<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
finalizer	B-Application
or	O
finalize	O
method	B-Language
is	O
a	O
special	O
method	B-Language
that	O
performs	O
finalization	B-Application
,	O
generally	O
some	O
form	O
of	O
cleanup	O
.	O
</s>
<s>
A	O
finalizer	B-Application
is	O
executed	O
during	O
object	O
destruction	O
,	O
prior	O
to	O
the	O
object	O
being	O
deallocated	B-General_Concept
,	O
and	O
is	O
complementary	O
to	O
an	O
initializer	O
,	O
which	O
is	O
executed	O
during	O
object	O
creation	O
,	O
following	O
allocation	B-General_Concept
.	O
</s>
<s>
Finalizers	B-Application
are	O
strongly	O
discouraged	O
by	O
some	O
,	O
due	O
to	O
difficulty	O
in	O
proper	O
use	O
and	O
the	O
complexity	O
they	O
add	O
,	O
and	O
alternatives	O
are	O
suggested	O
instead	O
,	O
mainly	O
the	O
dispose	B-Application
pattern	I-Application
(	O
see	O
problems	O
with	O
finalizers	B-Application
)	O
.	O
</s>
<s>
The	O
term	O
finalizer	B-Application
is	O
mostly	O
used	O
in	O
object-oriented	B-Language
and	O
functional	B-Language
programming	I-Language
languages	I-Language
that	O
use	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
of	O
which	O
the	O
archetype	O
is	O
Smalltalk	B-Application
.	O
</s>
<s>
This	O
is	O
contrasted	O
with	O
a	O
destructor	B-Language
,	O
which	O
is	O
a	O
method	B-Language
called	O
for	O
finalization	B-Application
in	O
languages	O
with	O
deterministic	O
object	O
lifetimes	O
,	O
archetypically	O
C++	B-Language
.	O
</s>
<s>
These	O
are	O
generally	O
exclusive	O
:	O
a	O
language	O
will	O
have	O
either	O
finalizers	B-Application
(	O
if	O
automatically	O
garbage	B-General_Concept
collected	I-General_Concept
)	O
or	O
destructors	B-Language
(	O
if	O
manually	O
memory	O
managed	O
)	O
,	O
but	O
in	O
rare	O
cases	O
a	O
language	O
may	O
have	O
both	O
,	O
as	O
in	O
C++	B-Language
/CLI	I-Language
and	O
D	B-Application
,	O
and	O
in	O
case	O
of	O
reference	B-General_Concept
counting	I-General_Concept
(	O
instead	O
of	O
tracing	O
garbage	B-General_Concept
collection	I-General_Concept
)	O
,	O
terminology	O
varies	O
.	O
</s>
<s>
In	O
technical	O
use	O
,	O
finalizer	B-Application
may	O
also	O
be	O
used	O
to	O
refer	O
to	O
destructors	B-Language
,	O
as	O
these	O
also	O
perform	O
finalization	B-Application
,	O
and	O
some	O
subtler	O
distinctions	O
are	O
drawn	O
–	O
see	O
terminology	O
.	O
</s>
<s>
The	O
term	O
final	O
is	O
also	O
used	O
to	O
indicate	O
a	O
class	O
that	O
cannot	O
be	O
inherited	B-Language
;	O
this	O
is	O
unrelated	O
.	O
</s>
<s>
The	O
terminology	O
of	O
finalizer	B-Application
and	O
finalization	B-Application
versus	O
destructor	B-Language
and	O
destruction	O
varies	O
between	O
authors	O
and	O
is	O
sometimes	O
unclear	O
.	O
</s>
<s>
In	O
common	O
use	O
,	O
a	O
destructor	B-Language
is	O
a	O
method	B-Language
called	O
deterministically	O
on	O
object	O
destruction	O
,	O
and	O
the	O
archetype	O
is	O
C++	B-Language
destructors	B-Language
;	O
while	O
a	O
finalizer	B-Application
is	O
called	O
non-deterministically	O
by	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
and	O
the	O
archetype	O
is	O
Java	B-Language
finalize	O
methods	O
.	O
</s>
<s>
For	O
languages	O
that	O
implement	O
garbage	B-General_Concept
collection	I-General_Concept
via	O
reference	B-General_Concept
counting	I-General_Concept
,	O
terminology	O
varies	O
,	O
with	O
some	O
languages	O
such	O
as	O
Objective-C	B-Language
and	O
Perl	B-Language
using	O
destructor	B-Language
,	O
and	O
other	O
languages	O
such	O
as	O
Python	B-Language
using	O
finalizer	B-Application
(	O
per	O
spec	O
,	O
Python	B-Language
is	O
garbage	B-General_Concept
collected	I-General_Concept
,	O
but	O
the	O
reference	O
CPython	B-Language
implementation	O
since	O
its	O
version	O
2.0	O
uses	O
a	O
combination	O
of	O
reference	B-General_Concept
counting	I-General_Concept
and	O
garbage	B-General_Concept
collection	I-General_Concept
)	O
.	O
</s>
<s>
This	O
reflects	O
that	O
reference	B-General_Concept
counting	I-General_Concept
results	O
in	O
semi-deterministic	O
object	O
lifetime	O
:	O
for	O
objects	O
that	O
are	O
not	O
part	O
of	O
a	O
cycle	O
,	O
objects	O
are	O
destroyed	O
deterministically	O
when	O
the	O
reference	B-General_Concept
count	I-General_Concept
drops	O
to	O
zero	O
,	O
but	O
objects	O
that	O
are	O
part	O
of	O
a	O
cycle	O
are	O
destroyed	O
non-deterministically	O
,	O
as	O
part	O
of	O
a	O
separate	O
form	O
of	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
In	O
certain	O
narrow	O
technical	O
