using Spring Expression Language (SpEL) in a blueprint container

Spring Expression Language (SpEL) can be used to configure beans in a spring application context. For example the Spring-batch framework makes heavy use of it for what they call late binding of properties. To get a feel of what SpEL can do, some examples from the spring documentation:

The prefix and suffix #{ and } in this case are just a convention and can be configured differently if you want or just leave them out

In this blog I will show how it can be used in an osgi-blueprint container. The idea is quite simple, we will use the type-converter feature from the blueprint spec to convert our SpEL expression we want to inject. If the expression is correct the result type should match the type expected by the bean. With the code given in this simple example it is not possible to access the beans from the blueprint container. That for a next post.
Here is the code:

OSGI is all about class loading so we have to configure the EvaluationContext to use the classloader associated with this bundle, this is done with a custom TypeLocator:

All this code is wired together in the blueprint.xml file:

Note the type-converters tags to register our type converter, our custom typeLocator and the SpEL expression #{ T(java.lang.Math).random() *100.0}.

Last but not least don’t forget to install the spring-expression and spring-core bundle in your osgi container when you want to deploy.


Author: Johan Huylebroeck



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