Write message in system.log file -
\Magento\Framework\App\ObjectManager::getInstance()->get('Psr\Log\LoggerInterface')->info('log message');
Print log in debug.log file
\Magento\Framework\App\ObjectManager::getInstance()->get('Psr\Log\LoggerInterface')->log(100,print_r($data,true));
OR
private $logger;
public function __construct(\Psr\Log\LoggerInterface $logger)
{
$this->logger = $logger;
}
public function mylogwritefunction()
{
try
{ /* Some logic that could throw an Exception */
} catch (\Exception $e)
{ $this->logger->critical($e->getMessage());
}
}