how to pass json object from java to javascript?

Question

I'm trying to pass data from spring controller to javascript but with no luck. Should I use ajax to do this? Please could you give me some hints on how do this? What is the best way?

In my controller I try to pass data:

@RequestMapping(value = "/map", method = RequestMethod.GET)
public String map(ModelMap model) {

...

model.addAttribute("trackpoints", json);


return "map";

}

where json is a gson object (JsonObject) containing:

{"trackpoints":[{"latitude":52.390556,"longitude":16.920295},
{"latitude":52.390606,"longitude":16.920262}]}

in my jsp file I have:

<script type="text/javascript">

var myJSON = {};

myJSON = ${trackpoints};

document.writeln(myJSON.trackpoints);

</script>

but the result is:

[object Object],[object Object]

I explain this in more detail: >

i want use google maps api to display map and draw path coordinated from many lat,longs. i reckon json will be better than list, but i can be wrong.

i try to adjust code from documentation - in code below i try to replace hardcoded coordinates with loop, and values from json object.

<script type="text/javascript">
function initialize() {
    var myLatLng = new google.maps.LatLng(0, -180);
    var myOptions = {
        zoom : 3,
        center : myLatLng,
        mapTypeId : google.maps.MapTypeId.TERRAIN
    };

    var map = new google.maps.Map(document.getElementById("map_canvas"),
            myOptions);
    var flightPlanCoordinates = [
            new google.maps.LatLng(37.772323, -122.214897),
            new google.maps.LatLng(21.291982, -157.821856),
            new google.maps.LatLng(-18.142599, 178.431),
            new google.maps.LatLng(-27.46758, 153.027892) ];
    var flightPath = new google.maps.Polyline({
        path : flightPlanCoordinates,
        strokeColor : "#FF0000",
        strokeOpacity : 1.0,
        strokeWeight : 2
    });

    flightPath.setMap(map);
}
</script>

i hope it's now more clear:)

Answer 1

myJSON.trackpoints is an array of two objects. If you want to write it as HTML you could do something like this:

function writeCoordinates(coords) {
    document.writeln('<div>lat = ' + coords.latitude);
    document.writeln(', lon = ' + coords.longitude + '</div>');
}

int len = myJSON.trackpoints.length;
for (int i = 0; i < len; i++) {
    writeCoordinates(myJSON.trackpoints[i]);
}

BTW: JSON is really useful when you're using AJAX requests, but for "normal" requests it's better to put plain Java objects into the model, for example:

Spring Controller:

List<Coordinate> trackpoints = ...
model.addAttribute("trackpoints", trackpoints);

JSP :

<c:forEach items="${trackpoints}" var="coord">
    <div>lat = ${coord.latitude}, lon = ${coord.longitude}</div>
</c:forEach>

given, that the Coordinate class has the methods getLatitude() and getLongitude() . That method in the Spring controller can even be used for both "normal" and AJAX requests, by using a JSON encoder, like Jackson , and the ContentNegotiatingViewResolver .

Answer 2

Take a look at: http://jsfiddle.net/AHue8/4/

I guess the take-away here is that you need to specify which element in the trackpoint object you want and you countrol how those are displayed in the browser. Since a "trackPoint" isn't a primitive data type (it's an object with 2 data elements latitude and longitude), the browser doesn't know how to interpret it, so you have to tell it how.