Find Lambda parameter types

FindParametersTypes.java

package test;

import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Arrays;
import java.util.function.Function;

import static java.util.stream.Stream.of;

public class FindParametersTypes {
  public static void main(String[] args) {
    Function<String, String> inner = new Function<String, String>() {
      @Override
      public String apply(String s) {
        return s;
      }
    };

    Function<String, String> lambda = s -> s;

    of(inner, lambda).forEach(f -> System.out.println(Arrays.toString(firstParameter(f))));
    of(inner, lambda).forEach(f -> System.out.println(Arrays.toString(firstParameterSmart(f))));
  }

  private static Class<?>[] firstParameter(Function<String, String> function) {
    return function.getClass().getMethods()[0].getParameterTypes();
  }

  // Lambda class name: test.Toto$$Lambda$1/1199823423
  // Implementation synthetic method: lambda$main$0
  //
  private static Class<?>[] firstParameterSmart(Function<String, String> function) {
    String functionClassName = function.getClass().getName();
    int lambdaMarkerIndex = functionClassName.indexOf("$$Lambda$");
    if (lambdaMarkerIndex == -1) { // Not a lambda
      return firstParameter(function);
    }

    String declaringClassName = functionClassName.substring(0, lambdaMarkerIndex);
    int lambdaIndex = Integer.parseInt(functionClassName.substring(lambdaMarkerIndex + 9, functionClassName.lastIndexOf('/')));

    Class<?> declaringClass;
    try {
      declaringClass = Class.forName(declaringClassName);
    } catch (ClassNotFoundException e) {
      throw new IllegalStateException("Unable to find lambda's parent class " + declaringClassName);
    }

    for (Method method : declaringClass.getDeclaredMethods()) {
      if (method.isSynthetic()
        && method.getName().startsWith("lambda$")
        && method.getName().endsWith("$" + (lambdaIndex - 1))
        && Modifier.isStatic(method.getModifiers())) {
        return method.getParameterTypes();
      }
    }

    throw new IllegalStateException("Unable to find lambda's implementation method");
  }
}

Thanks for the great example! But I found an exceptional case.
If the lambda for java.util.Function is declared in a Lambda expression, the index between the Lambda object and the method in the root class become different. (My environment is Oracle JDK 1.8.0_171 on MacOS)

public class FindParametersTypes {
    public static void main(String[] args) {
        Function<String, String> inner = new Function<String, String>() {
            @Override
            public String apply(String s) {
                return s;
            }
        };

        Function<String, String> lambda = s -> s;

        Runnable task = () -> {
            Function<String, String> lambdaInLambda = s -> s;
            of(inner, lambda, lambdaInLambda).forEach(f -> System.out.println(Arrays.toString(firstParameterSmart(f))));
        };
        task.run();
    }

In the example above, the name of class of lambdaInLambda is like test.FindParametersTypes$$Lambda$3/1349393271 but the corresponding method in the root class is lambda$null$1 and lambda$null$2 is actually the lambda for forEach so that the firstParameterSmart would cause a misditection.

[class java.lang.String]
[class java.lang.String]
[interface java.util.function.Function]

Although I have not found the solution of the case, just a heads up to you 😃

Tested on Temurin JDK 11 and 17 right now - not working for both static and non-static lambdas.