Re: Best way to halt Java process?

From:
"Mike Schilling" <mscottschilling@hotmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 12 Jun 2010 21:27:51 -0700
Message-ID:
<hv1mo9$845$1@news.eternal-september.org>
"ClassCastException" <zjkg3d9gj56@gmail.invalid> wrote in message
news:hv1kf6$rdk$1@news.eternal-september.org...

On Sat, 12 Jun 2010 01:07:42 -0700, Mike Schilling wrote:

"ClassCastException" <zjkg3d9gj56@gmail.invalid> wrote in message
news:huuvm3$c4e$1@news.eternal-september.org...

On Thu, 10 Jun 2010 22:55:59 -0700, Mike Schilling wrote:

"ClassCastException" <zjkg3d9gj56@gmail.invalid> wrote in message
news:husivb$hsb$3@news.eternal-september.org...

On Thu, 10 Jun 2010 20:38:23 -0700, Mike Schilling wrote:

"ClassCastException" <zjkg3d9gj56@gmail.invalid> wrote in message
news:hus9o8$g1n$6@news.eternal-september.org...

On Thu, 10 Jun 2010 19:54:51 -0700, Mike Schilling wrote:

"ClassCastException" <zjkg3d9gj56@gmail.invalid> wrote in message
news:hus76c$g1n$2@news.eternal-september.org...

I think the usual situation will be that a) you don't catch Error
and b) servlet containers etc. run servlets etc. in separate
threads and deal with it gracefully if any of these threads
abends. If you have multiple threads, provide some interrupt
mechanism that can be triggered.


A servlet container will handle each request in a separate thread,
and direct that thread to the servlet that should handle it. One
request's thread erroring horribly doesn't prevent further
requests from being processed by that same servlet. Preventing
any further processing would require some management operation
that disables the servlet, or more likely its containing web
application.


Euuww. That means you need to tell the servlet container to
unregister you, or else set a public static mutable error flag that
makes all subsequent instances prompt-fail, or something. Global
variables. Ick!

Does the servlet<->container API/protocol/whatever not include an
explicit way for a servlet to signal a fatal error to the
container? Let alone to perform just-once initialization, prior to
handling requests, which can fail?


Yeah, that you can do. There's an init() method, and if it throws
an exception the servlet is never enabled (at least, in containers I
know of. Not sure if that's part of the spec.)


Ah, then Todd should probably be doing these checks in the init()
method and throwing an exception if they fail.

But even without that, there's no need for anything but a private
field used by the (single) instance of the servlet class. Multiple
threads -- single instance.


Global variables by any other name ... *sigh*


A private instance field is a global variable?


Not normally, but in this case it's sort-of one in disguise.


It represents the state of the object (the servlet), which is what
instance fields are supposed to do. This works quite well for
intermittent problems, say losing the connection to an underlying DBMS.
The servlet can retry the connection every N seconds, while rejecting
all requests that come while it's down.


Okay, this looks like a reasonable use.

For unrecoverable initialization failures, throwing an exception from
init() makes more sense.


Yes, this arose elsewhere in the thread and seems like the best solution
to Todd's problem.

Let me clarify: a global mutable error flag that isn't actually supposed
to change more than once is an ugly hack to work around the lack of
something like init(), had it proved to be lacking.


You insist on calling a private instance field "global".

A shared mutable used to indicate a temporary error condition that will
be toggled when the condition arises and when it dissipates, repeatedly,
is a non-hacky use of a widely-visible mutable.

The former leaves the bad taste; the latter does not.

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