• Add slf4j-api jar as a dependency to your app

  <dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.6.1</version>
  </dependency>
  

• Include Logger and LoggerFactory interfaces:

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

• Create a Logger at the top of each class your write

  public class MyClass {
  
      Logger log = LoggerFactory.getLogger(HelloWorld.class);
  ...
  

• That's it! Use the logger anywhere you need to.

  public void anyFunction() {
  log.info("About to return Hello, World");
  }
    public class MyClass {
  
        Logger log = LoggerFactory.getLogger(HelloWorld.class);
    ...
  

• At this point, you can compile run your code. Anyone who uses your code as a dependency can use the log framework of their choice.
• To actually see log statements, you'll need to add a library (a slf4 binding) that actually implements the slf4j logging interface api.
• Pick an implementation, in this demo, we'll use log4j. Add corresponding slf4j "binding" dependency

  <dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.6.1</version>
    <scope>runtime</scope>
  </dependency>
  

• Since I chose to use Log4j, it's also necessary to add a log4j.properties file to the classpath so that log4j knows where and how to write logs
• Run the app again and now you should see log statements

Slf4j http://www.slf4j.org/

Log4j Framework http://logging.apache.org/log4j/1.2/

Log4j has been around since 1998 http://wiki.apache.org/logging-log4j/Log4jvsJDKLogging