Java 8: convert lambda to a Method instance with closure included

Question

(This is difficult to search because results are all about \"method reference\")

I want to get a Method instance for a lambda expression for use with a

Answer 1

In case you don't find an elegant way, here is the ugly way (Ideone). Usual warning when reflection is involved: may break in future releases etc.

public static void main(String[] args) throws Exception {
  Function sayHello = name -> "Hello, " + name;
  Method m = getMethodFromLambda(sayHello);
  registerFunction("World", m);
}

static void registerFunction(String name, Method method) throws Exception {
  String result = (String) method.invoke(null, name);
  System.out.println("result = " + result);
}

private static Method getMethodFromLambda(Function lambda) throws Exception {
  Constructor c = Method.class.getDeclaredConstructors()[0];
  c.setAccessible(true);
  Method m = (Method) c.newInstance(null, null, null, null, null, 0, 0, null, null, null, null);
  m.setAccessible(true); //sets override field to true

  //m.methodAccessor = new LambdaAccessor(...)
  Field ma = Method.class.getDeclaredField("methodAccessor");
  ma.setAccessible(true);
  ma.set(m, new LambdaAccessor(array -> lambda.apply((String) array[0])));

  return m;
}

static class LambdaAccessor implements MethodAccessor {
  private final Function lambda;
  public LambdaAccessor(Function lambda) {
    this.lambda = lambda;
  }

  @Override public Object invoke(Object o, Object[] os) {
    return lambda.apply(os);
  }
}