Implementing FeatureStore
¶
In this part we will complete the CSVDataStore
. At the end of this section we
will have a full functional CSVDataStore
supporting both read and write operations.
The DataStore
API provides three categories of public methods involved in making content writable.
DataStore.createSchema(featureType)
- sets up a new entry for content of the provided typeDataStore.getFeatureWriter(typeName)
- a low-level iterator that allows writingDataStore.getFeatureSource(typeName)
- read-onlyFeatureSource
orFeatureStore
for read-write
The infrastructure to support this functionality is quite extensive. A few highlights from the above diagram:
ContentState.listeners
- event notification for changed contentContentState.tx
- transaction object used by clients to stash session callbacksContentState.transactionState:
provides transaction independence between threads working on the same content.
CSVDataStoreFactory
¶
Now that we are going to be writing files we can fill in the createNewDataStore
method.
Open up
CSVDataStoreFactory
and fill in the methodcreateNewDataStore(Map params)
which we skipped over earlier.private static final Logger LOGGER = Logging.getLogger("org.geotools.data.csv"); public DataStore createNewDataStore(Map<String, ?> params) throws IOException { File file = (File) FILE_PARAM.lookUp(params); if (file.exists()) { LOGGER.warning("File already exsists: " + file); } return new CSVDataStore(file); }
The above code snippet introduces a GeoTools
Logger
we can use for warnings.Because GeoTools is a well mannered library it can be configured to use different logging engines. This allows it to integrate smoothly with larger projects.
To see this change in context review
CSVDataStoreFactory.java
from thegt-csv
plugin.
There is no DataStoreFinder
method for creating new content. We expect this method to be called
from a content creation wizard that already is making use of the DataStoreFactory
.
CSVDataStore
¶
Returning to CSVDataStore
we have a number of new methods to override to support write
functionality.
Introduce the
createSchema(featureType)
method used to set up a new file.Our CSV format has several limitations:
Representing
Points
, usingLAT
andLON
columnsAttributes are assumed to be
Strings
(this is a text format after all)CoordinateReferenceSystem
is limited to WGS84
Add
createSchema(featureType)
:@Override public void createSchema(SimpleFeatureType featureType) throws IOException { List<String> header = new ArrayList<>(); GeometryDescriptor geometryDescrptor = featureType.getGeometryDescriptor(); if (geometryDescrptor != null && CRS.equalsIgnoreMetadata( DefaultGeographicCRS.WGS84, geometryDescrptor.getCoordinateReferenceSystem()) && geometryDescrptor.getType().getBinding().isAssignableFrom(Point.class)) { header.add("LAT"); header.add("LON"); } else { throw new IOException("Unable use LAT/LON to represent " + geometryDescrptor); } for (AttributeDescriptor descriptor : featureType.getAttributeDescriptors()) { if (descriptor instanceof GeometryDescriptor) continue; header.add(descriptor.getLocalName()); } // Write out header, producing an empty file of the correct type CsvWriter writer = new CsvWriter(new FileWriter(file), ','); try { writer.writeRecord(header.toArray(new String[header.size()])); } finally { writer.close(); } }
And revise our implementation of
createFeatureSource(ContentEntry)
.While we will still return a
FeatureSource
, we have the option of returning the subclassFeatureStore
for read-write files.The
FeatureStore
interface provides additional methods allowing the modification of content.@Override protected ContentFeatureSource createFeatureSource(ContentEntry entry) throws IOException { if (file.canWrite()) { return new CSVFeatureStore(entry, Query.ALL); } else { return new CSVFeatureSource(entry, Query.ALL); } }
If you would like to review the
gt-csv
plugin has the completedCSVDataStore.java
file.
CSVFeatureStore
¶
CSVFFeatureStore
has an interesting design constraint:
It implements
FeatureStore
, which extendsFeatureSource
It extends the base class
ContentFeatureStore
, which handles most of the heavy lifting
So what is the trouble? Java only allows single inheritance - forcing us to account for all the
work we did reading features in CSVFeatureSource
.
Many first generation DataStore implementations practiced cut and paste coding, meaning fixes would often get applied in one spot and not another making for a frustrating debugging experience.
Instead we are going to use a delegate CSVFeatureSource
, hidden from public
view, simply to call its methods for reading. This prevents code duplication,
making the code easier to maintain, at the cost of some up front complexity.
We have to play a few tickets to ensure both the CSVFeatureStore
and its hidden CSVFeatureSource
are always on the same transaction, but other than that this approach is working well.
