Core Java

Log4j 2 Config Using a Prop File

Log4j 2, an extensively used logging framework developed in Java, addresses numerous architectural shortcomings of its predecessor, Log4j. Renowned for its thread safety, speed, and a plethora of enhancements, it is distributed under the open-source Apache Software License. Serving as the advanced iteration of the classic Log4j framework, Log4j 2 continues to be heavily utilized in numerous Java enterprise applications for logging purposes. Let us delve into understanding Log4j 2 and how to config it using a prop file via a simple example.

1. Understanding Log4j 2

Log4j 2 is a powerful and widely used logging framework for Java applications. It is designed to efficiently handle logging operations, providing developers with comprehensive tools to manage and analyze application logs.

1.1 Features

Log4j 2 boasts a range of features that make it a preferred choice among developers:

  • Thread safety: Log4j 2 ensures safe concurrent access to logging functionality, making it suitable for multi-threaded environments.
  • Performance: With optimized logging mechanisms, Log4j 2 offers excellent performance, minimizing overhead in logging operations.
  • Configurability: Developers can customize logging behavior through various configuration options, allowing for flexible adaptation to different project requirements.
  • Extensibility: Log4j 2 supports the creation of custom appenders, layouts, filters, and other components, enabling tailored logging solutions.
  • Pluggable architecture: The modular design of Log4j 2 facilitates integration with other frameworks and libraries, enhancing its versatility.

1.2 Usage

Integrating Log4j 2 into a Java project involves adding the Log4j 2 library to the project dependencies and configuring the logging behavior. Developers can choose from various configuration methods, including programmatic configuration using Java code, XML configuration files, JSON configuration files, and properties files.

1.3 Logging Levels

Log4j 2 supports different logging levels to categorize log messages based on their severity:

  • TRACE: Provides the most detailed logging information, typically used for debugging purposes.
  • DEBUG: Provides detailed information that is useful for debugging purposes.
  • INFO: Provides informational messages that highlight the progress of the application.
  • WARN: Indicates issues that could lead to problems if not addressed.
  • ERROR: Indicates errors that occurred during the execution of the application.
  • FATAL: Indicates critical errors that lead to the termination of the application.

Developers can specify the appropriate logging level for each log message to control the amount of information logged.

1.4 Important Components

  • Logger: Log4j 2 provides a robust logging system centered around the Logger class. This class serves as the primary entry point for logging operations within your Java applications. You can obtain an instance of the Logger using various methods provided by the LogManager class. Once obtained, you can utilize the Logger to output log messages at different severity levels, such as debug, info, warn, error, and fatal. Additionally, Log4j 2 supports hierarchical loggers, allowing for effective organization and customization of logging behavior across different components of your application.
  • Configuration: Configuring Log4j 2 involves defining how logging should behave in your application. This configuration typically includes specifying appenders, log levels, log formats, and any additional properties. Log4j 2 supports multiple configuration methods, including programmatic configuration using Java code, XML configuration files, JSON configuration files, and properties files.
  • Properties: The file is a commonly used configuration method for Log4j 2. It is a simple properties file where you define various logging configurations using key-value pairs. These configurations include specifying the logging level for different loggers, defining appenders and their properties, setting log formats, and configuring logging layouts. The file offers a straightforward and easily understandable way to configure Log4j 2 without needing to write XML or JSON configurations. The syntax of the file is quite intuitive. Each configuration is defined using a key-value pair format. The keys represent different aspects of logging configuration, such as loggers, appenders, and layouts, while the values specify corresponding properties or settings.

2. Installing Dependency and Code

2.1 Dependency

To use Log4j 2 in your Java project, you need to include the Log4j 2 dependency in your project’s build configuration. Add the following dependency to your Maven pom.xml file-


2.2 Code

Below is a complete example demonstrating the usage of Log4j 2 for console logging and logging to multiple destinations, including console and file.

package com.jcg.example;

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

public class Log4j2Example {
    // Obtain a logger instance
    private static final Logger logger = LogManager.getLogger(Log4j2Example.class);

    public static void main(String[] args) {
        // Console logging"This is an informational message.");
        logger.warn("This is a warning message.");
        logger.error("This is an error message.");
        logger.debug("This is a debug message.");

        // Logging to multiple destinations (console and file)

    private static void logToConsoleAndFile() {
        // Log to console and file"Logging to console and file.");
        try {
            int result = 10 / 0; // Intentional division by zero to generate an error
        } catch (ArithmeticException e) {
            // Log the exception
            logger.error("Exception occurred: ", e);

To configure Log4j 2 to log to both the console and a file, you need to provide a configuration file named

# Define appenders
appender.console.type = Console = STDOUT
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n

appender.file.type = File = File
appender.file.fileName = logs/application.log
appender.file.layout.type = PatternLayout
appender.file.layout.pattern = %d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n

# Root logger level and appenders
rootLogger.level = INFO
rootLogger.appenderRefs = console, file
rootLogger.appenderRef.console.ref = STDOUT
rootLogger.appenderRef.file.ref = File

With this configuration, log messages at the info level or higher will be logged to both the console and the logs/application.log file. Adjust the configuration as needed for your specific requirements.

2.2.1 Output

The output of the Java code provided in the Log4j 2 example will vary depending on the logging configuration and whether any exceptions occur during execution. Assuming there are no exceptions, and the logging configuration is set up to log to both the console and a file –

19:45:28.123 [main] WARN  Log4j2Example - This is a warning message.
19:45:28.123 [main] ERROR Log4j2Example - This is an error message.

And in the file logs/application.log

19:45:28.123 [main] INFO  Log4j2Example - This is an informational message.
19:45:28.123 [main] WARN  Log4j2Example - This is a warning message.
19:45:28.123 [main] ERROR Log4j2Example - This is an error message.
19:45:28.234 [main] INFO  Log4j2Example - Logging to console and file.
19:45:28.234 [main] ERROR Log4j2Example - Exception occurred: 
java.lang.ArithmeticException: / by zero
    at Log4j2Example.logToConsoleAndFile(
    at Log4j2Example.main(

The first three lines are standard logging messages produced by the main method of the Log4j2Example class. The last two lines are produced by the logToConsoleAndFile method which logs an informational message and then catches an ArithmeticException, logging it as an error along with the exception stack trace.

3. Conclusion

Log4j, with its versatile logging capabilities, plays a crucial role in modern software development. By efficiently managing log statements, it offers invaluable insights into application behavior and aids in debugging, monitoring, and performance analysis.

The three main components of Log4j – loggers, appenders, and layouts – work in harmony to produce customized log output. Loggers categorize log statements, appenders determine where the logs are sent, and layouts define the format of the log entries. This modular design allows developers to tailor logging behavior to meet specific requirements.

Moreover, Log4j provides various configuration options, including programmatic configuration and configuration files (e.g., XML, properties), offering flexibility and ease of setup. Its support for multiple logging levels enables developers to prioritize and filter log messages effectively.

In conclusion, Log4j stands as a reliable and indispensable tool for logging in Java applications. Its robust features, ease of use, and extensive documentation make it a preferred choice for developers worldwide. By leveraging Log4j, developers can streamline the logging process, improve application reliability, and enhance overall software quality.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
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