# 🚀 Complete Guide to Log4j2 in Java (Beginner to Advanced)

Logging is one of the most important parts of any application. Without logs, debugging issues in production becomes extremely difficult. In Java, one of the most powerful and widely used logging frameworks is Log4j2.

This blog will help you understand Log4j2 from basics to practical usage in a Spring Boot project.

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📌 What is Log4j2?

Log4j2 is a logging framework developed by Apache. It is used to record application events such as:

• Application start/stop
• Errors and exceptions
• User activity
• Debug information

👉 It is an improved version of Log4j (Log4j1).

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⚡ Why Use Log4j2?

Log4j2 is preferred because it offers:

✔ High performance
✔ Flexible configuration (XML, JSON, YAML)
✔ Multiple output options (Console, File, Database)
✔ Different log levels
✔ Asynchronous logging support

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📊 Log Levels in Log4j2

Log levels help categorize logs based on severity:

Level	Description
TRACE	Detailed debugging
DEBUG	Developer debugging
INFO	Normal application flow
WARN	Warning messages
ERROR	Something went wrong
FATAL	Critical failure

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📦 How to Add Log4j2 in Spring Boot

Add this dependency in pom.xml:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

👉 This replaces default logging with Log4j2.

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🧱 Basic Logger Example

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

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

    public static void main(String[] args) {
        logger.info("Application started");
        logger.error("Something went wrong");
    }
}

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⚙️ Log4j2 Configuration (log4j2.xml)

Log4j2 uses a configuration file:

src/main/resources/log4j2.xml

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🖥️ Console Logging

<Console name="Console" target="SYSTEM_OUT">
    <PatternLayout pattern="%d{yyyy-MM-dd HH:mm:ss} %-5level %logger - %msg%n"/>
</Console>

👉 Prints logs in terminal

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📁 File Logging

<RollingFile name="FileLogger"
             fileName="logs/app.log"
             filePattern="logs/app-%d{yyyy-MM-dd}.log">
    <PatternLayout pattern="%d{yyyy-MM-dd HH:mm:ss} %-5p %c - %m%n"/>
</RollingFile>

👉 Stores logs in files with daily rotation

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🗄️ Database Logging (Advanced)

<JDBC name="DatabaseAppender" tableName="logs">
    <DriverManager 
        connectionString="jdbc:postgresql://localhost:5432/arundb"
        userName="postgres"
        password="password"/>

    <ColumnMapping name="log_date" type="java.sql.Timestamp"/>
    <ColumnMapping name="level" pattern="%level"/>
    <ColumnMapping name="logger" pattern="%logger"/>
    <ColumnMapping name="message" pattern="%message"/>
</JDBC>

👉 Saves logs directly into database

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🔗 Connecting Appenders

<Root level="info">
    <AppenderRef ref="Console"/>
    <AppenderRef ref="FileLogger"/>
    <AppenderRef ref="DatabaseAppender"/>
</Root>

👉 Sends logs to multiple destinations

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🔄 Logging Flow

Application → Logger → Appenders → Output (Console/File/DB)

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🧪 Example in Spring Boot Controller

@RestController
public class UserController {

    private static final Logger logger = LogManager.getLogger(UserController.class);

    @GetMapping("/test")
    public String test() {
        logger.info("API called");
        return "Success";
    }
}

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