How to dry run Method.invoke()

Question

java.lang.reflect.Method.invoke() documentation states that it throws IllegalArgumentException

IllegalArgumentException - if the method is an instance method and the specified object argument is not an instance of the class or interface declaring the underlying method (or of a subclass or implementor thereof); if the number of actual and formal parameters differ; if an unwrapping conversion for primitive arguments fails; or if, after possible unwrapping, a parameter value cannot be converted to the corresponding formal parameter type by a method invocation conversion.

Is there a way to test if method throws IllegalArgumentException for given arguments without invoking it?

Answer 1

It is possible to check for correctness of instance, method and number of arguments manually and check for assignability of arguments by piggybacking on native java mechanism.

public class Methods {
  public static boolean isInvocable(Method method, Object instance,
      Object... arguments) {
    return correctInstance(instance, method)
        && correctArguments(arguments, method);
  }

  private static boolean correctInstance(Object instance, Method method) {
    return Modifier.isStatic(method.getModifiers())
        || method.getDeclaringClass().isInstance(instance);
  }

  private static boolean correctArguments(Object[] arguments, Method method) {
    Class<?>[] parameters = method.getParameterTypes();
    if (parameters.length != arguments.length) {
      return false;
    }
    for (int i = 0; i < parameters.length; i++) {
      if (!correctArgument(arguments[i], parameters[i])) {
        return false;
      }
    }
    return true;
  }

  private static boolean correctArgument(Object instance, Class<?> type) {
    return type.isPrimitive()
        ? correctPrimitive(instance, type)
        : instance == null || type.isAssignableFrom(instance.getClass());
  }

  private static boolean correctPrimitive(Object argument, Class<?> type) {
    try {
      Method method = Overloading.class.getDeclaredMethod("method", type);
      method.setAccessible(true);
      method.invoke(null, argument);
      return true;
    } catch (IllegalArgumentException e) {
      return false;
    } catch (NoSuchMethodException e) {
      throw new Error(e);
    } catch (IllegalAccessException e) {
      throw new Error(e);
    } catch (InvocationTargetException e) {
      throw new Error(e);
    }
  }

  @SuppressWarnings("unused")
  private static class Overloading {
    private static void method(byte argument) {}

    private static void method(short argument) {}

    private static void method(int argument) {}

    private static void method(long argument) {}

    private static void method(float argument) {}

    private static void method(double argument) {}

    private static void method(boolean argument) {}

    private static void method(char argument) {}
  }
}

Answer 2

Yes, you can use getDeclaredMethod from the Reflection API to verify that a certain method with given signature is declared. An example can be found in the docs (on the bottom): http://docs.oracle.com/javase/tutorial/reflect/member/methodInvocation.html

This might work as is, though I didn't test it:

public boolean hasMethod( Class<?> clazz, String methodName, Class[] argTypes ) {
    try {
        clazz.getDeclaredMethod( methodName, argTypes );
        return true;
    } catch( NoSuchMethodException e ) {
        return false;
    } catch( Exception e ) {
        // something else went wrong
        // you definitely wanna handle this case more gracefully
        throw new IllegalStateException( "Uh-Oh!", e );
    }
}

// example call for a method named someMethod that takes an array of strings
// as its only argument
Class[] argTypes = new Class[] { String[].class };
boolean hasMethod = hasMethod( yourClass, "someMethod", argTypes );

Answer 3

What you are asking

At runtime, first, method, instance and list of arguments is created and only some time later in different place in code it is invoked. I want to detect problems early on instead of wasting time and reporting exception with non-informative stack trace.

is not possible. For some of

IllegalArgumentException - if the method is an instance method and the specified object argument is not an instance of the class or interface declaring the underlying method (or of a subclass or implementor thereof); if the number of actual and formal parameters differ; if an unwrapping conversion for primitive arguments fails; or if, after possible unwrapping, a parameter value cannot be converted to the corresponding formal parameter type by a method invocation conversion.

you can check yourself. For example, you can do an instanceof on the instance or rather use Class#isInstance() on it. You can compare the number of arguments you have and the number of arguments the method requires. For type conversion of primitives, it's a little harder to test and might depend on actually knowing which types get converted.

Your best bet is to invoke it when you need to and handle the IllegalArgumentException .