Android and GSON throwing No-Args Constructor not found exception

问题

I am having a problem with GSON in my Android app.

JSON Input

[
    {"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"}
]

Container class for each array item

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;
}

Class to load category array

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;
}

When I run this code I get the following in the Log

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)

I tried adding a no-args constructor to the Category class, but this didn't help. No surprise since Java creates it automatically.

Does anyone have any idea how to fix this?

Thanks.

回答1:

Trying making the Category class a public static class...

public static class Category {/* fields */}

I bet what's happening is since you didn't make the class static it is trying to create your parent/outer class (com.signwarehouse.catalog.SWService) first which probably doesn't have a no-arg constructor.

来源：https://stackoverflow.com/questions/5148011/android-and-gson-throwing-no-args-constructor-not-found-exception

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