On Fri, Jan 29, 2016 at 4:00 PM, Justin Deoliveira <jdeol...@gmail.com>
wrote:
> These could be added as new methods to the GML facade class:
>> SimpleFeatureType GML.decodeSchema(InputStream in)
>> SimpleFeatureCollection GML.decodeFeatureCollection(InputStream in)
>>
>> I wonder if you could handle both the scan only single feature vs scan
> full feature collection with a single additional integer argument. The
> argument would be the number of features to “pre-scan” to compile the
> schema. This would allow for a bit more control for larger GML collections
> where you might not want to scan the whole collection but you want to scan
> more than just the first. Just a thought.
>
I'm a bit perplexed... like, it seems to me the two significant values
would be "1", aka "no null values, my collection is fully uniform" and "0",
aka "no clue, scan till the end", when you don't know
where the "odd one out" would be.
I do realize that most of the time scanning the first few features would
give you the final schema, but there is no reliability regarding the
number of features where this happens, the odd case out being exactly the
one where the attribute shows only in the last
features of the series (which happens if you do a "order by myAtt desc" in
a sql query, the null values are packed at
the beginning of the series).
So, let me show you what I did in my little experiment, this will also
explain why I was talking about pull
parsing :-)
My starting point is a desire to parse a feature collection in GML, and get
back a SimpleFeatureCollection
(streaming is not a requirement here). So I started using a normal Parser,
which gives me back the SimpleFeatureCollection.
However, noticed that the FeatureTypeCache was caching the type of the
first feature... I then injected in the
pico context a subclass that would not cache, and then
DefaultFeatureCollection would start complaining in
the logs that the features being inserted into it are not a match for its
feature type (determined from
the first feature).
So ok, I decided to give the pull parser a try, that does not use a
collection... here is the result:
-----------------------------------
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import javax.xml.stream.XMLStreamException;
import org.geotools.gml2.FeatureTypeCache;
import org.geotools.gml3.GMLConfiguration;
import org.geotools.xml.PullParser;
import org.opengis.feature.simple.SimpleFeature;
import org.opengis.feature.type.FeatureType;
import org.xml.sax.SAXException;
public class GMLPullParserReader {
/**
* Does not cache the feature type, allowing each feature to have its
own
*/
private static final class FeatureTypeCacheNone extends
FeatureTypeCache {
@Override
public void put(FeatureType type) {
// no op
}
}
public static void main(String[] args) throws XMLStreamException,
IOException, SAXException {
pullParse(new File("/tmp/GetFeatureResponse1.xml"));
pullParse(new File("/tmp/GetFeatureResponse2.xml"));
}
private static void pullParse(File input)
throws FileNotFoundException, XMLStreamException, IOException,
SAXException {
GMLConfiguration configuration = new GMLConfiguration();
PullParser parser = new PullParser(configuration, new
FileInputStream(input), SimpleFeature.class);
parser.setContextCustomizer(context -> {
Object instance =
context.getComponentInstanceOfType(FeatureTypeCache.class);
context.unregisterComponentByInstance(instance);
context.registerComponentInstance(new FeatureTypeCacheNone());
});
SimpleFeature f = null;
while((f = (SimpleFeature)parser.parse()) != null) {
System.out.println(f.getFeatureType());
}
}
}
---------------------------
The two collections have features in different orders, resulting in
different feature types in a normal
parse... but with this trick, no problem, each feature has its own full
feature type instead.
Now, extending this code one could at the same time accumulate the features
in a list,
determine an all encompassing feature type, and then return a
SimpleFeatureCollection.
And I wanted to do all this in a method in the GML facade, to hide the
details.
Thinking about it, it's not that I really need to use a pull parser, I
could use the normal one too,
provided that the feature collection binding stops using a
DefaultFeatureCollection, and
uses a new "GMLFeatureCollection" that does not complain about mixed
feature types :-p
Hope this clarifies things a bit
Cheers
Andrea
--
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