Serializing+deserializing lists breaks their hash


clojure.lang.PersistentList implements Serializable but a serialization roundtrip results in a hash of 0. This appears to be caused by ASeq marking its _hash property as transient; other collection base classes don't do this. I don't know if there is a rationale for the distinction, but the result is that serializing, then deserializing, a list results in its _hash property being 0 instead of either its previous, calculated value, or -1, which would trigger recalculation.

This means that any code that relies on a list's hash value can break in subtle ways.


1 2 3 4 5 6 7 8 9 10 11 (import '[ ByteArrayInputStream ByteArrayOutputStream ObjectInputStream ObjectOutputStream]) (defn obj->bytes [obj] (let [bytestream (ByteArrayOutputStream.)] (doto (ObjectOutputStream. bytestream) (.writeObject obj)) (.toByteArray bytestream))) (defn bytes->obj [bs] (.readObject (ObjectInputStream. (ByteArrayInputStream. bs)))) (defn round-trip [x] (bytes->obj (obj->bytes x)))
1 2 3 4 5 6 7 8 9 10 11 12 user=> (hash '(1)) -1381383523 user=> (hash (round-trip '(1))) 0 user=> #{'(1) (round-trip '(1))} #{(1) (1)} user=> (def m {'(1) 1}) #'user/m user=> (= m (round-trip m)) true user=> (= (round-trip m) m) false

Approach: Use 0 as the "uncomputed" hash value, not -1. The auto initialized value on serialization construction will then automatically trigger a re-computation.

Alternate approaches:

  • Implement a readObject method that sets the _hash property to -1. This is somewhat complicated in the face of subclasses, etc.

Note: Also need to consider other classes that use -1 instead of 0 as the uncomputed hash value: APersistentMap, APersistentSet, APersistentVector, PersistentQueue - however I believe in those cases they are not transient and thus may avoid the issue. Perhaps they should be transient though.

Patch: clj-1766-2.patch
Screened by: Alex Miller


Tested on OS X 10.10.3, Clojure 1.6.0, Java 1.8.0_45-b14, but don't think this is a factor.





Christopher Vermilion





Code and Test

Fix versions

Affects versions

Release 1.7
Release 1.6