I'm using Jackson 2.1.4 and want to serialize collections of objects that extend an Event class, and where the JSON has to include the class name for polymorphism purposes. I have put a #JsonTypeInfo annotation on Event, but unfortunately I can't use CLASS as strategy, because my object instantiation pattern is "new FooEvent(){{bar=someValue;}}", which will create anonymous inner classes. I want to put in "FooEvent" as the type in JSON, rather than the anonymous inner class.
It seems like what I need is to do this:
#JsonTypeInfo(use= JsonTypeInfo.Id.CUSTOM, include= JsonTypeInfo.As.PROPERTY, property="type")
But how do I register my own custom id serializer that will output the name of the class?
Figured out how to solve this: basically add a #JsonTypeIdResolver annotation on the same class as has #JsonTypeInfo, then implement a resolver which it can refer to. Here's what mine looks like. It essentially just removes anonymous inner class if present.
Related
I need to (de)serialize to and from JSON some of the data classes that are generated by a Gradle plugin. Normally I would just use a library like Moshi or kotlinx.serialization and add the proper annotation to the class I want to serialize but, since this data classes are autogenerated, this is a problem.
I would like to avoid manually to map all the fields of the generated data class to some other class that I can (de)serialize, or to write a custom adapter for all these data class so, I was wondering if there is another way to tell, for example, kotlinx.serialization that a class is #Serializable without having to put the annotation directly on top of the class itself.
Or, alternatively, is there a better way to convert to and from a string an instance of a generated data class?
kotlinx.serialization supports generating serializers for 3rd party classes. We need to use forClass parameter in #Serializer, for example:
data class MyData(val data: String)
#Serializer(forClass = MyData::class)
object MyDataSerializer
fun main() {
val data = MyData("foo")
println(Json.encodeToString(MyDataSerializer, data))
}
You can read more in the official documentation: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#deriving-external-serializer-for-another-kotlin-class-experimental
In JsonDeserialize annotation documentation the contentAs field is supposed to define the "Concrete type to deserialize content".
I tried to use this in combination, with either a Converter (via contentConverter field of the same annotation) or a JsonDeserializer (via contentUsing field of the same annotation), by extending either StdConverter or StdDeserializer, respectively, in an attempt to create an agnostic custom deserializer.
I cannot find a way to access the JsonDeserialize#contentAs information inside any of these two classes.
I am aware that the classes I extend from have a type parameter, I just put an Object class there. Documentation states
contentAs Concrete type to deserialize content (elements of a Collection/array, values of Maps) values as, instead of type otherwise declared. Must be a subtype of declared type; otherwise an exception may be thrown by deserializer.
Apparently I am applying the #JsonDeserializer annotation on a Collection of some persistable Class. I want to deserialize each such object, solely by knowing its id. Well, if I could only get that very type I defined in the #JsonDeserializer#contentAs field...
Can anyone tell me if this is possible anyhow?
I managed to implement the agnostic deserializer withou the use of #JsonDeserializer#contentAs after all.
After reading the javadocs of com.fasterxml.jackson.databind.JsonDeserializer I concluded that my custom deserializer should implement the com.fasterxml.jackson.databind.deser.ContextualDeserializer interface.
Inside the implementation of ContextualDeserializer#createContextual(DeserializationContext ctxt, BeanProperty property)
I could finally get access to the class type of the content of the collection, which I applied the #JsonDeserialize annotation on,
by calling:
ctxt.getContextualType().getRawClass()
NOTE that the same call inside the implementation of com.fasterxml.jackson.databind.JsonDeserializer#deserialize(com.fasterxml.jackson.core.JsonParser, com.fasterxml.jackson.databind.DeserializationContext) returned null, hence the need of the aforementioned interface.
All I had to do then is store the returned class in a member field (of type Class< ? >) of the custom deserializer and use it in the execution of JsonDeserializer#deserialize()
The only thing that remains to check is whether an instance of this custom deserializer is shared between threads. I only did some minor checks; I used the same implementation for two different collections of different types. I observed that ContextualDeserializer#createContextual(DeserializationContext ctxt, BeanProperty property) was called once (among multiple deserialization invokations), for each distinct type that was going to be deserialized. After checking during debugging, it seems that the same deserializer object is used for the same type. In my case, since what I store in the member field is this type itself, I don't mind if the same deserializer is used for the same java type to be deserialized because they should contain the same value. So we 're clear on this aspect as well.
EDIT: It appears all I have to do is update the com.fasterxml.jackson.databind.deser.std.StdDeserializer#_valueClass value to the now known class. Since it is final and since the ContextualDeserializer#createContextual(DeserializationContext ctxt, BeanProperty property) returns a JsonSerializer object, which is actually used,
instead of returning "this" serializer I can create a new one, passing the discovered class in the constructor, which actually sets the StdDeserializer#_valueClass to the class I actually want, and I'm all set!
Finally, NOTE that I didn't have to use the #JsonDeserializer#contentAs annotationfield as I get the value from the ctxt.getContextualType().getRawClass() statement inside ContextualDeserializer#createContextual(DeserializationContext ctxt, BeanProperty property) implementation
I know there are various capabilities in Java with reflection.
