EnumMap serialization ignores SerializationFeature.WRITE_ENUMS_USING_TO_STRING

[scubasau]

Discovered this when upgrading from Jackson 2.4.1 to 2.5.2. Starting in 2.4.4, EnumMapSerializer is deprecated in favor of MapSerializer. However MapSerializer modifies the expected behavior of Enum serialization by always using toString().

Failing unit tests added here:
scubasau@8e2c4cd

[cowtowncoder]

Ok. Thank you for reporting this.

[cowtowncoder]

Hmmh. This is a nasty one to fix actually, due to added dynamicity for root-level values.
I was able to fix one easier aspect, but serializer root-level EnumMaps gets trickier thanks to type erasure.

[cowtowncoder]

Ok. So, the problem here is due to the fact that at root-level, an EnumMap value does not have generic type information, and key type can not be detected before actual serialization. As a result, a StdKeySerializer is used, and while it knows how to handle some basic types, for most (including all Enums), it simply calls toString().

There are couple of ways to solve the issue. One would require MapSerializer to detect this, and try to find a better key serializer as needed. Since implementation is already complicated, this is not ideal.
On the other hand, maybe StdKeySerializer could be improved to be able to lookup proper serializer for Enum key. Alas, while there is EnumSerializer that works fine, there is no matching key serializer; and problem is that low-level write call to make is different.
So maybe there needs to be EnumKeySerializer to use.

Anyway, there are ways to solve this, but fix is somewhat more involved than I was hoping for, so it'll take bit longer.

In the mean time, the usual work-arounds for type erasure would allow working around the problem.
So, for example, using something like:

class EnumXMap<V> extends EnumMap<EnumX, V> { }

should let enum of type EnumX be detected and work as expected.
Similarly ObjectWriter allows definition of root value type with

ObjectWriter w = mapper.writerFor(new TypeReference<EnumMap<MyEnum, ValueType>>()  {});

While not ideal, one of these approaches should work to get things going until fix is available.

[bimargulies]

My variation on this is not using an EnumMap. Just using a HashMap that happens to have enum keys, the code in MapSerializer is not looking for custom key serializers based on the key type, at all.

I think I see how to do this in MapSerializer, I'm going to make a commit and a PR.

[bimargulies]

I've added an @ignored test case on the PR for #749. I thing this is too hard for me without more coaching.

[cowtowncoder]

Test case is welcome, I can try to see if I can find a solution.

[cowtowncoder]

On the new test: I'll have to see how it should go, but this may also be a limitation due to type erasure.
Problem being that no type information is passed on serialization, so values are considered just Objects, and not Map<TestEnum, Map<String, String>>. While for values serializer is still accessed dynamically, key serializer is fetched using static type I think.
Although it is possible there was some kludge for nominal type of java.lang.Object to do more dynamic lookup.
The reason for static lookup is, if I recall correctly, performance: there is no caching for key (de)serializers, and having to fetch (de)serializer for every entry is a major performance drain.

Work-around here would be to specify TypeReference when writing, something like:

mapper.writerFor(new TypeReference<Map<KeyEnum, Map<TestEnum,Object>>>() {})

which appears to fix observed behavior here.

But it has been a while since I had a look at the implementation so I may be underestimating how implementation works...

[bimargulies]

So, adding a cache for key deserializers, at least to the MapSerializer,
might make it plausible to respect individual key values?

On Thu, Sep 24, 2015 at 8:49 PM, Tatu Saloranta notifications@github.com
wrote:

On the new test: I'll have to see how it should go, but this may also be a
limitation due to type erasure.
Problem being that no type information is passed on serialization, so
values are considered just Objects, and not Map<TestEnum, Map<String,
String>>. While for values serializer is still accessed dynamically, key
serializer is fetched using static type I think.
Although it is possible there was some kludge for nominal type of
java.lang.Object to do more dynamic lookup.
The reason for static lookup is, if I recall correctly, performance: there
is no caching for key (de)serializers, and having to fetch (de)serializer
for every entry is a major performance drain.

Work-around here would be to specify TypeReference when writing,
something like:

mapper.writerFor(new TypeReference<Map<KeyEnum, Map<TestEnum,Object>>>() {})

But it has been a while since I had a look at the implementation so I may
be underestimating how implementation works...

—
Reply to this email directly or view it on GitHub
#749 (comment)
.

[bimargulies]

A little more detail on our application: We have an API that accepts a Map<X extends Enum<X>, Object> The appropriate type of the value depends on the key. We have one case, where the appropriate value for the top-level map is, in fact, Map<SpecialEnum, -something-> The special enum is this enum that is a perpetual pain for us with Jackson; years ago we make it with enum item names that are completely wrong for Json. So we're constantly using customization to make sure that we serialize and deserialize it to the replacement values that make sense. We can't go back in time and put the 'good' values in as the enum item names. If I'm missing some other approach, please let me know.

[cowtowncoder]

Right: so original reason for requiring Map key type to be (more) static was because it would have been hassle to replicate much of scaffolding. It's not a good reason, but it did not seem like a major limitation for most uses, esp. at the time, and since Map-valued properties are usually (fully) typed.

Now: in specific case of Enums, it may be easier to get root-level "untyped" Maps to still work as expected, wrt values. For serialization I think default handling would be to call toString() anyway, if that helps.

But like I said, I haven't re-read code to see how much of Enum magic might be supported even via "std" key deserializer.

One other possible work-around idea is that if you can use Map sub-classes that preserve generic declarations -- like, say, public class MyEnumStringMap extends HashMap<MyEnum,String>, it will not lose type information, even for root-values. Such approach is only needed for root values (or, in case of Collections, Maps, content type of root values as well).