SLF4J and Log4j – Understanding the Differences

Good logging isn’t optional when building Java applications—it’s critical. Logs are often the first place we turn to when something breaks and are essential for performance tuning, security audits, and long-term maintainability.

Two names come up in the Java logging conversation: Simple Logging Facade for Java (SLF4J) and Log for Java (Log4j). They sound similar and often work together, but they serve distinct roles. Choosing between them depends on your project’s structure, your long-term maintenance goals, and the level of flexibility you need.

In this post, we’ll discuss the key differences between SLF4J and Log4j, offer practical advice on when to use each, and show how to connect both to SolarWinds® Loggly® for centralized log management. Whether you’re working with an existing codebase or setting up a new application, this guide will help you make the right decision and configure logging that scales with your needs.

Choosing Between SLF4J and Log4j

Before deciding which logging tool to use, it’s important to understand what each one does.

SLF4J is not a logging implementation on its own. It’s an abstraction layer that lets you plug in a logging framework at runtime, such as Logback, Log4j, or java.util.logging. You write your code against the SLF4J API and then link it to a logging backend via the classpath.

Log4j, by contrast, is a complete logging implementation. It provides its own APIs, configuration mechanisms for generating logs, and powerful built-in features, including filtering, asynchronous loggers, custom appenders, and advanced formatting.

Use SLF4J When You Want Flexibility

• Swappable backends: SLF4J allows you to choose or change your logging backend without rewriting application code. This is particularly valuable if your application may be embedded in other systems or reused across multiple environments.
  
• Library compatibility: Many open-source libraries log using SLF4J, allowing the host application to control the final logging behavior. If you’re developing reusable components, SLF4J keeps you from imposing a logging framework on your users.
  
• Simplified migration: SLF4J makes it easier to migrate from one logging system to another—such as from Log4j 1.x to Logback or Log4j 2—by updating dependencies instead of refactoring code.

Use Log4j When You Want a Full-Featured Logging Framework

• Rich features out of the box: Log4j offers powerful configuration options, including filters, custom layouts, appenders for various output formats, and support for both synchronous and asynchronous logging.
  
• Fine-grained control: If you need to tune logging behavior with precise filters, thresholds, and custom log destinations, Log4j gives you more control than SLF4J plus a backend.
  
• Long-term familiarity: Many enterprise teams already have Log4j configurations and expertise in place. Sticking with Log4j may be simpler if you’re working in such an environment.

Advantages and Disadvantages

	SLF4J	Log4j
Advantages	Decouples APIs from implementation; promotes modular and portable code	Offers a full-featured framework with powerful configuration options
	Easy to switch or update logging backends	Direct access to logging features and behaviors
Disadvantages	Requires selecting and configuring a separate backend	Ties your code to a specific logging framework, reducing flexibility
	Doesn’t perform logging itself; depends entirely on external implementation	Harder to swap out if needs change later

Using Loggly With SLF4J

To connect SLF4J-based logging to Loggly, we’ll use Logback as the backend implementation. Logback is the default choice for many SLF4J users because it’s actively maintained and feature rich.

A Basic Java Application With SLF4J

Let’s start with a minimal Java application that logs some messages using SLF4J.

package com.example;

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

public class HelloWorld {
    private static final Logger logger = LoggerFactory.getLogger(HelloWorld.class);

    public static void main(String[] args) {
        logger.info("Hello from SLF4J + Logback!");
        logger.debug("This is a debug message");
        logger.warn("This is a warning message");
        logger.error("This is an error message");
        
        // Log an exception
        try {
            throw new RuntimeException("Test exception");
        } catch (Exception e) {
            logger.error("An error occurred", e);
        }
    }
}

Setting Up SLF4J and Logback With Maven

In your pom.xml, include the SLF4J API and Logback backend dependencies.

<repositories>
        <repository>
            <id>logback-extensions</id>
            <name>Logback Extensions Repository</name>
            <url>https://raw.githubusercontent.com/qos-ch/logback-extensions/master/maven-repo/releases</url>
        </repository>
    </repositories>
    <dependencies>
        <!-- SLF4J API -->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.36</version>
        </dependency>

        <!-- Logback Classic (SLF4J Implementation) -->
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.11</version>
        </dependency>

        <!-- Loggly Appender -->
        <dependency>
            <groupId>org.logback-extensions</groupId>
            <artifactId>logback-ext-loggly</artifactId>
            <version>0.1.2</version>
        </dependency>
    </dependencies>

Configuring Logback to Send Logs to Loggly

To route logs to Loggly, you’ll need to add a logback.xml file to your src/main/resources directory. This configuration uses the Loggly HTTP appender.

