Question

我有以下课程：

public class MeetingCenter {
    private String name;
    private List<MeetingRoom> meetingRoomList;
}

public class MeetingRoom {
    private MeetingCenter meetingCenter;
    private String code;
    private String name;
    private List<Reservation> reservationList;
}

public class Reservation {
    private MeetingRoom meetingRoom;
    private String owner;
}

我想使用以下架构创建JSON：

此方法在调用toJson（）方法的行上引发异常：

private static void exportToJson(List<MeetingCenter> mcs) throws IOException {
    Gson gson = new Gson();
    String data = gson.toJson(mcs);

    JsonWriter writer = new JsonWriter(new FileWriter("export.json"));
    writer.setIndent("    "); // set indent

    writer.beginObject(); // document start
    writer.name("schema").value("PLUS4U.EBC.MCS.MeetingRoom_Schedule_1.0");
    writer.name("uri").value("ues:UCL-BT:UCL.INF/DEMO_REZERVACE:EBC.MCS.DEMO/MR001/SCHEDULE");
    writer.name("data").value(data);

    writer.endObject(); // document end
    writer.close();
}

例外：

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.StringBuffer.append(StringBuffer.java:380)
    at java.io.StringWriter.write(StringWriter.java:77)
    at com.google.gson.stream.JsonWriter.beforeName(JsonWriter.java:614)
    at com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:401)
    at com.google.gson.stream.JsonWriter.beginArray(JsonWriter.java:287)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:95)
    ....

Answer 1

您的所有对象都引用了其父母。

GSON查看MeetingCenter，然后尝试序列化其子MeetingRoom。 MeetingRoom具有对MeetingCenter的引用，因此GSON会绕圈绕圈，直到出现堆栈溢出为止。

要解决此问题，您可以确保只暴露孩子而不暴露父母。已经有很多问题表明了这一点。参见Java Gson Exclude fields during serialization。

例如，您的Reservation可能看起来像这样：

class Reservation {
    MeetingRoom meetingRoom;
    @Expose
    String owner;
}

我把剩下的交给你。

此外，您还可以在未启动数组的情况下调用writer.endArray()。删除该行。

writer.beginObject(); // document start
writer.name("schema").value("PLUS4U.EBC.MCS.MeetingRoom_Schedule_1.0");
writer.name("uri").value("ues:UCL-BT:UCL.INF/DEMO_REZERVACE:EBC.MCS.DEMO/MR001/SCHEDULE");
writer.name("data").value(data);

//writer.endArray(); removed
writer.endObject(); // document end
writer.close();

Answer 2

@Expose是避免stackoverflow异常的解决方案，但该语句

writer.name("data").value(data);

无效，因为数据将使用转义字符丰富。对于示例，您可以在数据字段中输入

"data": "{\"name\": \"center 1\" ... }"

所以反序列化阶段可能会有问题。

我的实现为MeetingCenter类提议了一个Container类，可以在其中配置架构和URI。

/** Container class configures the schema and URI */
public class Container {
    @Expose
    private String schema;
    @Expose
    private String uri;
    @Expose
    private List<MeetingCenter> data;
}

public class Reservation {
    private MeetingRoom meetingRoom;
    @Expose
    private String owner;
}

public class MeetingRoom {
    private MeetingCenter meetingCenter;
    @Expose
    private String code;
    @Expose
    private String name;
    @Expose
    private List<Reservation> reservationList;
}

public class MeetingCenter {
    @Expose
    private String name;
    @Expose
    private List<MeetingRoom> meetingRoomList;
}

public class Main {
    public static void main(String[] args){
        Container container = meetingCenterInitialization();

        GsonBuilder builder = new GsonBuilder();
        builder.setPrettyPrinting();
        // it is necessary to avoid stackoverflow
        builder.excludeFieldsWithoutExposeAnnotation();

        Gson gson = builder.create();

        String jsonString = gson.toJson(container);
        System.out.println(jsonString);


        Container container1 = gson.fromJson(jsonString, Container.class);
        System.out.println("\n\n\n\n" + container1.getData().get(0).getName());
    }
}

main方法的输出是

{
  "schema": "PLUS4U.EBC.MCS.MeetingRoom_Schedule_1.0",
  "uri": "ues:UCL-BT:UCL.INF/DEMO_REZERVACE:EBC.MCS.DEMO/MR001/SCHEDULE",
  "data": [
    {
      "name": "center name",
      "meetingRoomList": [
        {
          "code": "room 1",

         ...

如何使用gson和JsonWriter反序列化对象数组

2 个答案: