ReportFragment cannot be cast to ReportFragment
环境
- Replugin插件框架
问题原因
问题原因的分析这段文字没有经过润色，纯粹是初稿，不一定易于阅读，这段看不下去的同学可以直接看文章最后的解决方案。
ReportFragment是android.arch.lifecycle组件中的类，lifecycle组件中的ProcessLifecycleOwnerInitializer继承自ContentProvider，利用ContentProvider#onCreate()方法初始化。
在初始化时，通过Application#registerActivityLifecycleCallbacks()注册Activity的生命周期监听，在onActivityCreated()回调中为每一个Activity注入一个ReportFragment
public static void injectIfNeededIn(Activity activity) {
// ProcessLifecycleOwner should always correctly work and some activities may not extend
// FragmentActivity from support lib, so we use framework fragments for activities
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
注入ReportFragment之后，ProcessLifecycleOwner中也会监听Activity的生命周期，并且在onActivityCreated()回调中通过ReportFragment.get()方法获取ReportFragment。
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
出错的代码就是get()方法中的转型。
ReportFragment为什么不能被转型成ReportFragment呢？因为它们的ClassLoader不同，但是从ReportFragment的注入方法:injectIfNeededIn()和使用方法:get()来看，这两处代码都是ProcessLifecycleOwnerInitializer在初始化时注册的Activity的生命周期监听中回调的。又因为在类A中通过new关键字创建类B，类B的ClassLoader就是类A的ClassLoader，那么这两处代码中的ReportFragment的Classloader应该是同一个，为什么会出现ClassLoader不一致的情况呢？
这就要从FragmentActivity的状态保存说起，来看代码，FragmentActivity#onSaveInstanceState()中，this.mFragments.saveAllState会保存Fragment。
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
this.markFragmentsCreated();
Parcelable p = this.mFragments.saveAllState();
if (p != null) {
outState.putParcelable("android:support:fragments", p);
}
...
}
}
最终通过new FragmentState()将Fragment序列化。
Parcelable saveAllState() {
...
if (this.mActive != null && this.mActive.size() > 0) {
int N = this.mActive.size();
FragmentState[] active = new FragmentState[N];
boolean haveFragments = false;
for(int i = 0; i < N; ++i) {
Fragment f = (Fragment)this.mActive.valueAt(i);
if (f != null) {
if (f.mIndex < 0) {
this.throwException(new IllegalStateException("Failure saving state: active " + f + " has cleared index: " + f.mIndex));
}
haveFragments = true;
FragmentState fs = new FragmentState(f);
active[i] = fs;
...
而序列化时，只保存了Fragment的类名，ClassLoader信息就丢失了。
FragmentState(Fragment frag) {
this.mClassName = frag.getClass().getName();
this.mIndex = frag.mIndex;
this.mFromLayout = frag.mFromLayout;
this.mFragmentId = frag.mFragmentId;
this.mContainerId = frag.mContainerId;
this.mTag = frag.mTag;
this.mRetainInstance = frag.mRetainInstance;
this.mDetached = frag.mDetached;
this.mArguments = frag.mArguments;
this.mHidden = frag.mHidden;
}
当后台App由于内存不足被kill后，从最近使用的App列表中返回App，系统会恢复切后台时所处的Activity，在Activity#onCreate()中通过this.mFragments.restoreAllState恢复Fragment。
protected void onCreate(@Nullable Bundle savedInstanceState) {
this.mFragments.attachHost((Fragment)null);
super.onCreate(savedInstanceState);
FragmentActivity.NonConfigurationInstances nc = (FragmentActivity.NonConfigurationInstances)this.getLastNonConfigurationInstance();
if (nc != null && nc.viewModelStore != null && this.mViewModelStore == null) {
this.mViewModelStore = nc.viewModelStore;
}
if (savedInstanceState != null) {
Parcelable p = savedInstanceState.getParcelable("android:support:fragments");
this.mFragments.restoreAllState(p, nc != null ? nc.fragments : null);
if (savedInstanceState.containsKey("android:support:next_request_index")) {
...
之前序列化了Fragment信息的FragmentState类，通过FragmentState#instantiate()方法，将Context, ClassName传递给Fragment#instantiate()方法，最终通过Context的ClassLoader加载Fragment的Class。
public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
try {
Class clazz = sClassMap.get(fname);
if (clazz == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = context.getClassLoader().loadClass(fname);
if (!Fragment.class.isAssignableFrom(clazz)) {
throw new InstantiationException("Trying to instantiate a class " + fname
+ " that is not a Fragment", new ClassCastException());
}
sClassMap.put(fname, clazz);
}
...
