Paul King created GROOVY-12281:
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Summary: Investigate mitigating class-loader pinning by
ClassInfo.globalClassValue under the default ClassValue implementation
Key: GROOVY-12281
URL: https://issues.apache.org/jira/browse/GROOVY-12281
Project: Groovy
Issue Type: Improvement
Reporter: Paul King
Follow-up from review discussion on [PR
#2798|https://github.com/apache/groovy/pull/2798] (GROOVY-12142), preserving
the analysis from that review. A {{ClassValue}} realizes the chain _key class →
association → value → everything reachable from it_, and the association lives
as long as the key class
([JDK-8136353|https://bugs.openjdk.org/browse/JDK-8136353], working as
intended). The {{ClassValue}} implementation class itself prevents no
unloading; what matters is the key's origin: a Groovy-loaded key class dies
with Groovy's loader (fine), but a JDK/platform key class is effectively
immortal, so any value reachable from it that was loaded by Groovy's loader
pins that loader forever. Indirection counts — a JDK-typed value (e.g. an
{{ArrayList}}) whose _elements_ are Groovy-loaded re-creates the pin one level
down.
{{ClassInfo.globalClassValue}} is static with unknown keys, including platform
classes ({{String}} receives a {{ClassInfo}} in essentially every Groovy
program), so the default {{ClassValue}} path pins the loader; today's only
remedy is the global {{groovy.use.classvalue=false}} escape hatch, which trades
away the per-class fast path for all keys.
The general mitigation — {{SoftReference}}-wrapped values with a
check-remove-recompute protocol — requires that recomputation be legal, and for
{{ClassInfo}} it is not in general: a {{ClassInfo}} can carry non-recomputable
state (modified metaclasses, category state), so a softly-collected value could
silently discard user metaclass customizations. Approaches to investigate:
* soft values while a {{ClassInfo}} is pristine, hardening the reference on
first mutation — only classes with customized metaclasses would then pin, a far
smaller set;
* per-key policy: platform-loader keys get soft/map treatment, Groovy-loader
keys stay strong (the key-origin rule applied mechanically);
* splitting {{ClassInfo}} into recomputable and stateful parts;
* revisiting whether the map-based implementation should become the default,
with {{ClassValue}} as the opt-in fast path.
Recompute cost and dispatch-path performance need measurement for any candidate.
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