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Table of Contents
- Basic Syntax
- Relevant methods
- Ensure
- retry
- Rescue modifier
- Exception in methods
- Multiple Exceptions
- Custom Exception
- References
Basic Syntax
The basic syntax of ruby’s exception handling looks like this.
begin
# Code that potentially raises an error
rescue => e # variable
# Excecute if there was any error between begin and rescue.
endExample
begin
raise
rescue => e
p e #=> RuntimeError
endRelevant methods
| Method | Explanation |
|---|---|
| Object#class | Returns the class of the receiver. |
| Kernel#raise | Raise an error. |
| Exception#backtrace | Returns the backtrace of the error. |
| Exception#message | Returns the error message. |
Example
begin
raise StandardError.new("Message")
rescue => e
p e.class #=> StandardError
p e.message #=> "Message"
p e.backtrace #=> ["(repl):4:in `<main>'", "/run_dir/repl.rb:41:in `eval'", "/run_dir/repl.rb:41:in `run'", "/run_dir/repl.rb:57:in `handle_eval'", "/run_dir/repl.rb:170:in `start'", "/run_dir/repl.rb:177:in `start'", "/run_dir/repl.rb:181:in `<main>'"]
endEnsure
You can run something regardless of the existence of any error with ensure
Example
begin
"no Error"
rescue => e
p e.message
ensure
p "Ensured" #=> Ensured
endExample2
begin
raise StandardError.new('error')
rescue => e
p e.message #=> error
ensure
p "Ensured" #=> Ensured
endretry
Literally re-try the execution.
Example
file = ARGV[0]
begin
# Open a file.
io = File.open( file )
rescue
# If there was an error during opening the file, this part gets executed.
sleep( 10 )
# Goes back to begin and re-try the execution.
retry
endRescue modifier
You can write begin ~ rescue in one sentence just like if ~ end in Ruby.
Example
raise "Error" rescue p "rescued" #=> rescuedException in methods
You don’t need to write begin and end to do exception handling in methods.
Example
def method
raise 'error'
rescue => e
p e.message #=> error
ensure
p 'ensured' #=> ensured
end
method #=> "error"Multiple Exceptions
You can write rescue multiple times for each corresponding error.
begin
rescue Exception1, Exception2 => e
# For Exception1 or Exception2
rescue Exception3 => e
# For Exception3
rescue => e
# For other errors.
endExample
begin
raise StandardError
rescue StandardError, RangeError
p 'Standard or Ranage Error'
rescue RuntimeError
p 'Runtime Error'
rescue => e
p 'some other error'
end
#=> "Standard or Ranage Error"Custom Exception
Simply Follow the steps below.
(1) Make a new Class
class CustomError < StandardError
end(2) Add a message
class CustomError < StandardError
def initialize(msg="My Error")
super
end
end(3) Add custom data attributes to your exception
class CustomError < StandardError
attr_reader :atr
def initialize(msg="My Error", atr="atrribute")
@atr = atr
super(msg)
end
endExample
class CustomError < StandardError
attr_reader :atr
def initialize(msg="My Error", atr="atrribute")
@atr = atr
super(msg)
end
end
begin
raise CustomError.new("CustomMessage", "Custom Attribute")
rescue => e
p e.message #=> "CustomMessage"
p e.atr #=> "Custom Attribute"
end