这个Context是Activity#mFragments中的HostCallbacks通过onGetHost获取的，就是Activity自身。
final FragmentController mFragments = FragmentController.createController(new HostCallbacks());
class HostCallbacks extends FragmentHostCallback {
@Override
public Activity onGetHost() {
return Activity.this;
}
}
下面了解一下Replugin的ClassLoader机制，Replugin中有宿主和插件之分，通过RePluginClassLoader加载宿主的类，通过PluginDexClassLoader加载插件的类，插件中的Activity就是PluginDexClassLoader加载的。如上所述，在内存不足的情况下，Activity中的ReportFragment是在状态恢复时，由PluginDexClassLoader加载的。
再回到出问题的代码，此时findfragmentByTag得到的ReportFragment就是由PluginDexClassLoader加载的，而转型后的ReportFragment是由系统的ClassLoader加载的，ClassLoader不同，自然就不能转型。
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
解决方案
知道了问题的来龙去脉，就好找解决方案了，只要保证LifeCycle使用的ReportFragment都是由LifeCycle组件加载，就不会有转型问题，但是Activity的状态保存步骤又不能去除，综上得到的方案就是：在Activity创建了ReportFragment之后，LifeCycle插入ReportFragment之前，移除Activity创建的ReportFragment，这样LifeCycle就会自行创建ReportFragment，就能保证LifeCycle使用的ReportFragment都是自己加载的。
方案一
在所有Activity的公共父类的onCreate中执行removeReportTag方法移除ReportFragment。
private static void removeReportTag(Activity activity) {
try {
XLLog.d(TAG, "removeReportTag");
FragmentManager manager = activity.getFragmentManager();
if (manager != null) {
Fragment fragment = manager.findFragmentByTag(REPORT_FRAGMENT_TAG);
XLLog.d(TAG, "removeReportTag--fragment=" + fragment);
if (fragment != null) {
XLLog.d(TAG, "removeReportTag--fragment.classLoader=" + fragment.getClass().getClassLoader());
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
manager.beginTransaction().remove(fragment).commitNow();
}
}
}
} catch (Throwable e) {
e.printStackTrace();
}
}
这种方法的缺点是，第三方的Activity不会继承我们的公共父类，无法执行这段代码，那就看方案二。
方案二
在Application#onCreate中执行HookActivityLifecycle#hook方法，通过反射在Activity生命周期的监听器list的第一位中插入一个监听器，在监听器中的onActivityCreated回调中移除ReportFragment。这个方案的缺点是，Activity中的FragmentManager中的commitNow方法在api 24及以上版本才实现，如果不用commitNow方法，在LifeCycle判断是否插入ReportFragment时，ReportFragment还没来得及被移除。
public class HookActivityLifeCycle {
private static final String TAG = "HookActivityLifeCycle";
private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
public static void hook(Application application) {
try {
Field[] fields = Application.class.getDeclaredFields();
for (Field field : fields) {
XLLog.d(TAG, "field=" + field.getName());
}
Field mActivityLifecycleCallbacksField = Application.class.getDeclaredField("mActivityLifecycleCallbacks");
mActivityLifecycleCallbacksField.setAccessible(true);
List list = (List) mActivityLifecycleCallbacksField.get(application);
if (list != null) {
list.add(0, new hook());
}
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
static class hook implements Application.ActivityLifecycleCallbacks {
@Override
public void onActivityCreated(@NonNull Activity activity, @Nullable Bundle savedInstanceState) {
removeReportTag(activity);
}
@Override
public void onActivityStarted(@NonNull Activity activity) {
}
@Override
public void onActivityResumed(@NonNull Activity activity) {
}
@Override
public void onActivityPaused(@NonNull Activity activity) {
}
@Override
public void onActivityStopped(@NonNull Activity activity) {
}
@Override
public void onActivitySaveInstanceState(@NonNull Activity activity, @NonNull Bundle outState) {
}
@Override
public void onActivityDestroyed(@NonNull Activity activity) {
}
}
private static void removeReportTag(Activity activity) {
if (activity instanceof FragmentActivity) {
removeReportTagV4((FragmentActivity) activity);
} else {
try {
XLLog.d(TAG, "removeReportTag");
FragmentManager manager = activity.getFragmentManager();
if (manager != null) {
Fragment fragment = manager.findFragmentByTag(REPORT_FRAGMENT_TAG);
XLLog.d(TAG, "removeReportTag--fragment=" + fragment);
if (fragment != null) {
XLLog.d(TAG, "removeReportTag--fragment.classLoader=" + fragment.getClass().getClassLoader());
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
manager.beginTransaction().remove(fragment).commitNow();
}
}
}
} catch (Throwable e) {
e.printStackTrace();
}
}
}
/**
* FragmentActivity中的FragmentManager的commitNow方法没有版本限制。
*/
private static void removeReportTagV4(FragmentActivity activity) {
try {
XLLog.d(TAG, "removeReportTagV4");
android.support.v4.app.FragmentManager manager = activity.getSupportFragmentManager();
if (manager != null) {
android.support.v4.app.Fragment fragment = manager.findFragmentByTag(REPORT_FRAGMENT_TAG);
XLLog.d(TAG, "removeReportTagV4--fragment=" + fragment);
if (fragment != null) {
XLLog.d(TAG, "removeReportTagV4--fragment.classLoader=" + fragment.getClass().getClassLoader());
manager.beginTransaction().remove(fragment).commitNow();
}
}
} catch (Throwable e) {
e.printStackTrace();
}
}
}
测试验证
这个问题的复现场景就是内存不足，那么如果模拟内存不足的情况呢，参考模拟内存不足