21 ruby exceptions
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Ruby provides mechanisms like begin/rescue/ensure to handle exceptions gracefully. The begin block contains code that could raise exceptions, rescue clauses specify the types of exceptions to handle, and ensure ensures some code is always executed. Exceptions can be raised manually using raise and caught using catch/throw. Common exception classes include StandardError, while custom exceptions should inherit from Exception.
![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 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 following result:
I am before the raise.
I am rescued.
I am after the begin block.
One more example showing 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 following result:
A test exception.
["test.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](https://image.slidesharecdn.com/21rubyexceptions-130204175512-phpapp01/85/21-ruby-exceptions-3-320.jpg)
![#.. 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 following result:
A test exception.
["test.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 following result:
I'm not raising exception
Congratulations-- no errors!
Ensuring execution
Raised error message can be captured using $! variable.](https://image.slidesharecdn.com/21rubyexceptions-130204175512-phpapp01/85/21-ruby-exceptions-4-320.jpg)
21 ruby exceptions
- 1. RUBY EXCEPTIONS http://www.tutorialspoint.com/ruby/ruby_exceptions.htm Copyright © tutorialspoint.com 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 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.
- 2. 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. Program 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.
- 3. 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 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 following result: I am before the raise. I am rescued. I am after the begin block. One more example showing 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 following result: A test exception. ["test.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
- 4. #.. 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 following result: A test exception. ["test.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 following result: I'm not raising exception Congratulations-- no errors! Ensuring execution Raised error message can be captured using $! variable.
- 5. 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: Interrupt NoMemoryError SignalException
- 6. 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.