..note:: This is clearly a contrived example, simply making the constructor public would alleviate the issues.
However, for the purposes of this example we are assuming that for external reasons this cannot be done.
To be serializable by Corda this would require a custom serializer to be written that can transform the unserializable
class into a form we can serialize. Continuing the above example, this could be written as follows:
..container:: codeset
..sourcecode:: java
/**
* The class lacks a public constructor that takes parameters it can associate
* with its properties and is thus not serializable by the CORDA serialization
* framework.
*/
class Example {
private int a;
private int b;
public int getA() { return a; }
public int getB() { return b; }
public Example(List<int> l) {
this.a = l.get(0);
this.b = l.get(1);
}
}
/**
* This is the class that will Proxy instances of Example within the serializer
*/
public class ExampleProxy {
/**
* These properties will be serialized into the byte stream, this is where we choose how to
* represent instances of the object we're proxying. In this example, which is somewhat
* contrived, this choice is obvious. In your own classes / 3rd party libraries, however, this
* may require more thought.
*/
private int proxiedA;
private int proxiedB;
/**
* The proxy class itself must be serializable by the framework, it must thus have a constructor that
* can be mapped to the properties of the class via getter methods.
*/
public int getProxiedA() { return proxiedA; }
public int getProxiedB() { return proxiedB; }
public ExampleProxy(int proxiedA, int proxiedB) {
this.proxiedA = proxiedA;
this.proxiedB = proxiedB;
}
}
/**
* Finally this is the custom serializer that will automatically loaded into the serialization
* framework when the CorDapp Jar is scanned at runtime.
*/
public class ExampleSerializer implements SerializationCustomSerializer<Example, ExampleProxy> {
/**
* Given an instance of the Example class, create an instance of the proxying object ExampleProxy.
*
* Essentially convert Example -> ExampleProxy
*/
public ExampleProxy toProxy(Example obj) {
return new ExampleProxy(obj.getA(), obj.getB());
}
/**
* Conversely, given an instance of the proxy object, revert that back to an instance of the
* type being proxied.
*
* Essentially convert ExampleProxy -> Example
*/
public Example fromProxy(ExampleProxy proxy) {
List<int> l = new ArrayList<int>(2);
l.add(proxy.getProxiedA());
l.add(proxy.getProxiedB());
return new Example(l);
}
}
..sourcecode:: kotlin
class ExampleSerializer : SerializationCustomSerializer<Example, ExampleSerializer.Proxy> {
/**
* This is the actual proxy class that is used as an intermediate representation
* of the Example class
*/
data class Proxy(val a: Int, val b: Int)
/**
* This method should be able to take an instance of the type being proxied and
* transpose it into that form, instantiating an instance of the Proxy object (it
* is this class instance that will be serialized into the byte stream.
*/
override fun toProxy(obj: Example) = Proxy(obj.a, obj.b)
/**
* This method is used during deserialization. The bytes will have been read
* from the serialized blob and an instance of the Proxy class returned, we must
* now be able to transform that back into an instance of our original class.
*
* In our example this requires us to evoke the static "of" method on the
* Example class, transforming the serialized properties of the Proxy instance
* into a form expected by the construction method of Example.
*/
override fun fromProxy(proxy: Proxy) : Example {
val constructorArg = IntArray(2);
constructorArg[0] = proxy.a
constructorArg[1] = proxy.b
return Example.of(constructorArg)
}
}
In the above examples
-``ExampleSerializer`` is the actual serializer that will be loaded by the framework to serialize instances of the ``Example`` type.
-``ExampleSerializer.Proxy``, in the Kotlin example, and ``ExampleProxy`` in the Java example, is the intermediate representation used by the framework to represent instances of ``Example`` within the wire format.
The Proxy Object
----------------
The proxy object should be thought of as an intermediate representation that the serialization framework
can reason about. One is being written for a class because, for some reason, that class cannot be
introspected successfully but that framework. It is therefore important to note that the proxy class must
only contain elements that the framework can reason about.
The proxy class itself is distinct from the proxy serializer. The serializer must refer to the unserializable
type in the ``toProxy`` and ``fromProxy`` methods.
For example, the first thought a developer may have when implementing a proxy class is to simply *wrap* an
instance of the object being proxied. This is shown below
