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

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, for example this FAQ entry says that any types you want to serialize "must be included in the serialization policy whitelist", and that the list is generated at compile time, but doesn't explain how the compiler decides what goes on the whitelist.

The generated list contains a number of types that are part of the standard library, such as java.lang.String and java.util.HashMap. I get an error when trying to serialize java.sql.Date, which implements the Serializable interface, but is not on the whitelist. How can I add this type to the list?

Solution 1:

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;
}

Solution 2:

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.