Ruby Exceptions


The execution and the exception always go together. If you are opening a file which does not exist then if you did not handle this situation properly then your program is considered to be of bad quality.

The program stops if an exception occurs. So exceptions are used to handle various type of errors which may occur during a program execution and take appropriate action instead of halting program completely.

Ruby provide a nice mechanism to handle exceptions. We enclose the code that could raise an exception in a begin/end block and use rescue clauses to tell Ruby the types of exceptions we want to handle.

Syntax :

      begin  
      # -  
      rescue OneTypeOfException  
      # -  
      rescue AnotherTypeOfException  
      # -  
      else  
      # Other exceptions
      ensure
      # Always will be executed
      end
      

Everything from begin to rescue is protected. If an exception occurs during the execution of this block of code, control is passed to the block between rescue and end.

For each rescue clause in the begin block, Ruby compares the raised Exception against each of the parameters in turn. The match will succeed if the exception named in the rescue clause is the same as the type of the currently thrown exception, or is a superclass of that exception.

In an event that an exception does not match any of the error types specified, we are allowed to use an else clause after all the rescue clauses.

Example:

      begin
         file = open("/nonexistent_file")
         if file
            puts "File opened successfully"
         end
      rescue
            file = STDIN
      end
      print file, "==", STDIN, "\n"
      

This will produce following result. You can see that STDIN is substituted to file because open failed.

      #<IO:0xb7d16f84>==#<IO:0xb7d16f84>
      

 

Using retry Statement:

You can capture an exception using rescue block and then use retry statement to execute begin block from the beginning.

Syntax:

      begin
          # Exceptions raised by this code will 
      	# be caught by the following rescue clause
      rescue
          # This block will capture all types of exceptions
          retry  # This will move control to the beginning of begin
      end
      

Example:

      begin
         file = open("/nonexistent_file")
         if file
            puts "File opened successfully"
         end
      rescue
         fname = "existant_file"
         retry
      end
      

The following is the flow of the process:

NOTE: Notice that if the file of re-substituted name does not exist this example code retries infinitely. Be careful if you use retry for an exception process.

 

Using raise Statement:

You can use raise statement to raise an exception. The following method raises an exception whenever it's called. It's second message will never be printed.

Syntax:

      raise 
      
      OR
      
      raise "Error Message" 
      
      OR
      
      raise ExceptionType, "Error Message"
      
      OR
      
      raise ExceptionType, "Error Message" condition
      

The first form simply reraises the current exception (or a RuntimeError if there is no current exception). This is used in exception handlers that need to intercept an exception before passing it on.

The second form creates a new RuntimeError exception, setting its message to the given string. This exception is then raised up the call stack.

The third form uses the first argument to create an exception and then sets the associated message to the second argument.

The third form is similar to third form but you can add any conditional statement like unless to raise an exception.

Example:

      begin  
          puts 'I am before the raise.'  
          raise 'An error has occurred.'  
          puts 'I am after the raise.'  
      rescue  
          puts 'I am rescued.'  
      end  
      puts 'I am after the begin block.'
      

This will produce following result:

      I am before the raise.  
      I am rescued.  
      I am after the begin block.
      

One more example showing usage of raise:

      begin  
        raise 'A test exception.'  
      rescue Exception => e  
        puts e.message  
        puts e.backtrace.inspect  
      end
      puts "I'm after the rescue."
      

This will produce following result:

      A test exception.
      ["test.rb:2"]
      I'm after the rescue.
      

 

Using ensure Statement:

Sometimes you need to guarantee that some processing is done at the end of a block of code, regardless of whether an exception was raised. For example, you may have a file open on entry to the block, and you need to make sure it gets closed as the block exits.

The ensure clause does just this. ensure goes after the last rescue clause and contains a chunk of code that will always be executed as the block terminates. It doesn't matter if the block exits normally, if it raises and rescues an exception, or if it is terminated by an uncaught exception . the ensure block will get run.

