转换响应后 Retrofit2 中的 NullPointerException

Question

我正在尝试从 XML 中获取类别列表。我正在使用带有 SimpleXmlConverter 和 RxJavaCallAdapter 的 Retrofit2。获取响应成功，但它包含空对象引用。所以，当我试图将我的列表添加到 RecyclerView 的适配器时，我得到了 NPE。我认为，问题出在 POJO 类中。

这是我的可观察和对应的方法：

    restartableLatestCache(GET_CATEGORY_REQUEST,  //method from Nucleus MVP library,
            () -> api.getCategories("ukAXxeJYZN") //API method, returns Observable<Categories> (see below)
                    .subscribeOn(Schedulers.io())
                    .map(Categories::getCategories)
                    .observeOn(mainThread()),
            CategoriesFragment::onItems,
            CategoriesFragment::onNetworkError);

void onItems(List<Category> items) {
    adapter.add(items); //<- exception raises in this method
}

void onNetworkError(Throwable throwable) {
    adapter.hideProgress();
    if (throwable instanceof HttpException) {
        try {
            Log.e(TAG, "Something wrong with your query: " + ((HttpException) throwable).response().errorBody().string() );
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    if (throwable instanceof IOException) {
        Log.e(TAG, "Network Error or bad conversion: ", throwable.getCause());
    }

    Log.e(TAG, "Something goes wrong: ", throwable.getCause());
    Toast.makeText(getActivity(), throwable.getMessage(), Toast.LENGTH_LONG).show();
}

//adapter.add(items);
public void add(List<T> items) {
    int prevSize = this.items.size();
    List<T> list = new ArrayList<>(prevSize + items.size()); <- NPE raises exactly here. items is null, as debugger shows
    list.addAll(this.items);
    list.addAll(items);
    this.items = Collections.unmodifiableList(list);
    notifyItemRangeInserted(prevSize, items.size());
}

有一个例外：

FATAL EXCEPTION: main
              Process: com.example.testapplication, PID: 26290
              java.lang.IllegalStateException: Exception thrown on Scheduler.Worker thread. Add `onError` handling.
                  at rx.android.schedulers.LooperScheduler$ScheduledAction.run(LooperScheduler.java:112)
                  at android.os.Handler.handleCallback(Handler.java:739)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5438)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
               Caused by: rx.exceptions.OnErrorNotImplementedException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                  at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(InternalObservableUtils.java:386)
                  at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(InternalObservableUtils.java:383)
                  at rx.internal.util.ActionSubscriber.onError(ActionSubscriber.java:44)
                  at rx.observers.SafeSubscriber._onError(SafeSubscriber.java:157)
                  at rx.observers.SafeSubscriber.onError(SafeSubscriber.java:120)
                  at rx.exceptions.Exceptions.throwOrReport(Exceptions.java:204)
                  at rx.observers.SafeSubscriber.onNext(SafeSubscriber.java:144)
                  at rx.internal.operators.OperatorFilter$FilterSubscriber.onNext(OperatorFilter.java:73)
                  at rx.internal.operators.OnSubscribeCombineLatest$LatestCoordinator.drain(OnSubscribeCombineLatest.java:286)
                  at rx.internal.operators.OnSubscribeCombineLatest$LatestCoordinator.combine(OnSubscribeCombineLatest.java:228)
                  at rx.internal.operators.OnSubscribeCombineLatest$CombinerSubscriber.onNext(OnSubscribeCombineLatest.java:383)
                  at rx.observers.SafeSubscriber.onNext(SafeSubscriber.java:139)
                  at rx.internal.operators.OperatorFilter$FilterSubscriber.onNext(OperatorFilter.java:73)
                  at rx.internal.operators.OperatorMaterialize$ParentSubscriber.onNext(OperatorMaterialize.java:113)
                  at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.call(OperatorObserveOn.java:227)
                  at rx.android.schedulers.LooperScheduler$ScheduledAction.run(LooperScheduler.java:107)
                  at android.os.Handler.handleCallback(Handler.java:739) 
                  at android.os.Handler.dispatchMessage(Handler.java:95) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:5438) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629) 
               Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                  at com.example.testapplication.util.adapters.SimpleListAdapter.add(SimpleListAdapter.java:29)
                  at com.example.testapplication.ui.cat.CategoriesFragment.onItems(CategoriesFragment.java:74)
                  at com.example.testapplication.ui.cat.CategoriesPresenter.-com_example_testapplication_ui_cat_CategoriesPresenter-mthref-1(CategoriesPresenter.java:43)
                  at com.example.testapplication.ui.cat.CategoriesPresenter$-void_onCreate_android_os_Bundle_savedState_LambdaImpl1.call(CategoriesPresenter.java)
                  at nucleus.presenter.delivery.Delivery.split(Delivery.java:26)
                  at nucleus.presenter.RxPresenter$4.call(RxPresenter.java:266)
                  at nucleus.presenter.RxPresenter$4.call(RxPresenter.java:263)
                  at rx.internal.util.ActionSubscriber.onNext(ActionSubscriber.java:39)
                  at rx.observers.SafeSubscriber.onNext(SafeSubscriber.java:139)
                  at rx.internal.operators.OperatorFilter$FilterSubscriber.onNext(OperatorFilter.java:73) 
                  at rx.internal.operators.OnSubscribeCombineLatest$LatestCoordinator.drain(OnSubscribeCombineLatest.java:286) 
                  at rx.internal.operators.OnSubscribeCombineLatest$LatestCoordinator.combine(OnSubscribeCombineLatest.java:228) 
                  at rx.internal.operators.OnSubscribeCombineLatest$CombinerSubscriber.onNext(OnSubscribeCombineLatest.java:383) 
                  at rx.observers.SafeSubscriber.onNext(SafeSubscriber.java:139) 
                  at rx.internal.operators.OperatorFilter$FilterSubscriber.onNext(OperatorFilter.java:73) 
                  at rx.internal.operators.OperatorMaterialize$ParentSubscriber.onNext(OperatorMaterialize.java:113) 
                  at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.call(OperatorObserveOn.java:227) 
                  at rx.android.schedulers.LooperScheduler$ScheduledAction.run(LooperScheduler.java:107) 
                  at android.os.Handler.handleCallback(Handler.java:739) 
                  at android.os.Handler.dispatchMessage(Handler.java:95) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:5438) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629) 

