Geotools操作GeoJSON：解析FeatureCollection对象文件
GeoJSON是基于JavaScript的对象的地理信息数据格式。
GeoJSON格式示例：
{ "type":"FeatureCollection", "features":[ { "type":"Feature", "properties":{ "area": 3865207830, "text": null }, "id":"polygon.1", "geometry":{ "type":"Polygon", "coordinates":[ [ [ 116.19827270507814, 39.78321267821705 ], [ 116.04446411132814, 39.232253141714914 ], [ 116.89590454101562, 39.3831409542565 ], [ 116.86981201171876, 39.918162846609455 ], [ 116.19827270507814, 39.78321267821705 ] ] ] } } ], "crs":{ "type":"name", "properties":{ "name":"EPSG:4326" } } }
一、Maven中GeoTools的引入
引入的geojson模块
<dependency> <groupId>org.geotools</groupId> <artifactId>gt-geojson</artifactId> <version>19.0</version> </dependency>
然后添加GeoTools官方的仓库
如果在Maven中引入GeoTools时找不到jar包，则参考：https://www.cnblogs.com/SkyTreeDelivery/p/12667972.html<repository> <id>osgeo</id> <name>OSGeo Release Repository</name> <url>https://repo.osgeo.org/repository/release/</url> <snapshots><enabled>false</enabled></snapshots> <releases><enabled>true</enabled></releases> </repository> <repository> <id>osgeo-snapshot</id> <name>OSGeo Snapshot Repository</name> <url>https://repo.osgeo.org/repository/snapshot/</url> <snapshots><enabled>true</enabled></snapshots> <releases><enabled>false</enabled></releases> </repository>
二、解析FeatureCollection对象文件
一个FeatureCollection对象文本，包含一个Feature要素。
1.1 geotools操作GeoJSON过程中的问题及相关源码
public static void main(String[] a) throws Exception { // 坐标顺序是EAST_NORTH，即经度在前 String json = "{"type":"FeatureCollection","features":[{"type":"Feature","properties":{"area":3865207830, "text": null},"id":"polygon.1","geometry":{"type":"Polygon","coordinates":[[[116.19827270507814,39.78321267821705],[116.04446411132814,39.232253141714914],[116.89590454101562,39.3831409542565],[116.86981201171876,39.918162846609455],[116.19827270507814,39.78321267821705]]]}}],"crs":{"type":"name","properties":{"name":"EPSG:4326"}}}"; // 指定GeometryJSON构造器，15位小数 FeatureJSON fjson_15 = new FeatureJSON(new GeometryJSON(15)); // 读取为FeatureCollection FeatureCollection featureCollection = fjson_15.readFeatureCollection(json); // 获取SimpleFeatureType SimpleFeatureType simpleFeatureType = (SimpleFeatureType) featureCollection.getSchema(); // 第1个问题。坐标顺序与实际坐标顺序不符合 System.out.println(CRS.getAxisOrder(simpleFeatureType.getCoordinateReferenceSystem())); //输出：NORTH_EAST //第2个问题。查看空间列名称 System.out.println(simpleFeatureType.getGeometryDescriptor().getLocalName()); //输出：geometry //第3个问题。坐标精度丢失 //第4个问题。默认无坐标系和空值输出 OutputStream ostream = new ByteArrayOutputStream(); GeoJSON.write(featureCollection, ostream); // 输出：{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[116.1983,39.7832],[116.0445,39.2323],[116.8959,39.3831],[116.8698,39.9182],[116.1983,39.7832]]]},"properties":{"area":3865207830},"id":"polygon.1"}]} System.out.println(ostream); // 第5个问题。坐标变换问题，由坐标顺序引发 SimpleFeatureIterator iterator = (SimpleFeatureIterator) featureCollection.features(); SimpleFeature simpleFeature = iterator.next(); Geometry geom = (Geometry) simpleFeature.getDefaultGeometry(); iterator.close(); System.out.println(geom.getArea()); // 输出：0.4043554020447081 MathTransform transform_1 = CRS.findMathTransform(CRS.decode("EPSG:4326"), CRS.decode("EPSG:3857"),true); // 下面一行代码会报异常：Exception in thread "main" org.geotools.referencing.operation.projection.ProjectionException: Latitude 116°11.8'N is too close to a pole. /*Geometry geom_3857 = JTS.transform(geom, transform_1); System.out.println(geom_3857.getArea());*/ }
