用GSON反序列化为ImmutableMap [英] Deserialize to an ImmutableMap with GSON

问题描述

 starterItems：{
Appeltaart： 3，
Soap_50：3 
} 
  

...变成Guava ImmutableMap ：

  private ImmutableMap< String，Integer> starterItems; 
  

我以为我只是使用普通的GSON地图解析，然后创建一个不可变的结果副本，像这样：

  gb.registerTypeAdapter（ImmutableMap.class，new JsonDeserializer< ImmutableMap>（）{
 @SuppressWarnings unchecked）
 @Override public ImmutableMap deserialize（JsonElement json，Type typeOfT，JsonDeserializationContext context）throws JsonParseException {
返回ImmutableMap.copyOf（（Map）context.deserialize（json，Map.class））; 
} 
}）; 
  

但正如所料，这太简单了（没有类型信息）。我得到错误：

  com.google.gson.JsonParseException：JsonDeserializer MapTypeAdapter无法反序列化json object {Appeltaart： 3，Soap_50：3}给定类型接口java.util.Map 
  

我可以做什么我想要吗？

解决方案

这不是那么直截了当，因为您想要维护类型参数以构建包含正确的类型。要做到这一点，你可以使用 TypeAdapterFactory ，然后请求代理 TypeAdapter ，使用完全指定的 TypeToken 。

  public class ImmutableMapTypeAdapterFactory implements TypeAdapterFactory {
 
 public static final ImmutableMapTypeAdapterFactory INSTANCE = new ImmutableMapTypeAdapterFactory（）; 
 
 @Override 
 public< T> TypeAdapter< T>创建（Gson gson，TypeToken< T>类型）{
 if（！ImmutableMap.class.isAssignableFrom（type.getRawType（）））{
 return null; 
} 
 final TypeAdapter< T> delegate = gson.getDelegateAdapter（this，type）; 
返回新的TypeAdapter< T>（）{
 @Override 
 public void write（JsonWriter out，T value）throws IOException {
 delegate.write（out，value）; 
 
 $ b @Override 
 @SuppressWarnings（unchecked）
 public T read（JsonReader in）throws IOException {
 return（T）ImmutableMap。 copyOf（（Map）delegate.read（in））; 
} 
}; 
 
 
 
 
 
 
 $ b现在你还有另一个问题：默认的GSON  MapTypeAdapterFactory 将尝试创建一个 ImmutableMap 的实例，并对其进行修改。这显然是行不通的。您应该从 TypeToken< ImmutableMap< K，V>>  TypeToken< HashMap< K，V>> c>，但我真的不知道你是如何做到的。相反，当 ImmutableMap 时，可以使用 InstanceCreator 来欺骗GSON构建 HashMap  / code>实际上是必需的：
  public static  InstanceCreator< Map< K，V>> newCreator（）{
 return new InstanceCreator< Map< K，V>>（）{
 @Override 
 public Map< K，V> createInstance（Type type）{
返回新的HashMap< K，V>（）; 
} 
}; 
 
  
显然，您必须同时注册TypeAdapterFactory和InstanceCreator：
  GsonBuilder b = new GsonBuilder（）; 
 b.registerTypeAdapterFactory（new ImmutableMapTypeAdapterFactory（））; 
 b.registerTypeAdapter（ImmutableMap.class，ImmutableMapTypeAdapterFactory.newCreator（））; 
  
 
I'd like to use GSON to derialize:
"starterItems": {
    "Appeltaart": 3,
    "Soap_50": 3
}
...into a Guava ImmutableMap:
private ImmutableMap<String,Integer> starterItems;
I thought I'd just use regular GSON map parsing, then make an immutable copy of the result, like this:
    gb.registerTypeAdapter(ImmutableMap.class, new JsonDeserializer<ImmutableMap>() {
        @SuppressWarnings("unchecked")
        @Override public ImmutableMap deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            return ImmutableMap.copyOf((Map) context.deserialize(json, Map.class));
        }
    });
But as expected, that was too simple (there is no type information). I get the error:
com.google.gson.JsonParseException: The JsonDeserializer MapTypeAdapter failed to deserialized json object {"Appeltaart":3,"Soap_50":3} given the type interface java.util.Map
Can I do what I want?
解决方案
It's not that straightforward, since you would want to maintain the type parameters in order to build a map containing the right types. To do that, you can go with a TypeAdapterFactory, and ask there for a delegate TypeAdapter, using the fully specified TypeToken.
public class ImmutableMapTypeAdapterFactory implements TypeAdapterFactory {

    public static final ImmutableMapTypeAdapterFactory INSTANCE = new ImmutableMapTypeAdapterFactory();

    @Override
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        if (!ImmutableMap.class.isAssignableFrom(type.getRawType())) {
            return null;
        }
        final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
        return new TypeAdapter<T>() {
            @Override
            public void write(JsonWriter out, T value) throws IOException {
                delegate.write(out, value);
            }

            @Override
            @SuppressWarnings("unchecked")
            public T read(JsonReader in) throws IOException {
                return (T) ImmutableMap.copyOf((Map) delegate.read(in));
            }
        };
    }
}
Now you have another problem: the default GSON MapTypeAdapterFactory will try to create an instance of ImmutableMap, and modify it. This obviously won't work. You should create a TypeToken<HashMap<K, V>> from the TypeToken<ImmutableMap<K, V>>, but I honestly don't know how you can do that. Instead, you can use an InstanceCreator to trick GSON into building a HashMap when an ImmutableMap is actually required:
public static <K,V> InstanceCreator<Map<K, V>> newCreator() {
    return new InstanceCreator<Map<K, V>>() {
        @Override
        public Map<K, V> createInstance(Type type) {
            return new HashMap<K, V>();
        }
    };
}
Clearly you have to register both the TypeAdapterFactory and the InstanceCreator:
GsonBuilder b = new GsonBuilder();
b.registerTypeAdapterFactory(new ImmutableMapTypeAdapterFactory());
b.registerTypeAdapter(ImmutableMap.class, ImmutableMapTypeAdapterFactory.newCreator());


                        
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