For example:
Class<?> clazz = Class.forName("java.util.Date");
Object ins = clazz.newInstance();
I wonder if I could pass class dynamicaly in some method declaration in <> tags (or there is other way to do it if it must be fixed). I would like to change that class declaration dynamicaly; because I would like to write generic method for all types of classes.
In there, I have this:
List<Country>
Can I write it something diffrent with reflection? For example can it be somehow be achieved to pass class as parameter (or how else should be this done):
List<ins>
? I would appreciate examples.
This cannot be done because generics are a compile time feature. Once code is compiled, the only place where generics are exists are at method signatures, and they are only used for compiling new code.
When working with reflection, you are basicly working with raw types, and need to code according to that, that means, you can cast the returned result of newInstance() to the list type your need, for example:
List<Country> ins = (List<Country>)clazz.newInstance();
This is a safe operation to do, because you know at that point its empty, and isn't passed to any outside code.
I don't think this is possible. Generics in Java are implemented in a way that prohibits runtime access.
Generics are there so that the compiler can verify correct typing, but are no longer present at runtime (this is called "type erasure"). Reflection deals with the runtime representation of types only. As far as I know the only case where reflection has to deal with generics is to find out "fixed" type parameters of sub-classes, e.g. when you have class Bar<T> and class Foo extends Bar<String>, you can find out that the T of Bar is fixed to String in Foo using reflection. However, this is information found in the class file, too. Except that, reflection can only see or create raw-types.
I have a program ZPROG1_TEST where I define a local class LCL_PROG1_HELPER.
I have a second program ZPROG2_TEST where I'd like to define a variable reference to this class.
Isn't there a syntactic possibility for me to do this?
Or could this be in theory doable with the RTTI classes like CL_ABAP_CLASSDESCR ?
EXTRA
Why I'd like to do this is because I have a custom form ZMM_MEDRUCK that needs to know if the ME32N Document it's printing has been changed but not saved.
I've figures out the exact objects whose properties I need to interogate, but some of them are defined at design time as common interfaces, like IF_SERIALIZABLE_MM, and I need to cast them to the local classes whose instances I know these objects are going to be, like \FUNCTION-POOL=MEGUI\CLASS=LCL_APPLICATION.
I could of course try a dynamic method call and not care about anything, but since i'm here i thought i'd ask this thing first.
You could do it like that.
REPORT ZPROG1_TEST.
INTERFACE lif_prog1_helper.
METHODS:
test.
ENDINTERFACE.
CLASS LCL_PROG1_HELPER DEFINITION.
PUBLIC SECTION.
INTERFACES:
lif_prog1_helper.
ALIASES:
test FOR lif_prog1_helper~test.
ENDCLASS.
CLASS LCL_PROG1_HELPER IMPLEMENTATION.
METHOD test.
WRITE / sy-repid.
ENDMETHOD.
ENDCLASS.
REPORT ZPROG2_TEST.
DATA: g_test TYPE REF TO object.
START-OF-SELECTION.
CREATE OBJECT g_test TYPE ('\PROGRAM=ZPROG1_TEST\CLASS=LCL_PROG1_HELPER').
CALL METHOD g_test->('TEST').
CALL METHOD g_test->('LIF_PROG1_HELPER~TEST').
As far as I know, this is not possible. Accessing the local class dynamically is easy (well, relatively easy), but referring to it statically - not as far as I know. You'll probably have to call the methods dynamically.
I have been seeing some odd behaviour in an entity that I have for which I created a partial class to override the ToSting Method and provide some basic property setting when a new instance of that entity is created (for example I might set an order date to 'Now') in a constructor.
This odd behaviour led me to look closely at the partial class and I was surprised to see that even when a set of pre existing records was being retrieved the constructor was being called for each retrieved record.
below is a very simple example of what I might have:
Partial Public Class Product
Public Sub New()
CostPrice = 0.0
ListPrice = 0.0
End Sub
Public Overrides Function ToString() As String
Return ProductDescription
End Function
End Class
I have two questions that arise from this:
1) is this normal behaviour in the Entity Framework if you add a partial class to which you add a constructor?
2) if not then I must assume that I have done something wrong, so what would be the correct way to
override the constructor to do things similar to the example I mentioned above?
Thanks for any insights that you can give me.
This is using EF 5.0 in a vb project
think to the sequence of events leading to the retrieval of an entity from the database. Basically it should be something like:
query the database
for each row of the query result give an entity
The giving is then as follow for each retrieved row:
create a new instance of the retrieved entity
populate this new instance with the value of the row
Well with each instance creation, the constructor is called.
I think you are mixing:
instance initialization where you "allocate" the object, and
business initialization where you enforce business logic
both may be done, at least partially, in the constructor.
new is always called when a class is first instantiated and if you do not explicitly declare a constructor then a default constructor will be created by the compiler.
Unless the class is static, classes without constructors are given a public default constructor by the C# compiler in order to enable class instantiation.
When defining POCO classes for Entity Framework the class must have a default constructor and EF will always call this default constructor whether you have explicitly defined it or the compiler did it for you.
If for any reason you have need to pass anything into the class when it is instantiated you can use the ObjectContext.ObjectMaterialized event.