Re: Java 8 and type erasure

Posted by Stephan Ewen on Aug 18, 2015; 11:46am
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Java-8-and-type-erasure-tp2411p2424.html

Hi Kristoffer!

I looked through the code as well. In fact, Flink currently uses the trick mentioned for Serializable Lambdas in the gist you sent me.

This works well for lambdas that return simple types (primitives or classes without generics). The information for the generic parametrization is unfortunately really erased, it is in no signature or anything.

Java has the concept of "generic method signatures", which means that a method gets a signature string that includes the generic types. These signatures are generated for regular functions, but OpenJDK and OracleJDK do not generate them for synthetic methods (like lambdas).

We tried to submit a patch to OpenJDK to add these generic signatures to lambda methods, but they did not like the fact that we try to figure out the generic types of lambdas. I hope they change their minds at some point...

Stephan

On Tue, Aug 18, 2015 at 11:46 AM, Aljoscha Krettek <[hidden email]> wrote:

Unfortunately, this also doesn't work for the same reasons. The generic types of generic parameters of a lambda are not stored anywhere. Stephan mentioned to me that the only possibility right now would be to look at the code using something like ASM to find a cast in the code to the concrete type of the generic parameter.

On Tue, 18 Aug 2015 at 11:35 Kristoffer Sjögren <[hidden email]> wrote:

How about https://github.com/jhalterman/typetools?

On Tue, Aug 18, 2015 at 11:16 AM, Aljoscha Krettek <[hidden email]> wrote:
> Hi Kristoffer,
> I'm afraid not, but maybe Timo has some further information. In this
> extended example we can see the problem:
> https://gist.github.com/aljoscha/84cc363d13cf1dfe9364. The output is:
>
> Type is: class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
> class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
> Type is: class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
> class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
> Type is :
> org.apache.flink.examples.java8.wordcount.TypeTest.org.apache.flink.examples.java8.wordcount.TypeTest$Thing<java.lang.String>
> class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>
> So with the two lambda calls it does not correctly determine the generic
> parameter of Thing while it works with the anonymous class.
>
> I was exited when I saw the example because we've been trying for so long to
> get this to work. Maybe we have to wait for java 19 to get this to work. ;-)
>
> Thanks a lot for helping, though. :D
> Aljoscha
>
> On Tue, 18 Aug 2015 at 11:01 Kristoffer Sjögren <[hidden email]> wrote:
>>
>> Hi
>>
>> Potential fix for writing flink jobs using lamdas without Eclipse JDT?
>>
>> https://gist.github.com/aslakhellesoy/3678beba60c109eacbe5
>>
>> Cheers,
>> -Kristoffer