From: Joel VanderWerf <vjoel@...>
Date: 2007-06-08T03:23:03+09:00
Subject: Re: raise problem in Thread

oliver wrote:
> Define following class:
> class MyThread < Thread
>   def initialize()
>     super{
>       puts "started"
>       sleep 1000
>     }
>   end
> 
>   def test_raise()
>     raise StandardError.new()
>     puts "hello"
>   end
> end
> 
> Open irb and run:
> irb(main):082:0> thread = MyThread.new
> started=> #<MyThread:0xb7cec234 run>
> irb(main):093:0> thread.test_raise
> hello
> => nil
> 
> I think the line "puts "hello"" should not be run after raising an
> exception.
> What is the reason of this problem?

The problem is that test_raise is executed not by the MyThread thread, 
but by the main thread. The MyThread instance receives the exception and 
dies (silently because abort_on_exception is off), and the main thread 
continues executing the puts "hello".

Keep in mind that the following contexts are different:

(1) *self*, the object that receives a message and performs the lookup 
to determine which method handles the message

(2) Thread.current: the thread that the interpreter is currently running 
to execute a method

Let's look at the context when you call test_raise. Even though the 
*self* is your MyThread instance, the current thread is still the main 
thread.

If you want to send commands to a thread and have them executed _by_ the 
thread, you might want to set up a Queue that it reads.

Also, it's useful to have the following when debugging threads:

Thread.abort_on_exception = true

(do this at the beginning of your program--you can also set this for 
individual threads, but the global setting is nice for debugging)

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407