targetTypeMapping not working when executing a Java lambda

[polsotos-picnic]

Hi,

Is there an explanation why the first unit test works and the second one doesn't?
I would expect the mapping to be applied in both cases.

import java.util.function.Consumer;
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.HostAccess;
import org.junit.jupiter.api.Test;

public class TargetTypeMappingTest {

    @Test
    void testTargetTypeMappingWhenObject() {
        HostAccess hostAccess = HostAccess.newBuilder(HostAccess.ALL)
                .targetTypeMapping(
                        Integer.class, // Source type: Integer
                        String.class, // Target type: String
                        null, // Accept all
                        Object::toString // Converter function
                )
                .build();

        try (Context context = Context.newBuilder().
                allowAllAccess(true)
                .allowHostAccess(hostAccess).build()) {

            context.getBindings("js").putMember("object", new IntegerToString());
            context.eval("js", "object.print(2)"); // works!
        }
    }

    @Test
    void testTargetTypeMappingWhenFunction() {
        HostAccess hostAccess = HostAccess.newBuilder(HostAccess.ALL)
                .targetTypeMapping(
                        Integer.class, // Source type: Integer
                        String.class, // Target type: String
                        null, // Accept all
                        Object::toString // Converter function
                )
                .build();

        try (Context context = Context.newBuilder().
                allowAllAccess(true)
                .allowHostAccess(hostAccess).build()) {

            Consumer<String> function = System.out::println;
            context.getBindings("js").putMember("javaFunction", function);
            context.eval("js", "javaFunction(2)"); // ClassCast exception
        }
    }

    public static class IntegerToString {
        public void print(String stringValue) {
            System.out.println(stringValue);
        }
    }
}

[polsotos-picnic]

To add more clarity, the following it does work. So it seems an issue with the syntax...

@Test
    void testTargetTypeMappingWhenFunctionWithImpl() {
        HostAccess hostAccess = HostAccess.newBuilder(HostAccess.ALL)
                .targetTypeMapping(
                        Integer.class, // Source type: Integer
                        String.class, // Target type: String
                        null, // Accept all
                        Object::toString // Converter function
                )
                .build();

        try (Context context = Context.newBuilder().
                allowAllAccess(true)
                .allowHostAccess(hostAccess).build()) {

            context.getBindings("js").putMember("javaFunction", new ConsumerImpl());
            context.eval("js", "javaFunction(2)"); // Works
        }
    }

    public class ConsumerImpl implements Consumer<String> {
        @Override
        public void accept(String s) {
            System.out.println(s);
        }
    }

[iamstolis]

Note that the invocation of Java methods from JavaScript (including the type mapping) is based on Java Reflection. The source type is the type of the object being passed to the method (2 in this case). The target type is the type required by the target method. When we can see (via Java Relection) that the parameter of the target method is java.lang.String then we know that we need to map/convert to java.lang.String. This is the case for ConsumerImpl, that has a method that takes java.lang.String (as you can verify using ConsumerImpl.class.getDeclaredMethod("accept", new Class[] { String.class })). On the other hand, there is no such method in the Java class generated for the lambda. The generated class has just accept(java.lang.Object). So, we have no idea that the conversion to String is needed/desirable. Hence, we invoke the method with Integer and it is an internal implementation detail (not visible via Java Reflection) of the lambda that it performs the cast to String (that results in ClassCastException that you can see).