public JTSGeometryExpression<Geometry> boundary() {
        if (boundary == null) {
            boundary = JTSGeometryExpressions.geometryOperation(SpatialOps.BOUNDARY, mixin);
        }
        return boundary;
    }
Returns the closure of the combinatorial boundary of this geometric object

@return boundary
querydsl-spatial/src/main/java/com/querydsl/spatial/locationtech/jts/JTSGeometryExpression.java