use	O
,	O
constructor	O
and	O
destructor	B-Language
are	O
language-level	O
terms	O
,	O
meaning	O
methods	O
defined	O
in	O
a	O
class	O
,	O
while	O
initializer	O
and	O
finalizer	B-Application
are	O
implementation-level	O
terms	O
,	O
meaning	O
methods	O
called	O
during	O
object	O
creation	O
or	O
destruction	O
.	O
</s>
<s>
Thus	O
for	O
example	O
the	O
original	O
specification	O
for	O
the	O
C#	B-Application
language	O
referred	O
to	O
"	O
destructors	B-Language
"	O
,	O
even	O
though	O
C#	B-Application
is	O
garbage-collected	O
,	O
but	O
the	O
specification	O
for	O
the	O
Common	O
Language	O
Infrastructure	O
(	O
CLI	O
)	O
,	O
and	O
the	O
implementation	O
of	O
its	O
runtime	O
environment	O
as	O
the	O
Common	O
Language	O
Runtime	O
(	O
CLR	O
)	O
,	O
referred	O
to	O
"	O
finalizers	B-Application
"	O
.	O
</s>
<s>
This	O
is	O
reflected	O
in	O
the	O
C#	B-Application
language	O
committee	O
's	O
notes	O
,	O
which	O
read	O
in	O
part	O
:	O
"	O
The	O
C#	B-Application
compiler	O
compiles	O
destructors	B-Language
to	O
...	O
 [ probably ] 	O
instance	O
finalizer[s]	O
 "	O
.	O
</s>
<s>
This	O
terminology	O
is	O
confusing	O
,	O
and	O
thus	O
more	O
recent	O
versions	O
of	O
the	O
C#	B-Application
spec	O
refer	O
to	O
the	O
language-level	O
method	B-Language
as	O
"	O
finalizers	B-Application
"	O
.	O
</s>
<s>
Another	O
language	O
that	O
does	O
not	O
make	O
this	O
terminology	O
distinction	O
is	O
D	B-Application
.	O
Although	O
D	B-Application
classes	O
are	O
garbage	B-General_Concept
collected	I-General_Concept
,	O
their	O
cleanup	O
functions	O
are	O
called	O
destructors	B-Language
.	O
</s>
<s>
Finalization	B-Application
is	O
mostly	O
used	O
for	O
cleanup	O
,	O
to	O
release	O
memory	O
or	O
other	O
resources	O
:	O
to	O
deallocate	O
memory	O
allocated	O
via	O
manual	B-Application
memory	I-Application
management	I-Application
;	O
to	O
clear	O
references	O
if	O
reference	B-General_Concept
counting	I-General_Concept
is	O
used	O
(	O
decrement	O
reference	B-General_Concept
counts	I-General_Concept
)	O
;	O
to	O
release	O
resources	O
,	O
particularly	O
in	O
the	O
Resource	B-Application
Acquisition	I-Application
Is	I-Application
Initialization	I-Application
(	O
RAII	B-Application
)	O
idiom	O
;	O
or	O
to	O
unregister	O
an	O
object	O
.	O
</s>
<s>
The	O
amount	O
of	O
finalization	B-Application
varies	O
significantly	O
between	O
languages	O
,	O
from	O
extensive	O
finalization	B-Application
in	O
C++	B-Language
,	O
which	O
has	O
manual	B-Application
memory	I-Application
management	I-Application
,	O
reference	B-General_Concept
counting	I-General_Concept
,	O
and	O
deterministic	O
object	O
lifetimes	O
;	O
to	O
often	O
no	O
finalization	B-Application
in	O
Java	B-Language
,	O
which	O
has	O
non-deterministic	O
object	O
lifetimes	O
and	O
is	O
often	O
implemented	O
with	O
a	O
tracing	O
garbage	B-General_Concept
collector	I-General_Concept
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
for	O
there	O
to	O
be	O
little	O
or	O
no	O
explicit	O
(	O
user-specified	O
)	O
finalization	B-Application
,	O
but	O
significant	O
implicit	O
finalization	B-Application
,	O
performed	O
by	O
the	O
compiler	O
,	O
interpreter	O
,	O
or	O
runtime	O
;	O
this	O
is	O
common	O
in	O
case	O
of	O
automatic	B-Language
reference	I-Language
counting	I-Language
,	O
as	O
in	O
the	O
CPython	B-Language
reference	O
implementation	O
of	O
Python	B-Language
,	O
or	O
in	O
Automatic	B-Language
Reference	I-Language
Counting	I-Language
in	O
Apple	O
's	O
implementation	O
of	O
Objective-C	B-Language
,	O
which	O
both	O
automatically	O
break	O
references	O
during	O
finalization	B-Application
.	O
</s>
<s>
A	O
finalizer	B-Application
can	O
include	O
arbitrary	O
code	O
;	O
a	O
particularly	O
complex	O
use	O
is	O
to	O
automatically	O
return	O
the	O
object	O
to	O
an	O
object	O
pool	O
.	O
</s>
<s>
Memory	B-General_Concept
deallocation	I-General_Concept
during	O
finalization	B-Application
is	O
common	O
in	O
languages	O
like	O
C++	B-Language
where	O
manual	B-Application
memory	I-Application
management	I-Application
is	O
standard	O
,	O
but	O
also	O
occurs	O
in	O
managed	O
languages	O
when	O
memory	O
has	O
been	O
allocated	O
outside	O
of	O
the	O
managed	O
heap	O
(	O
externally	O
to	O
the	O
language	O
)	O
;	O
in	O
Java	B-Language
this	O
occurs	O
with	O
Java	B-Language
Native	I-Language
Interface	I-Language
(	O
JNI	B-Language
)	O
and	O
ByteBuffer	O
objects	O
in	O
New	B-Language
I/O	I-Language
(	O
NIO	O
)	O
.	O
</s>
<s>
This	O
latter	O
can	O
cause	O
problems	O
