Java: how to resolve generic type of lambda parameter?

This is currently possible to solve but only in a pretty hackie way, but let me first explain a few things:

When you write a lambda, the compiler inserts a dynamic invoke instruction pointing to the LambdaMetafactory and a private static synthetic method with the body of the lambda. The synthetic method and the method handle in the constant pool both contain the generic type (if the lambda uses the type or is explicit as in your examples).

Now at runtime the LambdaMetaFactory is called and a class is generated using ASM that implements the functional interface and the body of the method then calls the private static method with any arguments passed. It is then injected into the original class using Unsafe.defineAnonymousClass (see John Rose post) so it can access the private members etc.

Unfortunately the generated Class does not store the generic signatures (it could) so you can't use the usual reflection methods that allow you to get around erasure

For a normal Class you could inspect the bytecode using Class.getResource(ClassName + ".class") but for anonymous classes defined using Unsafe you are out of luck. However you can make the LambdaMetaFactory dump them out with the JVM argument:

java -Djdk.internal.lambda.dumpProxyClasses=/some/folder

By looking at the dumped class file (using javap -p -s -v), one can see that it does indeed call the static method. But the problem remains how to get the bytecode from within Java itself.

This unfortunately is where it gets hackie:

Using reflection we can call Class.getConstantPool and then access the MethodRefInfo to get the type descriptors. We can then use ASM to parse this and return the argument types. Putting it all together:

Method getConstantPool = Class.class.getDeclaredMethod("getConstantPool");
getConstantPool.setAccessible(true);
ConstantPool constantPool = (ConstantPool) getConstantPool.invoke(lambda.getClass());
String[] methodRefInfo = constantPool.getMemberRefInfoAt(constantPool.size() - 2);

int argumentIndex = 0;
String argumentType = jdk.internal.org.objectweb.asm.Type.getArgumentTypes(methodRef[2])[argumentIndex].getClassName();
Class<?> type = (Class<?>) Class.forName(argumentType);

UPDATED with Jonathan's suggestion

Now ideally the classes generated by LambdaMetaFactory should store the generic type signatures (I might see if I can submit a patch to the OpenJDK) but currently this is the best we can do. The code above has the following problems:

• It uses undocumented methods and classes
• It is extremely vulnerable to code changes in the JDK
• It doesn't preserve the generic types, so if you pass List<String> into a lambda it will come out as List

Use a TypeRef to wrap the lambda expression.

@FunctionalInterface
interface TypeRef<T> {
    T create();

    default Class<?> getGenericType() {
        return create().getClass();
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(getFactoryTypeParameter(() -> "hello"));
    }

    private static <T> Class<?> getFactoryTypeParameter(TypeRef<T> typeRef) {
        return typeRef.getGenericType();
    }
}

output:

class java.lang.String