上述事例代码给出了将GeoJSON解析成FeatureCollection时出现的一些问题。 第1个问题是得到的FeatureCollection坐标顺序是错误的，给出的GeoJSON坐标顺序是经度（EAST）在前，geotools读取时给出了默认的坐标顺序（纬度在前），看下面的org.geotools.geojson.feature.FeatureJSON的readFeatureCollection(Object input)方法源码：
// org.geotools.geojson.feature.FeatureJSON // input可以是File，Reader，InputStream等 public FeatureCollection readFeatureCollection(Object input) throws IOException { // 新建一个DefaultFeatureCollection对象， DefaultFeatureCollection features = new DefaultFeatureCollection(null, null); // FeatureCollectionIterator实现了FeatureIterator接口，是一个内部类，用于控制从geojson文本中读取要素和坐标系等信息。 FeatureCollectionIterator it = (FeatureCollectionIterator) streamFeatureCollection(input); while(it.hasNext()) { features.add(it.next()); } if (features.getSchema() != null && features.getSchema().getCoordinateReferenceSystem() == null && it.getHandler().getCRS() != null ) { try { // 只将坐标系信息写入，即只更改了坐标系 return new ForceCoordinateSystemFeatureResults(features, it.getHandler().getCRS()); } catch (SchemaException e) { throw (IOException) new IOException().initCause(e); } } return features; }
//org.geotools.geojson.feature.CRSHandler public boolean primitive(Object value) throws ParseException, IOException { if (state == 2) { try { try { crs = CRS.decode(value.toString()); //坐标顺序默认NORTH_EAST，与实际数据不符 } catch(NoSuchAuthorityCodeException e) { //try pending on EPSG try { crs = CRS.decode("EPSG:" + value.toString()); } catch(Exception e1) { //throw the original throw e; } } } catch(Exception e) { throw (IOException) new IOException("Error parsing " + value + " as crs id").initCause(e); } state = -1; } return true; }
String srs = CRS.lookupIdentifier(simpleFeatureType.getCoordinateReferenceSystem(),true); // 获取EPSG featureCollection = new ForceCoordinateSystemFeatureResults(featureCollection, CRS.decode(srs, true));
//org.geotools.geojson.feature.FeatureHandler void addGeometryType(SimpleFeatureTypeBuilder typeBuilder, Geometry geometry) { // 空间列名"geometry"，而不是"the_geom" typeBuilder.add("geometry", geometry != null ? geometry.getClass() : Geometry.class); typeBuilder.setDefaultGeometry("geometry"); }
// 构建新的SimpleFeatureType public static SimpleFeatureType retype(SimpleFeatureType oldType){ SimpleFeatureTypeBuilder typeBuilder = new SimpleFeatureTypeBuilder(); typeBuilder.init(oldType); // the_geom if("geometry".equals(oldType.getGeometryDescriptor().getLocalName())){ typeBuilder.remove("geometry"); typeBuilder.add("the_geom",oldType.getType("geometry").getBinding()); } //生成新的SimpleFeatureType return typeBuilder.buildFeatureType(); } // 新建一个方法，用于变换feature的type public static SimpleFeature retypeFeature(SimpleFeature feature,SimpleFeatureType newType) { SimpleFeatureBuilder builder = new SimpleFeatureBuilder(newType); // 遍历属性 for (AttributeDescriptor att : newType.getAttributeDescriptors()) { Object value = feature.getAttribute(att.getName()); // 空间列 if(Geometry.class.isAssignableFrom(att.getType().getBinding())){ builder.set("the_geom", feature.getDefaultGeometry()); continue; } builder.set(att.getName(), value); } return builder.buildFeature(feature.getID()); }
