How do I add a type to GWT's Serialization Policy whitelist?

Question

There's a workaround: define a new Dummy class with member fields of all the types that you want to be included in serialization. Then add a method to your RPC interface:

Dummy dummy(Dummy d);

The implementation is just this:

Dummy dummy(Dummy d) { return d; }

And the async interface will have this:

void dummy(Dummy d, AsyncCallback< Dummy> callback);

The GWT compiler will pick this up, and because the Dummy class references those types, it will include them in the white list.

Example Dummy class:

public class Dummy implements IsSerializable {
    private java.sql.Date d;
}

Any specific types that you include in your service interface and any types that they reference will be automatically whitelisted, as long as they implement java.io.Serializable, eg:

public String getStringForDates(ArrayList<java.util.Date> dates);

Will result in ArrayList and Date both being included on the whitelist.

It gets trickier if you try and use java.lang.Object instead of specific types:

public Object getObjectForString(String str);

Because the compiler doesn't know what to whitelist. In that case if the objects are not referenced anywhere in your service interface, you have to mark them explicitly with the IsSerializable interface, otherwise it won't let you pass them through the RPC mechanism.

The whitelist is generated by the GWT compiler and contains all the entries that are designated by the IsSerializable marker interface.

To add a type to the list you just need to make sure that the class implements the IsSerializable interface.

Additionally for serialization to work correctly the class must have a default no arg constructor (constructor can be private if needed). Also if the class is an inner it must be marked as static.

IMHO the simpliest way to access whitelist programmatically is to create a class similar to this:

public class SerializableWhitelist implements IsSerializable {
    String[] dummy1;
    SomeOtherThingsIWishToSerialize dummy2;
}

Then include it in the .client package and reference from the RPC service (so it gets analyzed by the compiler).

I couldn't find a better way to enable tranfer of unparameterized maps, which is obviously what you sometimes need in order to create more generic services...