due	O
to	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
being	O
unable	O
to	O
track	O
these	O
external	O
resources	O
,	O
so	O
they	O
will	O
not	O
be	O
collected	O
aggressively	O
enough	O
,	O
and	O
can	O
cause	O
out-of-memory	O
errors	O
due	O
to	O
exhausting	O
unmanaged	O
memory	O
–	O
this	O
can	O
be	O
avoided	O
by	O
treating	O
native	O
memory	O
as	O
a	O
resource	O
and	O
using	O
the	O
dispose	B-Application
pattern	I-Application
,	O
as	O
discussed	O
below	O
.	O
</s>
<s>
Finalizers	B-Application
are	O
generally	O
both	O
much	O
less	O
necessary	O
and	O
much	O
less	O
used	O
than	O
destructors	B-Language
.	O
</s>
<s>
They	O
are	O
much	O
less	O
necessary	O
because	O
garbage	B-General_Concept
collection	I-General_Concept
automates	O
memory	B-General_Concept
management	I-General_Concept
,	O
and	O
much	O
less	O
used	O
because	O
they	O
are	O
not	O
generally	O
executed	O
deterministically	O
–	O
they	O
may	O
not	O
be	O
called	O
in	O
a	O
timely	O
manner	O
,	O
or	O
even	O
at	O
all	O
,	O
and	O
the	O
execution	O
environment	O
cannot	O
be	O
predicted	O
–	O
and	O
thus	O
any	O
cleanup	O
that	O
must	O
be	O
done	O
in	O
a	O
deterministic	O
way	O
must	O
instead	O
be	O
done	O
by	O
some	O
other	O
method	B-Language
,	O
most	O
frequently	O
manually	O
via	O
the	O
dispose	B-Application
pattern	I-Application
.	O
</s>
<s>
Notably	O
,	O
both	O
Java	B-Language
and	O
Python	B-Language
do	O
not	O
guarantee	O
that	O
finalizers	B-Application
will	O
ever	O
be	O
called	O
,	O
and	O
thus	O
they	O
cannot	O
be	O
relied	O
on	O
for	O
cleanup	O
.	O
</s>
<s>
Due	O
to	O
the	O
lack	O
of	O
programmer	O
control	O
over	O
their	O
execution	O
,	O
it	O
is	O
usually	O
recommended	O
to	O
avoid	O
finalizers	B-Application
for	O
any	O
but	O
the	O
most	O
trivial	O
operations	O
.	O
</s>
<s>
In	O
particular	O
,	O
operations	O
often	O
performed	O
in	O
destructors	B-Language
are	O
not	O
usually	O
appropriate	O
for	O
finalizers	B-Application
.	O
</s>
<s>
A	O
common	O
anti-pattern	O
is	O
to	O
write	O
finalizers	B-Application
as	O
if	O
they	O
were	O
destructors	B-Language
,	O
which	O
is	O
both	O
unnecessary	O
and	O
ineffectual	O
,	O
due	O
to	O
differences	O
between	O
finalizers	B-Application
and	O
destructors	B-Language
.	O
</s>
<s>
This	O
is	O
particularly	O
common	O
among	O
C++	B-Language
programmers	O
,	O
as	O
destructors	B-Language
are	O
heavily	O
used	O
in	O
idiomatic	O
C++	B-Language
,	O
following	O
the	O
Resource	B-Application
Acquisition	I-Application
Is	I-Application
Initialization	I-Application
(	O
RAII	B-Application
)	O
idiom	O
.	O
</s>
<s>
Programming	O
languages	O
that	O
use	O
finalizers	B-Application
include	O
C++	B-Language
/CLI	I-Language
,	O
C#	B-Application
,	O
Clean	B-Operating_System
,	O
Go	B-Application
,	O
Java	B-Language
,	O
JavaScript	B-Language
and	O
Python	B-Language
.	O
</s>
<s>
In	O
Java	B-Language
,	O
a	O
finalizer	B-Application
is	O
a	O
method	B-Language
called	O
finalize	O
,	O
which	O
overrides	O
the	O
Object.finalize	O
method	B-Language
.	O
</s>
<s>
In	O
JavaScript	B-Language
,	O
allows	O
you	O
to	O
request	O
a	O
callback	O
when	O
an	O
object	O
is	O
garbage-collected	O
.	O
</s>
<s>
In	O
Python	B-Language
,	O
a	O
finalizer	B-Application
is	O
a	O
method	B-Language
called	O
__del__	O
.	O
</s>
<s>
In	O
Perl	B-Language
,	O
a	O
finalizer	B-Application
is	O
a	O
method	B-Language
called	O
DESTROY	O
.	O
</s>
<s>
In	O
C#	B-Application
,	O
a	O
finalizer	B-Application
(	O
called	O
"	O
destructor	B-Language
"	O
in	O
earlier	O
versions	O
of	O
the	O
standard	O
)	O
is	O
a	O
method	B-Language
whose	O
name	O
is	O
the	O
class	O
name	O
with	O
~	O
prefixed	O
,	O
as	O
in	O
~	O
Foo	O
–	O
this	O
is	O
the	O
same	O
syntax	O
as	O
a	O
C++	B-Language
destructor	B-Language
,	O
and	O
these	O
methods	O
were	O
originally	O
called	O
"	O
destructors	B-Language
"	O
,	O
by	O
analogy	O
with	O
C++	B-Language
,	O
despite	O
having	O
different	O
behavior	O
,	O
but	O
were	O
renamed	O
to	O
"	O
finalizers	B-Application
"	O
due	O
to	O
the	O
confusion	O
this	O
caused	O
.	O
</s>
<s>
In	O
C++	B-Language
/CLI	I-Language
,	O
which	O
has	O
both	O
destructors	B-Language
and	O
finalizers	B-Application
,	O
a	O
destructor	B-Language
is	O
a	O
method	B-Language
whose	O
name	O
is	O
the	O
class	O
name	O
with	O
~	O
prefixed	O
,	O
as	O
in	O
~	O
Foo	O
(	O
as	O
in	O
C#	B-Application
)	O
,	O
and	O
a	O
finalizer	B-Application
is	O
a	O
method	B-Language
whose	O
name	O
is	O
the	O
class	O
name	O
with	O
!	O
</s>
<s>
In	O
Go	B-Application