Create
CSVFeatureStore
:package org.geotools.tutorial.csv; import java.io.IOException; import org.geotools.api.data.FeatureReader; import org.geotools.api.data.FeatureWriter; import org.geotools.api.data.Query; import org.geotools.api.data.QueryCapabilities; import org.geotools.api.data.ResourceInfo; import org.geotools.api.data.Transaction; import org.geotools.api.feature.FeatureVisitor; import org.geotools.api.feature.simple.SimpleFeature; import org.geotools.api.feature.simple.SimpleFeatureType; import org.geotools.api.feature.type.Name; import org.geotools.data.store.ContentEntry; import org.geotools.data.store.ContentFeatureStore; import org.geotools.data.store.ContentState; import org.geotools.geometry.jts.ReferencedEnvelope; /** * Read-write access to CSV File. * * @author Jody Garnett (Boundless) * @author Ian Turton (Astun Technology) */ public class CSVFeatureStore extends ContentFeatureStore { public CSVFeatureStore(ContentEntry entry, Query query) { super(entry, query); }Our first responsibility is to implement a
CSVFeatureWriter
for internal use.Transaction
and Event Notification are handled by wrappers applied to ourCSVFeatureWriter
.// // CSVFeatureStore implementations // @Override protected FeatureWriter<SimpleFeatureType, SimpleFeature> getWriterInternal( Query query, int flags) throws IOException { return new CSVFeatureWriter(getState(), query); }Note
In general the “Gang of Four” decorator pattern is used to layer functionality around the raw
FeatureReader
andFeatureWriters
you provided. This is very similar to the design of thejava-io
library (where a BufferedInputStream can be wrapped around a raw FileInputStream).You can control which decorators/wrappers are applied, by as shown in the following table.
Handle
Override
reprojection
canReproject()
filtering
canFilter()
max feature limiting
canLimit()
sorting
canSort()
locking
canLock()
As an example if your data format supported an attribute index you would be in position to override
canSort()
to return true if an index was available for sorting.Next we can set up our delegate, taking care to ensure both use the same
Transaction
./** * Delegate used for FeatureSource methods (We do this because Java cannot inherit from both * ContentFeatureStore and CSVFeatureSource at the same time */ CSVFeatureSource delegate = new CSVFeatureSource(entry, query) { @Override public void setTransaction(Transaction transaction) { super.setTransaction(transaction); CSVFeatureStore.this.setTransaction( transaction); // Keep these two implementations on the same transaction } }; @Override public void setTransaction(Transaction transaction) { super.setTransaction(transaction); if (delegate.getTransaction() != transaction) { delegate.setTransaction(transaction); } }Use the delegate to implement the internal
ContentDataStore
methods. In Eclipse you can use to speed this up.// // Internal Delegate Methods // Implement FeatureSource methods using CSVFeatureSource implementation // @Override protected SimpleFeatureType buildFeatureType() throws IOException { return delegate.buildFeatureType(); } @Override protected ReferencedEnvelope getBoundsInternal(Query query) throws IOException { return delegate.getBoundsInternal(query); } @Override protected int getCountInternal(Query query) throws IOException { return delegate.getCountInternal(query); } @Override protected FeatureReader<SimpleFeatureType, SimpleFeature> getReaderInternal(Query query) throws IOException { return delegate.getReaderInternal(query); }We have to do one “fix” to allow handle visitor method to be called - add the following to
CSVFeatureSource
./** * Make handleVisitor package visible allowing CSVFeatureStore to delegate to this * implementation. */ @Override protected boolean handleVisitor(Query query, FeatureVisitor visitor) throws IOException { return super.handleVisitor(query, visitor); // WARNING: Please note this method is in CSVFeatureSource! }Warning
Double check you are adding the above to CSVFeatureSource.
Note
Why does this work - because Java visibility rules are insane. Even though the method is marked protected it now has package visibility can be called by its peer
CSVFeatureStore
.Use the delegate to implement the public
FeatureSource
methods.// // Public Delegate Methods // Implement FeatureSource methods using CSVFeatureSource implementation // @Override public CSVDataStore getDataStore() { return delegate.getDataStore(); } @Override public ContentEntry getEntry() { return delegate.getEntry(); } public Transaction getTransaction() { return delegate.getTransaction(); } public ContentState getState() { return delegate.getState(); } public ResourceInfo getInfo() { return delegate.getInfo(); } public Name getName() { return delegate.getName(); } public QueryCapabilities getQueryCapabilities() { return delegate.getQueryCapabilities(); }You can see what this looks like in context by reviewing the
CSVFeatureStore.java
from thegt-csv
plugin.
CSVFeatureWriter
¶
This class uses an interesting trick to simulate updating a file in place, while still supporting streaming operation. We will be outputting content to a temporary file, leaving the original for concurrent processes such as rendering. When streaming is closed the temporary file is moved into the correct location to effect the change.
To avoid duplicating all the work we put into CSVFeatureReader
this code uses the same delegate
trick encountered earlier.
