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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
#!/usr/bin/ruby
begin
file = open("/unexistant_file")
if file
puts "File opened successfully"
end
rescue
file = STDIN
end
print file, "==", STDIN, "n"
This will produce the 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
#!/usr/bin/ruby
begin
file = open("/unexistant_file")
if file
puts "File opened successfully"
end
rescue
fname = "existant_file"
retry
end
The following is the flow of the process −
- An exception occurred at open.
- Went to rescue. fname was re-assigned.
- By retry went to the beginning of the begin.
- This time file opens successfully.
- Continued the essential 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 be printed.
Syntax
raise OR raise "Error Message" OR raise ExceptionType, "Error Message" OR raise ExceptionType, "Error Message" condition
The first form simply re-raises 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 fourth form is similar to the third form but you can add any conditional statement like unless to raise an exception.
Example
#!/usr/bin/ruby 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 the following result −
I am before the raise. I am rescued. I am after the begin block.
One more example showing the usage of raise −
#!/usr/bin/ruby begin raise ''A test exception.'' rescue Exception => e puts e.message puts e.backtrace.inspect end
This will produce the following result −
A test exception. ["main.rb:4"]
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
This will produce the following result −
A test exception. ["main.rb:4"] Ensuring execution
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 the following result −
I''m not raising exception Congratulations-- no errors! Ensuring execution
Raised error message can be captured using $! variable.
Catch and Throw
While the exception mechanism of raise and rescue is great for abandoning the 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:")
You should try the above program on your machine because it needs manual interaction. This will produce the 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 −
- Interrupt
- NoMemoryError
- SignalException
- ScriptError
- StandardError
- SystemExit
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.
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