问OpenLayers.Layer.Markers的问题-地图上仅显示上次添加的标记

EN

function init_map() {
    var map, layer;
    map = new OpenLayers.Map('map');
    layer = new OpenLayers.Layer.WMS( "OpenLayers WMS",
        "http://vmap0.tiles.osgeo.org/wms/vmap0", {layers: 'basic'} );
    map.addLayer(layer);
    map.setCenter(new OpenLayers.LonLat(43.5, 40), 10);
    var size = new OpenLayers.Size(21,25);
    var offset = new OpenLayers.Pixel(-(size.w/2), -size.h);
    var icon = new OpenLayers.Icon('http://www.openlayers.org/dev/img/marker.png',size,offset);
    var markers = new OpenLayers.Layer.Markers("Objects");
    map.addControl(new OpenLayers.Control.LayerSwitcher());
    map.addLayer(markers);
    $.ajax({
        type: "GET",
        url: "/rest/displaymap/getMarkers/",
        dataType: "json",
        error: function() {
            console.log('failed to retrieve data');
        },
        success: function(d) {
            console.log(d);
            var i;
            for(i=0; i<d.length; i++) {
                var mrk = new OpenLayers.Marker(new OpenLayers.LonLat(d[i]['objectLon'],d[i]['objectLat']), icon);
                markers.addMarker(mrk);
            }    
        }
    });
}