ConversionService cannot convert from Object to String anymore by default [SPR-11693]

[spring-projects-issues]

Oliver Drotbohm opened SPR-11693 and commented

On Spring 3.2.8 a default ConversionService instance (e.g. DefaultFormattingConversionService will answer the following call with true:

conversionService.canConvert(Object.class, String.class);

As of Spring 4.0.1 the same call returns false which is due to that change (introduced for #15884). From the commit however, it's not quite clear why the implementation of ObjectToObjectConverter now explicitly excludes String.valueOf(…).

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Affects: 4.0.1, 4.0.2, 4.0.3

Issue Links:

• Add Converter implementations that convert legacy Date instances into JDK 8 date/time types [SPR-11259] #15884 Add Converter implementations that convert legacy Date instances into JDK 8 date/time types

[spring-projects-issues]

Juergen Hoeller commented

While that change did come as part of #15884 where a general "toX()" convention was baked into ObjectToObjectConverter, it is intentional in that we never meant to return true for the generic Object-to-String case. Instead, our FallbackObjectToStringConverter always had specific rules when it would say true here, such as a specifically declared constructor or valueOf method that can perform specific String conversion here. However, due to the generic "String.valueOf(Object)" method, that convention unfortunately made it return true for the super-generic Object-to-String case... and that basically got corrected as part of #15884.

In fact, if a caller wants to accept any toString() result, he shouldn't call the ConversionService in the first place but rather simply call toString() himself.

Juergen