你可以看看我的 XML here。

我的 POJO：

@Root(strict = false, name = "categories")
public class Categories {

  @Path("yml_catalog/shop")
  @ElementList(inline = true, entry="category", type = Category.class)
  private List<Category> categories;
  //getters, setters
}

@Root(strict = false)
@Path("yml_catalog/shop/categories")
public class Category {
  @Attribute
  private int id;

  @Text
  private String categoryName;
  //getters, setters
}

那么，我怎样才能修复我的 POJO 以获得正确转换的对象？

Answer 1

获取响应成功

引用语句的问题在于，使用 response.isSuccessful() 返回 true 时的响应实际上可能返回状态代码范围。

例如：

response.isSuccessful() 返回 true 但 response.code() 返回 389 表示某种错误，并为您返回 null 对象。

所以通常您希望以这种方式处理响应：

            @Override
            public void onResponse(Call<Login> call, Response<Login> response) {

                if (response.code() == 200 && response.isSuccessful()) {
                    // all is good
                } else {
                    // something went wrong;
                }

            }

            @Override
            public void onFailure(Call<Login> call, Throwable t) {
                // Log error here since request failed
            }

但这显然需要您进一步调查。 你能确认你从响应中得到的对象是好的吗？

Answer 2

我已经找到了我的问题的解决方案，但在这里我尝试更广泛地解释这个问题。

首先，当您像我一样拥有包含要从中提取的分离模型的巨大 XML 文件时，您必须创建自定义转换器。默认情况下，SimpleXML 不适用于大型 XML。

应该是这样的：

public class CategoriesConverter implements Converter<Categories> {

@Override
public Categories read(InputNode node) throws Exception {
    Categories categories = new Categories(); 
    InputNode childNode = node.getNext(); //InputNode passed there points to the document root, so we're starting from the next one. 
    List<Category> categoriesList = new ArrayList<>(); 
    while( childNode != null ) { // while we're in the document tree
        if (childNode.getName().equals("categories")) {
            InputNode innerChild = childNode.getNext();

            while (innerChild != null) {
                Category category = new Category();
                if (innerChild.getName().equals("category")) {
                    category.setId(Integer.parseInt(innerChild.getAttribute("id").getValue()));
                    category.setText(innerChild.getValue());
                    categoriesList.add(category);
                }

                innerChild = childNode.getNext();
            }
        }

        childNode = node.getNext();
    }

    categories.setCategories(categoriesList);
    return categories;
}

    @Override
    public void write(OutputNode node, Categories value) throws Exception {
        //we don't need to serialize objects now, so it does nothing
    }
}

简而言之，我们需要通过文档树，手动获取我们需要的标签的属性和值。需要时，我们也可以填写列表或地图。这种方法只有在您准确了解文档的结构并在模型中重复时才有用。

模型类中所有带注释的字段（@Attribute、@Element 等）都应将参数 required 设置为 false，并明确指定 name 参数，例如：

  @Element(required = false, name = "text")
  private String text;

并且模型类应该用@Root(strict = false)和@Convert(MyConverter.class)注解，其中MyConverter是你为这个模型编写的转换器。每个模型都应该有自己的转换器。

要使其工作，您需要为 SimpeXML 指定 AnnotationStrategy。它关注@Convert 注释。你可以这样做：

Serializer serializer = new Persister(new AnnotationStrategy());

如果你将 Retrofit2 与 SimpleXML 转换器和 Dagger 一起使用，你可以编写以下代码：

@Provides
@Singleton
ServerAPI provideServerAPI() {
    return new Retrofit.Builder()
            .baseUrl(ServerAPI.ENDPOINT)
            .client(client)
            .addConverterFactory(SimpleXmlConverterFactory.createNonStrict(new Persister(new AnnotationStrategy())))
            .build().create(ServerAPI.class);
}

就是这样！希望这会有所帮助。