finalizers	B-Application
are	O
applied	O
to	O
a	O
single	O
pointer	O
by	O
calling	O
the	O
runtime.SetFinalizer	O
function	O
in	O
the	O
standard	O
library	O
.	O
</s>
<s>
A	O
finalizer	B-Application
is	O
called	O
when	O
an	O
object	O
is	O
garbage	B-General_Concept
collected	I-General_Concept
–	O
after	O
an	O
object	O
has	O
become	O
garbage	O
(	O
unreachable	O
)	O
,	O
but	O
before	O
its	O
memory	O
is	O
deallocated	B-General_Concept
.	O
</s>
<s>
Finalization	B-Application
occurs	O
non-deterministically	O
,	O
at	O
the	O
discretion	O
of	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
and	O
might	O
never	O
occur	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
destructors	B-Language
,	O
which	O
are	O
called	O
deterministically	O
as	O
soon	O
as	O
an	O
object	O
is	O
no	O
longer	O
in	O
use	O
,	O
and	O
are	O
always	O
called	O
,	O
except	O
in	O
case	O
of	O
uncontrolled	O
program	O
termination	O
.	O
</s>
<s>
Finalizers	B-Application
are	O
most	O
frequently	O
instance	B-Language
methods	I-Language
,	O
due	O
to	O
needing	O
to	O
do	O
object-specific	O
operations	O
.	O
</s>
<s>
The	O
garbage	B-General_Concept
collector	I-General_Concept
must	O
also	O
account	O
for	O
the	O
possibility	O
of	O
object	B-General_Concept
resurrection	I-General_Concept
.	O
</s>
<s>
Most	O
commonly	O
this	O
is	O
done	O
by	O
first	O
executing	O
finalizers	B-Application
,	O
then	O
checking	O
whether	O
any	O
objects	O
have	O
been	O
resurrected	O
,	O
and	O
if	O
so	O
,	O
aborting	O
their	O
destruction	O
.	O
</s>
<s>
This	O
additional	O
check	O
is	O
potentially	O
expensive	O
–	O
a	O
simple	O
implementation	O
re-checks	O
all	O
garbage	O
if	O
even	O
a	O
single	O
object	O
has	O
a	O
finalizer	B-Application
–	O
and	O
thus	O
both	O
slows	O
down	O
and	O
complicates	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
objects	O
with	O
finalizers	B-Application
may	O
be	O
collected	O
less	O
frequently	O
than	O
objects	O
without	O
finalizers	B-Application
(	O
only	O
on	O
certain	O
cycles	O
)	O
,	O
exacerbating	O
problems	O
caused	O
by	O
relying	O
on	O
prompt	O
finalization	B-Application
,	O
such	O
as	O
resource	O
leaks	O
.	O
</s>
<s>
If	O
an	O
object	O
is	O
resurrected	O
,	O
there	O
is	O
the	O
further	O
question	O
of	O
whether	O
its	O
finalizer	B-Application
is	O
called	O
again	O
,	O
when	O
it	O
is	O
next	O
destroyed	O
–	O
unlike	O
destructors	B-Language
,	O
finalizers	B-Application
are	O
potentially	O
called	O
multiple	O
times	O
.	O
</s>
<s>
If	O
finalizers	B-Application
are	O
called	O
for	O
resurrected	O
objects	O
,	O
objects	O
may	O
repeatedly	O
resurrect	O
themselves	O
and	O
be	O
indestructible	O
;	O
this	O
occurs	O
in	O
the	O
CPython	B-Language
implementation	O
of	O
Python	B-Language
prior	O
to	O
Python	B-Language
3.4	O
,	O
and	O
in	O
CLR	O
languages	O
such	O
as	O
C#	B-Application
.	O
</s>
<s>
To	O
avoid	O
this	O
,	O
in	O
many	O
languages	O
,	O
including	O
Java	B-Language
,	O
Objective-C	B-Language
(	O
at	O
least	O
in	O
recent	O
Apple	O
implementations	O
)	O
,	O
and	O
Python	B-Language
from	O
Python	B-Language
3.4	O
,	O
objects	O
are	O
finalized	O
at	O
most	O
once	O
,	O
which	O
requires	O
tracking	O
if	O
the	O
object	O
has	O
been	O
finalized	O
yet	O
.	O
</s>
<s>
In	O
other	O
cases	O
,	O
notably	O
CLR	O
languages	O
like	O
C#	B-Application
,	O
finalization	B-Application
is	O
tracked	O
separately	O
from	O
the	O
objects	O
themselves	O
,	O
and	O
objects	O
can	O
be	O
repeatedly	O
registered	O
or	O
deregistered	O
for	O
finalization	B-Application
.	O
</s>
<s>
Depending	O
on	O
the	O
implementation	O
,	O
finalizers	B-Application
can	O
cause	O
a	O
significant	O
number	O
of	O
problems	O
,	O
and	O
are	O
thus	O
strongly	O
discouraged	O
by	O
a	O
number	O
of	O
authorities	O
.	O
</s>
<s>
Finalizers	B-Application
may	O
not	O
be	O
called	O
in	O
a	O
timely	O
manner	O
,	O
or	O
at	O
all	O
,	O
so	O
they	O
cannot	O
be	O
relied	O
upon	O
to	O
persist	O
state	O
,	O
release	O
scarce	O
resources	O
,	O
or	O
do	O
anything	O
else	O
of	O
importance	O
.	O
</s>
<s>
Finalizers	B-Application
may	O
result	O
in	O
object	B-General_Concept
resurrection	I-General_Concept
,	O
which	O
is	O
often	O
a	O
programming	O
error	O
and	O
whose	O
very	O
possibility	O
significantly	O
slows	O
down	O
and	O
complicates	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
Finalizers	B-Application
are	O
run	O
based	O
on	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
which	O
is	O
generally	O
based	O
on	O
managed	O
memory	O
pressure	O
