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, 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

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.

Solution 2

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.

Solution 3

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.

-- Andrej

This is probably the easiest solution. The only thing to remember with this is that all the classes that you want to serialize should have "public, no-argument" constructor, and (depending upon requirements) setter methods for the member fields.