How to invoke default constructor using LambdaMetafactory

Question

In the link: How to instantiate an object using LambdaMetaFactory? it is mentioned how to instantiate a one-arg contrcutor using LambdaMetafactory.

I am trying to do the same for default-constructor but is failing with the below error:

Exception in thread "main" java.lang.AbstractMethodError: Method com/test/Main$$Lambda$1.apply(Ljava/lang/Object;)Ljava/lang/Object; is abstract
at com.test.Main$$Lambda$1/186370029.apply(Unknown Source)
at com.test.Main.test2(Main.java:29)
at com.test.Main.main(Main.java:14)

Code I am trying to run:

import java.lang.invoke.LambdaMetafactory;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.function.Function;

public class Main {

  public static void main(String[] args) throws Throwable {
    long t2= System.nanoTime();
    for(int i=0;i<10000;i++){
      test2(TestClass.class.getName());
    }
    long t3= System.nanoTime();

    System.out.println((t3-t2)*1e-9);
  }

  private static TestClass test2(String objclass)
      throws Throwable {
    Class clazz = Class.forName(objclass);
    MethodHandles.Lookup lookup = MethodHandles.lookup();
    MethodHandle mh = lookup.findConstructor(clazz, MethodType.methodType(void.class));
    Function<String, TestClass> constructor = (Function<String, TestClass>) LambdaMetafactory
        .metafactory(lookup, "apply",MethodType.methodType(Function.class),
            mh.type().generic(), mh, mh.type()).getTarget().invokeExact();
    TestClass testClass = constructor.apply(objclass);
    return testClass;
  }

}

TestClass

import java.util.Collections;
import java.util.Map;

public class TestClass {

  public Map<String, String> getContextMap() {
    return Collections.emptyMap();
  }
}

What am I doing wrong in order to invoke the default constructor using LambdaMetafactory?

Answer 1

Your default constructor takes no parameter. Change Function to Supplier:

private static TestClass test2(String objclass)
        throws Throwable {
    Class clazz = Class.forName(objclass);
    MethodHandles.Lookup lookup = MethodHandles.lookup();
    MethodHandle mh = lookup.findConstructor(clazz, MethodType.methodType(void.class));
    Supplier<TestClass> constructor = (Supplier<TestClass>) LambdaMetafactory.metafactory(
            lookup, "get", MethodType.methodType(Supplier.class), mh.type().generic(), mh, mh.type()
    ).getTarget().invokeExact();
    TestClass testClass = constructor.get();
    return testClass;
}