geotools操作GeoJSON过程中的问题及相关源码（上）

       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;
    }

        所以第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));

       再看第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();

       listFeatureCollection即是最终的正确的对象。本回就介绍上述两个问题，下回介绍剩下的。

转载自：https://blog.csdn.net/aliasone/article/details/80186301