How to log error message in Drupal 8 (watchdog in druapl 8)

hook_watchdog() and watchdog() removed

in drupal 8

In Drupal 8 watchdog has been refactored to an OO, PSR-3 compatible framework.

In the paragraphs below, a "channel" refers to what used to be the $type argument to watchdog(). For logging you would have done with simple watchdog() statements in Drupal 7, look at the example for injecting a specific channel.

The switch to PSR-3 logging has resulted in the following API changes:

1. hook_watchdog() is gone. For a module to implement a logger, it has to register a service tagged as logger. eg
   
   services:

  logger.mylog:
    class: Drupal\mylog\Logger\MyLog
    tags:
      - { name: logger }
   This class must implement \Psr\Log\LoggerInterface. like this:

   <?php
   namespace Drupal\mylog\Logger;
   
   use Drupal\Core\Logger\RfcLoggerTrait;
   use Psr\Log\LoggerInterface;
   
   class MyLog implements LoggerInterface {
     use RfcLoggerTrait;
   
     /**
      * {@inheritdoc}
      */
     public function log($level, $message, array $context = array()) {
       // Do stuff
     }
   
   }
   ?>

2. watchdog($type, $message, $variables, $severity, $link) has been removed in favor of \Drupal::logger($type)->log($severity, $message, $variables)
   D7

   <?php
   // Logs a notice
   watchdog('my_module', $message, array());
   // Logs an error
   watchdog('my_module', $message, array(), WATCHDOG_ERROR);
   ?>

   
   D8 - procedural

   <?php
   // Logs a notice
   \Drupal::logger('my_module')->notice($message);
   // Logs an error
   \Drupal::logger('my_module')->error($message);
   ?>

   
   D8 - injecting the whole factory
   
   services:
  myservice_that_needs_to_log_to_multiple_channels:
    class: Drupal\mymodule\MyService
    arguments: ['@logger.factory']
   
   

   <?php
   class MyService {
     public function __construct($factory) {
       $this->loggerFactory = $factory;
     }
   
     public function doStuff() {
       // Logs a notice to "my_module" channel.
       $this->loggerFactory->get('my_module')->notice($message);
        // Logs an error to "my_other_module" channel.
       $this->loggerFactory->get('my_other_module')->error($message);
     }
   }
   ?>

   D8 - injecting a specific channel
   Drupal core registers only one channel in core.services.yml for "system" channel.
   There might be cases where you don't want to inject the whole factory to your class, but just a specific channel.
   In that case you can easily register this channel with something like this:
   services:
  logger.channel.mymodule:
    parent: logger.channel_base
    arguments: ['mymodule']
   
   And then inject logger.channel.mymodule to your services.
   
   services:
  myservice_that_needs_specific_channel:
    class: Drupal\mymodule\MyService
    arguments: ['@logger.channel.mymodule']
   
   

   <?php
   class MyService {
     public function __construct($logger) {
       $this->logger = $logger;
     }
   
     public function doStuff() {
       // Logs a notice.
       $this->logger->notice($message);
        // Logs an error.
       $this->logger->error($message);
     }
   }
   ?>

3. WATCHDOG_* constants and watchdog_severity_levels() are removed
   D7
   

   <?php
   $severity = WATCHDOG_EMERGENCY;
   $severity = WATCHDOG_ALERT;
   $severity = WATCHDOG_CRITICAL;
   $severity = WATCHDOG_ERROR;
   $severity = WATCHDOG_WARNING;
   $severity = WATCHDOG_NOTICE;
   $severity = WATCHDOG_INFO;
   $severity = WATCHDOG_DEBUG;
   $levels = watchdog_severity_levels();
   ?>

   
   D8
   

   <?php
   use Drupal\Core\Logger\RfcLogLevel;
   $severity = RfcLogLevel::EMERGENCY;
   $severity = RfcLogLevel::ALERT;
   $severity = RfcLogLevel::CRITICAL;
   $severity = RfcLogLevel::ERROR;
   $severity = RfcLogLevel::WARNING;
   $severity = RfcLogLevel::NOTICE;
   $severity = RfcLogLevel::INFO;
   $severity = RfcLogLevel::DEBUG;
   $levels = RfcLogLevel::getLevels();
   ?>

 Source : https://www.drupal.org/node/2270941