// 创建buffer
private String createBuff(String xcgj){
    StringWriter writer=new StringWriter();
    try {
        GeoJSONReader reader=new GeoJSONReader(xcgj);
        SimpleFeatureCollection featureCollection = reader.getFeatures();

        // 要素类型
        SimpleFeatureType featureType = DataUtilities.createType("MultiPolygon",
                "the_geom:MultiPolygon:srid=4326," +
                        "name:String");

        // 遍历要素进行处理
        SimpleFeatureIterator features = featureCollection.features();
        List<SimpleFeature> targetFeatures=new ArrayList<>();
        while (features.hasNext()) {
            SimpleFeature feature = features.next();
            // 获取要素几何图形
            Geometry defaultGeometry =(Geometry) feature.getDefaultGeometry();
            // 生成缓冲区
            Geometry buff=defaultGeometry.buffer(buff_distance);
            // 创建 feature 工厂
            SimpleFeatureBuilder featureBuilder=new SimpleFeatureBuilder(featureType);
            // 新建feature
            SimpleFeature buffFeature=featureBuilder.buildFeature(feature.getID());
            // 添加全部的属性，这个其实没有用
            buffFeature.setAttributes(feature.getAttributes());
            // 设置图形
            buffFeature.setDefaultGeometry(buff);
            // 加入最后的要素集合
            targetFeatures.add(buffFeature);
        }
        // 要素集迭代器用完记得关闭
        features.close();
        // 将新生成的 featureCoolection 转为 GeoJSON
        SimpleFeatureCollection targetCollection=new ListFeatureCollection(featureType,targetFeatures);
        FeatureJSON featureJSON=new FeatureJSON();
        featureJSON.writeFeatureCollection(targetCollection,writer);
    }catch (Exception e){
        log.error("createBuff",e);
    }
    return writer.toString();
}

// 生成缓冲区
Geometry buff=defaultGeometry.buffer(buff_distance);
// 为了解决空洞的问题,取最小外包矩形。
buff=buff.convexHull();

Geometry defaultGeometry =(Geometry) feature.getDefaultGeometry();
defaultGeometry.setSRID(4326);
result=defaultGeometry.getArea();

/**
 * 方法一
 */
Geometry defaultGeometry =(Geometry) feature.getDefaultGeometry();
defaultGeometry.setSRID(4326);

// 这里是以OGC WKT形式定义的是World Mercator投影，网页地图一般使用该投影
final String strWKTMercator = "PROJCS[\"World_Mercator\","
        + "GEOGCS[\"GCS_WGS_1984\","
        + "DATUM[\"WGS_1984\","
        + "SPHEROID[\"WGS_1984\",6378137,298.257223563]],"
        + "PRIMEM[\"Greenwich\",0],"
        + "UNIT[\"Degree\",0.017453292519943295]],"
        + "PROJECTION[\"Mercator_1SP\"],"
        + "PARAMETER[\"False_Easting\",0],"
        + "PARAMETER[\"False_Northing\",0],"
        + "PARAMETER[\"Central_Meridian\",0],"
        + "PARAMETER[\"latitude_of_origin\",0],"
        + "UNIT[\"Meter\",1]]";
CoordinateReferenceSystem mercatroCRS = CRS.parseWKT(strWKTMercator);
// 做投影转换，将WCG84坐标转换成世界墨卡托投影转
MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, mercatroCRS);
defaultGeometry= JTS.transform(defaultGeometry, transform);
double area=defaultGeometry.getArea();


/**
 * 方法二
 */
CoordinateReferenceSystem source = null;
source = CRS.decode("CRS:84");
CoordinateReferenceSystem target = null;
target = CRS.decode("EPSG:3857");
MathTransform transform = null;
transform = CRS.findMathTransform(source, target, true);
Geometry transformGeom= null;
transformGeom = JTS.transform(defaultGeometry, transform);
double area = transformGeom.getArea();

/**
 * 方法三，这里的代码有些问题
 */
CoordinateReferenceSystem srcCRS = CRS.decode("EPSG:4326");
CoordinateReferenceSystem crsTarget = CRS.decode("EPSG:3857");
// 投影转换
MathTransform transform = CRS.findMathTransform(crsTarget,srcCRS,true);
Geometry res = JTS.transform(defaultGeometry, transform);
double area=defaultGeometry.getArea();

 try{
    Geometry[] geos= xcfwGeomList.toArray(new Geometry[xcfwGeomList.size()]);
    GeometryFactory geometryFactory = new GeometryFactory();
    GeometryCollection geometryCollection = geometryFactory.createGeometryCollection(geos);
    Geometry resultGeom=geometryCollection.union();

    // 转为Feature
    SimpleFeatureType featureType = DataUtilities.createType("MultiPolygon",
            "the_geom:MultiPolygon:srid=4326," +
                    "name:String");
    SimpleFeatureBuilder featureBuilder=new SimpleFeatureBuilder(featureType);
    SimpleFeature buffFeature=featureBuilder.buildFeature("10000");
    buffFeature.setDefaultGeometry(resultGeom);
    StringWriter writer = new StringWriter();
    writer.append("{\"type\": \"FeatureCollection\",\"features\":[");
    FeatureJSON geojson = new FeatureJSON();
    geojson.writeFeature(buffFeature, writer);
    writer.append("]}");
    // 保存结果
    result.put("buffer",writer.toString());
}catch (Exception e){
    log.error("getReport",e);
}

Geometry polyline=new GeometryFactory().createLineString(polygonCoordinateList.toArray(new Coordinate[0]));

// 进行抽稀,距离为 5 米
double distance=5*200/Math.PI/6371000;
polyline=DouglasPeuckerSimplifier.simplify(polyline,distance);