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、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(转载自：Shanks7529)

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;
}


ForceCoordinateSystemFeatureResults是FeatureCollection接口的一个子类，直接更改坐标系信息，原数据中的坐标信息不变。坐标系的生成是在org.geotools.geojson.feature.CRSHandler类中，相关代码如下：

//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;
}


第1个问题可做如下修改，使坐标系正常：

String srs = CRS.lookupIdentifier(simpleFeatureType.getCoordinateReferenceSystem(),true); // 获取EPSG
featureCollection = new ForceCoordinateSystemFeatureResults(featureCollection, CRS.decode(srs, true));


第2个问题，空间列名称不是我们想要的"the_geom"，而是"geometry"，这使和另外一些数据源的FeatureCollection一起做操作时空间列不一致。相关代码在org.geotools.geojson.feature.FeatureHandler类中：

//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");
}


SimpleFeatureTypeBuilder类用于构建SimpleFeatureType，SimpleFeatureType描述了FeatureCollection对象属性、数据类型、坐标系等信息。第2个问题可做如下修改，使空间列变为"the_geom"：

// 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());
}


在测试代码中加入如下，得到最终的FeatureCollection：

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());
}
}
}
}


该类除写Geometry外都是调用FeatureJSON的方法，在看下FeatureJSON的构造器和实例变量：

// 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();
}


GeometryJSON的相关代码就不列出来了。解决第3（坐标精度丢失）、第4（默认无坐标系和空值输出）问题我们只需做一些设置。如果想统一用GeoJSON.write()方法写json文本，可以重写该类，设置精度，代码如下：

// 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());
}
}
}
}


如果就想用FeatureJSON操作输出，可以在测试代码中添加如下代码解决：

// 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。

原创作者：Shanks7529

文章链接：geotools操作GeoJSON过程中的问题及相关源码

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();
}
}


二、Java Code Examples

Java Code Examples for org.geotools.data.simple.simplefeaturecollection.features()

一些Java的示例代码

三、API

SimpleFeatureCollection

FeatureJSON