使用Gson在Java中实现HashMap的序列化/反序列化

Question

我试图使用Google的Gson库在Java中序列化和反序列化一个HashMap。我希望序列化这里所示的hashmap，将其保存到一个文件中，然后在以后的阶段从一个文件中读取并反序列化它。

HashMap如下所示

public static HashMap<Integer,HashMap <Integer, Type5Val>> type5Model = new HashMap<Integer, HashMap<Integer, Type5Val>>();

反序列化类的反序列化逻辑，以及我目前基于Web上找到的示例实现的对象类。IS：

public class Type5Val implements Serializable,JsonDeserializer<Type5Val>{

    public int modelLogLines;
    public int modelTime;       

    @Override
    public Type5Val deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {

        final JsonObject jsonObject = jsonElement.getAsJsonObject();

        System.out.println("Print JSON Object" + jsonObject.getAsString());

        final int t_modelLogLines = jsonObject.get("modelLogLines").getAsInt();
        final int t_modelTime = jsonObject.get("modelTime").getAsInt();

        //Populating
        Type5Val val = new Type5Val();
        val.modelLogLines = t_modelLogLines;
        val.modelTime = t_modelTime;

        return val;
    }
}

下面是在test函数中调用的序列化和反序列化代码。测试函数序列化上面HashMap的一个对象，创建一个字符串，然后尝试反序列化它。反序列化当前失败：

//serialization code
public static String dump(){

    // Configure Gson
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(Type5Val.class, new Type5Val());

    Gson g = gsonBuilder.create();
    String json = g.toJson(type5Model);
    return json;
}

//deserialization code
public static HashMap<Integer, HashMap<Integer,Type5Val>> undump(String json){

    // Configure Gson
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(Type5Val.class, new Type5Val());

    Gson g = gsonBuilder.create();
    HashMap<Integer,HashMap <Integer, Type5Val>> newModel =  g.fromJson(json, type5Model.getClass());
    return newModel;
}

public static void printMap(HashMap<Integer,HashMap<Integer,Type5Val>> modelInput){

    Iterator<HashMap<Integer, Type5Val>> it = modelInput.values().iterator();

    while(it.hasNext()){
        HashMap<Integer,Type5Val> temp = it.next();
        Iterator<Type5Val> it2 = temp.values().iterator();

        while(it2.hasNext()){
            Type5Val temp2 = it2.next();
            System.out.println("Time: " + temp2.modelTime + " Lines: " + temp2.modelLogLines);
            System.out.println(" Queue Size " + temp2.q.size() + "\n");
        }
    }
}

public static void testcase(){

    System.out.println("Printing Initial Map");
    printMap(type5Model);

    //serialization
    String s = dump();
    System.out.println(s);

    //deserialization
    HashMap<Integer, HashMap<Integer,Type5Val>> newModel = undump(s);

    System.out.println("Printing Map After Serialization/Deserialization");
    printMap(newModel);

}

Printing Initial Map
Time: 0 Lines: 39600000
 Queue Size 0

Time: 0 Lines: 46800000
 Queue Size 0

//Json output after serialization
{"1":{"1":{"modelLogLines":39600000,"modelTime":0,"q":[]},"2":{"modelLogLines":46800000,"modelTime":0,"q":[]}}}
Printing Map After Serialization/Deserialization
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at com.simontuffs.onejar.Boot.run(Boot.java:340)
        at com.simontuffs.onejar.Boot.main(Boot.java:166)
Caused by: java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to java.util.HashMap
        at violationChecker.type5Convert.printMap(type5Convert.java:207)
        at violationChecker.type5Convert.testcase(type5Convert.java:259)
        at violationChecker.type5Convert.main(type5Convert.java:294)
        ... 6 more

Answer 1

在这条线上：

HashMap<Integer,HashMap <Integer, Type5Val>> newModel =  g.fromJson(json, type5Model.getClass());

您告诉Gson将字符串反序列化为Hashmap。只是一个hashmap，而不是hashmap > ，因为所有关于泛型类型参数的信息都在运行时被删除，并且传递给它的是运行时已知的type5model类型。这迫使Gson猜测应该在hashmap中作为键和值的类型，并为值中的内部映射生成LinkedTreemap而不是hashmap。

要给Gson提供完整的类型，包括泛型类型参数，必须使用typeToken。要创建适当的typeToken，请执行以下操作：

new TypeToken<HashMap<Integer, HashMap<Integer, Type5Val>>>(){}.getType()

但是，如果没有typeToken，您仍然会在其他地方得到另一个错误，因为Gson不会在没有被告知某些对象应该属于type5val类的情况下猜测。