–	O
they	O
are	O
not	O
run	O
in	O
case	O
of	O
other	O
resource	O
scarcity	O
,	O
and	O
thus	O
are	O
not	O
suited	O
for	O
managing	O
other	O
scarce	O
resources	O
.	O
</s>
<s>
Finalizers	B-Application
do	O
not	O
run	O
in	O
a	O
specified	O
order	O
,	O
and	O
cannot	O
rely	O
on	O
class	B-Application
invariants	I-Application
(	O
as	O
they	O
may	O
refer	O
to	O
other	O
objects	O
that	O
have	O
already	O
been	O
finalized	O
)	O
.	O
</s>
<s>
Slow	O
finalizers	B-Application
may	O
delay	O
other	O
finalizers	B-Application
.	O
</s>
<s>
Exceptions	O
within	O
finalizers	B-Application
generally	O
cannot	O
be	O
handled	O
,	O
because	O
the	O
finalizer	B-Application
is	O
run	O
in	O
an	O
unspecified	O
environment	O
,	O
and	O
may	O
be	O
either	O
ignored	O
or	O
cause	O
uncontrolled	O
program	O
termination	O
.	O
</s>
<s>
Finalizers	B-Application
may	O
reference	O
and	O
accidentally	O
finalize	O
live	O
objects	O
,	O
violating	O
program	O
invariants	O
.	O
</s>
<s>
Finalizers	B-Application
may	O
cause	O
synchronization	O
issues	O
,	O
even	O
in	O
otherwise	O
sequential	O
(	O
single-threaded	O
)	O
programs	O
,	O
when	O
finalization	B-Application
is	O
done	O
in	O
a	O
separate	O
threads	O
.	O
</s>
<s>
Finalizers	B-Application
may	O
cause	O
deadlocks	O
if	O
synchronization	O
mechanisms	O
such	O
as	O
locks	O
are	O
used	O
,	O
due	O
to	O
not	O
being	O
run	O
in	O
a	O
specified	O
order	O
and	O
possibly	O
being	O
run	O
concurrently	O
.	O
</s>
<s>
Finalizers	B-Application
that	O
are	O
run	O
during	O
program	O
termination	O
cannot	O
rely	O
on	O
the	O
usual	O
runtime	O
environment	O
,	O
and	O
thus	O
may	O
fail	O
due	O
to	O
incorrect	O
assumptions	O
–	O
for	O
this	O
reason	O
finalizers	B-Application
are	O
often	O
not	O
run	O
during	O
termination	O
.	O
</s>
<s>
Further	O
,	O
finalizers	B-Application
may	O
fail	O
to	O
run	O
due	O
to	O
objects	O
remaining	O
reachable	O
beyond	O
when	O
they	O
are	O
expected	O
to	O
be	O
garbage	O
,	O
either	O
due	O
to	O
programming	O
errors	O
or	O
due	O
to	O
unexpected	O
reachability	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
Python	B-Language
catches	O
an	O
exception	O
(	O
or	O
an	O
exception	O
is	O
not	O
caught	O
in	O
interactive	O
mode	O
)	O
,	O
it	O
keeps	O
a	O
reference	O
to	O
the	O
stack	O
frame	O
where	O
the	O
exception	O
was	O
raised	O
,	O
which	O
keeps	O
objects	O
referenced	O
from	O
that	O
stack	O
frame	O
alive	O
.	O
</s>
<s>
In	O
Java	B-Language
,	O
finalizers	B-Application
in	O
a	O
superclass	O
can	O
also	O
slow	O
down	O
garbage	B-General_Concept
collection	I-General_Concept
in	O
a	O
subclass	O
,	O
as	O
the	O
finalizer	B-Application
can	O
potentially	O
refer	O
to	O
fields	O
in	O
the	O
subclass	O
,	O
and	O
thus	O
the	O
field	O
cannot	O
be	O
garbage	B-General_Concept
collected	I-General_Concept
until	O
the	O
following	O
cycle	O
,	O
once	O
the	O
finalizer	B-Application
has	O
run	O
.	O
</s>
<s>
This	O
can	O
be	O
avoided	O
by	O
using	O
composition	B-Library
over	I-Library
inheritance	I-Library
.	O
</s>
<s>
A	O
common	O
anti-pattern	O
is	O
to	O
use	O
finalizers	B-Application
to	O
release	O
resources	O
,	O
by	O
analogy	O
with	O
the	O
Resource	B-Application
Acquisition	I-Application
Is	I-Application
Initialization	I-Application
(	O
RAII	B-Application
)	O
idiom	O
of	O
C++	B-Language
:	O
acquire	O
a	O
resource	O
in	O
the	O
initializer	O
(	O
constructor	O
)	O
,	O
and	O
release	O
it	O
in	O
the	O
finalizer	B-Application
(	O
destructor	B-Language
)	O
.	O
</s>
<s>
Most	O
basically	O
,	O
finalizers	B-Application
may	O
never	O
be	O
called	O
,	O
and	O
even	O
if	O
called	O
,	O
may	O
not	O
be	O
called	O
in	O
a	O
timely	O
manner	O
–	O
thus	O
using	O
finalizers	B-Application
to	O
release	O
resources	O
will	O
generally	O
cause	O
resource	O
leaks	O
.	O
</s>
<s>
Further	O
,	O
finalizers	B-Application
are	O
not	O
called	O
in	O
a	O
prescribed	O
order	O
,	O
while	O
resources	O
often	O
need	O
to	O
be	O
released	O
in	O
a	O
specific	O
order	O
,	O
frequently	O
the	O
opposite	O
order	O
in	O
which	O
they	O
were	O
acquired	O
.	O
</s>
<s>
Also	O
,	O
as	O
finalizers	B-Application
are	O
called	O
at	O
the	O
discretion	O
of	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
they	O
will	O
often	O
only	O
be	O
called	O
under	O
managed	O
memory	O
pressure	O
(	O
when	O
there	O
is	O
little	O
managed	O
memory	O
available	O
)	O
,	O
regardless	O
of	O
resource	O
pressure	O
–	O
if	O
scarce	O
resources	O
are	O
held	O
by	O
garbage	O
but	O
there	O
is	O
plenty	O
of	O
managed	O
memory	O
