GWT Serialize Ordering/Comparator of TreeMap

Question

I have a serializable object with a TreeMap.

Map<String, Dogs> dogsByNames = Maps.newTreeMap(); // guava style

So far, everything is fine. Now it's nessassary to ignore the case of the keys:

Map<String, Dogs> dogsByNames = Maps.newTreeMap(String.CASE_INSENSITIVE_ORDER);

This do not work because String.CASE_INSENSITIVE_ORDER isn't serialzable:

com.google.gwt.user.client.rpc.SerializationException: Type 'java.lang.String$CaseInsensitiveComparator' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = java.lang.String$CaseInsensitiveComparator@f26f68

So I create my own one:

private static abstract class MyComparator<T>
    implements
        Comparator<T>, // extends Ordering<T> do not work
        IsSerializable { // Serializable do not work
}

public static final MyComparator<String> CASE_INSENSITIVE_ORDER = new MyComparator<String>() {
    @Override
    public int compare(final String left, final String right) {
        if (null == left) {
            return (null == right) ? 0 : -1;
        } else if (null == right) {
            return 1;
        }
        return left.compareToIgnoreCase(right);
    }
};

But this still do not work.

Whats wrong?

Solution:

protected static class MyOrdering // no-private!!!
    extends
        Ordering<String>
    implements
        IsSerializable {
    @Override
    public int compare(final String left, final String right) {
        if (null == left) {
            return (null == right) ? 0 : -1;
        } else if (null == right) {
            return 1;
        }
        return left.compareToIgnoreCase(right);
    }
}

public static final Ordering<String> CASE_INSENSITIVE_ORDER = new MyOrdering();

Accoring to docs `CASE_INSENSITIVE_ORDER` **is** serializable: _A Comparator that orders String objects as by compareToIgnoreCase. This comparator is serializable._ What error do you get then? — Grzegorz Rożniecki, Apr 12 '12 at 17:40

score 5 · Accepted Answer · answered Apr 13 '12 at 00:44

5

The CASE_INSENSITIVE_ORDER you created is an anonymous class, which cannot be serialized in GWT. To make it GWT serializable, you need to

Create a custom top-level class (like

CaseInsensitiveOrder implements Comparator<..>, Serializable {
    ...
}

Create a constant CASE_INSENSITIVE_ORDER like you did.
Create a custom field serializer for that class (CaseInsensitiveOrder_CustomFieldSerializer), which returns the CASE_INSENSITIVE_ORDER instance on deserialize().

answered Apr 13 '12 at 00:44

Hayward Chan

166
2

This was the right hint. After implementing the comparator as a nested class. I needed to make it non-private. So your 3 is not really necessary. – Marcel Jaeschke Apr 13 '12 at 08:09
3 is necessary to make sure the deserialized order is the same instance as the original. If you can live with multiple instances of MyOrdering around, then it can be skipped. – Hayward Chan Apr 14 '12 at 00:45

score 0 · Answer 2 · edited May 23 '17 at 11:44

A stacktrace of the error would be helpful. But I'll hazard the guess that you get an exception like:

com.google.gwt.user.client.rpc.SerializationException: Type 'com.foo.Bar.MyComparator' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized

This happens because GWT creates a list of all the types it might need to serialize, at compile-time. This is the "serialization policy". To do so, GWT inspects all your RPC methods for arguments and return types, and searches for all the "reachable" types.

The compiler might not realize that your Comparator will be serialized, and it is not added to the serialization policy.

A workaround is to declare a dummy method, which takes as parameter all the types that might need to be serialized, as described in this other SO answer. Something like:

// in your RPC interface
SerializationWhitelist dummyMethodForSerializationPolicy(SerializationWhitelist serializationWhiteList);

// the SerializableWhiteList class
public class SerializationWhitelist {
    public Bar.MyComparator<String> myComparator;
}

GWT Serialize Ordering/Comparator of TreeMap

2 Answers2