SimpleFeatureType newType = retype(simpleFeatureType); // ListFeatureCollection是FeatureCollection的一个子类 ListFeatureCollection listFeatureCollection = new ListFeatureCollection(newType); SimpleFeatureIterator iterator_3 = (SimpleFeatureIterator) featureCollection.features(); while (iterator_3.hasNext()){ SimpleFeature newFeature = retypeFeature(iterator_3.next(),newType); listFeatureCollection.add(newFeature); } iterator_3.close();
第3（坐标精度丢失）、第4（默认无坐标系和空值输出）、第5（由坐标顺序引发坐标变换）这三个问题。我用GeoJSON的static void write(Object obj, Object output)静态方法将FeatureCollection转化成了json文本输出，先看org.geotools.geojson.GeoJSON源码：
// 该类用于FeatureCollection、Feature和坐标系的JSON输出 public class GeoJSON { static GeometryJSON gjson = new GeometryJSON(); static FeatureJSON fjson = new FeatureJSON(); // 用的默认构造器 public static Object read(Object input) throws IOException { throw new UnsupportedOperationException(); } public static void write(Object obj, Object output) throws IOException { if (obj instanceof Geometry) { gjson.write((Geometry)obj, output); } else if (obj instanceof Feature || obj instanceof FeatureCollection || obj instanceof CoordinateReferenceSystem) { if (obj instanceof SimpleFeature) { fjson.writeFeature((SimpleFeature)obj, output); } else if (obj instanceof FeatureCollection) { fjson.writeFeatureCollection((FeatureCollection)obj, output); } else if (obj instanceof CoordinateReferenceSystem) { fjson.writeCRS((CoordinateReferenceSystem)obj, output); } else { throw new IllegalArgumentException("Unable able to encode object of type " + obj.getClass()); } } } }
// org.geotools.geojson.feature.FeatureJSON GeometryJSON gjson; // 决定坐标保留的位数 SimpleFeatureType featureType; AttributeIO attio; boolean encodeFeatureBounds = false; // true表示json文本中Feature输出bbox boolean encodeFeatureCollectionBounds = false; // true表示json文本中FeatureCollection输出bbox boolean encodeFeatureCRS = false; // true表示json文本中Feature输出坐标系 boolean encodeFeatureCollectionCRS = false; // true表示json文本中FeatureCollection输出坐标系 boolean encodeNullValues = false; // true表示识别值为null的属性 public FeatureJSON() { this(new GeometryJSON()); // GeometryJSON默认保留4为小数 } public FeatureJSON(GeometryJSON gjson) { // 自定义GeometryJSON，可控制小数位数 this.gjson = gjson; attio = new DefaultAttributeIO(); }
// 重写后的GeoJSON public class GeoJSON { static GeometryJSON gjson = new GeometryJSON(15); // 15位小数 static FeatureJSON fjson = new FeatureJSON(gjson); // 指定GeometryJSON public static Object read(Object input) throws IOException { throw new UnsupportedOperationException(); } public static void write(Object obj, Object output) throws IOException { if (obj instanceof Geometry) { gjson.write((Geometry)obj, output); } else if (obj instanceof Feature || obj instanceof FeatureCollection || obj instanceof CoordinateReferenceSystem) { // 值为null的属性也识别 fjson.setEncodeNullValues(true); // 输出坐标系文本 fjson.setEncodeFeatureCollectionCRS(true); if (obj instanceof SimpleFeature) { fjson.writeFeature((SimpleFeature)obj, output); } else if (obj instanceof FeatureCollection) { fjson.writeFeatureCollection((FeatureCollection)obj, output); } else if (obj instanceof CoordinateReferenceSystem) { fjson.writeCRS((CoordinateReferenceSystem)obj, output); } else { throw new IllegalArgumentException("Unable able to encode object of type " + obj.getClass()); } } } }
// fjson_15已经保留15位 fjson_15.setEncodeFeatureCollectionCRS(true); fjson_15.setEncodeNullValues(true); fjson_15.writeFeatureCollection(featureCollection,System.out); // 控制台输出和原始geojson一致