available	O
,	O
garbage	B-General_Concept
collection	I-General_Concept
may	O
not	O
occur	O
,	O
thus	O
not	O
reclaiming	O
these	O
resources	O
.	O
</s>
<s>
Thus	O
instead	O
of	O
using	O
finalizers	B-Application
for	O
automatic	O
resource	O
management	O
,	O
in	O
garbage-collected	O
languages	O
one	O
instead	O
must	O
manually	O
manage	O
resources	O
,	O
generally	O
by	O
using	O
the	O
dispose	B-Application
pattern	I-Application
.	O
</s>
<s>
In	O
this	O
case	O
resources	O
may	O
still	O
be	O
acquired	O
in	O
the	O
initializer	O
,	O
which	O
is	O
called	O
explicitly	O
on	O
object	O
instantiation	O
,	O
but	O
are	O
released	O
in	O
the	O
dispose	B-Application
method	B-Language
.	O
</s>
<s>
The	O
dispose	B-Application
method	B-Language
may	O
be	O
called	O
explicitly	O
,	O
or	O
implicitly	O
by	O
language	O
constructs	O
such	O
as	O
C#'s	O
using	O
,	O
Java	B-Language
's	O
try-with-resources	O
,	O
or	O
Python	B-Language
's	O
with	O
.	O
</s>
<s>
However	O
,	O
in	O
certain	O
cases	O
both	O
the	O
dispose	B-Application
pattern	I-Application
and	O
finalizers	B-Application
are	O
used	O
for	O
releasing	O
resources	O
.	O
</s>
<s>
This	O
is	O
mostly	O
found	O
in	O
CLR	O
languages	O
such	O
as	O
C#	B-Application
,	O
where	O
finalization	B-Application
is	O
used	O
as	O
a	O
backup	O
for	O
disposal	O
:	O
when	O
a	O
resource	O
is	O
acquired	O
,	O
the	O
acquiring	O
object	O
is	O
queued	O
for	O
finalization	B-Application
so	O
that	O
the	O
resource	O
is	O
released	O
on	O
object	O
destruction	O
,	O
even	O
if	O
the	O
resource	O
is	O
not	O
released	O
by	O
manual	O
disposal	O
.	O
</s>
<s>
In	O
languages	O
with	O
deterministic	O
object	O
lifetimes	O
,	O
notably	O
C++	B-Language
,	O
resource	O
management	O
is	O
frequently	O
done	O
by	O
tying	O
resource	O
possession	O
lifetime	O
to	O
object	O
lifetime	O
,	O
acquiring	O
resources	O
during	O
initialization	O
and	O
releasing	O
them	O
during	O
finalization	B-Application
;	O
this	O
is	O
known	O
as	O
Resource	B-Application
Acquisition	I-Application
Is	I-Application
Initialization	I-Application
(	O
RAII	B-Application
)	O
.	O
</s>
<s>
This	O
ensures	O
that	O
resource	O
possession	O
is	O
a	O
class	B-Application
invariant	I-Application
,	O
and	O
that	O
resources	O
are	O
released	O
promptly	O
when	O
the	O
object	O
is	O
destroyed	O
.	O
</s>
<s>
However	O
,	O
in	O
languages	O
with	O
non-deterministic	O
object	O
lifetimes	O
–	O
which	O
include	O
all	O
major	O
languages	O
with	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
such	O
as	O
C#	B-Application
,	O
Java	B-Language
,	O
and	O
Python	B-Language
–	O
this	O
does	O
not	O
work	O
,	O
because	O
finalization	B-Application
may	O
not	O
be	O
timely	O
or	O
may	O
not	O
happen	O
at	O
all	O
,	O
and	O
thus	O
resources	O
may	O
not	O
be	O
released	O
for	O
a	O
long	O
time	O
or	O
even	O
at	O
all	O
,	O
causing	O
resource	O
leaks	O
.	O
</s>
<s>
In	O
these	O
languages	O
resources	O
are	O
instead	O
generally	O
managed	O
manually	O
via	O
the	O
dispose	B-Application
pattern	I-Application
:	O
resources	O
may	O
still	O
be	O
acquired	O
during	O
initialization	O
,	O
but	O
are	O
released	O
by	O
calling	O
a	O
dispose	B-Application
method	B-Language
.	O
</s>
<s>
Nevertheless	O
,	O
using	O
finalization	B-Application
for	O
releasing	O
resources	O
in	O
these	O
languages	O
is	O
a	O
common	O
anti-pattern	O
,	O
and	O
forgetting	O
to	O
call	O
dispose	B-Application
will	O
still	O
cause	O
a	O
resource	O
leak	O
.	O
</s>
<s>
In	O
some	O
cases	O
both	O
techniques	O
are	O
combined	O
,	O
using	O
an	O
explicit	O
dispose	B-Application
method	B-Language
,	O
but	O
also	O
releasing	O
any	O
still-held	O
resources	O
during	O
finalization	B-Application
as	O
a	O
backup	O
.	O
</s>
<s>
This	O
is	O
commonly	O
found	O
in	O
C#	B-Application
,	O
and	O
is	O
implemented	O
by	O
registering	O
an	O
object	O
for	O
finalization	B-Application
whenever	O
a	O
resource	O
is	O
acquired	O
,	O
and	O
suppressing	O
finalization	B-Application
whenever	O
a	O
resource	O
is	O
released	O
.	O
</s>
<s>
If	O
user-specified	O
finalizers	B-Application
are	O
allowed	O
,	O
it	O
is	O
possible	O
for	O
finalization	B-Application
to	O
cause	O
object	B-General_Concept
resurrection	I-General_Concept
,	O
as	O
the	O
finalizers	B-Application
can	O
run	O
arbitrary	O
code	O
,	O
which	O
may	O
create	O
references	O
from	O
live	O
objects	O
to	O
objects	O
being	O
destroyed	O
.	O
</s>
<s>
For	O
languages	O
without	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
this	O
is	O
a	O
severe	O
