We Keep Coding

Problem with Deserialization of Generic Classes

I came across a problem with Generics and Jackson recently and ended up with not using it.
I have an interface MonetaryType:
public interface MonetaryType implements Serializable {}
which is implemented by multiple enums like:
public enum IncomeType implements MonetaryType {
FULL_TIME_SALARY,
PART_TIME_SALARY,
CHILD_BENEFIT
}
public enum ExpenseType implements MonetaryType {
HEAT,
CONDO_FEES,
ELECTRICITY
}
I created a Generic Class:
public MonetaryValidation<T extends MonetaryType> {
private T monetaryType;
private boolean isPassed;
private String message;
// Getters and Setters
}
This object is not deserialized by Jackson library. Meaning that if any Spring REST endpoints are called while the payload contains MonetaryValidation object, it throws below exception:
java.lang.IllegalArgumentException: Cannot construct instance of
**.enumeration.MonetaryType (no Creators, like default construct,
exist): abstract types either need to be mapped to concrete types,
have custom deserializer, or contain additional type information
I do not want to solve the issue with Jackson polymorphic deserialization approach since it requires the client to pass an extra flag specifying the concrete implementation of the interface or abstract class, as far as I understood.
Unfortunately I ended up creating multiple sub classes of non-generic MonetaryValidation (one subclass per each MonetaryType subclass), which I know it is not a decent solution.
It is much appreciated if you could help me out to understand where the problem is and whether there is an approach to use #JsonSubTypes while passing an extra field is not needed.

There is an idea, try accept monetaryType as the String type parameter, and you can custom converter in the Generic class for handling the generic type field, such as:
public void setMonetaryType(String monetaryType) {
Optional<IncomeType> incomeType = Arrays.stream(IncomeType.values()).filter(i -> i.name().equals(monetaryType)).findFirst();
incomeType.ifPresent(i -> {
this.monetaryType = (T)i;
});
Optional<ExpenseType> expenseType = Arrays.stream(ExpenseType.values()).filter(i -> i.name().equals(monetaryType)).findFirst();
expenseType.ifPresent(i -> {
this.monetaryType = (T)i;
});
}
I think this is a simply way to achieve other than using JsonSubTypes or custom Converters, since it's really a generic parameter.

Separate jackson feature from classes

I am using jackson to handle JSON (de)-serialization. I have a bunch of classes which are annotated with are essentially objects holding properties and associated getters and setters.
However, I often find that at some point I want to add additional properties which I don't want to include in the (de)-serialization process. This does work using #JsonIgnore, but it strikes me as relatively ugly since I have to add the annotation everywhere and things break down as soon as I forget.
I would like to know if there is a better way to separate the ignored and serialized properties. I have the following two ideas:
Use inheritance, add the new properties to the inherited class:
// everything here should be (de)-serialized
class Base {
public int getJSONProperty() {...}
}
// nothing specific to the class Derived should be (de)-serialized
class Derived extends Base {
// *not* to be included
public SomeClass getAdditionalProperty() {...}
}
However, I don't know how to tell jackson to deserialize the Derived
objects as Bases. Is this possible (Does jackson make guarantees
regarding (non)-polymorphic serialization of classes)?
Use MixIn annotations. This would require an additional abstract
class for each existing class. Also I am not sure whether this solves
the problem. Are getters which do not appear in the MixIn base class
ignored automatically or do I need to #JsonIgnore them manually?

I've seen that you don't like the previous solution i've provided, so I'm again here to provide another way to do what you want using Gson Library. I hope to help you this time.
This is The Base Class that you want to serialize
public class Base {
public int getJSONProperty() {
return jsonProperty;
}
private int jsonProperty = 2;
}
This is The Derived Class that you don't want to serialize
public class Derived extends Base{
public String getAdditionalProperty(){
return additionalProperty;
}
private String additionalProperty = "value-not-to-serialize";
}
Using Type type = new TypeToken<Base>(){}.getType(); you can define the class to use for serialization so you get the JSON String using:
Derived derived = new Derived();
Gson gson = new Gson();
Type type = new TypeToken<Base>(){}.getType();
String jsonString = gson.toJson(derived, type);

You know Gson? it's a good library to handle JSON.
You can use transient keyword to define variable thats not must be serialized, this works with Gson (It should work well with jackson, but i'm not sure)...
class Base {
// (de)-serialized
private int jsonProperty;
// not (de)-serialized
private transient SomeClass additionalProperty;
}
I think the best approach is to add annotation or use the transient variable.
Create the inheritance only for the purpose of serialization an object complicates the application uselessly in my point of view...

