[jira] Commented: (FELIX-227) iPOJO should allow configuration and service properties to be bound via setter/getter methods, not just via direct fields

["Richard S. Hall (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FELIX-227?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12478062 ] 

Richard S. Hall commented on FELIX-227:
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I am not sure I totally agree with your second issue above. iPOJO's approach tells components not to worry about the values of fields managed by the container, but just to use them assuming that the values will be correct at runtime.

Regardless, you raise some valid points and I think supporting method invocations too could be worthwhile when setting properties.

> iPOJO should allow configuration and service properties to be bound via setter/getter methods, not just via direct fields
> -------------------------------------------------------------------------------------------------------------------------
>
>                 Key: FELIX-227
>                 URL: https://issues.apache.org/jira/browse/FELIX-227
>             Project: Felix
>          Issue Type: Improvement
>          Components: iPOJO
>         Environment: Not relevant.
>            Reporter: Steven E. Harris
>
> iPOJO's binding of configuration properties and service properties directly to component fields causes a few difficult situations that could be eased by optionally binding the reading and writing to getter and setter methods instead.
> First, consider a component class with invariants that span multiple fields. iPOJO can change the value of a field without the owning class knowing, depriving it the chance to update dependent fields that participate in the invariant. Examples include recalculating and caching an expensive result whenever some input field changes, or validating a changed value and updating other fields in response.
> Second, at present iPOJO sets the value of bound fields before the component class constructor runs. If the constructor attempts to initialize some fields, it may be inadvertantly overwriting the initial values supplied by iPOJO. Writing the constructor to deliberately ignore values that might be bound by iPOJO flies in the face of the very name of the project: a POJO isn't supposed to know that it's being silently manipulated like this, and hence a POJO should be written in standard form: Initialize variables to sane defaults, including constructor parameters, expecting them to be overwritten later.
> Take for example a class that has two fields, one of which is bound to a configuration property:
> public class Example {
>   private String bound;
>   private String dependent;
>   public Example() {
>     bound = null;
>     dependent = "empty";
>   }
>   public void setBound(String s) {
>     bound = s;
>     dependent = null == s || 0 == s.length() ? "empty" : "full";
>   }
>   public String getDependent() {
>     return dependent;
>   }
> }
> If written in this manner, the constructor mistakenly overwrites the initial iPOJO-provided value for "bound" by initializing it to null. But to resist initializing "bound' is also dangerous; how would one reading this code have any idea that "bound" might get set to a different value before the constructor runs, or while the instance is live?
> Also, consider that if "bound' changes silently once the instance is live, "dependent" will fall out of step, as the invariant maintained in setBound() can be violated.
> If iPOJO would allow property binding to optionally work by way of getter and setter methods, one and a half of these problems could be avoided. The missing half relates to construction. If we ask iPOJO to defer setting the "initial values" until the constructor completes, we may have to defer some initialization that would use the values not yet available.
> Trying to write a POJO class that gets manipulated on the sly by iPOJO is proving to be more tricky than just writing some of the ManagedServiceFactory code myself, as I'm forced to adjust my would-be POJO service class to deal with these weird initialization and invariant maintenance problems normally solved by member variable encapsulation. Perhaps we should look at Spring's example that better acknowledges not just the technical possibilities, but the logical difficulties in iPOJO's kind of silent injection and manipulation.

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