java - Android 和 GSON 抛出 No-Args Constructor not found 异常

Question

我的 Android 应用程序中出现 GSON 问题。

JSON 输入

[
    {"id":"1202","parent_id":"0","agrpcount":"0","disp_order":"1","code":"PROD-NEW","name":"New Products","active":"1"},
    {"id":"1119","parent_id":"0","agrpcount":"0","disp_order":"2","code":"PROD-VID","name":"\\\"How To\\\" Videos","active":"1"},
    {"id":"1685","parent_id":"0","agrpcount":"0","disp_order":"3","code":"SUPP-RESOUR","name":"Support Resources","active":"1"},
    {"id":"49","parent_id":"0","agrpcount":"0","disp_order":"109","code":"EQ","name":"Equipment for Sign Making","active":"1"},
    {"id":"22","parent_id":"0","agrpcount":"0","disp_order":"110","code":"application-tape","name":"Application Tapes","active":"1"}
]

每个数组项的容器类

public class Category {
    public int id;
    public int parent_id;
    public int agrpcount;
    public int disp_order;
    public String code;
    public String name;
    public int active;
}

加载类别数组的类

public static List<Category> getCategories(){
    Category[] cats;

    Gson gson = new Gson();

    cats = gson.fromJson(strJSON, Category[].class);

    List<Category> CatsList = Arrays.asList(cats);
    return CatsList;
}

当我运行这段代码时，我在日志中得到以下内容

02-28 15:15:42.471: ERROR/AndroidRuntime(777): Caused by: java.lang.RuntimeException: No-args constructor for class com.signwarehouse.catalog.SWService$Category does not exist. Register an InstanceCreator with Gson for this type to fix this problem.
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.MappedObjectConstructor.constructWithNoArgConstructor(MappedObjectConstructor.java:64)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.MappedObjectConstructor.construct(MappedObjectConstructor.java:53)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonObjectDeserializationVisitor.constructTarget(JsonObjectDeserializationVisitor.java:40)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonDeserializationVisitor.getTarget(JsonDeserializationVisitor.java:56)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:109)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonDeserializationVisitor.visitChild(JsonDeserializationVisitor.java:107)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonDeserializationVisitor.visitChildAsObject(JsonDeserializationVisitor.java:95)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonArrayDeserializationVisitor.visitArray(JsonArrayDeserializationVisitor.java:72)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:117)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonDeserializationContextDefault.fromJsonArray(JsonDeserializationContextDefault.java:64)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:49)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.Gson.fromJson(Gson.java:568)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.Gson.fromJson(Gson.java:515)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.Gson.fromJson(Gson.java:484)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.Gson.fromJson(Gson.java:434)
02-28 15:15:42.471: ERROR/AndroidRuntime(777):     at com.google.gson.Gson.fromJson(Gson.java:406)

我尝试向 Category 类添加一个无参数构造函数，但这没有帮助。不足为奇，因为 Java 会自动创建它。

有人知道如何解决这个问题吗？

谢谢。

Answer 1

尝试将类别类设为公共(public)静态类...

public static class Category {/* fields */}

我敢打赌，由于您没有将类设为静态，所以它首先尝试创建您的父类/外部类 (com.signwarehouse.catalog.SWService)，这可能没有无参数构造函数。