Syntax:

      begin 
         #.. process 
         #..raise exception
      rescue 
         #.. handle error 
      ensure 
         #.. finally ensure execution
         #.. This will always execute.
      end
      

Example:

      begin
        raise 'A test exception.'
      rescue Exception => e
        puts e.message
        puts e.backtrace.inspect
      ensure
        puts "Ensuring execution"
      end
      puts "I'm after the ensure."
      

This will produce following result:

      A test exception.
      ["test.rb:2"]
      Ensuring execution
      I'm after the ensure.
      

 

Using else Statement:

If the else clause is present, it goes after the rescue clauses and before any ensure.

The body of an else clause is executed only if no exceptions are raised by the main body of code.

Syntax:

      begin 
         #.. process 
         #..raise exception
      rescue 
         # .. handle error
      else
         #.. executes if there is no exception
      ensure 
         #.. finally ensure execution
         #.. This will always execute.
      end
      

Example:

      begin
       # raise 'A test exception.'
       puts "I'm not raising exception"
      rescue Exception => e
        puts e.message
        puts e.backtrace.inspect
      else
         puts "Congratulations-- no errors!"
      ensure
        puts "Ensuring execution"
      end
      

This will produce following result:

      I'm not raising exception
      Congratulations-- no errors!
      Ensuring execution
      

Raised error message can be captured using $! variable.

Notice: to check this accurately, you would need to use IRB.

      C:\Ruby>irb
      irb(main):001:0> raise "test error"
      RuntimeError: test error
              from (irb):1
      irb(main):002:0> begin
      irb(main):003:1* raise "test2"
      irb(main):004:1> rescue
      irb(main):005:1> puts "An error occurred: #{$!}"
      irb(main):006:1> end
      An error occurred: test2
      => nil
      irb(main):007:0> exit
      

 

Catch and Throw:

While the exception mechanism of raise and rescue is great for abandoning execution when things go wrong, it's sometimes nice to be able to jump out of some deeply nested construct during normal processing. This is where catch and throw come in handy.

The catch defines a block that is labeled with the given name (which may be a Symbol or a String). The block is executed normally until a throw is encountered.

Syntax:

      throw :lablename
      #.. this will not be executed
      catch :lablename do
      #.. matching catch will be executed after a throw is encountered.
      end
      
      OR
      
      throw :lablename condition
      #.. this will not be executed
      catch :lablename do
      #.. matching catch will be executed after a throw is encountered.
      end
      

Example:

The following example uses a throw to terminate interaction with the user if '!' is typed in response to any prompt.

      def promptAndGet(prompt)
         print prompt
         res = readline.chomp
         throw :quitRequested if res == "!"
         return res
      end
      
      catch :quitRequested do
         name = promptAndGet("Name: ")
         age = promptAndGet("Age: ")
         sex = promptAndGet("Sex: ")
         # ..
         # process information
      end
      promptAndGet("Name:")
      

This will produce following result:

      Name: Ruby on Rails
      Age: 3
      Sex: !
      Name:Just Ruby
      

 

Class Exception:

Ruby's standard classes and modules raise exceptions. All the exception classes form a hierarchy, with the class Exception at the top. The next level contains seven different types:

There is one other exception at this level, Fatal, but the Ruby interpreter only uses this internally.

Both ScriptError and StandardError have a number of subclasses, but we do not need to go into the details here. The important thing is that if we create our own exception classes, they need to be subclasses of either class Exception or one of its descendants.

Let's look at an example:

      class FileSaveError < StandardError
         attr_reader :reason
         def initialize(reason)
            @reason = reason
         end
      end
      

 

Now look at the following example which will use this exception:

      File.open(path, "w") do |file|
        begin
            # Write out the data ...
        rescue
            # Something went wrong!
            raise FileSaveError.new($!)
        end
      end
      

The important line here is raise FileSaveError.new($!). We call raise to signal that an exception has occurred, passing it a new instance of FileSaveError, with the reason being that specific exception caused the writing of the data to fail.