...
In
...
extension
...
of
...
my
...
work
...
on
...
refactoring
...
the
...
HarvestScheduler
...
assignment
...
in
...
the
...
unit
...
test
...
class,
...
which
...
I
...
described
...
in the More informative naming,
...
...
...
...
thread,
...
I've
...
struggled
...
with
...
the
...
instantiation,
...
assignment
...
and
...
cleanup
...
of
...
the
...
HarvestSchedule'er.
...
It
...
has
...
finally
...
dawned
...
on
...
me
...
that
...
the
...
the
...
problem
...
is
...
2
...
conflicting
...
characteristics
...
of
...
the
...
HarvestScheduler:
...
- The
...
- class
...
- is
...
- meant
...
- to
...
- be
...
- a
...
...
- .
...
- This
...
- means
...
- that
...
- only
...
- one
...
- instance
...
- of
...
- the
...
- class
...
- should
...
- exist
...
- a
...
- one
...
- time,
...
- which
...
- the
...
- Singleton
...
- pattern
...
- enforces
...
- by
...
- hiding
...
- the
...
- possibility
...
- of
...
- construction/creating
...
- instances
...
- from
...
- users
...
- of
...
- the
...
- class.
...
- The
...
- class
...
- extends
...
- the
...
...
- :
...
- This
...
- interface
...
- implies
...
- that
...
- it
...
- is
...
- possible
...
- to
...
- invoke
...
- this
...
- operation
...
- when
...
- closing
...
- down
...
- the
...
- class.
...
- This
...
- works
...
- fine
...
- in
...
- normal
...
- class,
...
- where
...
- the
...
- operation
...
- is
...
- symmetric
...
- to
...
- the
...
- construction
...
- of
...
- the
...
- class.
...
- Eg.
...
- an
...
- objects
...
- life
...
- cycle
...
- is:
...
- create
...
- ->
...
- living
...
- ->
...
- cleanup.
...
The
...
conflict
...
between
...
the
...
2
...
aspects
...
of
...
the
...
HarvestScheduler
...
arises
...
because
...
a
...
Singleton
...
pattern
...
hides
...
the
...
object
...
creation
...
control
...
from
...
its
...
API
...
control,
...
where
...
the
...
CleanupIF
...
is
...
logically
...
a
...
functionality
...
associated
...
with
...
the
...
destruction
...
of
...
a
...
object.
...
The
...
problem
...
can
...
be
...
seen
...
in
...
the
...
follow
...
HarvestScheduler
...
code:
...
| Code Block |
|---|
public void cleanup() {
hsmon = null;
instance = null;
}{code}
|
This
...
code
...
enables
...
a
...
user
...
to
...
create
...
multiple
...
instances
...
of
...
the
...
HarvestScheduler
...
as
...
simple
...
as
...
this:
...
| Code Block |
|---|
HarvestScheduler h1 = HarvestScheduler.getInstance(); h1.cleanup(); HarvestScheduler h2 = HarvestScheduler.getInstance(); |
This
...
means
...
the
...
that
...
the
...
contract
...
for
...
the
...
Singleton-pattern
...
has
...
been
...
broken.
...
It
...
also
...
means
...
that
...
application
...
behaviour
...
will
...
now
...
depend
...
on
...
when
...
the
...
created
...
HarvestScheduler
...
instances
...
are
...
garbage
...
collect.
...
What
...
is
...
even
...
worse
...
is
...
that
...
this
...
is
...
used
...
in
...
the
...
...
...
to
...
emulate
...
contruction
...
of
...
a
...
new
...
HarvestScheduler
...
instance
...
in
...
each
...
test
...
case.
...
Eg.
...
| Code Block |
|---|
public class HarvestSchedulerTester extends TestCase {
...
public void tearDown() {
if (harvestScheduler \!= null) {
harvestScheduler.close();
}
...
}
public void testGetHoursPassedSince() throws Exception {
harvestScheduler = submitNewJobsAndGetSchedulerInstance();
}
{code}
|
Note:
...
The
...
HarvestSchedulers
...
close
...
call
...
delegated
...
to
...
the
...
Cleanup
...
method.
...
Conclusion:
...
The
...
CleanupIF
...
should
...
never
...
be
...
used
...
on
...
Singletonic
...
classes
...
as
...
this
...
leads
...
to
...
complicated
...
and
...
brittle
...
code,
...
and
...
may
...
lead
...
to
...
inconsistent
...
application
...
behavior.
...
A solution could
...
be
...
to
...
split
...
the
...
HarvestScheduler
...
into
...
a
...
plain
...
thread
...
execution
...
scheduler
...
singleton,
...
and
...
a
...
'JobManager'
...
class containing the business logicextending the CleanupIF.
...
Another
...
solution
...
might
...
be
...
to
...
replace
...
the
...
singleton
...
pattern
...
with
...
a
...
...
...
,
...
delegation
...
the
...
creation
...
control
...
to
...
a
...
class/container
...
external
...
to
...
the
...
HarvestScheduler
...
and
...
its
...
users.