A couple common questions:
Q: How do you make a
Transaction
out of our simple reader?ContentFeatureSource
uses wrappers (or delegates) to process the data on the fly as it is read off disk. Wrappers can do all kinds of work, from cutting off reading when “max feature limit” is hit, or in the case of transactions modifying the content as it is read to match any edits that have been made.Q: How do you know what wrappers to use?
ContentFeatureSource
checks to see if a wrapper is needed.For example a query with a “max feature limit” is implemented using the
MaxFeatureReader
wrapper.// max feature limit if (!canLimit()) { if (query.getMaxFeatures() != -1 && query.getMaxFeatures() < Integer.MAX_VALUE) { reader = new MaxFeatureReader<SimpleFeatureType, SimpleFeature>(reader, query.getMaxFeatures()); } }
MaxFeatureWrapper
counts the features that are returned, and returnshasNext()
false once the configured limit has been reached:public boolean hasNext() throws IOException { return (featureReader.hasNext() && (counter < maxFeatures)); }
Q: How does that work with transactions?
ContentState
manages aDiffTransactionState
used to capture each modification. Each change is recorded byFeatureId
(a feature recorded for each add or modification, or null for a delete).As CSVFeatureReader is used to access the file contents, a wrapper checks the Diff and dynamically modifies the content to match any outstanding edits. When it reaches the end of your file, it keeps going listing any features that were added.
Q: That is fine for transaction independence, what if two
FeatureSources
are using the sameTransaction
?Those two
FeatureSources
would be configured with the sameContentState
, and thus have the same view of the world.The
ContentDataStore
API is divided into two levels:Public classes focused on ease of use for client code - examples include
DataStore
,FeatureSource
, Transaction.Private classes focused on ease of development for
DataStore
developers - examples includeContentEntry
,ContentEntry
,BatchFeatureEvent
Note
Our first generation
DataStore
implementations tried to produce a similar effect using a series ofHashMaps
, with suitably scary consequences for code readability. If any additional information is required by yourDataStore
implementation you are actively encouraged to subclassContentState
.Q: Wait what about when I am not using a
Transaction
? How do I get aContentState
?When the use has not specified a
Transaction
we default to the use ofTransaction.AUTO_COMMIT
.This makes the
ContentState
recorded forTransaction.AUTO_COMMIT
special in that it represents the point of truth on the files current status. The bounds recorded forTransaction.AUTO_COMMIT
are the bounds of the file. The number of features recorded forTransaction.AUTO_COMMIT
are the number of features recorded in the file.
Now that we have some idea of what is riding on top, lets implement our CSVFeatureWriter
:
Create the file
CSVFeatureWriter.java
:package org.geotools.tutorial.csv; import com.csvreader.CsvWriter; import java.io.File; import java.io.FileWriter; import java.io.IOException; import java.nio.file.Files; import java.nio.file.StandardCopyOption; import java.util.NoSuchElementException; import org.geotools.api.data.FeatureWriter; import org.geotools.api.data.Query; import org.geotools.api.feature.Property; import org.geotools.api.feature.simple.SimpleFeature; import org.geotools.api.feature.simple.SimpleFeatureType; import org.geotools.api.referencing.cs.AxisDirection; import org.geotools.data.DataUtilities; import org.geotools.data.store.ContentState; import org.geotools.feature.simple.SimpleFeatureBuilder; import org.geotools.referencing.crs.DefaultGeographicCRS; import org.locationtech.jts.geom.Point; /** * Iterator supporting writing of feature content. * * @author Jody Garnett (Boundless) * @author Lee Breisacher */ public class CSVFeatureWriter implements FeatureWriter<SimpleFeatureType, SimpleFeature> { /** State of current transaction */ private ContentState state; /** Delegate handing reading of original file */ private CSVFeatureReader delegate; /** Temporary file used to stage output */ private File temp; /** CsvWriter used for temp file output */ private CsvWriter csvWriter; /** Current feature available for modification, may be null if feature removed */ private SimpleFeature currentFeature; /** Flag indicating we have reached the end of the file */ private boolean appending = false; /** flag to keep track of lat/lon order */ private boolean latlon = DefaultGeographicCRS.WGS84 .getCoordinateSystem() .getAxis(0) .getDirection() .equals(AxisDirection.NORTH); int latIndex = 0; int lngIndex = 0; /** Row count used to generate FeatureId when appending */ int nextRow = 0; }
Our construct is responsible for a lot of activities:
Setting up a temporary file for output
Creating a
CsvWriter
for outputQuickly making a copy of the file if we are just interested in appending
Starting the file off with a copy of the headers
Creating a delegate to read the original file
Check if the columns are
lat,lon
orlon,lat
for when we write them out.
