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https://issues.apache.org/jira/browse/FELIX-227?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12478110
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Clement Escoffier commented on FELIX-227:
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iPOJO follow the dependency injection pattern (as Spring). So when you read
your code, you don't have to focus on which value a container-managed field
has. You should guess : "it will have a consistent value". The JEE annotation
are another good (and famous) example of dependency injections. But the
advantages, is that these annotation are in the code. So user know that the
field will be container-managed. iPOJO does not provide annotations because it
brings compilation dependency that I don't want (it implies that each handler
must provide its annotation classes and be in the classpath). A parser using
"doclet" is currently under study. Doctlet have the advantage to not create
compilation dependency but cannot be checked statically.
However, you point up an interisting limitation. There is no way in the POJO to
be notified of the injection of a value in a property. Perhaps callback can
help.
Imagine something like :
<property name="foo.bound" field="bound" method="setBound'>
and in your code
setBound(String newvalue) { ...}
When the container injects a value inside the "bound" property, it should
notify the pojo by invoking the setBound method. What do you think of this
solution ?
> 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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