调用RecyclerView更新方法时getContext()致App崩溃求助

问题现象 #

FATAL EXCEPTION: main
Process: com.akstudios.kindergarten, PID: 15680
java.lang.NullPointerException: 尝试对空对象引用调用虚拟方法 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)'
at com.akstudios.kindergarten.GroupbookKrippeFragment.setRecyclerView(GroupbookKrippeFragment.java:56)

访问GroupbookKrippeFragment时频繁触发上述空指针崩溃，尝试过getContext()、onAttach()、GroupbookKrippeFragment.this.getContext()等方式获取上下文，问题依旧。

问题排查 #

App原本运行稳定，修改以下代码后出现崩溃：

1. 在GroupbookKrippeFragment.java中封装setRecyclerView方法用于更新RecyclerView：

public void setRecyclerView(List<ChildrenTable> childrenTableList) {
    recycler_view.setLayoutManager(new LinearLayoutManager(getContext()));
    childAdapter = new ChildAdapter(getContext(), childrenTableList);
//        childAdapter = new ChildAdapter(getContext());
    recycler_view.setAdapter(childAdapter);
}

2. 在DatabaseHelper.java的AllChildren AsyncTask的onPostExecute()中调用该方法：

public class DatabaseHelper {
Context context;
GroupbookKrippeFragment groupbookKrippeFragment = new GroupbookKrippeFragment();

// ... 其他代码省略

// Show all data from ChildrenTable
public void getAllChildrenData() {
    class AllChildren extends AsyncTask<Void, Void, List<ChildrenTable>> {

        @Override
        protected List<ChildrenTable> doInBackground(Void... voids) {
            List<ChildrenTable> list = DatabaseClient.getInstance(context)
                    .getChildrenDatabase()
                    .childrenDAO()
                    .selectAll();
            return list;
        }

        @Override
        protected void onPostExecute(List<ChildrenTable> childrenTable) {
            super.onPostExecute(childrenTable);
            if (childrenTable != null && childrenTable.size() > 0) {
                groupbookKrippeFragment.setRecyclerView(childrenTable);
            }

        }
    }

    AllChildren allChildren = new AllChildren();
    allChildren.execute();
}
}

注释掉该调用，将代码移到GroupbookKrippeFragment的onViewCreated中，App恢复稳定，但RecyclerView无法显示和更新数据库数据。

核心原因 #

DatabaseHelper中直接new GroupbookKrippeFragment()创建的实例，不是当前界面正在显示的Fragment实例。这个新实例的onCreateView从未被调用，recycler_view自然没有被初始化，调用setRecyclerView时就会触发空指针。

解决方案 #

方案1：接口回调传递数据 #

1. 定义数据回调接口：

public interface OnChildrenDataLoadedListener {
    void onDataLoaded(List<ChildrenTable> dataList);
}

2. 修改DatabaseHelper，持有接口引用而非Fragment实例：

public class DatabaseHelper {
    Context context;
    OnChildrenDataLoadedListener listener;

    // 修改构造方法，传入回调接口
    public DatabaseHelper(Context context, OnChildrenDataLoadedListener listener) {
        this.context = context;
        this.listener = listener;
    }

    public static DatabaseHelper getInstance(Context context, OnChildrenDataLoadedListener listener) {
        return new DatabaseHelper(context, listener);
    }

    // ... 其他代码省略

    @Override
    protected void onPostExecute(List<ChildrenTable> childrenTable) {
        super.onPostExecute(childrenTable);
        if (childrenTable != null && childrenTable.size() > 0) {
            // 通过接口回调传递数据
            listener.onDataLoaded(childrenTable);
        }
    }
}

3. 在GroupbookKrippeFragment中实现接口并初始化DatabaseHelper：

public class GroupbookKrippeFragment extends Fragment implements OnChildrenDataLoadedListener {

    // ... 成员变量省略

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_groupbook_krippe, container, false);

        // ... 初始化控件代码省略

        // 传入当前Fragment作为回调实例
        helper = DatabaseHelper.getInstance(getContext(), this);
        helper.getAllChildrenData();

        return view;
    }

    // 实现接口方法
    @Override
    public void onDataLoaded(List<ChildrenTable> dataList) {
        setRecyclerView(dataList);
    }

    public void setRecyclerView(List<ChildrenTable> childrenTableList) {
        recycler_view.setLayoutManager(new LinearLayoutManager(getContext()));
        childAdapter = new ChildAdapter(getContext(), childrenTableList);
        recycler_view.setAdapter(childAdapter);
    }
}

方案2：LiveData自动观察数据库数据（推荐） #

如果使用Room数据库，推荐用LiveData自动监听数据变化，无需手动维护AsyncTask：

1. 修改DAO层，返回LiveData：

@Dao
public interface ChildrenDAO {
    @Insert
    void insertData(ChildrenTable childrenTable);

    @Query("SELECT * FROM children_table")
    LiveData<List<ChildrenTable>> selectAllLive();
}

2. 在GroupbookKrippeFragment中观察数据：

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    // 初始化RecyclerView基础配置
    recycler_view.setLayoutManager(new LinearLayoutManager(getContext()));
    childAdapter = new ChildAdapter(getContext(), new ArrayList<>());
    recycler_view.setAdapter(childAdapter);

    // 观察数据库数据变化，自动更新RecyclerView
    DatabaseClient.getInstance(getContext())
            .getChildrenDatabase()
            .childrenDAO()
            .selectAllLive()
            .observe(getViewLifecycleOwner(), childrenTableList -> {
                if (childrenTableList != null && !childrenTableList.isEmpty()) {
                    childAdapter.updateData(childrenTableList);
                }
            });
}

3. 给ChildAdapter添加数据更新方法：

public class ChildAdapter extends RecyclerView.Adapter<ChildAdapter.ViewHolder> {
    private Context context;
    private List<ChildrenTable> dataList;

    public ChildAdapter(Context context, List<ChildrenTable> dataList) {
        this.context = context;
        this.dataList = dataList;
    }

    // 添加更新数据方法
    public void updateData(List<ChildrenTable> newData) {
        dataList.clear();
        dataList.addAll(newData);
        notifyDataSetChanged();
    }

    // ... 其他Adapter代码省略
}

内容的提问来源于stack exchange，提问作者EXTSY