Allow for non-void method calls in outbound channel adapter expression [INT-3815]

Mauro Molinari opened INT-3815 and commented

Right now, the SpEL expression I specify on an <int:outbound-channel-adapter> requires a void-returning expression, otherwise an exception is thrown.
However, this is limiting. What if I just want to call a method and ignore its return value?

In my case, I have a flow that ends in an outbound adapter that should just delete a temporary file. However File.delete() returns a boolean value, which I'm not interested in. Hence, an expression like payload.file.delete() throws a ConversionNotSupportedException, so now I have to wrap that expression into a ad-hoc service method call.

Is there a strong reason for requiring a void returning expression?

Affects: 4.1.6

Start by tracing how the int:outbound-channel-adapter evaluates its SpEL expression and handles non-void results, using the reported ConversionNotSupportedException as the failure point. Reproduce the payload.file.delete() case and verify that a non-void method call can have its return value ignored without changing existing void-expression behavior.