Logging is a very important part of software development. A well-written logging code offers quick debugging, easy maintenance, and structured storage of an application’s runtime information.

Logging does have its drawbacks also. It can slow down an application. If too verbose, it can cause scrolling blindness. To alleviate these concerns, log4j is designed to be reliable, fast and extensible.

Since logging is rarely the main focus of an application, the log4j API strives to be simple to understand and to use.

Log4j is fast, threadsafe, uses multiple levels, namely ALL, TRACE, DEBUG, INFO, WARN, ERROR and FATAL and more.

In this tutorial, we are going to learn how to use log4j in your application.

Include the log4j jar file in your classpath or include this dependency in your maven file

<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>

Create a log4j property file and put it into the root folder.

log4j.rootLogger=TRACE, consoleAppender, fileAppender
 
log4j.appender.consoleAppender=org.apache.log4j.ConsoleAppender
log4j.appender.consoleAppender.layout=org.apache.log4j.PatternLayout
log4j.appender.consoleAppender.layout.ConversionPattern=[date: %d{yyyy-MM-dd HH:mm:ss}] [priority: %-5p] [thread: %t] [memberUuid: %X{memberUuid}]  [class&line: %c{2}.%M:%L] [msg: %m]%n

# log to file
log4j.appender.fileAppender=org.apache.log4j.RollingFileAppender
log4j.appender.fileAppender.layout=org.apache.log4j.PatternLayout
log4j.appender.fileAppender.layout.ConversionPattern=[date: %d{yyyy-MM-dd HH:mm:ss}] [priority: %-5p] [thread: %t] [memberUuid: %X{memberUuid}]  [class&line: %c{2}.%M:%L] [msg: %m]%n
log4j.appender.fileAppender.File=application.log

But let’s break down the ConversionPattern layout. You can refer to its documentation here.

%d{yyyy-MM-dd HH:mm:ss} = Date and time format, refer to SimpleDateFormat JavaDoc.
%-5p = The logging priority, like DEBUG or ERROR. The -5 is optional, for the pretty print format.
%c{1} = The logging name we set via getLogger(), refer to log4j PatternLayout guide.
%M = Used to output the method name where the logging request was issued. WARNING Generating caller location information is extremely slow and should be avoided unless execution speed is not an issue.
%L = The line number from where the logging request.
%t = Used to output the name of the thread that generated the logging event.
%m%n = The message to log and line break.
%X = Used to output the MDC (mapped diagnostic context) associated with the thread that generated the logging event. The X conversion character must be followed by the key for the map placed between braces, as in %X{clientNumber} where clientNumber is the key. The value in the MDC corresponding to the key will be output.

For an application to have appropriate logs, you must have at least a console logs and a file containing logs. You can see the logs at real time while debugging and also you can go back to a previous log for a specific point in time.

You can even save your logs into a database

# Define the root logger with appender file
log4j.rootLogger = DEBUG, DB

# Define the DB appender
log4j.appender.DB=org.apache.log4j.jdbc.JDBCAppender

# Set JDBC URL
log4j.appender.DB.URL=jdbc:mysql://localhost/DBNAME

# Set Database Driver
log4j.appender.DB.driver=com.mysql.jdbc.Driver

# Set database user name and password
log4j.appender.DB.user=user_name
log4j.appender.DB.password=password

# Set the SQL statement to be executed.
log4j.appender.DB.sql=INSERT INTO LOGS VALUES('%x','%d','%C','%p','%m')

# Define the layout for file appender
log4j.appender.DB.layout=org.apache.log4j.PatternLayout

create a database table

CREATE TABLE LOGS
   (USER_ID VARCHAR(20)    NOT NULL,
    DATED   DATE           NOT NULL,
    LOGGER  VARCHAR(50)    NOT NULL,
    LEVEL   VARCHAR(10)    NOT NULL,
    MESSAGE VARCHAR(1000)  NOT NULL
);

Create a Logger field.

Logger log = Logger.getLogger(MainLog.class);

Log levels

Trace – Designates finer-grained informational events than the DEBUG.

Debug – Designates fine-grained informational events that are most useful to debug an application.

Info – Designates informational messages that highlight the progress of the application at coarse-grained level.

Warn – Designates potentially harmful situations.

Error – Designates error events that might still allow the application to continue running.

Fatal – Designates very severe error events that will presumably lead the application to abort.

All – All levels including custom levels.

Off – The highest possible rank and is intended to turn off logging.

import org.apache.log4j.PropertyConfigurator;

import org.apache.log4j.Logger;
import org.apache.log4j.PropertyConfigurator;

public class MainLog {

    static Logger log = Logger.getLogger(MainLog.class);

    public static void main(String[] args) {

        PropertyConfigurator.configure("log4j.properties");

        log.trace("Trace Message Logged !!!");
        log.debug("Debug Message Logged !!!");
        log.info("Info Message Logged !!!");
        log.error("Error Message Logged !!!");
        log.fatal("Fatal Message Logged !!!");
        
    }

}

[date: 2020-10-18 01:58:20] [priority: TRACE] [thread: main] [memberUuid: ]  [class&line: lovemesomecoding.MainLog.main:16] [msg: Trace Message Logged !!!]
[date: 2020-10-18 01:58:20] [priority: DEBUG] [thread: main] [memberUuid: ]  [class&line: lovemesomecoding.MainLog.main:17] [msg: Debug Message Logged !!!]
[date: 2020-10-18 01:58:20] [priority: INFO ] [thread: main] [memberUuid: ]  [class&line: lovemesomecoding.MainLog.main:18] [msg: Info Message Logged !!!]
[date: 2020-10-18 01:58:20] [priority: ERROR] [thread: main] [memberUuid: ]  [class&line: lovemesomecoding.MainLog.main:19] [msg: Error Message Logged !!!]
[date: 2020-10-18 01:58:20] [priority: FATAL] [thread: main] [memberUuid: ]  [class&line: lovemesomecoding.MainLog.main:20] [msg: Fatal Message Logged !!!]