How to configure SLF4J with different logger implementations

Zemian DengAugust 26th, 2013Last Updated: August 22nd, 2013

1 87 3 minutes read

There are many good benefits in using slf4j library as your Java application logging API layer. Here I will show few examples on how to use and configure it with different loggers.

You can think of slf4j as an Java interface, and then you would need an implementation (ONLY ONE) at runtime to provide the actual logging details, such as writing to STDOUT or to a file etc. Each logging implementation (or called binding) would obviously have their own way of configuring the log output, but your application will remain agnostic and always use the same org.slf4j.Logger API. Let’s see how this works in practice.

Using `slf4j` with Simple logger

Create a Maven based project and this in your pom.xml.

<dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-api</artifactId>
        <version>1.7.5</version>
    </dependency>

Now you may use Logger in your Java code like this.

package deng;
import org.slf4j.*;
public class Hello {
    static Logger LOGGER = LoggerFactory.getLogger(Hello.class);
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++)
            if (i % 2 == 0)
                LOGGER.info("Hello {}", i);
            else
                LOGGER.debug("I am on index {}", i);
    }
}

The above will get your program compiled, but when you run it, you will see these output.

bash> java deng.Hello
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

What it’s saying is that at runtime, you are missing the logging “implementation” (or the logger binding), so slf4j simply use a “NOP” implmentation, which does nothing. In order to see the output properly, you may try use an simple implementation that does not require any configuration at all! Just go back to your pom.xml and add the following:

<dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-simple</artifactId>
        <version>1.7.5</version>
    </dependency>

Now you see logging output on STDOUT with INFO level. This simple logger will default show any INFO level message or higher. In order to see DEBUG messages, you would need to pass in this System Property -Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG at your Java startup.

Using `slf4j` with Log4j logger

Now we can experiment and swap different logger implementations, but your application code can remain the same. All we need is ot replace slf4j-simple with another popular logger implementation, such as the Log4j.

<dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
        <version>1.7.5</version>
    </dependency>

Again, we must configure logging per implementation that we picked. In this case, we need an src/main/resources/log4j.properties file.

log4j.rootLogger=DEBUG, STDOUT
    log4j.logger.deng=INFO
    log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
    log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
    log4j.appender.STDOUT.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

Re-run your program, and you should see similar output.

Using `slf4j` with JDK logger

The JDK actually comes with a logger package, and you can replace pom.xml with this logger implementation.

<dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-jdk14</artifactId>
        <version>1.7.5</version>
    </dependency>

Now the configuration for JDK logging is a bit difficult to work with. Not only need a config file, such as src/main/resources/logging.properties, but you would also need to add a System properties -Djava.util.logging.config.file=logging.properties in order to have it pick it up. Here is an example to get you started:

level=INFO

handlers=java.util.logging.ConsoleHandler
java.util.logging.ConsoleHandler.level=FINEST
deng.level=FINEST

Using `slf4j` with Logback logger

The logback logger implementation is a super dupa quality implementation. If you intend to write serious code that go into production, you may want to evaluate this option. Again modify your pom.xml to replace with this:

<dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.0.13</version>
    </dependency>

Here is a sample of configuration src/main/resources/logback.xml to get things started.

<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
    </encoder>
  </appender>

<logger name="deng" level="DEBUG"/>

<root level="INFO">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

Writing your own library with `slf4j` logger

If you are providing an Java library for large end users consumption, it’s good idea to set your project to depend on slf4j-api only, and then let your user choose any logger implementation at their development or runtime environment. As end users, they may quickly select one of option above and take advatage of their own favorite logging implementation features.