# Introduction

## 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

In order to run the examples in this introduction, you need to make sure that certain jar files are present on the classpath. Please refer to the setup page for further details.

### 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 retreived by invoking the static method `getLogger` in 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".

You will note that the above example does not reference any logback classes. In most cases, as far as logging is concerned, your classes will need to import only SLF4J classes. In principle, you will have to import logback classes only for configuring logback. Thus, the vast majority of your classes will only 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 file is found to configure logback explicitely, 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.

Example 1.2: Printing Logger Status (logback-examples/src/main/java/chapter1/HelloWorld2.java)

    package chapter1;

    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    import ch.qos.logback.classic.util.LoggerStatusPrinter;

    public class HelloWorld2 {

      public static void main(String[] args) {
        Logger logger = LoggerFactory.getLogger("chapter1.HelloWorld2");
        logger.debug("Hello world.");
        LoggerStatusPrinter.printStatusInDefaultContext();
      }
    }

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. However, actual logging in a larger application would not be any different. The general pattern logging statements will not change. Only the configuration process will be different since you will certainly need a more specific configuration than what logback provides by default. As you will see later on in this document, configuring logback can be done in different flexible and powerfull ways. Note that, normally, you won't need to invoke `LoggerStatusPrinter` after your log statements.

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 the `org.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

Like many java applications today, logback relies on Maven 2 as its build tool. Maven 2 is a free open source build tool that requires one or more build files names pom.xml which already ship with logback distributions.

Building all logback modules is mostly done by issuing the mvn package line in a terminal or command window. Maven will automatically download the required external libraries and use them. However, certain artefacts cannot be downloaded from the Maven2 repository. Libraries such as `JMS` from SUN Inc. need to be downloaded and installed separately into your local repository.

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.