Fix/collection element field transformer null regression#654
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fborriello merged 4 commits intomasterfrom Mar 31, 2026
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…ents with flat field name mapping
When a CollectionPopulator transforms list elements (e.g. List<PersonSvc> -> List<Person>),
it calls transformer.transform(elem, targetClass) without a breadcrumb. In resolveEffectiveSource,
evalBreadcrumb("fieldName", null) returns just "fieldName", which could match a simple flat
field mapping (e.g. "name" -> "fullName"). This caused the lookup to incorrectly redirect to
the root source object instead of the current element, and getSourceFieldValue silently returned
null (swallowing the MissingFieldException because a transformer was defined), ultimately
passing null into the user's transformer function and triggering a NullPointerException.
Fix: guard the breadcrumb-based mapping lookup with isNotEmpty(breadcrumb) so root-source
redirection only occurs when there is an actual breadcrumb prefix in the path.
Add tests verifying that field transformers using flat field name transformation correctly use the collection element as the source, not the root object. New sample classes: - FromFooWithFullName: source item with a fullName field - FromFooContainingList: root source with id + List<FromFooWithFullName> - ImmutableToFooWithSplitName: target item with name + surname fields - ImmutableToFooContainingList: root target with id + List<ImmutableToFooWithSplitName> New tests in MixedObjectTransformationTest: - testFieldTransformerOnCollectionElementsUsesElementNotRootAsSource: verifies that withFieldMapping + withFieldTransformer on collection elements reads from the element, not the root (would have NPEd before the fix) - testAllCollectionElementsAreIndependentlyTransformedFromTheirOwnSource: verifies each element is transformed from its own source independently Also adds resetFieldsMapping() to AbstractBeanTransformerTest.beforeMethod() so field name mappings are always cleared between tests, preventing state leakage when a test throws before calling underTest.reset().
Test Coverage Report
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Description
Guard the breadcrumb-based mapping lookup with
isNotEmpty(breadcrumb). When breadcrumb is null (collection element transforms and root-level transforms), the method falls through togetSourceFieldName(), which already performs the same mapping lookup correctly using the element as the source.This has no impact on root-level or nested-object transforms: at root level
rootSourceObj == sourceObjso both paths were equivalent; at nested level the breadcrumb is always non-null so the guard has no effect.How Has This Been Tested?
Checklist:
feature: Feature I'm adding or expandingbug: Bugfix or experimentwip: Work in progress; stuff I know won't be finished soon