Logger in Java – Java Logging Example

Filed Under: Java

Today we will look into Logger in Java. Java Logger provides logging in java programming.

Logger in Java

logger in java, java logging example

Java Logging API was introduced in 1.4 and you can use java logging API to log application messages. In this java logging tutorial, we will learn basic features of Java Logger. We will also look into Java Logger example of different logging levels, Logging Handlers, Formatters, Filters, Log Manager and logging configurations.

Java logging, logger in java, java logger example

Java Logger

java.util.logging.Logger is the class used to log application messages in java logging API.

We can create java Logger with very simple one line code as;


Logger logger = Logger.getLogger(MyClass.class.getName());

Java Logging Levels

java.util.logging.Level defines the different levels of java logging. There are seven levels of logging in java.

  1. SEVERE (highest)
  2. WARNING
  3. INFO
  4. CONFIG
  5. FINE
  6. FINER
  7. FINEST

There are two other logging levels, OFF that will turn off all logging and ALL that will log all the messages.

We can set the logger level using following code:


logger.setLevel(Level.FINE);

The logs will be generated for all the levels equal to or greater than the logger level. For example if logger level is set to INFO, logs will be generated for INFO, WARNING and SEVERE logging messages.

Java Logging Handlers

We can add multiple handlers to a java logger and whenever we log any message, every handler will process it accordingly. There are two default handlers provided by Java Logging API.

  1. ConsoleHandler: This handler writes all the logging messages to console
  2. FileHandler: This handler writes all the logging messages to file in the XML format.

We can create our own custom handlers also to perform specific tasks. To create our own Handler class, we need to extend java.util.logging.Handler class or any of it’s subclasses like StreamHandler, SocketHandler etc.

Here is an example of a custom java logging handler:


package com.journaldev.log;

import java.util.logging.LogRecord;
import java.util.logging.StreamHandler;

public class MyHandler extends StreamHandler {

    @Override
    public void publish(LogRecord record) {
        //add own logic to publish
        super.publish(record);
    }


    @Override
    public void flush() {
        super.flush();
    }


    @Override
    public void close() throws SecurityException {
        super.close();
    }

}

Java Logging Formatters

Formatters are used to format the log messages. There are two available formatters in java logging API.

  1. SimpleFormatter: This formatter generates text messages with basic information. ConsoleHandler uses this formatter class to print log messages to console.
  2. XMLFormatter: This formatter generates XML message for the log, FileHandler uses XMLFormatter as a default formatter.

We can create our own custom Formatter class by extending java.util.logging.Formatter class and attach it to any of the handlers. Here is an example of a simple custom formatter class.


package com.journaldev.log;

import java.util.Date;
import java.util.logging.Formatter;
import java.util.logging.LogRecord;

public class MyFormatter extends Formatter {

    @Override
    public String format(LogRecord record) {
        return record.getThreadID()+"::"+record.getSourceClassName()+"::"
                +record.getSourceMethodName()+"::"
                +new Date(record.getMillis())+"::"
                +record.getMessage()+"\n";
    }

}

Logger in Java – Java Log Manager

java.util.logging.LogManager is the class that reads the logging configuration, create and maintains the logger instances. We can use this class to set our own application specific configuration.


LogManager.getLogManager().readConfiguration(new FileInputStream("mylogging.properties"));

Here is an example of Java Logging API Configuration file. If we don’t specify any configuration, it’s read from JRE Home lib/logging.properties file.

mylogging.properties


handlers= java.util.logging.ConsoleHandler

.level= FINE

# default file output is in user's home directory.
java.util.logging.FileHandler.pattern = %h/java%u.log
java.util.logging.FileHandler.limit = 50000
java.util.logging.FileHandler.count = 1
java.util.logging.FileHandler.formatter = java.util.logging.XMLFormatter

# Limit the message that are printed on the console to INFO and above.
java.util.logging.ConsoleHandler.level = INFO
java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

com.journaldev.files = SEVERE

Here is a simple java program showing usage of Logger in Java.


package com.journaldev.log;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.logging.ConsoleHandler;
import java.util.logging.FileHandler;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.LogManager;
import java.util.logging.Logger;

public class LoggingExample {

    static Logger logger = Logger.getLogger(LoggingExample.class.getName());
    
    public static void main(String[] args) {
        try {
            LogManager.getLogManager().readConfiguration(new FileInputStream("mylogging.properties"));
        } catch (SecurityException | IOException e1) {
            e1.printStackTrace();
        }
        logger.setLevel(Level.FINE);
        logger.addHandler(new ConsoleHandler());
        //adding custom handler
        logger.addHandler(new MyHandler());
        try {
            //FileHandler file name with max size and number of log files limit
            Handler fileHandler = new FileHandler("/Users/pankaj/tmp/logger.log", 2000, 5);
            fileHandler.setFormatter(new MyFormatter());
            //setting custom filter for FileHandler
            fileHandler.setFilter(new MyFilter());
            logger.addHandler(fileHandler);
            
            for(int i=0; i<1000; i++){
                //logging messages
                logger.log(Level.INFO, "Msg"+i);
            }
            logger.log(Level.CONFIG, "Config data");
        } catch (SecurityException | IOException e) {
            e.printStackTrace();
        }
    }

}

When you will run above java logger example program, you will notice that CONFIG log is not getting printed in file, that is because of MyFilter class.


package com.journaldev.log;

import java.util.logging.Filter;
import java.util.logging.Level;
import java.util.logging.LogRecord;

public class MyFilter implements Filter {

	@Override
	public boolean isLoggable(LogRecord log) {
		//don't log CONFIG logs in file
		if(log.getLevel() == Level.CONFIG) return false;
		return true;
	}

}

Also the output format will be same as defined by MyFormatter class.


