloWorld1"); logger.debug("Hello world."); } }
The HelloWorld
class is defined in the
chapter1
package. It starts by importing the Logger
and LoggerFactory
classes defined in the SLF4J API, more specifically within the org.slf4j
package.
On the first line of the main() method, the variable named
logger
is assigned a Logger
instance
retrieved by invoking the static getLogger
method
from the LoggerFactory
class. This logger is named
"chapter1.HelloWorld1". The main method proceeds to call the
debug
method of this logger passing "Hello World"
as an argument. We say that the main method contains a logging
statement of level DEBUG with the message "Hello world".
Note that the above example does not reference any logback classes. In most cases, as far as logging is concerned, your classes will only need to import SLF4J classes. Thus, the vast majority, if not all, of your classes will be cognizant of SLF4J API and oblivious to the existence of logback.
You can launch the first sample application, chapter1.HelloWord1 with the command:
java chapter1.HelloWorld1
Launching the HelloWorld1
application will output
a single line on the console. By virtue of to logback's default
configuration policy, when no default configuration file is found,
logback will add a ConsoleAppender
to the root
logger.
20:49:07.962 [main] DEBUG chapter1.HelloWorld1 - Hello world.
Logback can report information about its internal state
using a built-in status system. Important events occuring
during logback's lifetime can be accessed through a
StatusManager
. For the time being, let us instruct logback to print its
internal state. This is accomplished by a static method in
the LoggerStatusPrinter
class.
package chapter1; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import ch.qos.logback.classic.LoggerContext; import ch.qos.logback.core.util.StatusPrinter; public class HelloWorld2 { public static void main(String[] args) { Logger logger = LoggerFactory.getLogger("chapter1.HelloWorld2"); logger.debug("Hello world."); // print internal state LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory(); StatusPrinter.print(lc); } }
Running the HelloWorld2
application will produce
the following output:
20:49:07.962 [main] DEBUG chapter1.HelloWorld2 - Hello world. |-INFO in ch.qos.logback.classic.BasicConfigurator@1c1ea29 - Setting up default configuration.
Logback explains that it configured itself using its default
policy, which is a basic ConsoleAppender
. An
Appender
is a class that can be seen as an output
destination. Appenders exist for many different destinations
including the console, files, Syslog, TCP Socket, JMS and many
more. Users can also easily create their own Appenders as
appropriate for their specific situation.
The previous examples are rather simple. Actual logging in a larger application would not be that different. The general pattern for logging statements would not change. nly the configuration process would be different since However, you would probably want to customize or configure logback for according to your needs. Logback configuration will be covered in subsequent chapters.
Note that in the above example we have instructed logback to
print its internal state by invoking
StatusPrinter.print()
method. Logback's internal status
information can be very useful in diagnosing logback-related
problems.
Here is a list of the three required steps in order to enable logging in your application.
- Configure the logback environment. You can do so in several more or less sophisticated ways. More on this later.
- In every class where you wish to perform logging, retrieve a
Logger
instance by invoking theorg.slf4j.LoggerFactory
class'getLogger()
method, passing the current class name or the class itself as parameter. - Use this logger instance by invoking its printing methods, namely the debug(), info(), warn() and error(). This will produce logging output on the configured appenders.
Building logback
Logback relies on Maven2 as its build tool. Maven2 is a widely-used open-source build tool.
Once you have installed Maven2, building the logback project,
including all its modules, should be as easy as issuing a mvn
package
command from within the directory where you
unarchived the logback distribution. Maven will automatically
download the required external libraries. However, certain artefacts
cannot be downloaded from the Maven2 repository. At present, time
only the JMS API from from SUN Inc. needs to be downloaded and
installed separately into your local repository.
You can manually download the JMS API from Sun. Once you have downloaded the jms.jar file, you can install it in your local Maven 2 repository by issuing the command:
mvn install:install-file -DgroupId=javax.jms -DartifactId=jms -Dversion=1.1 / -Dpackaging=jar -Dfile=/path/to/jms.jar.file
Logback distributions contain complete source code such that you can modify parts of logback library and build your own version of it. You may even redistribute the modified version, as long as you adhere to the conditions of the LGPL License.