Prototyping in Java instead of extending

Is Javascript-like prototyping anyhow achievable, even using Reflection? Can I wrap my object inside another one, just to extend its functionality with one or two more methods, without wiring all its original nonprivate methods to the wrapper class, or extends is all I get?

If you are looking for extension methods, you could try Xtend. Xtend is language that compiles to java code and eliminates boilerplate code.
The following text is stolen from the Xtend Docs for extensions:
By adding the extension keyword to a field, a local variable or a parameter declaration, its instance methods become extension methods.
Imagine you want to have some layer specific functionality on a class Person. Let us say you are in a servlet-like class and want to persist a Person using some persistence mechanism. Let us assume Person implements a common interface Entity. You could have the following interface
interface EntityPersistence {
public save(Entity e);
public update(Entity e);
public delete(Entity e);
}
And if you have obtained an instance of that type (through a factory or dependency injection or what ever) like this:
class MyServlet {
extension EntityPersistence ep = Factory.get(typeof(EntityPersistence))
...
}
You are able to save, update and delete any entity like this:
val Person person = ...
person.save // calls ep.save(person)
person.name = 'Horst'
person.update // calls ep.update(person)
person.delete // calls ep.delete(person)

I don't think you can do this in Java. You can though in Groovy, using metaclasses
String.metaClass.world = {
return delegate + " world!"
}
println "Hello".world()

How do I use Local Variable Annotations for Wicket Authorization?

I'm rolling my own IAuthorizationStrategy for Wicket 1.5.x I've setup type annotation for pages to use with isInstantiationAuthorized(). It works well and I'd like to use annotations for isActionAuthorized() as well. Ideally I'd like to be able annotate local variables and then check the annotations in my AuthStrategy. From what I've read Local variable Annotation doesn't work that way.
Is there any kind of known work around, maybe some sort of Compile time annotation processing to turn an annotated local variable into an "anonymous" subclass with the annotation as a type annotation?
For the record, the annotation I'm trying to use looks like this:
#Retention(RetentionPolicy.Runtime)
#Target(ElementType.Type, ElementType.LOCAL_VARIABLE)
public #interface AdminOnly
{
int isVisible() default 0;
int isEnabled() default 1;
}
UPDATE
So based on #Xavi López'es answer what I was hoping to do isn't exactly possible.
Annotated LocalVariables should be available at compile time though. Is there some way maybe I could use them as a shortcut for boiler-plating the meta-data code examples that are available in Wicket Examples or the excellent Apache Wicket Cookbook?

I've struggled with a similar issue some time ago with Wicket 1.3.x, and didn't find any way to achieve this with annotations. Annotations on local variables can't be retained at run-time, as explained in the JLS (9.6.3.2. #Retention):
An annotation on a local variable declaration is never retained in the binary representation.
In this related question: How can I create an annotation processor that processes a Local Variable? they talked about LAPT-javac, a patched javac version to allow this. On their site there's a link to the Type Annotations Specification (JSR 308), which will hopefully address this subject (JDK 8 ?).
I ended up defining a plain old interface with a related functionality code:
public interface RestrictedComponent {
Integer getFunction();
}
The main problem with this approach is that it's not possible to make instant anonymous subclasses of a specific class implement other interfaces (such as Component c = new TextField() implements AdminOnly { }) , but you can always define Component extensions that just implement RestrictedComponent in a class:
public abstract class RestrictedTextField extends TextField implements RestrictedComponent { }
Finally, I ended up implementing a RestrictedContainer that just subclassed WebMarkupContainer and put every secured component inside one, modelling it with a <wicket:container> in the markup.
public class RestrictedContainer extends WebMarkupContainer implements RestrictedComponent {
private final Integer function;
public RestrictedContainer(String id, IModel model, final Integer function) {
super(id, model);
this.function = function;
}
public RestrictedContainer(String id, final Integer funcionalitat) {
super(id);
this.function = function;
}
public Integer getFunction() {
return function;
}
}
And then in the Authorization Strategy checked for component instanceof RestrictedComponent and returned true or false depending on user permissions on the associated function.

