GSON：使用java.util.TreeSet序列化对象

Question

How can I serialize a TreeSet properly? In order to give you an idea of what's not working I've set up this little demo project. The main goal is to print a JSON string of my QData object.

App.java

package de.company.gsonserializer;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;

public class App 
{
    public static void main( String[] args )
    {
        QData qdata = new QData();

        ArrayList<LData> arrayList = new ArrayList<LData>(1);

        LData l = new LData();
        Map<String, String> unsortedBuabList = new HashMap<String, String>();
        for (int i = 0; i < 5; i++) {
            unsortedBuabList.put("Key-" + i, "Value" + i);
        }
        SortedSet<Map.Entry<String, String>> sortedBuabList = new TreeSet<Map.Entry<String, String>>(
                new Comparator<Map.Entry<String, String>>() {
                    public int compare(Map.Entry<String, String> e1, Map.Entry<String, String> e2) {
                        return e1.getValue().compareTo(e2.getValue());
                    }
                });
        sortedBuabList.addAll(unsortedBuabList.entrySet());
        l.setBuabList(sortedBuabList);

        arrayList.add(l);
        qdata.setLocations(arrayList);

        System.out.println( qdata.toString() );
    }
}

QData.java

package de.it2media.gsonserializer;

import java.util.ArrayList;

import com.google.gson.Gson;

public class QData {

    private ArrayList<LData> locations = new ArrayList<LData>(0);

    public ArrayList<LData> getLocations() {
        return locations;
    }

    public void setLocations(ArrayList<LData> locations) {
        this.locations = locations;
    }

    @Override
    public String toString(){
        Gson gson = new Gson();
        String thisObj = gson.toJson(this);
        return thisObj;     
    }

}

LData.java

package de.it2media.gsonserializer;

import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.Map.Entry;

public class LData {

    private SortedSet<Entry<String, String>> buabList = new TreeSet<Map.Entry<String, String>>();

    public SortedSet<Entry<String, String>> getBuabList() {
        return buabList;
    }

    public void setBuabList(SortedSet<Entry<String, String>> buabList) {
        this.buabList = buabList;
    }

}

The output: {"locations":[{"buabList":[{},{},{},{},{}]}]}

Expected output would be something like: {"locations":[{"buabList":[{"key":"Key-0","value":"Value0"},{"key":"Key-1","value":"Value1"},{"key":"Key-2","value":"Value2"},{"key":"Key-3","value":"Value3"},{"key":"Key-4","value":"Value4"}]}]}

Do you might know why GSON is not working as I'd expect it to work?

Thanks for any help, highly appreciated!

Answer 1

The problem you are running into has nothing to do with the TreeSet , but rather with the fact that GSON does not know how to serialize a map Entry in the way that you would like. You therefore need to write a custom serializer for it, which looks something like this:

public static class EntrySerializer implements JsonSerializer<Entry<String, String>> {
    @Override
    public JsonElement serialize(Entry<String, String> entry, Type typeOfSrc, JsonSerializationContext context) {
        JsonElement serializedKey = context.serialize(entry.getKey());
        JsonElement serializedValue = context.serialize(entry.getValue());

        JsonObject jsonObject = new JsonObject();
        jsonObject.add("key", serializedKey);
        jsonObject.add("value", serializedValue);
        return jsonObject;
    }
}

When you create the Gson object, you then need to register this custom serializer:

Gson gson = new GsonBuilder()
        .registerTypeAdapter(Entry.class, new EntrySerializer())
        .create();

You can read more about custom serializers and deserializers in the GSON documentation .