In magento2, You can also write to the logs using the Zend library like below :

$writer = new ZendLogWriterStream(BP . '/var/log/test.log');

$logger = new ZendLogLogger();

$logger->addWriter($writer);

$logger->info('Your text message');

Edited

You can also print PHP objects and arrays like below :

$logger->info(print_r($yourArray, true));

MagentoFrameworkAppObjectManager::getInstance()

    ->get(PsrLogLoggerInterface::class)->debug('message');

Temporary print log with new file

$writer = new ZendLogWriterStream(BP . '/var/log/logfile.log');

$logger = new ZendLogLogger();

$logger->addWriter($writer);

$logger->info('Simple Text Log'); // Simple Text Log

$logger->info('Array Log'.print_r($myArrayVar, true)); // Array Log

Factory Method

You need to inject PsrLogLoggerInterface class into constructor to call logger object

protected $_logger;

public function __construct(

...

PsrLogLoggerInterface $logger

...

) {

    $this->_logger = $logger;

}



public function logExample() {



    //To print string Output in debug.log

    $this->_logger->addDebug('Your Text Or Variables'); 



    // To print array Output in system.log

    $this->_logger->log('600', print_r($yourArray, true));



}

Or you directly use this code in phtml file:

To print string Output in debug.log

MagentoFrameworkAppObjectManager::getInstance()

   ->get('PsrLogLoggerInterface')->debug('Your Message');

To print array Output in system.log

$myArray = array('test1'=>'123', 'test2'=>'123', 'test3'=>'123');

$level = '100'; // use one of: 100, 200, 250, 300, 400, 500, 550, 600

MagentoFrameworkAppObjectManager::getInstance()

    ->get('PsrLogLoggerInterface')

    ->log($level, print_r($myArray, true));

If you want to use default logger but custom file for logging (or other custom logic) you need to use custom logger handler:

class Logger extends MagentoFrameworkLoggerHandlerBase

{

  /**

   * @var string

   */

  protected $fileName = '/var/log/my-log-file.log';



  /**

   * @var int

   */

  protected $loggerType = MonologLogger::DEBUG;

}

Then add it as handler somewhere within your code:

protected function addCustomLogHandler()

{

    $logger = Data::getCustomLogger();

    if(isset($this->_logger)){

        $this->_logger->pushHandler($logger);

    }

}

A step back in convenience IMO

No, there is no direct equivalent. It's a bit complicated now.

See: Logging to a custom file in Magento 2

Include psr logger class in your file using use and then call addDebug() method. This will print log message in var/log/debug.log file

use PsrLogLoggerInterface;



class demo {

  function demo()

  {

    //EDIT: Using debug instead of addDebug for PSR compatiblity

    $this->_objectManager->get('PsrLogLoggerInterface')->debug("your message goes here");

  }



}

In a simple way if you don't want to create dependency injection or anything else use below code it will store log in system.log file

$logger = MagentoFrameworkAppObjectManager::getInstance()->get(PsrLogLoggerInterface::class);

$logger->info('message');

That's all..

If you are looking for elegant custom log handler, I recommend you to create a helper (which able to use anywhere in your project by injecting it in your constructors).

Inspired from answer of Petar Dzhambazov and halk, ladies and gentlement I introduced you a better and shorter way instead of duplicated custom log code all the time.

StackoverflowCoreHelperData.php

<?php

/**

 * Copyright © 2016 Toan Nguyen <https://nntoan.github.io>. All rights reserved.

 * See COPYING.txt for license details.

 */



namespace StackoverflowCoreHelper;



use MagentoFrameworkAppHelperAbstractHelper;

use MagentoFrameworkAppHelperContext;

use MagentoFrameworkObjectManagerInterface;

use MonologHandlerStreamHandler;

use MonologLogger;



/**

 * Stackoverflow Core Helper

 *

 * @package StackoverflowCoreHelper

 * @author  Toan Nguyen <https://github.com/nntoan>

 */

class Data extends AbstractHelper

{

    /**

     * @var MagentoFrameworkObjectManagerInterface

     */

    protected $objectManager;

    /**

     * @var PsrLogLoggerInterface

     */

    protected $defaultLogger;

    /**

     * @var string

     */

    protected $channelName;

    /**

     * @var string

     */

    protected $logFile;



    /**

     * Data constructor.

