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get rid of String.intern

See original GitHub issue

InternCache is used in jackson for json property names only and solves the problem of extra memory footprint. In the same time it uses String.intern - that is not an issue rather than misusage of it. The purpose of using string.intern is jvm specific: usually it maintains string literal pool (and covers internal jvm cases).

There are several known drawbacks of using intern:

  • String pool Hashtable is non-resizable table. That means: we suffer of hash code collisions when table will have items more than its size (suffer as much as many items in string.intern pool)
  • The other drawback is involve GC into that: string intern pool is subject for minor collection too. So, app suffers of using string.intern implicitly via GC phases.

There is no use cases of check the equality of strings using == and therefore no any reasons of using String.intern - the biggest profit is to have string deduplication (already achieved).


--- src/main/java/com/fasterxml/jackson/core/util/  (revision 489becbfb28a41980f0d5147d6069b30fa3b5864)
+++ src/main/java/com/fasterxml/jackson/core/util/  (revision )
@@ -58,7 +58,7 @@
-        result = input.intern();
+        result = input;
         put(result, result);
         return result;

Test case

The general idea behind syntenic json is used in test (a flat json structure with 10000 properties with prefix someQName) is provide lots of different property names to trigger usage of string.intern within InternCache - that is close to real apps use cases where amount of unique property names is on a scale from hundreds to thousands.

    "someQName0": 0,
    "someQName1": 1,
    "someQName9999": 9999

JMH test

Let’s measure performance of a single json parse with interting and w/o it:

Benchmark                   Mode  Cnt     Score     Error  Units
-PerfInternCache.intern      avgt    5  4098.696 ± 164.484  us/op
+PerfInternCache.noIntern    avgt    5  2320.159 ± 204.301  us/op


Another test is to measure how intern is affecting implicitly via GC: handle 10k the same jsons as we use in previous test (the use case is very very close to real one in real apps like web service / microsevices): InternCache GC timings java test

Run it with -verbose:gc -XX:+PrintGCDetails -Xloggc:gc.log

and after that get the total GC pause time $ grep "GC" gc.log | grep "Times: " | awk '{S+=$8}END{print S}'

-intern      0.1907254 +- 0.00469 sec
+w/o intern  0.07665   +- 0.00498 sec


Using intern harms application performance as explicitly via more expencive InternCache.intern and implicitly via GC. In the same time we keep memory footprint on the same low possible level.

Issue Analytics

  • State:closed
  • Created 7 years ago
  • Reactions:14
  • Comments:38 (15 by maintainers)

github_iconTop GitHub Comments

cowtowncodercommented, Nov 14, 2016

There is JsonFactory.Feature.INTERN_FIELD_NAMES to use if you prefer no intern()ing.]

vladimirdolzhenkocommented, Nov 14, 2016

@cowtowncoder I know about that flag - the problem that it is enabled by default and that kind of optimisation harms app more neither having it switched off.

What’s the point to have such kind of degradation flag rather to fix it and make apps (bearing in mind the spread around of json) using jackon feel a bit better ?

Read more comments on GitHub >

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