Force Singleton Pattern on a Class implementing an Interface

I better explain the question with an example.
I have an Interface Model which can be used to access data.
There can be different implementations of Model which can represent the data in various format say XMl , txt format etc. Model is not concerned with the formats.
Lets say one such implementation is myxmlModel.
Now i want to force myxmlModel and every other implementation of Model to follow Singleton Pattern.The usual way is to make myxmlModels constructor private and provide a static factory method to return an instance of myModel class.But the problem is interface cannot have static method definitions and a result i cannot enforce a particular Factory method definition on all implementation of Model. So one implementation may end with providing getObject() and other may have getNewModel()..
One work around is to allow package access to myxmlModel's constructor and create a Factory class which creates the myxmlModel object and cache it for further use.
I was wondering if there is a better way to achieve the same functionality .

Make a factory that returns
instances of your interface, Model.
Make all concrete implementations of the model package-private classes
in the same package as your factory.
If your model is to be a singleton, and you are using java
5+, use enum instead of traditional
singleton, as it is safer.
public enum MyXMLModel{
INSTANCE();
//rest of class
};
EDIT:
Another possibility is to create delegate classes that do all the work and then use an enum to provide all of the Model Options.
for instance:
class MyXMLModelDelegate implements Model {
public void foo() { /*does foo*/}
...
}
class MyJSONModelDelegate implements Model {
public void foo() { /*does foo*/ }
...
}
public enum Models {
XML(new MyXMLModelDelgate()),
JSON(new MyJSONModelDelegate());
private Model delegate;
public Models(Model delegate) { this.delegate=delegate; }
public void foo() { delegate.foo(); }
}

You can use reflection. Something like this:
public interface Model {
class Singleton {
public static Model instance(Class<? extends Model> modelClass) {
try {
return (Model)modelClass.getField("instance").get(null);
} catch (blah-blah) {
blah-blah
}
}
}
public class XmlModel implements Model {
private static final Model instance = new XmlModel();
private XmlModel() {
}
}
usage:
Model.Singleton.instance(XmlModel.class)
Actually, I don't like this code much :). First, it uses reflection - very slow, second - there are possibilities of runtime errors in case of wrong definitions of classes.

Can you refactor the interface to be an abstract class? This will allow you to force a particular factory method down to all implementing classes.

I used to ask myself the same question. And I proposed the same answer ;-)
Now I normally drop the "forcing" behavior, I rely on documentation.
I found no case where the Singleton aspect was so compelling that it needed to be enforced by all means.
It is just a "best-practice" for the project.
I usually use Spring to instanciate such an object,
and it is the Spring configuration that makes it a Singleton.
Safe, and so easy ... plus additionnal Spring advantages (such as Proxying, substituing a different object once to make some tests etc...)

This is more an answer to your comment/clarification to kts's answer. Is it so, that the real problem is not using the Singleton pattern but instead defining an eclipse (equinox) extension point schema that allows contributing a singleton?
I think, this can't be done, because everytime you call IConfigurationElement.createExecutableExtension you create a new instance. This is quite incompatible with your singleton requirement. And therefore you need the public default constructor so that everybody can create instances.
Unless you can change the extension point definition so that plugins contribute a ModelFactory rather than a model, like
public interface ModelFactory {
public Model getModelInstance();
}
So the extension user will instantiate a ModelFactory and use it to obtain the singleton.
If I guessed wrong, leave a comment and I delete the answer ;)