1::com.journaldev.log.LoggingExample::main::Sat Dec 15 01:42:43 PST 2012::Msg977
1::com.journaldev.log.LoggingExample::main::Sat Dec 15 01:42:43 PST 2012::Msg978
1::com.journaldev.log.LoggingExample::main::Sat Dec 15 01:42:43 PST 2012::Msg979
1::com.journaldev.log.LoggingExample::main::Sat Dec 15 01:42:43 PST 2012::Msg980

If we don’t add our own Formatter class to FileHandler, the log message will be printed like this.


<record>
  <date>2012-12-14T17:03:13</date>
  <millis>1355533393319</millis>
  <sequence>996</sequence>
  <logger>com.journaldev.log.LoggingExample</logger>
  <level>INFO</level>
  <class>com.journaldev.log.LoggingExample</class>
  <method>main</method>
  <thread>1</thread>
  <message>Msg996</message>
</record>

Console log messages will be of following format:


Dec 15, 2012 1:42:43 AM com.journaldev.log.LoggingExample main
INFO: Msg997
Dec 15, 2012 1:42:43 AM com.journaldev.log.LoggingExample main
INFO: Msg998
Dec 15, 2012 1:42:43 AM com.journaldev.log.LoggingExample main
INFO: Msg998

Below image shows the final Java Logger example project.
logger in java, java logging example

That’s all for Logger in Java and Java Logger Example. You can download the project from below link.

Reference: Java Logging API

Comments

  1. Abhishek says:

    Logs are mixing under heavy load. also sometimes getting GC overload exception. Can you guide me

  2. Marco Scarpa says:

    I saw that each logger has a name that usually is the same of the class (Logger.getLogger(LoggingExample.class.getName());)

    Is it possible to set a different logging level for each logger in logging.properties?

    Something similar to spring boot that allows to set a different logging level for each package like this:

    logging.level.root=INFO
    logging.level.my.package.stuff=DEBUG
    logging.level.my.package.otherstuff = WARN

  3. vinod says:

    If generates a new log file, How should we provide the permission like Read / Write permissions in to the particular file.

  4. Arthur says:

    I found the examples useful. Just do some tailoring and they fits with my application.. Thanks.
    However I cannot find the MyFilter class example code. Did I miss something?

    1. Pankaj says:

      I forgot to put MyFilter code, I have added it now. Also you can download the Java Logging Example project now.

  5. Akhil R says:

    Very helpful buddy and great article. Keep going. I am already a follower of journaldev, and its really good for understanding programming concepts.

  6. Rajeev says:

    Good article.
    What does the levels FINE, FINER, FINEST specify? I mean In which context we can use these? whether the Level FINE indicates debug?

  7. Prasanti says:

    Properties file has the log file set as below:
    java.util.logging.FileHandler.pattern = %h/java%u.log
    and again file handler has the log file set as below:
    Handler fileHandler = new FileHandler(“/Users/pankaj/tmp/logger.log”, 2000, 5);

    – Aren’t both contradicting each other? Where will the logs ultimately get logged?

  8. Manoj Jawalkar says:

    in the statement:
    LogManager.getLogManager().readConfiguration(new FileInputStream(“mylogging.properties”));

    Where does the LogManager search for the properties file “mylogging.properties”?

    1. Pankaj says:

      it should be in the classpath.

  9. Rajveer says:

    how can we display every log to text file i need this code,and how can we take input through port from 2D barcode scanner

  10. Alan Freeman says:

    Thanks for an informative tutorial pitched at exactly the right level; it tells you what to do without an excess of technical details

  11. dima says:

    Sorry, in the line “Formatters are used to format the log messages. There are two available handlers in java logging API.”
    Maybe, must be … two available formatters ?

    1. Pankaj says:

      Yes, it was typo error. Thanks for pointing out, I have corrected the post.

  12. vinuta says:

    how to print receipt in struts2??? thanks in advance

  13. Andrew says:

    Does anybody know how I can make the java.util logger display the date in the european format:
    “24-06-2014 23:21:08”
    and not the american?
    “Jun 24, 2014 10:21:08 PM”

    I would like to specify that somehow in the logging .properties file.
    In all examples on the web I can find

    java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

    There are no examples of a custom format of the message log

    1. Pankaj says:

      Check the documentation, there must be a way to do it.

  14. Saurabh says:

    Hi ,
    I have one application which send email to multiple person email id.It is a scheduler which automatically send email to all person upon a triggering event.
    I want to create a log file which will contain logs of email ids ,date on which mail has auto triggered.
    Advance thanks.

  15. Phil Wright says:

    The ‘Formatter’ class can not be subclassed, as it has been declared final.

    Declaration in Formatter.java:


    public final class Formatter implements Closeable, Flushable ...

    1. Pankaj says:

      You are getting confused with Formatter class in java.util package. The one we are extending is in java.util.logging package.

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