对封闭类实例的内部类引用为空

Question

我有一个具有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 
        }
    }
}

在另一个类中，我遍历实例的项Outer并调用getInnerX()每个项：

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

ANullPointerException被扔在里面，getInnerX()因为Outer.this是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。所以它的一些东西我没有包含在问题中。

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

的 item 属性Outer具有GSon转换器的注释：

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

Outer 没有定义构造函数，Inner 也没有。

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 作弊。（在撰写本文时）它使用sun.misc.Unsafe该类来实例化您的类allocateInstance。这完全跳过了任何构造函数。

在内部类的情况下，任何声明的构造函数都会隐式声明它的第一个参数作为封闭类型来表示封闭类的实例。Gson 无法创建该封闭类的实例，但即使创建了，也无所谓。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改变你的设计，去掉内部类。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。

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