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Editorial Team
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Asked: 2026-05-14T00:54:22+00:00

The opposite of this question: How do I add a type to GWT's Serialization

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The opposite of this question: How do I add a type to GWT's Serialization Policy whitelist?

GWT is adding undesired types to the serialization policy and bloating my JS. How do I trim my GWT whitelist by hand? Or should I at all?

For example, if I put the interface List on a GWT RPC service class, GWT has to generate Javascript that handles ArrayList, LinkedList, Stack, Vector, … even though my team knows we’re only ever going to return an ArrayList. I could just make the method’s return type ArrayList, but I like relying on an interface rather than a specific implementation. After all, maybe one day we will switch it up and return e.g. a LinkedList. In that case, I’d like to force the GWT serialization policy to compile for only ArrayList and LinkedList. No Stacks or Vectors.

These implicit restrictions have one huge downside I can think of: a new member of the team starts returning Vectors, which will be a runtime error. So besides the question in the title, what is your experience designing around this?

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1 Answer

  1. Editorial Team

    There is a property that can do this class blacklisting. For example, to blacklist non-ArrayList Collections, add these lines to your *.gwt.xml:

    <extend-configuration-property name="rpc.blacklist" value="java.util.HashSet"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.LinkedHashSet"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.LinkedList"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.Stack"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.TreeMap"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.TreeSet"/>
<extend-configuration-property name="rpc.blacklist" value="java.util.Vector"/>

    This was necessary for me to reduce JS size when sending GWT’s built-in com.google.gwt.user.client.ui.SuggestOracle$Response objects over the wire. Such objects contain a java.util.Collection, but I knew I would only ever be sending back an ArrayList.

    I still respect the advantages of compile-time checks as discussed in the other responses, comments, and my original question. Indeed, this is a flaky solution if GWT starts picking up additional implementations to serialize (why is this not a whitelist?). However this “rpc.blacklist” property saved me from rolling my own SuggestOracle just to get a more specific collection type.

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