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我正在尝试反序列化一个包含 a 的对象，ConcurrentMap但出现异常。

Caused by: java.lang.IllegalArgumentException: Can not set java.util.concurrent.ConcurrentMap field com.example.Row.doubleValues to java.util.LinkedHashMap

我的函数看起来像这样：

T deserialise(final InputStream input, final Class<T> type) {
  GsonBuilder gsonBuilder = new GsonBuilder();
  Gson gson = gsonBuilder.create();
  InputStreamReader isr = new InputStreamReader(input);
  return gson.fromJson(isr, type);
}

如何让它正确反序列化地图？我需要提供自定义反序列化器吗？

请注意，我既有普通班Maps，也有ConcurrentMaps同班同学。我也是对类进行序列化的人，因此，如果我可以提供一个自定义序列化器来指定并发映射的类型，然后提供一个反序列化器来指定应该如何将它们创建为应该工作的对象。

更新：我反序列化的类如下所示：

public class Row implements Serializable {
    private static final long serialVersionUID = 1L; //there's a diff value here

    private final String key;
    private final ConcurrentMap<String, Double> doubleValues = Maps.newConcurrentMap();
    private Map<String, String> writeOnceValues = Maps.newHashMap();
    ...
}
4

1 回答 1

-1

这是我提出的解决方案TypeAdapter。在向您展示代码之前，我不得不说，final如果字段存在于已解析的字符串中，则不应将其用于字段，因为 Gson 将尝试为该字段分配值。

我改变Row的班级，只是为了展示解决方案。

public class Row implements Serializable {
    private static final long serialVersionUID = 1L; //there's a diff value here

    private final String key = "myKey";
    private ConcurrentMap<String, Double> doubleValues = Maps.newConcurrentMap();
    private Map<String, String> writeOnceValues = Maps.newHashMap();

    @Override
    public String toString() {
        return "Row [key=" + key.getClass() + ", doubleValues=" + doubleValues.getClass()
                + ", writeOnceValues=" + writeOnceValues.getClass() + "]";
    }

}

和类型适配器，我基本上使用标准解析但返回一个ConcurrentHashMap.

public final class ConcurrentHashMapTypeAdapter<K,V> extends TypeAdapter<ConcurrentMap<K,V>> {

  @Override
  public synchronized ConcurrentMap<K,V> read(JsonReader in) throws IOException {
    if (in.peek() == JsonToken.NULL) {
      in.nextNull();
      return null;
    }
    Type aType = new TypeToken<LinkedTreeMap<K,V>>() {}.getType();
    Gson g = new Gson();
    LinkedTreeMap<K,V> ltm = g.fromJson(in, aType);
    return new ConcurrentHashMap<K,V>(ltm); 
  }

  @Override
  public synchronized void write(JsonWriter out, ConcurrentMap<K, V> value) throws IOException {
     Gson g = new Gson();
     out.value(g.toJson(value));
  }
}

以及我向您展示它有效的测试方法。

public static void main(String[] args) {
    Gson simpleGson = new Gson();

    // deserialize row once to get a suitable JSON string
    String s = simpleGson.toJson(new Row());
    System.out.println("JSON string: " + s);

    Row r;
    try{
        r = simpleGson.fromJson(s, Row.class);
    } catch(Exception e){
        System.out.println("Oh no, assignment is not possible");
        e.printStackTrace();
    }


    GsonBuilder gb = new GsonBuilder();
    Type aType = new TypeToken<ConcurrentMap<String,Double>>() {}.getType();
    gb.registerTypeAdapter(aType, new ConcurrentHashMapTypeAdapter<String, Double>());

    Gson gsonWithTypeAdapter = gb.create();

    r = gsonWithTypeAdapter.fromJson(s, Row.class);
    System.out.println("Now it works");
    System.out.println("Row: " +r);
}

这是我的执行：

JSON string: {"key":"myKey","doubleValues":{},"writeOnceValues":{}}
Oh no, assignment is not possible
java.lang.IllegalArgumentException: Can not set java.util.concurrent.ConcurrentMap field stackoverflow.questions.q18856793.Row.doubleValues to com.google.gson.internal.LinkedTreeMap
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.set(Unknown Source)
    at java.lang.reflect.Field.set(Unknown Source)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:95)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:172)
    at com.google.gson.Gson.fromJson(Gson.java:803)
    at com.google.gson.Gson.fromJson(Gson.java:768)
    at com.google.gson.Gson.fromJson(Gson.java:717)
    at com.google.gson.Gson.fromJson(Gson.java:689)
    at stackoverflow.questions.q18856793.Q18856793.main(Q18856793.java:23)
Now it works
Row: Row [key=class java.lang.String, doubleValues=class java.util.concurrent.ConcurrentHashMap, writeOnceValues=class com.google.gson.internal.LinkedTreeMap]
于 2013-09-23T19:09:48.547 回答