     *

     * @param Context                $context       Context

     * @param ObjectManagerInterface $objectManager Object manager

     */

    public function __construct(

        Context $context,

        ObjectManagerInterface $objectManager

    ) {

        parent::__construct($context);

        $this->objectManager = $objectManager;

        $this->defaultLogger = $context->getLogger();

        $this->channelName = 'stackoverflow-default-channel';

        $this->logFile = BP . '/var/log/stackoverflow.log';

    }



    /**

     * Get default/custom logger instance

     *

     * @param string $type

     * @param string|null $channelName

     * @param string|null $logFile

     *

     * @return PsrLogLoggerInterface

     */

    public function getLogger($type = 'default', $channelName = null, $logFile = null)

    {

        switch ($type) {

            case 'custom':

                $logger = $this->getCustomLogger($channelName, $logFile);

                break;

            case 'default':

            default:

                $logger = $this->getDefaultLogger();

                break;

        }



        return $logger;

    }



    /**

     * Return default logger instance

     *

     * @return PsrLogLoggerInterface

     */

    private function getDefaultLogger()

    {

        return $this->defaultLogger;

    }



    /**

     * Create new logger instance with custom channel name

     * and log file name

     *

     * @param string|null $channelName

     * @param string|null $logFile

     *

     * @return MonologLogger

     */

    private function getCustomLogger($channelName = null, $logFile = null)

    {

        if (!empty($channelName) && !empty($logFile)) {

            $this->channelName = $channelName;

            $this->logFile = BP . $logFile;

        }



        /** @var MonologLogger $logger */

        $logger = $this->objectManager->create(Logger::class, [$this->channelName]);



        /** @var MonologHandlerStreamHandler $streamHandler */

        $streamHandler = $this->objectManager->create(StreamHandler::class, [$this->logFile]);

        $logger->pushHandler($streamHandler);



        return $logger;

    }

}

USAGE

VendorSomethingModelDonaldTrump.php

<?php

/**

 * Copyright © 2016 Toan Nguyen <https://nntoan.github.io>. All rights reserved.

 * See COPYING.txt for license details.

 */



namespace VendorSomethingModel;



use StackoverflowCoreHelperData as SoHelper;



/**

 * DonaldTrump business logic file

 *

 * @package VendorSomethingModel

 * @author  Toan Nguyen <https://github.com/nntoan>

 */

class DonaldTrump

{

    /**

     * @var PsrLogLoggerInterface

     */

    private $logger;



    const LOG_CHANNEL_NAME = 'donald-trump';

    const LOG_FILEPATH = '/var/log/donald-trump/make-america-great-again.log';



    /**

     * DonaldTrump constructor.

     *

     * @param SoHelper    $soHelper

     */

    public function __construct(

        SoHelper $soHelper

    ) {

        $this->logger = $soHelper->getLogger('custom', self::LOG_CHANNEL_NAME, self::LOG_FILEPATH);

    }



    /**

     * Test my custom log

     *

     * @return void

     */

    public function test()

    {

        $this->logger->error('MAKE AMERICA GREAT AGAIN !?');

    }

}

As you see, whenever test() method called, the string MAKE AMERICA GREAT AGAIN !? will be log as error into a custom file (create folder automatically if doesn't exist) in <magento_dir>/var/log/donald-trump/make-america-great-again.log

In other model, e.g HillaryClinton you can create simple declare new custom log and so on...

Hope this helps ;)

There is one update for logger in 2.2. You can enable logger for production mode by run SQL:

 "INSERT INTO core_config_data (scope, scope_id, path, value) VALUES ('default', '0', 'dev/debug/debug_logging', '1');"

Then you can use PsrLogLoggerInterface for print log just like above answers:

protected $logger;



public function __construct(

  PsrLogLoggerInterface $logger

) {

    $this->logger = $logger;

  }



public function yourFunction() {

    $data = ["test" => "testing"];

    $this->logger->debug(var_export($data, true));

}

1. Inject $logger class in constructor PsrLogLoggerInterface $logger
   
   This is achieved by passing $logger as argument.



2. 
   

   Initialize $logger in constructor

   
   
   

   $this->logger = $logger
   
   

   
   


3. 
   

   In function within the class you want to log use the below line

   
   
   

   $this->logger->debug($message);
   
   $this->logger->log($level, $message);
   
   

   
   

If you need it within your single class with custom log file:

public function __construct(PsrLogLoggerInterface $logger, MagentoFrameworkAppFilesystemDirectoryList $dir) 

{

    $this->logger = $logger;

    $this->dir = $dir;



    $this->logger->pushHandler(new MonologHandlerStreamHandler($this->dir->getRoot().'/var/log/custom.log'));

}

Place PSR logger code in your constructor:

protected $logger;

public function __construct(PsrLogLoggerInterface $logger)

{

    $this->logger = $logger;

}

then you can use in your function like:

$this->logger->info($message);