[Springframework-developer] FactoryBeans, BeanFactoryPostProcessors and finding beans by type

[David H. <dh...@ob...>]

Have=20just=20come=20across=20an=20interesting=20and=20quite=20subtle=20go=
tcha=20that=20is=20probably=20worth=20sharing.
=20
If=20a=20BeanPostProcessor=20attempts=20to=20locate=20collaborators=20by=20=
type=20in=20the=20ListableBeanFactory=20,=20and=20follows=20the=20standard=
=20method=20of=20preventing=20circular=20references=20using=20ControlFlow,=
=20then=20it=20will=20fail=20to=20process=20dependencies=20of=20non-lazy=20=
FactoryBeans.
=20
I'll=20elaborate=20on=20that=20mouthful=20of=20a=20sentence=20with=20an=20=
example.=20We=20use=20a=20BeanPostProcessor=20to=20auto-register=20JMX=20m=
anageable=20beans,=20which=20does=20the=20following=20in=20postProcessAfte=
rInitialization:
*=09Checks=20that=20we=20are=20not=20in=20a=20circular=20dependency=20usin=
g=20the=20ControlFlow=20method,=20as=20in=20AbstractAdvisorAutoProxyCreato=
r.shouldSkip
*=09If=20this=20test=20succeeds,=20it=20locates=20a=20number=20of=20collab=
orating=20handlers=20by=20type=20that=20define=20different=20strategies=20=
for=20registering=20MBeans=20(ie=20registering=20standard=20MBeans,=20or=20=
POJOs=20with=20attributes).=20=20
*=09The=20found=20handlers=20are=20then=20applied=20to=20the=20bean=20bein=
g=20processed
=20
This=20seems=20to=20be=20a=20perfectly=20sane=20and=20valid=20series=20of=20=
calls.=20In=20fact,=20the=20only=20real=20difference=20between=20this=20an=
d=20DefaultAdvisorAutoProxyCreator=20is=20that=20collaborators=20are=20loc=
ated=20by=20type=20and=20not=20by=20name,=20which=20is=20not=20unreasonabl=
e.=20However,=20an=20issue=20occurs=20when=20a=20bean=20you=20expect=20to=20=
be=20post-processed=20is=20itself=20a=20dependency=20of=20a=20FactoryBean=20=
that=20is=20not=20lazily=20instantiated.=20All=20FactoryBeans=20are=20auto=
matically=20instantiated=20when=20we=20locate=20beans=20by=20type,=20to=20=
determine=20what=20type=20of=20object=20they=20return.=20However,=20this=20=
in=20turn=20instantiates=20all=20of=20the=20FactoryBean's=20dependencies=20=
-=20one=20of=20which=20is=20a=20bean=20we=20expect=20to=20be=20handled=20b=
y=20our=20BeanPostProcessor,=20which=20does=20not=20process=20it=20since=20=
we=20are=20already=20in=20a=20call=20stack=20protected=20by=20ControlFlow.=
=20The=20result=20is=20that=20some=20beans=20are=20registered=20with=20our=
=20JMX=20server,=20and=20some=20aren't=20-=20and=20the=20ones=20that=20are=
n't=20are=20those=20that=20happen=20to=20be=20dependencies=20of=20non-lazy=
=20FactoryBeans.=20
=20
All=20of=20the=20above=20is=20exactly=20the=20behaviour=20you=20would=20ex=
pect=20if=20you=20were=20paying=20really=20close=20attention=20-=20except=20=
I=20wasn't=20and=20it=20surprised=20the=20hell=20out=20of=20me.=20If=20the=
=20TODO=20in=20AbstractAdvisorAutoProxyCreator=20is=20to=20be=20believed=20=
(in=20the=201.0.2=20source),=20the=20ControlFlow=20check=20is=20likely=20t=
o=20end=20up=20in=20a=20base=20class=20for=20broader=20re-use.=20Thus,=20i=
s=20it=20worth=20explicitly=20documenting=20(assuming=20that=20it=20isn't=20=
already)=20that=20ListableBeanFactory.getBeansOfType=20can=20cause=20probl=
ems=20like=20this=20with=20FactoryBeans=20and=20should=20be=20used=20with=20=
care?
=20
Dave=20Hewitt
------------------
Systems=20Engineer
OBJECTIVITY
Tel:=20024=207642=200000
Fax:=20024=207642=200001
Email:=20d...@ob...=20<mailto:dh...@ob...>=20=

www.objectivity.co.uk=20<http://www.objectivity.co.uk/>=20
=20

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