code: public static ResolvableType forField(Field field, Class<?> implementationClass) {
  		Assert.notNull(field, "Field must not be null");
  		ResolvableType owner = forType(implementationClass).as(field.getDeclaringClass());
  		return forType(null, new FieldTypeProvider(field), owner.asVariableResolver());
  	}
docs: Return a {@link ResolvableType} for the specified {@link Field} with a given
  implementation.
  <p>Use this variant when the class that declares the field includes generic
  parameter variables that are satisfied by the implementation class.
  @param field the source field
  @param implementationClass the implementation class
  @return a {@link ResolvableType} for the specified field
  @see #forField(Field)