java - 对封闭类实例的内部类引用为空

Question

我有一个包含内部类和内部实例集合的类:

public class Outer {

    private List<Inner> items = new ArrayList<Inner>(); 

    ... 

    public List<Inner> getItems(){
        return this.items;
    }

    public int getOuterX() {
        ...
    }
    ...

    public class Inner {
        ... 
        public int getInnerX(){     
            return Outer.this.getOuterX(); // this line throws the exception 
        }
    }
}

在其他类中，我遍历 Outer 实例的项目，并对每个项目调用 getInnerX():

for (Outer.Inner item : outer.getItems()) {
    item.getInnerX();
}

NullPointerException 在 getInnerX() 中抛出，因为 Outer.this 为 null。

我不知道这怎么会发生。

编辑:

我正在准备@Sotirios Delimanolis 要求的示例，但我无法重现该错误。所以它是我没有包含在问题中的东西。

我正在使用 retrofit2 并且 Outer 对象是根据来自服务器的 json 响应创建的。

Outer 的 item 属性有 GSon 转换器的注解:

@SerializedName("items")
@Expose
private List<Inner> items = new ArrayList<Inner>(); 

Outer 和 Inner 都没有定义构造函数。

这是堆栈跟踪:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String 
com.example.server_comunication.datatypes.OrderResponse$Order.getStatus()' on a null object reference
      at com.example.server_comunication.datatypes.OrderResponse$Order$Supply.forPlacement(OrderResponse.java:408)
      at com.example.server_comunication.datatypes.OrderResponse$Order$Supply.getRemaining(OrderResponse.java:423)
      at com.example.uiHelpers.UIFactoryPanels.drawPanelSuppliesEditor(UIFactoryPanels.java:142)
      at com.example.OrderFragment.onCreateView(OrderFragment.java:85)
      at android.app.Fragment.performCreateView(Fragment.java:2053)
      at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:894)
      at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
      at android.app.BackStackRecord.run(BackStackRecord.java:833)
      at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1452)
      at android.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:483)
      at android.support.v13.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:145)
      at android.support.v4.view.ViewPager.populate(ViewPager.java:1177)
      at android.support.v4.view.ViewPager.populate(ViewPager.java:1025)
      at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1545)
      at android.view.View.measure(View.java:17430)
      at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:727)
      at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:463)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:668)
      at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:735)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
      at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
      at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
      at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
      at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
      at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
      at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
      at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
      at android.view.View.measure(View.java:17430)
      at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
      at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
      at android.view.Choreographer.doCallbacks(Choreographer.java:580)
      at android.view.Choreographer.doFrame(Choreographer.java:550)
      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)

Answer 1

Retrofit 使用 Gson 和 Gson 秘籍。 (截至撰写本文时)它使用 sun.misc.Unsafe 类通过 allocateInstance 实例化您的类。这将完全跳过任何构造函数。

对于内部类，any declared constructor will implicitly declare its first parameter as the enclosing type to represent the enclosing class' instance . Gson 无法创建该封闭类的实例，但即使创建了也没关系。 Gson 完全跳过构造函数，将隐式 Outer.this 引用保留为 null。

最简单的解决方案是更改设计，摆脱内部类。或者，您可以注册并使用自定义解串器。下面的帖子中有示例。

相关帖子:

• > GSON does not deserialize reference to outer class
• > Is default no-args constructor mandatory for Gson?