ObjectMapper.readValue fails for TreeSet for byte[] (and other types that are not Comparable)

[webbean]

We use jackson-databind-2.2.2 to serialize and deserialize NavigableSet<byte[]>.
Serializing works fine, but deserializing throws java.lang.ClassCastException:

Exception in thread "main" java.lang.ClassCastException: [B cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(TreeMap.java:1188)
    at java.util.TreeMap.put(TreeMap.java:531)
    at java.util.TreeSet.add(TreeSet.java:255)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:234)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:207)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:23)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2120)

Below is the code:

public static void main(String[] args) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    NavigableSet<byte[]> set = new TreeSet<>(Bytes.BYTES_COMPARATOR);
    set.add(Bytes.toBytes("ROW1"));
    set.add(Bytes.toBytes("ROW2"));
    set.add(Bytes.toBytes("ROW3"));
    byte[] array = mapper.writeValueAsBytes(set);
    NavigableSet<byte[]> result = mapper.readValue(array,
            new TypeReference<NavigableSet<byte[]>>() {
            });
    for (byte[] value : result) {
        System.out.println(Bytes.toString(value));
    }
}

It seems that we need provide a Comparator parameter(such as Bytes.BYTES_COMPARATOR) when we create a new TreeSet object, but how can we do that in TypeReference when deserializing by mapper.readValue method.

[cowtowncoder]

Interesting.

One work-around that might work (but one I have not tried) would be to use 'updating reader':

NavigableSet<byte[]> result = ...;
mapper.readerForUpdating(result).read(array);

in which case existing map will be used.

I will see if I can find a fix for the underlying issue.

[webbean]

Thank you very much.
We solved this problem by first deserializing it to a Set, and then converting it to NavigableSet, like below:

public static void main(String[] args) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    NavigableSet<byte[]> set = new TreeSet<>(Bytes.BYTES_COMPARATOR);
    set.add(Bytes.toBytes("ROW1"));
    set.add(Bytes.toBytes("ROW2"));
    set.add(Bytes.toBytes("ROW3"));
    byte[] array = mapper.writeValueAsBytes(set);
    Set<byte[]> result = mapper.readValue(array,
            new TypeReference<Set<byte[]>>() {
            });

    NavigableSet<byte[]> temp = new TreeSet<>(Bytes.BYTES_COMPARATOR);
    temp.addAll(result);

    for (byte[] value : temp) {
        System.out.println(Bytes.toString(value));
    }
}

But the following code would be the best we think (if it can work).

    NavigableSet<byte[]> result = mapper.readValue(array,
            new TypeReference<TreeSet<byte[]>>() {
            });

[cowtowncoder]

Oh. There is actually something that should help you solve your problem. Just create a sub-class like:

 MyByteArraySet extends TreeSet<byte[]> {
     public MyByteArraySet() { super(Bytes.BYTES_COMPARATOR); }
 }

and then either use this type instead of TreeSet<byte[]>. This would allow proper deserialization.
It's not a particularly elegant solution, but would make things work.

[webbean]

Thanks, we have changed the solution to the one you provided, it's more effective than ours.

[cowtowncoder]

Ok good that it works. I hope that eventually there could be better solutions; I will edit title of this entry a bit, but leave it open.

[webbean]

hi, cowtowncoder
Unfortunately, we put the MyByteArraySet sub-class into our test envionment, but the following exception thrown.

Exception in thread "main" java.lang.IllegalStateException: No default constructor for [collection type; class Test$MyByteArraySet, contains [array type, component type: [simple type, class byte]]]
    at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createUsingDefault(StdValueInstantiator.java:221)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:207)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:23)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2120

The demo source is below:

import java.util.NavigableSet;
import java.util.TreeSet;

import org.apache.hadoop.hbase.util.Bytes;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Test {

class MyByteArraySet extends TreeSet<byte[]> {

    public MyByteArraySet() {
        super(Bytes.BYTES_COMPARATOR);
    }
}

public static void main(String[] args) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    NavigableSet<byte[]> set = new TreeSet<>(Bytes.BYTES_COMPARATOR);
    set.add(Bytes.toBytes("ROW1"));
    set.add(Bytes.toBytes("ROW2"));
    set.add(Bytes.toBytes("ROW3"));
    byte[] array = mapper.writeValueAsBytes(set);
    NavigableSet<byte[]> result = mapper.readValue(array,
            new TypeReference<MyByteArraySet>() {
            });

    for (byte[] value : result) {
        System.out.println(Bytes.toString(value));
    }
}
}

Any idea about this?

[cowtowncoder]

Oh that's simple. Your inner class must have static qualifier: otherwise it can not be constructed without implicit "this" argument. So either create it as main-level class (no static used), or add static in there.

[kdkeck]

Related issue: #509.

[cowtowncoder]

Actually I think that #1399 is a solution for this: this does require setting of initial value (and either global default for merging, or per-property annotation), but should work fine.
Will close as I do not have a better general-purpose solution in mind; custom deserializer would be next logical choice.