使用Gson与比较器序列化设置

Question

I need to serialize a class that contains a TreeSet for which it has a custom comparator (byte[] not being comparable).

When I attempt to deserialize the class with Gson (v2.3.1) I get:

Exception in thread "main" java.lang.ClassCastException: [B cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(TreeMap.java:1290)
    at java.util.TreeMap.put(TreeMap.java:538)
    at java.util.TreeSet.add(TreeSet.java:255)....

So the TreeSet is not deserialized using the comparator.

The class looks like this:

public class In {

    private final SortedSet<byte[]> keys = new TreeSet<>(new ByteKeyComparator());

}

Is there a way to make Gson understand that it needs a comparator for this set?

Answer 1

You're right, it's a serialization bug. However, it has nothing to do with Gson, and actually your custom comparator is being retained.

The problem lies in the implementation of TreeMap.put(K,V) combined with the deserialization behavior of an empty TreeSet, which results in the backing map having a null root . If the TreeMap's root node is null and you put a key/value pair, it ignores the configured comparator.

To solve your problem, skip serializing empty TreeSet s.