bug	O
,	O
and	O
causes	O
dangling	B-Error_Name
references	I-Error_Name
and	O
memory	B-Application
safety	I-Application
violations	O
;	O
for	O
languages	O
with	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
this	O
is	O
prevented	O
by	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
most	O
commonly	O
by	O
adding	O
another	O
step	O
to	O
garbage	B-General_Concept
collection	I-General_Concept
(	O
after	O
running	O
all	O
user-specified	O
finalizers	B-Application
,	O
check	O
for	O
resurrection	O
)	O
,	O
which	O
complicates	O
and	O
slows	O
down	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
Further	O
,	O
object	B-General_Concept
resurrection	I-General_Concept
means	O
that	O
an	O
object	O
may	O
not	O
be	O
destroyed	O
,	O
and	O
in	O
pathological	O
cases	O
an	O
object	O
can	O
always	O
resurrect	O
itself	O
during	O
finalization	B-Application
,	O
making	O
itself	O
indestructible	O
.	O
</s>
<s>
To	O
prevent	O
this	O
,	O
some	O
languages	O
,	O
like	O
Java	B-Language
and	O
Python	B-Language
(	O
from	O
Python	B-Language
3.4	O
)	O
only	O
finalize	O
objects	O
once	O
,	O
and	O
do	O
not	O
finalize	O
resurrected	O
objects	O
.	O
</s>
<s>
Objective-C	B-Language
also	O
tracks	O
finalization	B-Application
(	O
at	O
least	O
in	O
recent	O
Apple	O
versions	O
)	O
for	O
similar	O
reasons	O
,	O
treating	O
resurrection	O
as	O
a	O
bug	O
.	O
</s>
<s>
A	O
different	O
approach	O
is	O
used	O
in	O
the	O
.NET	B-Application
Framework	I-Application
,	O
notably	O
C#	B-Application
and	O
Visual	B-Language
Basic	I-Language
.NET	I-Language
,	O
where	O
finalization	B-Application
is	O
tracked	O
by	O
a	O
"	O
queue	O
"	O
,	O
rather	O
than	O
by	O
object	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
if	O
a	O
user-specified	O
finalizer	B-Application
is	O
provided	O
,	O
by	O
default	O
the	O
object	O
is	O
only	O
finalized	O
once	O
(	O
it	O
is	O
queued	O
for	O
finalization	B-Application
on	O
creation	O
,	O
and	O
dequeued	O
once	O
it	O
is	O
finalized	O
)	O
,	O
but	O
this	O
can	O
be	O
changed	O
via	O
calling	O
the	O
GC	O
module	O
.	O
</s>
<s>
Finalization	B-Application
can	O
be	O
prevented	O
by	O
calling	O
GC.SuppressFinalize	O
,	O
which	O
dequeues	O
the	O
object	O
,	O
or	O
reactivated	O
by	O
calling	O
GC.ReRegisterForFinalize	O
,	O
which	O
enqueues	O
the	O
object	O
.	O
</s>
<s>
These	O
are	O
particularly	O
used	O
when	O
using	O
finalization	B-Application
for	O
resource	O
management	O
as	O
a	O
supplement	O
to	O
the	O
dispose	B-Application
pattern	I-Application
,	O
or	O
when	O
implementing	O
an	O
object	O
pool	O
.	O
</s>
<s>
Finalization	B-Application
is	O
formally	O
complementary	O
to	O
initialization	O
–	O
initialization	O
occurs	O
at	O
the	O
start	O
of	O
lifetime	O
,	O
finalization	B-Application
at	O
the	O
end	O
–	O
but	O
differs	O
significantly	O
in	O
practice	O
.	O
</s>
<s>
Both	O
variables	O
and	O
objects	O
are	O
initialized	O
,	O
mostly	O
to	O
assign	O
values	O
,	O
but	O
in	O
general	O
only	O
objects	O
are	O
finalized	O
,	O
and	O
in	O
general	O
there	O
is	O
no	O
need	O
to	O
clear	O
values	O
–	O
the	O
memory	O
can	O
simply	O
be	O
deallocated	B-General_Concept
and	O
reclaimed	O
by	O
the	O
operating	O
system	O
.	O
</s>
<s>
These	O
actions	O
have	O
symmetric	O
release	O
or	O
unregister	O
actions	O
,	O
and	O
these	O
can	O
symmetrically	O
be	O
handled	O
in	O
a	O
finalizer	B-Application
,	O
which	O
is	O
done	O
in	O
RAII	B-Application
.	O
</s>
<s>
However	O
,	O
in	O
many	O
languages	O
,	O
notably	O
those	O
with	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
object	O
lifetime	O
is	O
asymmetric	O
:	O
object	O
creation	O
happens	O
deterministically	O
at	O
some	O
explicit	O
point	O
in	O
the	O
code	O
,	O
but	O
object	O
destruction	O
happens	O
non-deterministically	O
,	O
in	O
some	O
unspecified	O
environment	O
,	O
at	O
the	O
discretion	O
of	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
.	O
</s>
<s>
This	O
asymmetry	O
means	O
that	O
finalization	B-Application
cannot	O
be	O
effectively	O
used	O
as	O
the	O
complement	O
of	O
initialization	O
,	O
because	O
it	O
does	O
not	O
happen	O
in	O
a	O
timely	O
manner	O
,	O
in	O
a	O
specified	O
order	O
,	O
or	O
in	O
a	O
specified	O
environment	O
.	O
</s>
<s>
The	O
symmetry	O
is	O
partially	O
restored	O
by	O
also	O
disposing	O
of	O
the	O
object	O
at	O
an	O
explicit	O
point	O
,	O
but	O
in	O
this	O
case	O
disposal	O
and	O
destruction	O
do	O
not	O
happen	O
at	O
the	O
same	O
point	O
,	O
and	O
an	O
object	O
may	O
be	O
in	O
a	O
"	O
disposed	O
