Jody Garnett a écrit :
> FeatureTypeFactory (the geotools interface) cannot be used with
> FactoryRegistry and thus made available through CommonFactoryFinder ...
> it is too "stateful".
Note that this is not a blocker. FeatureTypeFactory can be as statefull as you
want, providing that the state can be specified by Hints (adding new hints key
if needed).
Note that there is one advantage in specifying the state through hints compared
to explicit arguments to constructor: we can add or remove hints in future
version without API break. The inconvenient is that the set of hints must be
documented in javadoc.
The ability of adding or removing hints without compatibility break is useful
when the hints cascade through many factories. Use case:
- Factory A depends on factory B.
- Factory A constructor passes the user hints to factory B. Example:
FactoryA(Hints userHints) {
myDependency = FactoryFinder.getFactoryB(userHints);
}
- A new hints (Hints.NEW_HINT) is added for factory B.
- A user try to use Hints.NEW_HINT while instantiating factory A.
He expects the Hints.NEW_HINT to be passed to factory B. Example:
Hints userHints = new Hints(Hints.NEW_HINTS, someValue);
myFactory = FactoryFinder.getFactoryA(userHints);
- With the hints framework, Hints.NEW_HINTS is passed gracefully
to factory B without any change in the code.
- If values were specified explicitly as contructor arguments instead
of a map of Hints, as below:
myDependency = FactoryFinder.getFactoryB(arg0, arg1, arg2);
then the new user hint would be ignored or unusable until every
occurence of FactoryFinder.getFactoryB(...) in the whole Geotools
code base is updated to something like:
FactoryA(Foo0 arg0, Foo1 arg1, Foo2 arg2, Foo3 newArg) {
myDependency = FactoryFinder.getFactoryB(arg0, arg1, arg2, newArg);
}
In summary, explicit constructor argument means that any change in arguments
may
cascade in many places because of dependencies, while a single Hints argument
cascade gently.
> However:
> - DefaultFeatureTypeFactory extends
> - FeatureTypeFactory extends
> - FeatureTypeBuilder extends
> - FeatureTypes
This is not a problem if:
- FeatureTypes has a FeatureTypes(Hints userHints) constructor.
- FeatureTypeBuilder(Hints) constructor invoke super(userHints)
- FeatureTypeFactory(Hints) constructor invoke super(userHints)
- etc.
> All of which has way more state then just "name" ... and I cannot
> figure out how to ask FactoryRegistry not to cache (so I get the same
> instance back each time).
You don't need to ask FactoryRegistry to not cache. Quite the opposite; let him
cache.
Your factory should extends, directly or indirectly, AbstractFactory. Suppose
that FeatureTypes use a name, an other factory and a foo (I add explicit
"this."
for clarity):
FeatureTypes(Hints userHints) {
super(userHints);
if (userHints != null) {
this.name = userHints.get(Hints.NAME);
this.foo = userHints.get(Hints.FOO);
this.anOtherFactory = FactoryFinder.getAnOtherFactory(userHints);
}
if (this.name == null) {
this.name = someDefaultValue;
}
if (this.foo == null) {
this.foo = someDefaultValue;
}
if (this.anOtherFactory == null) {
this.anOtherFactory = someDefaultValue;
}
this.hints.put(Hints.NAME, this.name);
this.hints.put(Hints.FOO, this.foo);
this.hints.put(Hints.AN_OTHER_FACTORY, this.anOtherFactory);
}
The keys elements are the 3 "hints.put(key, value)" instructions at the end of
the above snipset. They copy the values in a hints HashMap inherited from
AbstractFactory. FactoryRegistry uses this information in order to determine if
an existing Factory match the requested hints.
In summary:
* Do not tell FactoryRegistry to not cache. Let him cache.
* Be sure to gives to AbstractFactory.hints every hints that your factory
uses. Do not copy the whole "userHints" map; only the hints that are really
used with the values that are actually used (this is why the above code put
the hints after the default values have been taken in account).
Then everything should work well...
> FeatureTypeBuilder asks CommonFactoryFinder for an instance of
> FeatureTypeFactory. Right now we will just create one using "new" ...
> unless someone can tell me how to turn off FactoryRegistry caching.
See above. Do not turn it off. It should work fine if AbstractFactory.hints has
the right info.
Martin
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