本次排查Bug的fragment基于AndroidX1.1.0
線上bugly報了一個Could not find Fragment constructor，先看一下Fragment的構造函數

/**
     * Constructor used by the default {@link FragmentFactory}. You must
     * {@link FragmentManager#setFragmentFactory(FragmentFactory) set a custom FragmentFactory}
     * if you want to use a non-default constructor to ensure that your constructor
     * is called when the fragment is re-instantiated.
     *
     * <p>It is strongly recommended to supply arguments with {@link #setArguments}
     * and later retrieved by the Fragment with {@link #getArguments}. These arguments
     * are automatically saved and restored alongside the Fragment.
     *
     * <p>Applications should generally not implement a constructor. Prefer
     * {@link #onAttach(Context)} instead. It is the first place application code can run where
     * the fragment is ready to be used - the point where the fragment is actually associated with
     * its context. Some applications may also want to implement {@link #onInflate} to retrieve
     * attributes from a layout resource, although note this happens when the fragment is attached.
     */
    public Fragment() {
        initLifecycle();
    }

主要看一下注釋說明，如果你想要的使用非默認的構造函數，需要自己實現一個FragmentFactory去初始化，然后強烈推薦使用setArguments和getArguments存取參數。看了一下報錯的類，可以基本確定是使用了帶參數的構造函數

然后看一下報錯的位置，在Fragment里搜索could not find Fragment constructor

 public static Fragment instantiate(@NonNull Context context, @NonNull String fname,
            @Nullable Bundle args) {
        try {
            Class<? extends Fragment> clazz = FragmentFactory.loadFragmentClass(
                    context.getClassLoader(), fname);
            Fragment f = clazz.getConstructor().newInstance();
            if (args != null) {
                args.setClassLoader(f.getClass().getClassLoader());
                f.setArguments(args);
            }
            return f;
        } catch (java.lang.InstantiationException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": make sure class name exists, is public, and has an"
                    + " empty constructor that is public", e);
        } catch (IllegalAccessException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": make sure class name exists, is public, and has an"
                    + " empty constructor that is public", e);
        } catch (NoSuchMethodException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": could not find Fragment constructor", e);
        } catch (InvocationTargetException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": calling Fragment constructor caused an exception", e);
        }
    }

可以看到這里通過反射，調用了空的構造函數

Fragment f = clazz.getConstructor().newInstance();

但是對應的類沒有空的構造函數，所以拋出了這個異常。然后看一下初始的地方，是FragmentActivity在onCreate中調用的mFragments.restoreSaveState(p);

結論：
Fragment必須有一個無參public的構造函數，否則在數據恢復的時候，會報crash
ps：手動測試發現私有的構造函數無法通過該Constructor.newInstance方法調用