Java - 调用 echarts 提供的地图压缩方法压缩地图

蔚落 2022-06-01 12:13 303阅读 0赞

由于默认情况下的 GeoJSON 文件太大，ECharts 提供了方法可以压缩地图。

> GeoJSON 介绍：[http://geojson.org/][http_geojson.org]

ECharts 提供了压缩算法，但是代码是 JavaScript 的，参考代码写个 Java 的也可以，但是通过 Java 提供的脚本引擎可以很方便的直接执行 JS。

本文压缩代码参考下面的项目：

> [https://github.com/giscafer/mapshaper-plus][https_github.com_giscafer_mapshaper-plus]  
> 这里的算法也是来源于 ECharts

从该项目提取出压缩地图的 JS 代码，简化后如下：

一个河北省的 GeoJSON 示例（文件大小为 55 kb）：

下面使用 Java 8 中的脚本引擎执行该代码。

package tk.mybatis;
    
    import javax.script.Invocable;
    import javax.script.ScriptEngine;
    import javax.script.ScriptEngineManager;
    import javax.script.ScriptException;
    import java.io.FileNotFoundException;
    import java.io.FileReader;
    
    public class Encoder { 
    
        public static final ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
    
        static {
            try {
                //注意这里的 encode.js 路径，文件内容就是上面的 js 代码
                engine.eval(new FileReader(Encoder.class.getResource("/encode.js").getPath()));
            } catch (ScriptException e) {
                e.printStackTrace();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }
    
        public static final Invocable invocable = (Invocable) engine;
    
        /** * 压缩为 echarts 格式的地图 * * @param geoJson * @param fileName * @param type json 或 其他（按js处理） * @return */
        public static String convert2Echarts(String geoJson, String fileName, String type) {
            try {
                return (String) invocable.invokeFunction("convert2Echarts", geoJson, fileName, type);
            } catch (ScriptException e) {
                throw new RuntimeException(e);
            } catch (NoSuchMethodException e) {
                throw new RuntimeException(e);
            }
        }
    
    }

用法：

String json = Encoder.convert2Echarts(
        "上面提供的河北省的GeoJSON内容", "河北省", "json");

