Descendants of class Exception are used to communicate between Kernel#raise and rescue statements in begin ... end blocks. Exception objects carry information about the exception – its type (the exception's class name), an optional descriptive string, and optional traceback information. Exception subclasses may add additional information like NameError#name.

Programs may make subclasses of Exception, typically of StandardError or RuntimeError, to provide custom classes and add additional information. See the subclass list below for defaults for raise and rescue.

When an exception has been raised but not yet handled (in rescue, ensure, at_exit and END blocks) the global variable $! will contain the current exception and $@ contains the current exception's backtrace.

It is recommended that a library should have one subclass of StandardError or RuntimeError and have specific exception types inherit from it. This allows the user to rescue a generic exception type to catch all exceptions the library may raise even if future versions of the library add new exception subclasses.

For example:

class MyLibrary
  class Error < RuntimeError

  class WidgetError < Error

  class FrobError < Error


To handle both WidgetError and FrobError the library user can rescue MyLibrary::Error.

The built-in subclasses of Exception are:

Equality—If obj is not an Exception, returns false. Otherwise, returns true if exc and obj share same class, messages, and backtrace.

Returns a hash, that will be turned into a JSON object and represent this object.

Returns any backtrace associated with the exception. The backtrace is an array of strings, each containing either “filename:lineNo: in `method''' or “filename:lineNo.''

def a
  raise "boom"

def b

rescue => detail
  print detail.backtrace.join("\n")


prog.rb:2:in `a'
prog.rb:6:in `b'

Returns any backtrace associated with the exception. This method is similar to Exception#backtrace, but the backtrace is an array of Thread::Backtrace::Location.

Now, this method is not affected by Exception#set_backtrace().

Returns the previous exception ($!) at the time this exception was raised. This is useful for wrapping exceptions and retaining the original exception information.

With no argument, or if the argument is the same as the receiver, return the receiver. Otherwise, create a new exception object of the same class as the receiver, but with a message equal to string.to_str.

Returns formatted string of exception. The returned string is formatted using the same format that Ruby uses when printing an uncaught exceptions to stderr.

If highlight is true the default error handler will send the messages to a tty.

order must be either of :top or :bottom, and places the error message and the innermost backtrace come at the top or the bottom.

The default values of these options depend on $stderr and its tty? at the timing of a call.

Return this exception's class name and message.

Deserializes JSON string by constructing new Exception object with message m and backtrace b serialized with to_json

Returns the result of invoking exception.to_s. Normally this returns the exception's message or name.

Construct a new Exception object, optionally passing in a message.

Sets the backtrace information associated with exc. The backtrace must be an array of String objects or a single String in the format described in Exception#backtrace.

Stores class name (Exception) with message m and backtrace array b as JSON string

Returns exception's message (or the name of the exception if no message is set).

Returns true if exception messages will be sent to a tty.