On Thu, 13 Jan 2022 08:25:22 GMT, Andrey Turbanov <aturba...@openjdk.org> wrote:
> Method `Class.isAssignableFrom` is often used in form of:
>
> if (clazz.isAssignableFrom(obj.getClass())) {
> Such condition could be simplified to more shorter and performarnt code
>
> if (clazz.isInstance(obj)) {
>
> Replacement is equivalent if it's known that `obj != null`.
> In JDK codebase there are many such places.
>
> I tried to measure performance via JMH
>
> Class<?> integerClass = Integer.class;
> Class<?> numberClass = Number.class;
>
> Object integerObject = 45666;
> Object doubleObject = 8777d;
>
> @Benchmark
> public boolean integerInteger_isInstance() {
> return integerClass.isInstance(integerObject);
> }
>
> @Benchmark
> public boolean integerInteger_isAssignableFrom() {
> return integerClass.isAssignableFrom(integerObject.getClass());
> }
>
> @Benchmark
> public boolean integerDouble_isInstance() {
> return integerClass.isInstance(doubleObject);
> }
>
> @Benchmark
> public boolean integerDouble_isAssignableFrom() {
> return integerClass.isAssignableFrom(doubleObject.getClass());
> }
>
> @Benchmark
> public boolean numberDouble_isInstance() {
> return numberClass.isInstance(doubleObject);
> }
>
> @Benchmark
> public boolean numberDouble_isAssignableFrom() {
> return numberClass.isAssignableFrom(doubleObject.getClass());
> }
>
> @Benchmark
> public boolean numberInteger_isInstance() {
> return numberClass.isInstance(integerObject);
> }
>
> @Benchmark
> public boolean numberInteger_isAssignableFrom() {
> return numberClass.isAssignableFrom(integerObject.getClass());
> }
>
> @Benchmark
> public void numberIntegerDouble_isInstance(Blackhole bh) {
> bh.consume(numberClass.isInstance(integerObject));
> bh.consume(numberClass.isInstance(doubleObject));
> }
>
> @Benchmark
> public void integerIntegerDouble_isAssignableFrom(Blackhole bh) {
> bh.consume(integerClass.isAssignableFrom(integerObject.getClass()));
> bh.consume(integerClass.isAssignableFrom(doubleObject.getClass()));
> }
>
> `isInstance` is a bit faster than `isAssignableFrom`
>
> Benchmark Mode Cnt Score Error Units
> integerDouble_isAssignableFrom avgt 5 1,173 ± 0,026 ns/op
> integerDouble_isInstance avgt 5 0,939 ± 0,038 ns/op
> integerIntegerDouble_isAssignableFrom avgt 5 2,106 ± 0,068 ns/op
> numberIntegerDouble_isInstance avgt 5 1,516 ± 0,046 ns/op
> integerInteger_isAssignableFrom avgt 5 1,175 ± 0,029 ns/op
> integerInteger_isInstance avgt 5 0,886 ± 0,017 ns/op
> numberDouble_isAssignableFrom avgt 5 1,172 ± 0,007 ns/op
> numberDouble_isInstance avgt 5 0,891 ± 0,030 ns/op
> numberInteger_isAssignableFrom avgt 5 1,169 ± 0,014 ns/op
> numberInteger_isInstance avgt 5 0,887 ± 0,016 ns/op
I've stared at the javadoc for Class.isAssignableFrom and Class.isInstance and
if a non-null instance is used to get a non-null class they are PROBABLY the
same but it is far from clear. The implementations of both are at least native
and may be instrinsicified. The doc for Class.isAssignableFrom cites JLS 5.1.4
which in what I found is about primitives so I suspect it is woefully out of
date
https://docs.oracle.com/javase/specs/jls/se17/html/jls-5.html#jls-5.1.4
What client tests have you run that touch the code you are changing ?
In short I see insufficient value in the changes here and would prefer you drop
the client part so I don't have to worry about it.
-------------
PR: https://git.openjdk.java.net/jdk/pull/7061
