Regression in Collection to array conversion [SPR-7496]

[spring-projects-issues]

Rossen Stoyanchev opened SPR-7496 and commented

The following code doesn't work in 3.0.4 but used to work in 3.0.3:

FormattingConversionServiceFactoryBean factoryBean = new FormattingConversionServiceFactoryBean();
factoryBean.afterPropertiesSet();
ConversionService conversionService = factoryBean.getObject();
assertTrue(conversionService.canConvert(Collection.class, Integer[].class));

Output from 3.0.3:

TRACE: GenericConversionService - Checking if I can convert [TypeDescriptor java.util.Collection<?>] to [TypeDescriptor java.lang.Integer[]]
TRACE: GenericConversionService - Searching for converters indexed by sourceType [java.util.Collection]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Integer;]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Number;]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Object;]
TRACE: GenericConversionService - Found matchable converters CollectionToArrayConverter@b23b25c
TRACE: GenericConversionService - Matching CollectionToArrayConverter@b23b25c
TRACE: GenericConversionService - Checking if I can convert [TypeDescriptor.NULL] to [TypeDescriptor java.lang.Integer]
TRACE: GenericConversionService - Yes, I can convert
TRACE: GenericConversionService - Matched converter CollectionToArrayConverter@b23b25c
TRACE: GenericConversionService - Caching under ConverterCacheKey [sourceType = [TypeDescriptor java.util.Collection<?>], targetType = [TypeDescriptor java.lang.Integer[]]]
TRACE: GenericConversionService - Yes, I can convert

Output from 3.0.4 (truncated at the bottom):

TRACE: GenericConversionService - Checking if I can convert [TypeDescriptor java.util.Collection<java.lang.Object>] to [TypeDescriptor java.lang.Integer[]]
TRACE: GenericConversionService - Searching for converters indexed by sourceType [java.util.Collection]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Integer;]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Number;]
TRACE: GenericConversionService - and indexed by targetType [[Ljava.lang.Object;]
TRACE: GenericConversionService - Found matchable converters CollectionToArrayConverter@b23b25c
TRACE: GenericConversionService - Matching CollectionToArrayConverter@b23b25c
TRACE: GenericConversionService - Checking if I can convert [TypeDescriptor java.lang.Object] to [TypeDescriptor java.lang.Integer]
TRACE: GenericConversionService - Searching for converters indexed by sourceType [java.lang.Object]
TRACE: GenericConversionService - and indexed by targetType [java.lang.Integer]
TRACE: GenericConversionService - and indexed by targetType [java.lang.Comparable]
TRACE: GenericConversionService - and indexed by targetType [java.lang.Number]
TRACE: GenericConversionService - and indexed by targetType [java.io.Serializable]
TRACE: GenericConversionService - and indexed by targetType [java.lang.Object]
TRACE: GenericConversionService - Found matchable converters IdToEntityConverter@7546c1d4, ObjectToObjectConverter@785f8172
TRACE: GenericConversionService - Matching IdToEntityConverter@7546c1d4
TRACE: GenericConversionService - Did not match converter IdToEntityConverter@7546c1d4
TRACE: GenericConversionService - Matching ObjectToObjectConverter@785f8172
TRACE: GenericConversionService - Did not match converter ObjectToObjectConverter@785f8172
TRACE: GenericConversionService - Caching NO_MATCH under ConverterCacheKey [sourceType = [TypeDescriptor java.lang.Object], targetType = [TypeDescriptor java.lang.Integer]]
TRACE: GenericConversionService - No, I cannot convert
TRACE: GenericConversionService - Did not match converter CollectionToArrayConverter@b23b25c
...

Notice the difference in the TypeDescriptor built for the source in each case (line 1).

---

Affects: 3.0.4

Issue Links:

• SpEL regression with ArrayList to int array conversion [SPR-7519] #12177 SpEL regression with ArrayList to int array conversion

Referenced from: commits spring-projects/spring-webflow@cef35b6

[spring-projects-issues]

Keith Donald commented

This seems related to the introduction of TypeDescription.UNKNOWN where NULL was used previously...

[spring-projects-issues]

Juergen Hoeller commented

Hmm, arguably this is correct since we cannot convert a Collection of arbitrary Objects (lacking better knowledge of its actual element type) to an Integer array. canConvert will return true if you pass in a more specific source descriptor, referring to a typed collection parameter or to a collection object with actual elements to introspect.

Juergen

[spring-projects-issues]

Juergen Hoeller commented

I'm closing this as Won't Fix, since this new behavior is what we want - or at least what I want ;-)

Seriously, the old behavior breaks multi-step conversion algorithms that have a fallback after the ConversionService canConvert call (like our beans package's TypeConverterDelegate). The ConversionService must only return true if it knows that it can convert the actual element type. Passing in a full TypeDescriptor based on a MethodParameter or based on a full collection object (using TypeDescriptor.forObject) will provide the right context to make such a decision.

For an example of how this is supposed to be used (now), check out #12177 where we are passing through the full TypeDescriptors as of 3.0.5, whereas we were passing along a plain Class array before.

Juergen