压缩后的结果如下（文件大小为 9 kb）：

{
        "type": "FeatureCollection",
        "cp": [115.4004, 39.4688],
        "size": "1300",
        "features": [{ "type": "Feature", "properties": { "id": "1308", "name": "承德市", "cp": [117.5757, 41.4075], "childNum": 11 }, "geometry": { "type": "Polygon", "coordinates": 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40.8527], "childNum": 15 }, "geometry": { "type": "Polygon", "coordinates": ["@@k™ġÛ™al¥@wn@nml¹UWlaVknUVƒ„Kla„@„Ušƒ@_ma@ƒœ¥WwnaU‚wnƒmw@KXaVUVašUnmWUk°™lƒnUVUXWVw™IWVóKUI@WXƒxUU@mma@kUKWLkw@yk@ƒaVkUUċaUU@Wk@Unm@UVmLm±IUƒkJ™kWƒ@a„I@m@U„ƒVƒ„Ula„@VXVƒXmVwnkWKƒKU_k@m¥ƒmX_™JmnU@km@U@KmU™VƒU@U™@Umk@@LƒmW@Û£Wƒka@wk™@aƒI@mmk@mUa@UmUƒIƒwW@aWUƒbU@kbÇ@kwƒ@makVUk™U@aƒm@aU@mxkUƒbƒKUXUƒ±KXVWLUK@wkU@V™@WXUa@WbUxƒJIƒŽ@¦VèVVX@±ê¯KUIƒ`¯UULVx@Vƒ@UKƒIƒVkLmVkKmš@nUJÝbkIUJVXšVVxVbU„VJ„Un™°bV„mlU°„XnK@Ul@lVÈVUXšx@W„@VXšV‚KÞb„n@VnbV„m`ƒUx™kW@UVkL™Km¼@lUnUJVnV„XV@Vm@@LV„klƒIkl@VƒWlŽULWKUL@mJ„@blbUVUlmzUJUxm@UUbċÜk@Ub@VšLVV„¦ôbVŽmšUKUkU@m„„@VlVn¼WbUJ¯@@„°šnIllÈl˜@nXšWlLœk‚J@bkxlxkxlXUlklJƒšXL@bW„n`@nÆŽXxlL@xl@Xb‚LœKlVlIXblVUbUJW@lX@VL@VVŽšXšJšw„n@WnL°K„bVbl@VI@K„@U@nmVmV@XUWI@aXm@™VUUkWmn@lmUUk@mUmK@UnwVĉ@ƒƒmU_V@XJôVVUšLVUn@šllUnJl_n@šml@XŽlLlw²LVJUL@VmbVblVXmVnlš@Ť¦„nn@ÜŽ@bšl„@@XV`„Unb@VlLVb²J‚Xn¥ÆÑ@¥Þ@"], "encodeOffsets": [[118868, 42784]] } }, { "type": "Feature", "properties": { "id": "1306", "name": "保定市", "cp": [115.0488, 39.0948], "childNum": 23 }, "geometry": { "type": "Polygon", "coordinates": ["@@VbXWš@@UlV@xVLXKWU²LV„VW„L„alVnwV@@b„n@bšVVllUnb„@lxÈ@laV@„aXV@b‚X„x„J‚nV@VVb@nnl@n„J@blšl@„ašƒU_VWUwVUškUm™Ukb±mVwœU@VIUW@UWk„@VU@ynL„m@IV@‚bnK„LVaVmnIlaXwV@@WVL°@@xnX„@V`V@VbUVVLVKnwnL@ll@@_V@VVnaÆ@œKVXÆ@nƒ@wƒKmU—™Wm@km@kÜKXU@ÑW±nIUwVƒ„Kla@I°wU±kškmm¯mƒ_ƒJnƒaƒwW@IVaUama@wƒUƒmU@mVw@aXk@mWa@£km@aƒ_kVmUnWW@¯bƒkUmk@ƒVÇm@@kUU™KUU™@UVUamVUaWIkb@xU@@amUkKƒVkam@@kVUkUWmKmUkLUb@xmJƒ™U@UImVÛVmnUwƒJƒU@VƒX@UWm@Ub°¦UšmxklmX@`ULU@@UW@@xkn¯@makV™UmxUb™°ƒlUšƒbUbƒnUJƒUUVƒa™LkbUU›JUU@mUUUƒJkaƒ@™xUIWJƒUnƒJ@V™zƒ@kb@`@bln@l™bƒŽ@X@š@š„@Xl‚bnbVb„@„„VJlInlšbVw@U„K„l@lbnan@Vb‚JôLn‚UzlV@lÈLVbVK@LVx—VWXX`WxXz‚bV`UXV¤nx@„bVlVnVlUL"], "encodeOffsets": [[117304, 40512]] } }, { "type": "Feature", "properties": { "id": "1302", "name": "唐山市", "cp": [118.4766, 39.6826], "childNum": 11 }, "geometry": { "type": "Polygon", "coordinates": 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{ "id": "1311", "name": "衡水市", "cp": [115.8838, 37.7161], "childNum": 11 }, "geometry": { "type": "Polygon", "coordinates": ["@@„KVlV@X°xƒb@VnnmbVŽXblb@VkL@lV@Vbn@@l‚@XX@bWVXlmXnlV„V@@VUbƒK¯LUl@nmbV¤n@l‚LXnlVUV@ln@lb„UlLnV@bV@@wlaXJVbnUVbVUš@VVšLVVn@VVX@@U‚KXU˜U@wUK@U„wVnk@UUWlk„V@a„UVUÆ`X_ƒw@mlU@anUmK@UXal¥„UmƒÈLVbVxVL„a„bVW@nXU‚Vn„„V°UŤV@Uƒ¯Um@Uƒ@@U™UaƒWVUmUUƒU@k£Vw™W@wW@XKƒIUa@wU@@al@UK@_mKXKƒbUU@aVKmš@Xmƒƒ±@kbÇakLğVaUw@a@ƒmkUJƒk@ykw@£ƒWX@lknk@WVkbUŽVnUVƒL@‚mVkI@JUb›I@JXb™XllkLUmƒLmbV`kLƒx¯Lk„›VUV@VôXkVVL„V™V@xƒVUbW@Kxƒl™L¯kV`UnV¦°@"], "encodeOffsets": [[118024, 38549]] } }, { "type": "Feature", "properties": { "id": "1310", "name": "廊坊市", "cp": [116.521, 39.0509], "childNum": 9 }, "geometry": { "type": "MultiPolygon", "coordinates": 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        "UTF8Encoding": true }

[http_geojson.org]: http://geojson.org/
[https_github.com_giscafer_mapshaper-plus]: https://github.com/giscafer/mapshaper-plus