Performance of "instanceof"
John.Rose at Sun.COM
Thu Jan 15 08:09:43 PST 2009
It would be a good idea for some of us to make a page on instanceof
(If someone else starts it I will contribute.)
HotSpot has a 1-1 relation between java.lang.Class instances and Java
types; any array, instance class, or interface also has an internal
meta-data object called a Klass. It is a single indirection to get
from either to its sibling, so things like foo.class.isAssignableFrom
(bar.getClass()) tend to cook down to the right things. The server
compiler knows how to optimize away things like aConstantClass.klass
and foo.klass.Class.klass (=> foo.klass). To observe the
optimizations which handle this, look in the source code for uses of
But, in order to see if your use case is getting helped, there is no
substitute for examining the code:
On Jan 15, 2009, at 3:21 AM, Ulf Zibis wrote:
> Thanks Tom,
> I was asking because of my RFE 8 in http://bugs.sun.com/bugdatabase/
> So I can be sure, that my proposed shortcut is much faster than the
> saved HashMap lookup, and hopefully HotSpot remembers the result of
> "instanceof", so following cast to String could be skipped.
> I was in doubt, how the right part of "instanceof" would retrieve a
> reference pointer to the Class class from the given class name. So
> as I understand you right, MyClass.klass is static final available
> from bytecode.
> Am 15.01.2009 03:19, Tom Rodriguez schrieb:
>> instanceof is the fastest way of asking the question it's designed
>> to ask.
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