Ruby allows you to handle an exception when you want to recover from an error in your application. This is important if you decide to gracefully exist form the program without system generated error message being presented to the UI.

Rescue is one such method in Ruby. It is used with begin/rescue code block.

def error_handled
 begin
  puts “my text”
  raise
 rescue
  puts “no problem”
 end
end

The code will continue executing after rescue so that your program can run uninterrupted even if there is an error in the middle of it.

A rescue method always deal with exceptions of the StandardError class or it’s subclasses. You can rescue a specific class by mentioning it as well.

def error_handled
 begin
  puts “my text”
  raise
 rescue SyntaxError
  puts “no problem”
 end
end

a rescue clause also allows you to stack up Exceptions to be rescued in your code.

begin
rescue SyntaxError
rescue SystemError
end

Exception handling lets you view an exception details after you rescue method is called. Here is an example how Ruby does in code.

begin
 raise ‘An error’
rescue Exception => e
 puts e.message
end

Alternatively, you can use $! Notation to output most recent exception that occurred.

begin
 raise ‘An error’
rescue Exception
 puts $!.message
end

The later method is not recommended due to global nature of $! Notation.

You can also retry an exception within rescue clause so that code can re-execute the line that gave an error in a first place in hope that error was a result of network connection drop or something similar in nature. You can see from the example below that you can call retry method right after rescue.

begin
 raise ‘An error’
rescue Exception => e
 puts e.message
   retry
end

It is the same method Ruby check_connection method uses for retrying database connection certain number of times before giving up on it completely.

require “open-uri”
def check_url(max=3,url=”http://www.google.com”)
 tries = 0
 begin
  tries+=1
  open(url)
 rescue Exception
  retry unless tries >= max
 end
end