针对第5（由坐标顺序引发坐标变换）个问题，“org.geotools.referencing.operation.projection.ProjectionException: Latitude 116°11.8’N is too close to a pole”异常其实是由坐标顺序不正确导致，经纬度顺序调换后识别的坐标超出了范围，不是当前坐标系能表示的值了。这是一个隐藏问题，在处理另一些原数据或变换不同的坐标系时，不一定会产生这个异常，那用不合理的坐标顺序得到的结果是不正确的。调整代码如下即可以解决：
String srs = CRS.lookupIdentifier(simpleFeatureType.getCoordinateReferenceSystem(),true); // CRS.decode()方法可以设置经纬度顺序 MathTransform transform_2 = CRS.findMathTransform(CRS.decode(srs,true), CRS.decode("EPSG:3857",true),true); Geometry geom_3857 = JTS.transform(geom, transform_2); System.out.println(geom_3857.getArea()); // 输出：6.501222710260582E9
测试代码里输出了几何对象geom的面积，但这个面积很粗糙，只做测试用。给出的GeoJSON文本中的"area"值也只做参考，不是最精确的面积。大家都知道，EPSG:3857以EPSG:4326地理坐标系和投影方式为伪墨卡托的平面坐标系，给出的面积偏差较大。
1.2 方法二：读取本地txt文件进行解析
FeatJson.class
import com.geomesa.spark.SparkJTS.Operation; import com.vividsolutions.jts.geom.Coordinate; import com.vividsolutions.jts.geom.Geometry; import com.vividsolutions.jts.geom.GeometryFactory; import com.vividsolutions.jts.operation.distance.DistanceOp; import org.geotools.data.simple.SimpleFeatureIterator; import org.geotools.feature.FeatureCollection; import org.geotools.geojson.GeoJSON; import org.geotools.geojson.feature.FeatureJSON; import org.geotools.geojson.geom.GeometryJSON; import org.geotools.geometry.jts.JTSFactoryFinder; import org.opengis.feature.simple.SimpleFeature; import org.opengis.feature.simple.SimpleFeatureType; import java.io.*; public class FeatJson { static GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory(null); public static void main(String[] args) throws IOException { //读取本地文件 FileReader reader = new FileReader("D:/GitProjects/GeoMesa/GeoMesaSpark/src/main/resources/gsmc.txt"); BufferedReader bufferReader = new BufferedReader(reader); String dict = bufferReader.readLine(); //按行读取文件 //构造FeatureJSON对象，GeometryJSON保留15位小数 FeatureJSON featureJSON = new FeatureJSON(new GeometryJSON(15)); FeatureCollection featureCollection = featureJSON.readFeatureCollection(dict); SimpleFeatureType simpleFeatureType = (SimpleFeatureType) featureCollection.getSchema(); System.out.println(simpleFeatureType.getGeometryDescriptor().getLocalName()); OutputStream ostream = new ByteArrayOutputStream(); GeoJSON.write(featureCollection, ostream); System.out.println(ostream); SimpleFeatureIterator iterator = (SimpleFeatureIterator) featureCollection.features(); SimpleFeature simpleFeature = iterator.next(); Geometry geom = (Geometry) simpleFeature.getDefaultGeometry(); iterator.close(); System.out.println(geom.getLength()); System.out.println(geom.getCoordinate()); System.out.println(geom.getBoundary()); System.out.println(geom.getGeometryType()); //新建一个经纬度坐标对象 Coordinate coordinate1 = new Coordinate(1.357846020181606E7, 4505819.87283728); Coordinate[] coordinates2 = geom.getCoordinates(); Operation op = new Operation(); //求点到线的距离 System.out.println("距离："+op.distanceGeo(geometryFactory.createPoint(coordinate1),geometryFactory.createLineString(coordinates2))); //求点到线的最近一个点 System.out.println(DistanceOp.nearestPoints(geometryFactory.createPoint(coordinate1),geometryFactory.createLineString(coordinates2))); bufferReader.close(); reader.close(); } }
三、Java Code Examples
Java Code Examples for org.geotools.data.simple.simplefeaturecollection.features()
一些Java的示例代码：https://www.programcreek.com/java-api-examples/?class=org.geotools.data.simple.simplefeaturecollection&method=features
四、API
FeatureJSON：http://docs.geotools.org/stable/javadocs/org/geotools/geojson/feature/FeatureJSON.html