前言

Fragment，简称碎片，是 Android 3.0（API 11）提出的，为了兼容低版本，support-v4 库中也开发了一套 Fragment API，最低兼容 Android 1.6。

过去 support-v4 库是一个 jar 包，24.2.0 版本开始，将 support-v4 库模块化为多个 jar 包，包含：support-fragment, support-ui, support-media-compat 等，这么做是为了减少 APK 包大小，你需要用哪个模块就引入哪个模块。

如果想引入整个 support-v4 库，则 compile ‘com.android.support:support-v4:24.2.1’，如果只想引入 support-fragment 库，则 com.android.support:support-fragment:24.2.1。

因为 support 库是不断更新的，因此建议使用 support 库中的 android.support.v4.app.
Fragment，而不要用系统自带的 android.app.Fragment。而如果要使用 support 库的 Fragment，Activity 必须要继承 FragmentActivity（AppCompatActivity 是 FragmentActivity 的子类）。

问题归纳

Could not find Fragment constructor

本次排查 Bug 的 fragment 基于 AndroidX1.1.0
线上 bugly 报了一个 Could not find Fragment constructor，先看一下 Fragment 的构造函数

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/**
     * 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();
    }

主要看一下注释说明，如果你想要的使用非默认的构造函数，需要自己实现一个 FragmentFactory 去初始化，然后强烈推荐使用 setArguments 和 getArguments 存取参数。看了一下报错的类，可以基本确定是使用了带参数的构造函数

然后看一下报错的位置，在 Fragment 里搜索 could not find Fragment constructor

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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);
        }
    }

可以看到这里通过反射，调用了空的构造函数

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

但是对应的类没有空的构造函数，所以抛出了这个异常。然后看一下初始的地方，是 FragmentActivity 在 onCreate 中调用的 mFragments.restoreSaveState(p);

解决方案：
Fragment 必须有一个无参 public 的构造函数，否则在数据恢复的时候，会报 crash
ps：手动测试发现私有的构造函数无法通过该 Constructor.newInstance 方法调用

to be continued…