[CAMEL-10572] RefLanguage should support Predicates for Choice

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.18.1
• Fix Version/s: 2.19.0
• Component/s: camel-core
• Estimated Complexity: Novice
• Assignee: Claus Ibsen
• Reporter: Christoph Läubrich
• Created: 08/Dec/16 11:47
• Updated: 05/Jan/17 09:25
• Resolved: 05/Jan/17 09:25

Description

The documentation for the CBR (http://camel.apache.org/content-based-router.html) states

The following example shows how to route ... depending on the evaluation of various Predicate expressions

If you now look at the predicate section (http://camel.apache.org/predicate.html) it says:

Camel supports extensible Predicates using multiple Languages; the following languages are supported out of the box
...
Ref Language
...

But going to the Ref page (http://camel.apache.org/ref-language.html) it states

The Ref Expression Language is really just a way to lookup a custom Expression from the Registry.

So Ref Language does NOT support Predicates, it works in most cases since nearly all classes in Camel core implement Expresion and Predicate interface and there is a lot of converting between those, but if you have a custom expression that only implements predicate, you get in fact an (a bit misleading) exception:

java.lang.IllegalArgumentException: Cannot find expression in registry with ref: predicate
at org.apache.camel.language.ref.RefLanguage$1.evaluate(RefLanguage.java:50)
at org.apache.camel.support.ExpressionAdapter.evaluate(ExpressionAdapter.java:36)
at org.apache.camel.support.ExpressionSupport.matches(ExpressionSupport.java:32)
at org.apache.camel.util.ExpressionToPredicateAdapter.matches(ExpressionToPredicateAdapter.java:35)
at org.apache.camel.processor.FilterProcessor.matches(FilterProcessor.java:65)
at org.apache.camel.processor.ChoiceProcessor.process(ChoiceProcessor.java:93)
at org.apache.camel.management.InstrumentationProcessor.process(InstrumentationProcessor.java:77)
at org.apache.camel.processor.RedeliveryErrorHandler.process(RedeliveryErrorHandler.java:542)
at org.apache.camel.processor.CamelInternalProcessor.process(CamelInternalProcessor.java:197)
at org.apache.camel.processor.CamelInternalProcessor.process(CamelInternalProcessor.java:197)
at org.apache.camel.component.timer.TimerConsumer.sendTimerExchange(TimerConsumer.java:192)
at org.apache.camel.component.timer.TimerConsumer$1.run(TimerConsumer.java:76)

It takes me some time to find out that Camel is in fact finding my bean but can't use it, beside that its hard to work out why a predicate must also implement expression (Choice it self requires an Predicate in fact, so it converts all expresions to Predicates).

So the following improvements would be nice

1. Let RefLanguage send out some kind of TypeConverterException if it can't use the bean as the desired type
2. Let Ref, if it finds that the referenced bean is a predicate, convert this to an expression e.g with PredicateToExpressionAdapter