Can Jackson reuse Key-Strings?

[Fabian Lange]

Hi,

I was wondering if there is some kind of "String" pooling possible with Jackson, if it might be a good idea to add support for it, or if thats more of a case for a module?

The use case is to parse large amounts of similar json documents into memory. As the keys are always the same, they end up taking a lot of memory when de-serializing to Map<String, Object>, but I assume same holds true for JsonNodes.

I searched a bit and could not find if it was ever discussed to add some sort of caching level for keys (possibly also for values, but keys are much more obvious), which during de-serialization realizes that the (immutable) value is identical to one which has been read already.

I am not voting for String.intern() as it has its own problems, maybe some pluggable Key Cache?

Rationale is that this way one could hold on many more deserialized documents in memory.

Fabian

[brharr...@gmail.com]

Yes, see INTERN_FIELD_NAMES. 

Brian

[Tatu Saloranta]

I am also bit puzzled by the original question, so perhaps it is good to indicate that by default Jackson core (JsonParser) does two things to ALL json property keys:

1. Canonicalizes them so that there is only one instance of each distinct key (like "property"): no new Strings are allocated. This is controlled by `JsonFactory.Feature.CANONICALIZE_FIELD_NAMES)`

2. Each canonicalized instance is also intern()ed if `JsonFactory.Feature.INTERN_FIELD_NAMES` is enabled.

Both features are enabled by default, but may be disabled. Note, however, that if canonicalization is disabled, no intern()ing will occur (since intern()ing is done by symbol table; and since consistent intern()ing would be counter-productive).

Now: `ObjectNode` and `Map` will simply use property name as reported by `JsonParser`, and it being canonicalized (usually) there won't be duplicate Strings. There is nothing for node or map handling that participates in this.

-+ Tatu +-

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[Fabian Lange]

Hi Tatu,

ist this the KeyDeserializer used for maps?

com.fasterxml.jackson.databind.deser.std.StdKeyDeserializer.StringKD

So yeah it seems that I missed the intern feature.

Fabian

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[Fabian Lange]

Hi Again,

not sure what I did there, but of course you are correct. I now was able to verify the keys (also in maps) are indeed the same object reference.

Fabian