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[ Java Common Class Libraries](#)\n\n* * *\n\nLog4j is an open-source logging framework under the Apache Software Foundation, designed specifically for Java applications.\n\nLog4j provides flexible logging functionality, allowing developers to control the output format, level, and destination of log messages.\n\n**Main Features**:\n\n* Highly configurable\n* Supports multiple log levels\n* Can output logs to multiple destinations (console, file, database, etc.)\n* Thread-safe\n* Excellent performance\n\n* * *\n\n## Log4j Core Components\n\n### Logger\n\nLogger is the core component of Log4j, responsible for capturing log information. Applications log messages by calling methods on the Logger.\n\n**Log Levels (from highest to lowest)**:\n\n1. FATAL - Very serious errors that may cause the application to crash\n2. ERROR - Error events but the application can continue running\n3. WARN - Potentially harmful situations\n4. INFO - Coarse-grained runtime information\n5. DEBUG - Fine-grained debugging information\n6. TRACE - Even more fine-grained information than DEBUG\n\n### Appender\n\nAppender defines the destination for log output. Common Appenders include:\n\n* ConsoleAppender: Output to console\n* FileAppender: Output to file\n* RollingFileAppender: Output to file with file rolling support\n* JDBCAppender: Output to database\n\n### Layout\n\nLayout controls the format of log messages. Common Layouts include:\n\n* PatternLayout: Uses pattern strings to define format\n* SimpleLayout: Simple format (level + message)\n* HTMLLayout: HTML table format\n\n* * *\n\n## Log4j Basic Usage\n\n### Adding Dependency\n\nFor Maven projects, add to pom.xml:\n\n## Example\n\nlog4j\n\nlog4j\n\n1.2.17\n\n### Configuration File (log4j.properties)\n\n## Example\n\n# Set root logger level to INFO, with console and file appenders\n\nlog4j.rootLogger=INFO, console, file\n\n# Console appender configuration\n\nlog4j.appender.console=org.apache.log4j.ConsoleAppender\n\nlog4j.appender.console.layout=org.apache.log4j.PatternLayout\n\nlog4j.appender.console.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n\n\n# File appender configuration\n\nlog4j.appender.file=org.apache.log4j.RollingFileAppender\n\nlog4j.appender.file.File=logs/application.log\n\nlog4j.appender.file.MaxFileSize=10MB\n\nlog4j.appender.file.MaxBackupIndex=10\n\nlog4j.appender.file.layout=org.apache.log4j.PatternLayout\n\nlog4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n\n\n### Java Code Example\n\n## Example\n\nimport org.apache.log4j.Logger;\n\npublic class Log4jExample {\n\n// Get Logger instance\n\nprivate static final Logger logger = Logger.getLogger(Log4jExample.class);\n\npublic static void main(String[] args){\n\n// Log messages at different levels\n\n logger.trace("This is a TRACE message");\n\n logger.debug("This is a DEBUG message");\n\n logger.info("This is an INFO message");\n\n logger.warn("This is a WARN message");\n\n logger.error("This is an ERROR message");\n\n logger.fatal("This is a FATAL message");\n\ntry{\n\nint result =10/0;\n\n}catch(Exception e){\n\n logger.error("An error occurred: ", e);\n\n}\n\n}\n\n}\n\n* * *\n\n## Log4j Advanced Configuration\n\n### Setting Different Log Levels by Package/Class\n\n## Example\n\n# Set log level for com.example package to DEBUG\n\nlog4j.logger.com.example=DEBUG\n\n### Using Multiple Loggers\n\n## Example\n\n// Get different Logger instances\n\n Logger mainLogger = Logger.getLogger("main");\n\n Logger dbLogger = Logger.getLogger("database");\n\nmainLogger.info("Application started");\n\n dbLogger.debug("Database connection established");\n\n### Asynchronous Logging\n\nFor applications with high performance requirements, you can use AsyncAppender for asynchronous logging:\n\n## Example\n\nlog4j.appender.async=org.apache.log4j.AsyncAppender\n\nlog4j.appender.async.appender-ref=file\n\n* * *\n\n## New Features in Log4j 2.x\n\nLog4j 2.x is an upgraded version of Log4j, providing more improvements:\n\n* Significant performance improvement\n* Supports plugin architecture\n* More flexible configuration methods (XML, JSON, YAML)\n* Supports lambda expressions\n* Automatic reload of configuration files\n\n### Log4j 2.x Basic Configuration Example (log4j2.xml)\n\n## Example\n\n<AppenderRef ref="