Friday, 22 December 2017

Why use slf4j over log4j or logback for logging in Java


In last post we saw how we can use slf4j over log4j and logback -
 But the question is why would be use slf4j over any logging implementation and not use the actual implementation. In this post we will try to understand this question.

 SLF4J or Simple logging Facade is not really a logging implementation but an abstraction that can use any of the logging implementation like -
  • java.util.logging, 
  • Apache log4j, 
  • logback etc
So consider this - You have developed a project that uses log4j for logging. But your project is dependent on some other module/library that uses lets say logback for logging. In this case you will need to include logback jar in your application as well. This is just unnecessary overhead. If the module your project is dependent on used slf4j then it could have reused our existing log4j configurations and jar.

Other way to see this is let's say you are writing a library that you want someone else to use. In this case you can use slf4j and let the user of your library choose the actual logging implementation rather than using a actual logging implementation like log4j and  making the user of your library stick to the same.

In short slf4j makes your code independent of any logging implementation specially if your code is part of public api/library.

Now that we know the very basics of why one would use slf4j lets see some of it's advantages -

Why use slf4j over log4j or logback for logging in Java

Let us see how a log statement would look in a log4j implementation -

if (logger.isDebugEnabled()) {
    logger.debug("Inputs are input1 : " + input1 + " input2 : " + input2 );

Couple of quick observations -
  1. Lot of boiler plate. Need to check if debug level is enabled everytime we need to log a debug statement.
  2. Lot of string concatenation everytime we call this debug statement.
In slf4j it would be as simple as -

logger.debug("Inputs are input1 : {} , input2 : {}" , input1, input2 );

Here {} are the palceholders and are replaced by the comma separated arguments provided later in the call. Yes the method takes variable arguments. And this is cool because - No more string concatenations!

You also avoid the boiler plate code since sl4fj will internally take care of the logging levels and proceed only if debug level is enabled. So if debug is not enabled final string needed to be logged is not even created. This not only help save memory but also CPU.

Related Links

How to configure Slf4j logging in your web application with log4j or logback implementations


One of the important part of any application building is to implement proper logging.  Slf4j is widely used for this. Slf4j itself is not a logging implementation but it is kind of a wrapper over existing implementations like log4j etc. In this post we will see how we can configure our web application to use slf4j logging with -
  1. log4j
  2. logback
 For log4j I am going to use ivy as dependencies management tool and in case for logback I will use maven. But you can use any really as long as you include correct dependencies in your application.

Using slf4j with log4j

 To use log4j you need to include following dependencies in your application. Your ivy.xml file would look like -

<ivy-module version="2.0" xmlns:xsi=""
        <dependency org="org.slf4j" name="slf4j-api" rev="1.7.21"/>
        <!-- -->
        <dependency org="org.slf4j" name="slf4j-log4j12" rev="1.7.21"/>

You can see the complete xml here - 
and complete working app here -

Once you have dependencies in place you need to give it a configuration file to tell how your logging behaves. A sample configuration file would look like -

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration debug="true"

    <appender name="CONSOLE" class="org.apache.log4j.ConsoleAppender">
        <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern"
            value="%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n" />

    <appender name="FILE" class="org.apache.log4j.RollingFileAppender">
        <param name="append" value="false" />
        <param name="maxFileSize" value="10MB" />
        <param name="maxBackupIndex" value="10" />
        <param name="file" value="${catalina.home}/logs/webdynamo.log" />
        <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern"
            value="%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n" />
    <category name="org.springframework">
        <priority value="debug" />

    <category name="org.springframework.beans">
        <priority value="debug" />

    <category name="">
        <priority value="debug" />

        <level value="DEBUG" />
        <appender-ref ref="CONSOLE" />
        <appender-ref ref="FILE" />


Again you can see this file in the same project mentioned above -

This configuration file should be in the classpath. log4j implementation by default looks for a file called or log4j.xml in your classpath.

You can visualize this with following diagram -

Application code uses slf4j interface which in turn uses a log4j-slf4j bridge to talk to log4j implementation.

We will see how to actually use Sl4fj logger a bit later in this post. Let's look how to do the same with a logback implementation.

Using slf4j with logback

For this you need to add following dependencies. Your pom.xml dependencies section would look like -


NOTE : you do not need slf4j-api here as logback has it as a compile time dependency. You can see that here -

NOTE : The logback-classic module can be assimilated to a significantly improved version of log4j. Moreover, logback-classic natively implements the SLF4J API so that you can readily switch back and forth between logback and other logging frameworks such as log4j or java.util.logging (JUL).  (Source)

You can visualize this with following diagram -

As we saw in log4j implementation we need to supply a configuration file to the implementation to tell how logging should work. In case of logback it expects a file called logback.xml to be in the classpath. A sample file could be -

<?xml version="1.0" encoding="UTF-8"?>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
            <Pattern>%d{yyyy-MM-dd HH:mm:ss} %-5level %logger{36} - %msg%n</Pattern>
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <pattern>%date{HH:mm:ss.SSS} %-5p [%t] %c{1} - %m%n</pattern>
    <root level="DEBUG">
        <appender-ref ref="STDOUT" />

    <logger name="com.osfg" level="DEBUG" additivity="false">
        <appender-ref ref="STDOUT" />
        <appender-ref ref="FILE" />

This is in the same app memtioned above. You can see this file here -

And that's it your logging framework is all set to be used. We will now see how we can actually use this logger.

Using slf4j logger in your application

 Following is a simple controller that uses slf4j logging (implementation can be anything underneath - log4j, logback etc)

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Controller;
import org.springframework.ui.ModelMap;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
 * @author athakur
 * Test controller
public class TestController {
    Logger logger = LoggerFactory.getLogger(TestController.class);

    public String test(@PathVariable String data, ModelMap model,
            HttpServletRequest request, HttpServletResponse response) {
        logger.debug("Received request for test controller with data : {}", data);
        model.put("adminName", properties.getAdminName());
        return "test";    

You can just run the code and see that logging works. This is again part of the same maven app I mentioned above in case of logback implementation. You can see this file here -

Related Links

t> UA-39527780-1 back to top