Why @ResponseBody returns sorted LinkedHashMap as not sorted?

问题

Here is SpringMVC Controller code snippet:

@RequestMapping(value = "/getCityList", method = RequestMethod.POST)
public @ResponseBody LinkedHashMap<String, String> getCityList(@RequestParam(value = "countryCode") String countryCode, HttpServletRequest request) throws Exception {
    //gets ordered city list of country  [sorted by city name]
    LinkedHashMap<String, String> cityList = uiOperationsService.getCityList(countryCode); 

    for (String s : cityList.values()) {
        System.out.println(s); //prints sorted list  [sorted by name]
    }
    return cityList;
}

Here is ajax call:

function fillCityList(countryCode) {
        $.ajax({
            type: "POST",
            url: '/getCityList',
            data: {countryCode:countryCode},
            beforeSend:function(){
                $('#city').html("<option value=''>-- SELECT --</option>" );
            }
        }).done(function (data) {

            console.log(data); // UNSORTED JSON STRING  [Actually sorted by key... not by city name]

        })
    }

Sorted LinkedHashMap returns as unsorted JSON object from getCityList method. Why order is changed during return process ? Is LinkedHashMap converted to HashMap because of ResponseBody annotation? I can convert my sorted object to json string via Gson library and return json string from my getCityList method but i don't like this solution. What can i do to provide javascript callback method with sorted list?

回答1:

You're expecting a JSON object's entries to have the same order as the LinkedHashMap entries. That won't happen, because JavaScript object keys have no intrinsic order. They're just like Java HashMaps.

If you need a JavaScript data structure that maintains an order, you should use an array, not an object. Return a sorted List<City> from your method, where City has a key and a value.

回答2:

If you wanted to send sorted data then pass List of Your object or City as List<City> which have sorted data.

I have also face same problem when I have used HashMap in my application, it doesn't guarantee that it will receive in same order in which we add them. We have to access it via it's key.

so it's better to use List instead of LinkedHashMap.

Most implementations of JSON make no effort to preserve the order of an object's name/value pairs, since it is (by definition) not significant.

来源：https://stackoverflow.com/questions/26085353/why-responsebody-returns-sorted-linkedhashmap-as-not-sorted

标签

java

json

sorting

spring-mvc

linkedhashmap