reflection - 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 legacy reflection-based API. The clousure should be included, so calling thatMethod.invoke(null, ...) should have the same effect as calling the lambda.

I have looked at MethodHandles.Lookup, but it only seems to be relevant for the reverse transform. But I guess the bind method may help to include the clousure?

Edit:

Say I have am lambda experssion:

Function<String, String> sayHello = name -> "Hello, " + name;

and I have a legacy framework (SpEL) that has an API like

registerFunction(String name, Method method)

which will call the given Method with no this argument (i.e. Method assumed to be static). So I'll need to get a special Method instance that includes the lambda logic + the clousure data.

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<String, String> 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<String, String> 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<Object[], Object> lambda;
  public LambdaAccessor(Function<Object[], Object> lambda) {
    this.lambda = lambda;
  }

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