Magento 2 get current category Id using registry

You need to use this in your constructer 

/**
 * @var Magento\Framework\Registry
 */
private $registry;

public function __construct(\Magento\Framework\Registry $registry)

{
    $this->registry = $registry;
}

then you can get the current category by this code

$category = $this->registry->registry('current_category');