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="false">
    <!-- Set internal Logback logging to WARN -->
    <logger name="ch.qos.logback" level="WARN" />

    <!-- Loggly Appender -->
    <appender name="loggly" class="ch.qos.logback.ext.loggly.LogglyAppender">
        <endpointUrl>https://logs-01.loggly.com/inputs/LOGGLY-TOKEN/tag/logback</endpointUrl>
        <pattern>%d{"ISO8601", UTC}  %p %t %c{0}.%M - %m%n</pattern>
    </appender>

    <!-- Root Logger -->
    <root level="info">
        <appender-ref ref="loggly" />
    </root>
</configuration> 

Replace LOGGLY-TOKEN with your actual Loggly token.

Next, let’s look at doing the same with Log4j.

Using Loggly With Log4j

If you’re using Log4j directly, you can also route logs to Loggly with a simple setup. This approach works well for applications that are already standardized on Log4j or don’t need to abstract their logging backend.

A Basic Java Application With Log4j

Here’s a simple Java program that uses Log4j to log some messages.

package com.example;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class HelloWorld {
    private static final Logger logger = LogManager.getLogger(HelloWorld.class);

    public static void main(String[] args) {
        logger.info("Hello from Log4j2!");
        logger.debug("This is a debug message");
        logger.warn("This is a warning message");
        logger.error("This is an error message");
        
        // Log an exception
        try {
            throw new RuntimeException("Test exception");
        } catch (Exception e) {
            logger.error("An error occurred", e);
        }
    }
}

Setting up Log4j with Maven

Include the Log4j dependency in your pom.xml:

    <dependencies>
        <!-- Log4j2 Core -->
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-api</artifactId>
            <version>${log4j2.version}</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>${log4j2.version}</version>
        </dependency>
    </dependencies>

Note: At the time of writing, the log4j2.version used was 2.22.1.

Configuring Log4j to send logs to Loggly

To forward logs to Loggly, you can use the Loggly Log4j appender. Create a log4j.xml file in your src/main/resources directory:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Properties>
        <Property name="LOG_PATTERN">%d{ISO8601}{UTC} %p %t %c{1}.%M - %m%n</Property>
    </Properties>

    <Appenders>
        <!-- Loggly HTTP Appender -->
        <Http name="LogglyAppender" 
              url="https://logs-01.loggly.com/inputs/LOGGLY-TOKEN/tag/log4j2"
              method="POST">
            <Property name="Content-Type" value="text/plain"/>
            <PatternLayout pattern="${LOG_PATTERN}"/>
        </Http>

        <!-- Console Appender for local development -->
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="${LOG_PATTERN}"/>
        </Console>
    </Appenders>

    <Loggers>
        <!-- Set internal Log4j2 logging to WARN -->
        <Logger name="org.apache.logging.log4j" level="WARN"/>
        
        <!-- Root Logger -->
        <Root level="info">
            <AppenderRef ref="LogglyAppender"/>
            <AppenderRef ref="Console"/>
        </Root>
    </Loggers>
</Configuration> 

Be sure to replace LOGGLY-TOKEN with your actual Loggly token.

Using Log4j directly provides a consistent, streamlined way to manage logging for many projects. It also integrates smoothly with Loggly for centralized log analysis.

Test and Verify

To verify that your setup for each application is correct, run each application and then check the Log Explorer at Loggly.

For the SLF4J application, run the following commands:

~/slf4j$ mvn clean package
~/slf4j$ java \
  -jar target/loggly-slf4j-demo-1.0-SNAPSHOT-jar-with-dependencies.jar

Then, log in to Loggly and navigate to the Log Explorer. You’ll see the log events with messages indicating they came from SLF4J with Logback.

For the Log4j application, run the following commands:

~/log4j$ mvn clean package
~/log4j$ java \
  -jar target/loggly-log4j2-demo-1.0-SNAPSHOT-jar-with-dependencies.jar

Check the Log Explorer at Loggly, where you should find messages from the Log4j implementation.

Final Thoughts

SLF4J and Log4j each have their place in the Java logging ecosystem. If flexibility and abstraction are priorities—especially in shared libraries or evolving systems—SLF4J is the better fit. If you’re working within an environment where Log4j is the standard or you need deep control over logging behavior, going with Log4j directly makes sense.

No matter which route you choose, integrating with Loggly gives you centralized access to your logs, enabling easier searching, alerting, and analyzing. Logs scattered across servers aren’t useful—pulling them into a single, searchable location strengthens your observability strategy.

Pick the tool that fits your needs now, but build with extensibility and visibility in mind. Logging isn’t a debugging tool only—it’s your window into the health and behavior of your application in production.

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