I have a Kotlin class whose primary (and only) constructor is empty.
I have a reference to this class:
val kClass: KClass<MyClass> = MyClass::class
How do I create an instance of this class using reflection?
In Java I would do myClass.newInstance() but it seems in Kotlin I need to find the constructor first:
kClass.constructors.first().call()
I have seen mention of primaryConstructor in some bug reports but it's not showing up in my IDE.
In your case, Java reflection might be enough: you can use MyClass::class.java and create a new instance in the same way as you would with Java reflection (see #IngoKegel's answer).
But in case there's more than one constructor and you really need to get the primary one (not the default no-arg one), use the primaryConstructor extension function of a KClass<T>. It is a part of Kotlin reflection, which is not shipped within kotlin-stdlib.
To use it, you have to add kotlin-reflect as a dependency, e.g. a in Gradle project:
dependencies {
compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
}
Assuming that there is ext.kotlin_version, otherwise replace $kotlin_version with the version you use.
Then you will be able to use primaryConstructor, for example:
fun <T : Any> construct(kClass: KClass<T>): T? {
val ctor = kClass.primaryConstructor
return if (ctor != null && ctor.parameters.isEmpty())
ctor.call() else
null
}
You can use the Java class to create new instance:
MyClass::class.java.newInstance()
For those checking this question now, since Kotlin 1.1 there's also createInstance() extension method on KClass
Much like the accepted answer, this function works only in case class has an empty constructor or constructor with all default arguments.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect.full/create-instance.html
Expanding on Alexeys Answer, to include a primary constructor call with parameters:
/* Example class with no-args constructor */
class MyClass
/* Example class requiring parameters */
class MyClassWithParams(parameter1: String, parameter2: MyClass)
val myKClass: KClass<MyClass> = MyClass::class
val myKClassWithParameters: KClass<MyClassWithParams> = MyClassWithParams::class
/* We can create an object by calling createInstance when no constructor parameters are required as explained in other answers. */
val myObject: MyClass = myKClass.createInstance()
/* To create an object with parameters, we need to get the constructor first, and call it with the parameters instead, similarly to how we would do in Java. */
val myObjectWithParameters: MyClassWithParams? =
myKClassWithParameters.primaryConstructor?.call(
"StringParameter", myObject
)
Related
I am new to Flutter and Dart, coming from native Android.
Android has a very nice database abstraction architecture called the Room Persistence Library. As far as I am aware, no such database abstraction architecture exists for Flutter using the MVVM / MVC design patterns.
My solution was to create a Dart version of it myself. I got it pretty much done after a few headaches, but I cannot seem to get LiveData to work properly using generics.
I set up my class like this:
class LiveData<T> {
...
}
Now when I want to return some data, it can either be an Object or List<Object>. I found a neat hack for differentiating the two from T:
...
// Parse response
// This checks if the type is an instance of a single entity or a list.
if (entity is T) {
cachedData = rawData.isEmpty ? null : entity.fromMap(rawData.first) as T;
} else {
cachedData = rawData.map((e) => entity.fromMap(e)).toList() as T;
}
...
The problem lies in the second block:
cachedData = rawData.map((e) => entity.fromMap(e)).toList() as T;
With the error:
- Unhandled Exception: type 'List<Entity>' is not a subtype of type 'List<Vehicle>' in type cast
The question then becomes: How can I cast Entity to Vehicle when I do not have access to the Vehicle class. Only an instance of it is assigned to an Entity entity variable.
Here's a snippet to demonstrate my access to Vehicle:
final Entity entity;
...assign Vehicle instance to entity...
print(entity is Vehicle) // True
I've tried using .runtimeType to no avail. I have also thought about splitting LiveData into two classes, the second one being LiveDataList. Although this seems to be the easiest solution to not bug the code- it would bug me (bad pun is intentional) and break the otherwise pretty direct port of Room.
As a temporary solution, I have abstracted out the build logic into a generic function to be passed to the LiveData in the constructor.
final T Function(List<Map<String, dynamic>> rawData) builder;
And now I call that instead of the previous code to build the cachedData.
// Parse response
cachedData = builder(rawData);
With the constructor for the LiveData<List<Vehicle>> called when accessing all vehicles in the Dao<Vehicle> being:
class VehicleDao implements Dao<Vehicle> {
...
static LiveData<List<Vehicle>> get() {
return LiveData<List<Vehicle>>(
...
(rawData) => rawData.map((e) => Vehicle.fromMap(e)).toList(),
...
);
}
}
In Dart (and indeed in many languages) generics screws with the concept of inheritance. You would think that if Bar inherits from Foo, that List<Bar> would also be castable to List<Foo>.
