If we use SOP (System.out.print()) statements to print log messages, then we can run into some disadvantages:

1. We can print log messages on the console only. So, when the console is closed, we will lose all of those logs.
2. We can’t store log messages in any permanent place. These messages will print one by one on the console because it is a single-threaded environment.

To overcome these problems, the Log4j framework came into the picture. Log4j is an open source framework provided by Apache for Java projects.

Log4j Components

Log4j has three main components, which are the following:

1. Logger
2. Appender
3. Layout

Logger

Logger is a class in the org.apache.log4j.* package. We have to initialize one Logger object for each Java class. We use Logger’s methods to generate log statements. Log4j provides the factory method to get Logger objects.

Syntax to get Logger objects:

static Logger logger = Logger.getLogger(CurrentClass.class.getName()).

Note: CurrentClass is a Java class name for which we are getting logger object.

Example

public class Student{

    private static final Logger LOGGER = Logger.getLogger(Student.class);
    public void getStudentRecord() {
    }
}

The Logger class has some methods that are used to print application status.

We have five methods in the Logger class

1. info()
2. debug()
3. warn()
4. fatal()
5. error()

How and when to use these methods depends on us. Here, the method names are different, but the process is the same for all of them: all will print a message only.

Levels

Level is a class in the org.apache.log4j.* package. We can also make a custom level by extending the Level class. Each level has a different priority order, like this:

debug < info < warn < error < fatal

It means fatal is the highest priority error, like if/when the database is closed.

Appender

Appender is used to write messages into a file or DB or SMTP.

Log4j has different types of appenders:

1. SyslogAppendersends
2. SMTPAppender
3. JDBCAppender
4. FileAppender
5. SocketHubAppender
6. SocketAppender
7. TelnetAppender
8. ConsoleAppender

Layout

This is used to define the formatting in which logs will print in a repository.

We have different types of layouts:

1. PatternLayout
2. SimpleLayout
3. XMLLayout
4. HTMLLayout

Log4j: Configuration

log4j.properties

# Root logger option
log4j.rootLogger=INFO, file, stdout

# configuration to print into file
log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.File=D:\\log\\logging.log
log4j.appender.file.MaxFileSize=12MB
log4j.appender.file.MaxBackupIndex=10
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# configuration to print on console
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

Description of log4j.properties file :

• log4j.appender.file=org.apache.log4j.RollingFileAppender

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

These will define appender types: That means they will specify where we want to store application logs. RollingFileAppender will print all logs in a file, and ConsoleAppender will print all logs in the console.

• log4j.appender.file.File=D:\\log\\logging.log

That specifies the log file location.

• log4j.appender.file.layout=org.apache.log4j.PatternLayout

• log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

These specify the pattern in which logs will print to the log file.

Example:

import org.apache.log4j.Logger;

public class Student {
    static Logger logger = Logger.getLogger(Student.class);

    public static void main(String[] args) {        
        logger.debug("This is debug message");
        logger.info("This is info message");
        logger.warn("This is warn message");
        logger.fatal("This is fatal message");
        logger.error("This is error message");

        System.out.println("Logic executed successfully....");

    }

}

logging.log(log file):

2018-05-02 16:01:45 INFO  Student:12 - This is info message
2018-05-02 16:01:45 WARN  Student:13 - This is warn message
2018-05-02 16:01:45 FATAL Student:14 - This is fatal message
2018-05-02 16:01:45 ERROR Student:15 - This is error message

It will not print debug level error logs because we defined our root logger as INFO-level in our log4j.properties file. Error messages with a priority greater than INFO will print.

Console logs:

16:01:45,511  INFO Student:12 - This is info message
16:01:45,517  WARN Student:13 - This is warn message
16:01:45,517 FATAL Student:14 - This is fatal message
16:01:45,518 ERROR Student:15 - This is error message

program executed successfully....