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

Question

GWT\'s serializer has limited java.io.Serializable support, but for security reasons there is a whitelist of types it supports. The documentation I\'ve found,

Answer 1

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.

Answer 2

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;
}

Answer 3

I found that just putting it in the client package or using it in a dummy service interface was not sufficient as it seemed the system optimized it away.

I found it easiest to create a class that derived from one of the types already used in the service interface and stick it in the client package. Nothing else needed.

public class GWTSerializableTypes extends SomeTypeInServiceInterface implements IsSerializable {
    Long l;
    Double d;
    private GWTSerializableTypes() {}
}