Introduce a compiler option to store generic type information about a lambda expression using the Signature Attribute

[Maurizio Cimadamore]

Hi Timo,
thanks for the patch - if I'm not mistaken, this would be the very first 
case in which a method marked with ACC_SYNTHETIC also gets a signature 
attribute. I will consult with the rest of the team and see if there's 
any objection.

Cheers
Maurizio

On 07/01/15 15:22, Timo Walther wrote:
> Hi all,
>
> the Java Reflection API allows to access the generic signature of 
> methods through the java.lang.reflect.Method#getGenericReturnType() 
> method. However, this is not yet supported for Lambda expressions.
>
> Given the following classes:
>
> class Tuple2<F0,F1> {
>   F0 field0;
>   F1 field1;
>
>   public Tuple2(F0 f0, F1 f1) {
>     this.field0 = f0;
>     this.field1 = f1;
>   }
> }
>
> interface Map<IN, OUT> {
>   OUT map(IN in);
> }
>
> Currently, there is no solution that allows to get further type 
> information about expressions like:
>
> Map<String, Tuple2<String, Integer>> map = (str) -> new Tuple2<>(str, 1);
> System.out.println(getReturnType(map)) // can only print Tuple2 => 
> information about the Tuple2's fields are always lost
>
> Especially data-intensive runtimes (like Apache Flink[0] where I am 
> working on) need as much type information as possible to be efficient. 
> Therefore, we searched for a way to also extract type information from 
> lambda expressions. Adding a generic signature to the generated 
> "lambda$XXX" static methods (at least by using a compiler option) 
> would be the perfect solution. It seems that the JVM Specification 
> does not prohibit such an implementation. An implementation of a later 
> extraction is described here[1].
>
> The Eclipse JDT compiler team is introducing a compiler option 
> "-genericsignature" with version 4.5 M4[2].
>
> I have implemented a patch prototype. It can be found at: 
> http://people.apache.org/~twalthr/patches/lambdaSignature.patch
>
> The patch postpones the type erasure of the lambda function's 
> parameters after the generic descriptor has been saved in a newly 
> introduced variable in MethodSymbol (if compiler option 
> "-lambdasignature" is set). The contents of the variable is read in 
> ClassWriter and written to method's Signature attribute in the 
> classfile. Tests are also included.
>
> No change to the class file format is required. The change is guarded 
> by a compiler option, so without that addition, nothing should behave 
> differently.
>
>
> [0] http://flink.apache.org/
> [1] 
> http://stackoverflow.com/questions/21887358/reflection-type-inference-on-java-8-lambdas
> [2] https://bugs.eclipse.org/bugs/show_bug.cgi?id=449063
>
>
> Regards,
> Timo Walther