SLF4J - Simple Logging Facade for Java | How to identify which logging library SLF4J using for logging?

The Simple Logging Façade for Java (SLF4J) serves as a simple façade or abstraction for various logging frameworks, such as java.util.logging, logback and log4j. SLF4J allows the end-user to plug in the desired logging framework at deployment time. It enables a generic API making the logging independent of the actual implementation.

SimpleLogger(org.slf4j.simple.SimpleLoggerFactory) - sends all log messages to the console using the “standard” error output stream (System.err).

NOPLogger(org.slf4j.helpers.NOPLoggerFactory) - All logging will be silently discarded. Starting with version 1.6.0, if no binding(logger implementation) is found on the classpath, this one will be used by default. 

SLF4J: No SLF4J providers were found.

SLF4J: Defaulting to no-operation (NOP) logger implementation

SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.

Log4j(org.slf4j.log4j12.Log4jLoggerFactory/org.apache.logging.slf4j.Log4jLogger) - A wrapper over the Log4j 1.2.2 Logger's

Java Util Logging(org.slf4j.jul.JDK14LoggerFactory) - wrapper for the Java Util Logging logger

Logback Logging Framework(ch.qos.logback.classic.LoggerContext) - Wrapper for Logback Logging Framework

Dependencies:

SLF4J API Dependency:

<dependency>

    <groupId>org.slf4j</groupId>

    <artifactId>slf4j-api</artifactId>

    <version>2.0.0-alpha5</version>

</dependency>

Implementation Dependencies:

SimpleLogger:

<dependency>

<groupId>org.slf4j</groupId>

<artifactId>slf4j-simple</artifactId>

<version>2.0.0-alpha5</version>

<scope>runtime</scope>

</dependency>

The logger configuration can be enabled by adding "simplelogger.properties" file to the class path(src/main/resources)

simplelogger.properties

org.slf4j.simpleLogger.logFile=System.out

org.slf4j.simpleLogger.defaultLogLevel=warn

Log4j 1.2:

<dependency>

    <groupId>org.slf4j</groupId>

    <artifactId>slf4j-log4j12</artifactId>

    <version>2.0.0-alpha5</version>

    <scope>runtime</scope>

</dependency>

While enabling Log4j logging Framework, the log4j.proerties should be added into the classpath(src/main/resources), sample log4j file

log4j.properties

log4j.rootLogger=info, stdout

log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

log4j.appender.stdout.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

Log4j 2:

<!-- SLF4J Bridge -->

<dependency>

<groupId>org.apache.logging.log4j</groupId>

<artifactId>log4j-slf4j-impl</artifactId>

<version>2.15.0</version>

</dependency>

In this case remove SLF4J API dependency(slf4j-api) - only add the above dependency 

While enabling Log4j 2 logging Framework, the log4j2.xml should be added into the classpath(src/main/resources), sample log4j2.xml file

log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>

<Configuration status="INFO">

    <Appenders>

        <Console name="console" target="SYSTEM_OUT">

            <PatternLayout

                pattern="[%-5level] %d{yyyy-MM-dd HH:mm:ss.SSS} [%t] %c{1} - %msg%n" />

        </Console>

    </Appenders>

    <Loggers>

        <Root level="debug" additivity="false">

            <AppenderRef ref="console" />

        </Root>

    </Loggers>

</Configuration>

Java Util Logging:

<dependency>

    <groupId>org.slf4j</groupId>

    <artifactId>slf4j-jdk14</artifactId>

    <version>2.0.0-alpha5</version>

    <scope>runtime</scope>

</dependency>

The default logger configuration is available in $JAVA_HOME/conf/logging.properties, custom logging.properties can be enabled if required(there are multiple ways to load the custom logging.properties - JVM parameter - -Djava.util.logging.config.file=logging.properties, System.setProperty("java.util.logging.config.file", "logging.properties"); etc)

logging.properties

handlers= java.util.logging.ConsoleHandler

.level= INFO

java.util.logging.ConsoleHandler.level = INFO

java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

com.demo.logging.level=INFO

Logback Logging Framework

<dependency>

    <groupId>ch.qos.logback</groupId>

    <artifactId>logback-classic</artifactId>

    <version>1.3.0-alpha10</version>

</dependency>

The logger configuration can be enabled by adding "logback.xml" file to the class path(src/main/resources)

<configuration>

<appender name="CONSOLE"

class="ch.qos.logback.core.ConsoleAppender">

<layout class="ch.qos.logback.classic.PatternLayout">

<Pattern>

%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n

</Pattern>

</layout>

</appender>

<logger name="com.demo" level="debug" additivity="false">

<appender-ref ref="CONSOLE" />

</logger>

<root level="error">

<appender-ref ref="CONSOLE" />

</root>

</configuration>

Ensure only one implementation is enabled to the project, the latest SLF4J versions use the first implementation loaded as the actual implementation.

SLF4J: Class path contains multiple SLF4J providers.

SLF4J: Found provider [ch.qos.logback.classic.spi.LogbackServiceProvider@34b7bfc0]

SLF4J: Found provider [org.slf4j.jul.JULServiceProvider@366e2eef]

SLF4J: Found provider [org.slf4j.simple.SimpleServiceProvider@6df97b55]

SLF4J: Found provider [org.slf4j.log4j12.Log4j12ServiceProvider@3cbbc1e0]

SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.

SLF4J: Actual provider is of type [ch.qos.logback.classic.spi.LogbackServiceProvider@34b7bfc0]

Some of the time, we may have the use case to identify the logging implementation SLF4J is using to log the messages, this can be easily identified by looking into the logger implementation dependency added to the project. But sometimes the dependencies may come from external projects, the below APIs can be used to identify the same.

//

org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(Application.class);

System.out.println(logger.getClass());

Output - class org.slf4j.simple.SimpleLogger

//

LoggerFactory.getILoggerFactory().getClass().getName()

Output - ch.qos.logback.classic.LoggerContext

//

In earlier versions, even the below API is supported

org.slf4j.impl.StaticLoggerBinder.getSingleton().getLoggerFactory()

Output - ch.qos.logback.classic.LoggerContext[default]

Sample Java Class:

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

public class Sample {

private static final Logger log = LoggerFactory.getLogger(VulnerableLog4jExampleHandlerLocal.class);

public static void main(String[] args) {

 

      log.info("test");  

log.info(log.getClass().toString());

log.info(LoggerFactory.getILoggerFactory().getClass().getName());

}

}