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
/**
* 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?？戳艘幌卤ù淼睦?，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
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
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);
结论：
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