This is not actually going to be the case because of how generics work. When you have a generic class, every time you use that class with a different type, that type is treated as a completely separate class. This is because when the compiler compiles those types, class MyGenericType<Foo> extends BaseClass and class MyGenericType<Bar> extends BaseClass are basically converted to something like class MyGenericType_Foo extends BaseClass and class MyGenericType_Bar extends BaseClass.
Do you see the problem? MyGenericType_Foo and MyGenericType_Bar are not descendants of one another. They are siblings of each other, both extending from BaseClass. This is why when you try to convert a List<Entity> to List<Vehicle>, the cast doesn't work because they are sibling types, not a supertype and subtype.
With all this being said, while you cannot directly cast one generic type to another based on the relationship of the generic type parameter, in the case of List there is a way to convert one List type to another: the cast method.
List<Entity> entityList = <Entity>[...];
List<Vehicle> vehicleList = entityList.cast<Vehicle>(); // This cast will work
One thing to note though, if you are casting from a supertype generic to a sub-type generic and not all the elements of the list are that new type, this cast will throw an error.
I got a class which accepts multiple Consumer implementations as constructor arguments.
I want to "fill in" all my Consumers via the Symfony DI-Container.
I tried injection tagged services.
final class SynchronousMessageDispatcher implements MessageDispatcher
{
/**
* #var Consumer[]
*/
private $consumers;
public function __construct(Consumer ...$consumers)
{
$this->consumers = $consumers;
}
}
So I tried to Tag the services in the services.yml like that:
services:
_instanceof:
EventSauce\EventSourcing\Consumer:
tags: ['eventsauce.consumer']
And then inject it like this:
eventsauce.message_dispatcher:
class: EventSauce\EventSourcing\SynchronousMessageDispatcher
arguments: [!tagged eventsauce.consumer]
Now I'm getting the following error:
Argument 1 passed to EventSauce\EventSourcing\SynchronousMessageDispatcher::__construct() must implement interface EventSauce\EventSourcing\Consumer, instance of Symfony\Component\DependencyInjection\Argument\RewindableGenerator given
I fully understand why. Is there a way to unpack services
In other words: Is it possible to modify [!tagged eventsauce.consumer] somehow. Or is the ...$consumers syntax incompatible with the Tagged service Injection in Symfony.
Don't get me wrong. I know that I can easily implement MessageDispatcher myself. Just wanted to know ;-)
My original solution:
As "Tomáš Votruba" mentioned you'd have to rewrite your own !tagged functionality. e.g. !tagged-variadic.
This is not worth the effort for me. I'd rather implement the class using an iteratable ("nifr" explained the benefits, thanks).
For further reading, there is a closed issue on symfony/symfony#23608
My new solution
I used Argument unpacking and the Delegation pattern to use the class the library provided with my tagged services.
Work :-) Hurray.
final class TaggedMessageDispatcher implements MessageDispatcher {
public function __construct(iterable $consumers)
{
$this->dispatcher = new SynchronousMessageDispatcher(... $consumers);
}
public function dispatch(Message ...$messages): void
{
$this->dispatcher->dispatch(... $messages);
}
}
You're using a wrong typehint here.
With the [!tagged <tag>] syntax a single iterable will be injected - not an undefined number of arguments as expected by the splat operator.
You're actually typehinting for multiple Consumer objects as arguments with the splat (...$arguments) operator here.
So the answer to your question is:
The splat operator is not compatible with the [!tagged ..] syntax.
You'd indeed need to write your own injection type that splits up the tagged services when using a new notation like [!tagged-call_user_func ..].
That said it doesn't really make sense to collect a list of objects, extract them to be function arguments just to let PHP put them back into a list again. I get your idea behind it in terms of code cleanliness though.
Another limitation is the fact that you can't pass multiple variadic arguments to a function. So ...
public function __construct(Alpha ...$alphas, Beta ...$betas)
... is not possible.
A possible solution/workaround allowing you to keep the typehinting for the collection would be the following:
final class SynchronousMessageDispatcher implements MessageDispatcher
{
/**
* #var Consumer[]
*/
private $consumers;
public function __construct(iterable $consumers)
{
foreach($consumers as $consumer) {
assert($consumer instanceof Consumer, \InvalidArgumentException('..'));
}
$this->consumers = $consumers;
}
}
I've been trying some stuff from kotlin.reflection during my project, and got stuck on something what occurs to me as hard to understand, I have declared object as follows:
object WebsiteMapping
{
const val ADMIN = "/admin"
}
once I call:
Arrays
.stream(WebsiteMapping::class.java.declaredFields)
.forEach { field -> println(field.type) }
what I get is:
class java.lang.String
class mapping.WebsiteMapping
When I looked a little bit into what is behind declaredFields invocation I grasped why it works as it is, but is there any convenient way of taking only declared consts within that object without getting also root of the whole structure?
The field with the type class mapping.WebsiteMapping is, basically, not the root of the structure but a special field generated in the object type that holds the reference to the singleton object.