but	O
still	O
alive	O
"	O
state	O
,	O
which	O
weakens	O
the	O
class	B-Application
invariants	I-Application
and	O
complicates	O
use	O
.	O
</s>
<s>
In	O
some	O
cases	O
variables	O
are	O
also	O
finalized	O
:	O
GCC	O
extensions	O
allow	O
finalization	B-Application
of	O
variables	O
.	O
</s>
<s>
As	O
reflected	O
in	O
the	O
naming	O
,	O
"	O
finalization	B-Application
"	O
and	O
the	O
finally	O
construct	O
both	O
fulfill	O
similar	O
purposes	O
:	O
performing	O
some	O
final	O
action	O
,	O
generally	O
cleaning	O
up	O
,	O
after	O
something	O
else	O
has	O
finished	O
.	O
</s>
<s>
They	O
differ	O
in	O
when	O
they	O
occur	O
–	O
a	O
finally	O
clause	O
is	O
executed	O
when	O
program	O
execution	O
leaves	O
the	O
body	O
of	O
the	O
associated	O
try	O
clause	O
–	O
this	O
occurs	O
during	O
stack	O
unwind	O
,	O
and	O
there	O
is	O
thus	O
a	O
stack	O
of	O
pending	O
finally	O
clauses	O
,	O
in	O
order	O
–	O
while	O
finalization	B-Application
occurs	O
when	O
an	O
object	O
is	O
destroyed	O
,	O
which	O
happens	O
depending	O
on	O
the	O
memory	B-General_Concept
management	I-General_Concept
method	B-Language
,	O
and	O
in	O
general	O
there	O
is	O
simply	O
a	O
set	O
of	O
objects	O
awaiting	O
finalization	B-Application
–	O
often	O
on	O
the	O
heap	O
–	O
which	O
need	O
not	O
happen	O
in	O
any	O
specific	O
order	O
.	O
</s>
<s>
In	O
C++	B-Language
,	O
object	O
destruction	O
is	O
deterministic	O
,	O
and	O
the	O
behavior	O
of	O
a	O
finally	O
clause	O
can	O
be	O
produced	O
by	O
having	O
a	O
local	O
variable	O
with	O
an	O
object	O
as	O
its	O
value	O
,	O
whose	O
scope	O
is	O
a	O
block	O
corresponds	O
to	O
the	O
body	O
of	O
a	O
try	O
clause	O
–	O
the	O
object	O
is	O
finalized	O
(	O
destructed	O
)	O
when	O
execution	O
exits	O
this	O
scope	O
,	O
exactly	O
as	O
if	O
there	O
were	O
a	O
finally	O
clause	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
C++	B-Language
does	O
not	O
have	O
a	O
finally	O
construct	O
–	O
the	O
difference	O
being	O
that	O
finalization	B-Application
is	O
defined	O
in	O
the	O
class	O
definition	O
as	O
the	O
destructor	B-Language
method	B-Language
,	O
rather	O
than	O
at	O
the	O
call	O
site	O
in	O
a	O
finally	O
clause	O
.	O
</s>
<s>
Conversely	O
,	O
in	O
the	O
case	O
of	O
a	O
finally	O
clause	O
in	O
a	O
coroutine	B-Architecture
,	O
like	O
in	O
a	O
Python	B-Language
generator	O
,	O
the	O
coroutine	B-Architecture
may	O
never	O
terminate	O
–	O
only	O
ever	O
yielding	O
–	O
and	O
thus	O
in	O
ordinary	O
execution	O
the	O
finally	O
clause	O
is	O
never	O
executed	O
.	O
</s>
<s>
If	O
one	O
interprets	O
instances	O
of	O
a	O
coroutine	B-Architecture
as	O
objects	O
,	O
then	O
the	O
finally	O
clause	O
can	O
be	O
considered	O
a	O
finalizer	B-Application
of	O
the	O
object	O
,	O
and	O
thus	O
can	O
be	O
executed	O
when	O
the	O
instance	O
is	O
garbage	B-General_Concept
collected	I-General_Concept
.	O
</s>
<s>
In	O
Python	B-Language
terminology	O
,	O
the	O
definition	O
of	O
a	O
coroutine	B-Architecture
is	O
a	O
generator	O
function	O
,	O
while	O
an	O
instance	O
of	O
it	O
is	O
a	O
generator	O
iterator	O
,	O
and	O
thus	O
a	O
finally	O
clause	O
in	O
a	O
generator	O
function	O
becomes	O
a	O
finalizer	B-Application
in	O
generator	O
iterators	O
instantiated	O
from	O
this	O
function	O
.	O
</s>
<s>
The	O
notion	O
of	O
finalization	B-Application
as	O
a	O
separate	O
step	O
in	O
object	O
destruction	O
dates	O
to	O
,	O
by	O
analogy	O
with	O
the	O
earlier	O
distinction	O
of	O
initialization	O
in	O
object	O
construction	O
in	O
.	O
</s>
<s>
Literature	O
prior	O
to	O
this	O
point	O
used	O
"	O
destruction	O
"	O
for	O
this	O
process	O
,	O
not	O
distinguishing	O
finalization	B-Application
and	O
deallocation	B-General_Concept
,	O
and	O
programming	O
languages	O
dating	O
to	O
this	O
period	O
,	O
like	O
C++	B-Language
and	O
Perl	B-Language
,	O
use	O
the	O
term	O
"	O
destruction	O
"	O
.	O
</s>
<s>
The	O
terms	O
"	O
finalize	O
"	O
and	O
"	O
finalization	B-Application
"	O
are	O
also	O
used	O
in	O
the	O
influential	O
book	O
Design	O
Patterns	O
(	O
1994	O
)	O
.	O
</s>
<s>
The	O
introduction	O
of	O
Java	B-Language
in	O
1995	O
contained	O
finalize	O
methods	O
,	O
which	O
popularized	O
the	O
term	O
and	O
associated	O
it	O
with	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
and	O
languages	O
from	O
this	O
point	O
generally	O
make	O
this	O
distinction	O
and	O
use	O
the	O
term	O
"	O
finalization	B-Application
"	O
,	O
particularly	O
in	O
the	O
context	O
of	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