Putting all that together:
public CSVFeatureWriter(ContentState state, Query query) throws IOException { this.state = state; String typeName = query.getTypeName(); File file = ((CSVDataStore) state.getEntry().getDataStore()).file; File directory = file.getParentFile(); this.temp = File.createTempFile(typeName + System.currentTimeMillis(), "csv", directory); this.csvWriter = new CsvWriter(new FileWriter(this.temp), ','); this.delegate = new CSVFeatureReader(state, query); this.csvWriter.writeRecord(delegate.reader.getHeaders()); String latField = "lat"; String lngField = "lon"; String[] headers = delegate.reader.getHeaders(); for (int i = 0; i < headers.length; i++) { if (headers[i].equalsIgnoreCase(latField)) { latIndex = i; } if (headers[i].equalsIgnoreCase(lngField)) { lngIndex = i; } } }
Add
FeatureWriter.getFeatureType()
implementation:@Override public SimpleFeatureType getFeatureType() { return state.getFeatureType(); }
Add
hasNext()
implementation, making use of delegate before switching over to returning false when appending.@Override public boolean hasNext() throws IOException { if (csvWriter == null) { return false; } if (this.appending) { return false; // reader has no more contents } return delegate.hasNext(); }
The
next()
method is used for two purposes:To access
Features
for modification or removal (when working through existing content)To create new
Features
(when working past the end of the file)
The
next()
implementation has a couple of interesting tricks:Care is taken to write out the
currentFeature
if requiredThe next feature is fetched from the delegate; or
when appending a new feature is created for the user to fill in with attributes
Here is what that looks like:
@Override public SimpleFeature next() throws IOException, IllegalArgumentException, NoSuchElementException { if (csvWriter == null) { throw new IOException("FeatureWriter has been closed"); } if (this.currentFeature != null) { this.write(); // the previous one was not written, so do it now. } try { if (!appending) { if (delegate.reader != null && delegate.hasNext()) { this.currentFeature = delegate.next(); return this.currentFeature; } else { this.appending = true; } } SimpleFeatureType featureType = state.getFeatureType(); String fid = featureType.getTypeName() + "." + nextRow; Object[] values = DataUtilities.defaultValues(featureType); this.currentFeature = SimpleFeatureBuilder.build(featureType, values, fid); return this.currentFeature; } catch (IllegalArgumentException invalid) { throw new IOException("Unable to create feature:" + invalid.getMessage(), invalid); } }
Note
There are a large number of utility classes to perform common functions, take a look around before building something yourself.
DataUtilities
: Mix of methods helping developers use DataStore, with a few methods to help implementers perform common tasks. Acts as Facade for a wide range of servicesSimpleFeatureBuilder
: used to ease interaction withFeatureFactory
Add
remove()
implementation, marking thecurrentFeature
as null./** * Mark our {@link #currentFeature} feature as null, it will be skipped when written effectively * removing it. */ public void remove() throws IOException { this.currentFeature = null; // just mark it done which means it will not get written out. }
Add
write()
implementation, note how we have to keep track of the axis order (surprisingly this is not fixed):public void write() throws IOException { if (this.currentFeature == null) { return; // current feature has been deleted } for (Property property : currentFeature.getProperties()) { Object value = property.getValue(); if (value == null) { this.csvWriter.write(""); } else if (value instanceof Point) { Point point = (Point) value; if (latlon && latIndex <= lngIndex) { this.csvWriter.write(Double.toString(point.getX())); this.csvWriter.write(Double.toString(point.getY())); } else { this.csvWriter.write(Double.toString(point.getY())); this.csvWriter.write(Double.toString(point.getX())); } } else { String txt = value.toString(); this.csvWriter.write(txt); } } this.csvWriter.endRecord(); nextRow++; this.currentFeature = null; // indicate that it has been written }
Note
Previous implementations would make a copy of the feature to return. When write was called copy would be compared to the original to see if any change had been made. Why? So that an appropriate event notification could be sent out.
This is another case where a wrapper has been created, and applied by
ContentFeatureStore
.
Like the constructor the implementation of
close()
has a number of responsibilities.To implement
close()
we must remember to write out any remaining features in the delegate before closing our new file.The last thing our
FeatureWriter
must do is replace the existingFile
with our new one.@Override public void close() throws IOException { if (csvWriter == null) { throw new IOException("Writer alread closed"); } if (this.currentFeature != null) { this.write(); // the previous one was not written, so do it now. } // Step 1: Write out remaining contents (if applicable) while (hasNext()) { next(); write(); } csvWriter.close(); csvWriter = null; if (delegate != null) { this.delegate.close(); this.delegate = null; } // Step 2: Replace file contents File file = ((CSVDataStore) state.getEntry().getDataStore()).file; Files.copy(temp.toPath(), file.toPath(), StandardCopyOption.REPLACE_EXISTING); }
You can see what this looks like in context by reviewing the
CSVFeatureWriter.java
from thegt-csv
plugin.