In Kotlin, this field is named INSTANCE by convention. You can therefore filter the fields that you get from the class as follows:
WebsiteMapping::class.java.declaredFields
.filter { it.name != "INSTANCE" }
.forEach { println(it.type) }
Another solution is to switch from java.reflect.* to the Kotlin reflection API kotlin.reflect (needs a dependency on the kotlin-reflect module), which automatically filters the property:
WebsiteMapping::class.memberProperties
.forEach { println(it.returnType) }
I have a big set of classes (like more that 100) and they are all extend from some abstract class, let's call it ParentClass. Let's call child classes ChildA,ChildB, etc. How can I register custom deserializer for all children and get class type inside my Deserializer?
I tried:
module.addDeserializer(ParentClass.class, new MyObjectDeserializer());
but it does not work.
I want to skip doing (what is working):
module.addDeserializer(ChildA.class, new MyObjectDeserializer(ChildA.class));
module.addDeserializer(ChildB.class, new MyObjectDeserializer(ChildB.class));
module.addDeserializer(ChildC.class, new MyObjectDeserializer(ChildC.class));
//etc......
Class type should be known, as I am use Jackson for spring #RequestBody method, what have defined class name there.
Any ideas how this can be done?
As far as I know, I don't think there is a mechanism in jackson that will address your exact needs.
However, there are a couple alternatives you can try.
Deserializing polymorphic types with Jackson describes one such alternative, however, you would still need to explicitly define all of the supported subtypes.
Another alternative that would not require you to explicitly define deserialization relationships would be to change your class hierarchy from one of inheritance to that of a container.
For example, converting your abstract parent class to a container like so:
public class DataContainer<T> {
String commonString;
Integer commonInteger;
T subData;
}
Would allow you to simply define in your controller input function as
public String controllerFunction(DataContainer<ClassA> classA);
without a need to define all these subclass deserializations.
Late to the party but I had a similar problem which I solved by registering a custom Deserializers to my SimpleModule. The code is in Kotlin but it should be easy to port it to Java.
The class itself:
class UseBaseClassSimpleDeserializers(
private val baseClass: Class<*>,
private val baseClassDeserializer: JsonDeserializer<*>
) : SimpleDeserializers() {
#Throws(JsonMappingException::class)
override fun findBeanDeserializer(
type: JavaType?,
config: DeserializationConfig?,
beanDesc: BeanDescription?
): JsonDeserializer<*>? {
val beanDeserializer = super.findBeanDeserializer(type, config, beanDesc)
return if (beanDeserializer == null && baseClass.isAssignableFrom(type!!.rawClass)) {
baseClassDeserializer
} else {
beanDeserializer
}
}
}
How to register the custom Deserializers class to a SimpleModule:
val simpleModule = SimpleModule()
simpleModule.setDeserializers(UseBaseClassSimpleDeserializers(ParentClass::class.java, ParentClassDeserializer()))
I'm testing a private method of a class used in Symfony2 project with PHPUnit.
I'm using the private methods testing strategy (through reflection) described by many developers such as http://aaronsaray.com/blog/2011/08/16/testing-protected-and-private-attributes-and-methods-using-phpunit/
But unfortunately, I got the following error:
There was 1 error: 1) My\CalendarBundle\Tests\Calendar\CalendarTest::testCalculateDaysPreviousMonth
ReflectionException: Class Calendar does not exist /Library/WebServer/Documents/calendar/src/My/CalendarBundle/Tests/Calendar/CalendarTest.php:47
<?php
namespace My\CalendarBundle\Tests\Calendar;
use My\CalendarBundle\Calendar\Calendar;
class CalendarTest
{
//this method works fine
public function testGetNextYear()
{
$this->calendar = new Calendar('12', '2012', $this->get('translator'));
$result = $this->calendar->getNextYear();
$this->assertEquals(2013, $result);
}
public function testCalculateDaysPreviousMonth()
{
$reflectionCalendar = new \ReflectionClass('Calendar'); //this is the line
$method = $reflectionCalendar->getMethod('calculateDaysPreviousMonth');
$method->setAccessible(true);
$this->assertEquals(5, $method->invokeArgs($this->calendar, array()));
}
}
Why?
Thank you in advance
You need to use the whole namespaced class name when creating your reflection method, even if you include a use statement.
new \ReflectionClass('My\CalendarBundle\Calendar\Calendar');
This is because you are passing the class name as a string to the constructor, so it doesn't know about your use statement and is looking for the class name in the global namespace.
Also, for what it's worth, you don't actually need to create a ReflectionClass, then call getMethod() on it. Rather, you can directly create a ReflectionMethod object.
new \ReflectionMethod('My\CalendarBundle\Calendar\Calendar', 'calculateDaysPreviousMonth');
That should be essentially the same, but a bit shorter.