Resolve type argument of a Java generics typed field

resolve-field-type-argument.groovy

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface TestPojo {
  String file()
}

@TestPojo(file = "src/test/resources/foo-response.json")
FooResponse fooResponse
  
// invoke processSpecAnnotations()

static def processSpecAnnotations(Object objectWithAnno) {
    for (Field field : objectWithAnno.getClass().declaredFields) {
        TestPojo testPojo = field.getAnnotation(TestPojo)
        if (testPojo) {
            Object deserializedValue
            if (List.class.isAssignableFrom(field.type)) {
                def pt = (ParameterizedType) field.getGenericType() // like List<Foo>
                assert pt.actualTypeArguments.size() == 1
                Class typeArg = (Class)pt.actualTypeArguments[0] // like Foo
                JavaType javaType = objectMapper.getTypeFactory().constructCollectionType(List.class, typeArg)
                deserializedValue = objectMapper.readValue(new File(testPojo.file()), javaType)
            } else {
                deserializedValue = objectMapper.readValue(new File(testPojo.file()), (Class)field.genericType)
            }
            assert deserializedValue != null
            validatePojo(deserializedValue)
            field.setAccessible(true)
            field.set(spec, deserializedValue)
        }
    }
}