Chapter 1: 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 of experience gained in designing industrial-strength logging systems. The resulting product, i.e. logback, is faster and has a smaller footprint than all existing logging systems, sometimes by a wide margin. Just as importantly, logback offers unique and rather useful features missing in other logging systems.

First Baby Step

Requirements

Logback-classic module requires the presence of slf4j-api.jar and logback-core.jar in addition to logback-classic.jar on the classpath.

The logback-*.jar files are part of the logback distribution whereas slf4j-api-1.7.7.jar ships with SLF4J, a separate project.

Let us now begin experimenting with logback.

package chapters.introduction;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class HelloWorld1 {

  public static void main(String[] args) {

    Logger logger = LoggerFactory.getLogger("chapters.introduction.HelloWorld1");
    logger.debug("Hello world.");

  }
}

HelloWorld1 class is defined in the chapters.introduction package. It starts by importing the Logger and LoggerFactory classes defined in the SLF4J API, 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 "chapters.introduction.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 use the SLF4J API and will be oblivious to the existence of logback.

You can launch the first sample application, chapters.introduction.HelloWorld1 with the command:

java chapters.introduction.HelloWorld1

Launching the HelloWorld1 application will output a single line on the console. By virtue of 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 chapters.introduction.HelloWorld1 - Hello world.

Logback can report information about its internal state using a built-in status system. Important events occurring during logback's lifetime can be accessed through a component called StatusManager. For the time being, let us instruct logback to print its internal state by invoking the static print() method of the StatusPrinter class.

package chapters.introduction;

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("chapters.introduction.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:

12:49:22.203 [main] DEBUG chapters.introduction.HelloWorld2 - Hello world.
12:49:22,076 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback.groovy]
12:49:22,078 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback-test.xml]
12:49:22,093 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback.xml]
12:49:22,093 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Setting up default configuration.

Logback explains that having failed to find the logback-test.xml and logback.xml configuration files (discussed later), 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 Sockets, JMS and many more. Users can also easily create their own Appenders as appropriate for their specific situation.

Note that in case of errors, logback will automatically print its internal state on the console.

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. Only the configuration process would be different. However, you would probably want to customize or configure logback 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 the 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.

1. Configure the logback environment. You can do so in several more or less sophisticated ways. More on this later.
2. 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 a parameter.
3. Use this logger instance by invoking its printing methods, namely the debug(), info(), warn() and error() methods. This will produce logging output on the configured appenders.

Building logback

As its build tool, logback relies on Maven, a widely-used open-source build tool.

Once you have installed Maven, building the logback project, including all its modules, should be as easy as issuing a mvn install command from within the directory where you unarchived the logback distribution. Maven will automatically download the required external libraries.

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 or the EPL license.

For building logback under an IDE, please see the relevant section on the class path setup page.