環境
- Replugin插件框架
問題原因
問題原因的分析這段文字沒有經過潤色，純粹是初稿，不一定易於閱讀，這段看不下去的同學可以直接看文章最後的解決方案。
ReportFragment是android.arch.lifecycle組件中的類，lifecycle組件中的ProcessLifecycleOwnerInitializer繼承自ContentProvider，利用ContentProvider#onCreate()方法初始化。
在初始化時，通過Application#registerActivityLifecycleCallbacks()註冊Activity的生命週期監聽，在onActivityCreated()回調中爲每一個Activity注入一個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之後，ProcessLifecycleOwner中也會監聽Activity的生命週期，並且在onActivityCreated()回調中通過ReportFragment.get()方法獲取ReportFragment。
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
出錯的代碼就是get()方法中的轉型。
ReportFragment爲什麼不能被轉型成ReportFragment呢？因爲它們的ClassLoader不同，但是從ReportFragment的注入方法:injectIfNeededIn()和使用方法:get()來看，這兩處代碼都是ProcessLifecycleOwnerInitializer在初始化時註冊的Activity的生命週期監聽中回調的。又因爲在類A中通過new關鍵字創建類B，類B的ClassLoader就是類A的ClassLoader，那麼這兩處代碼中的ReportFragment的Classloader應該是同一個，爲什麼會出現ClassLoader不一致的情況呢？
這就要從FragmentActivity的狀態保存說起，來看代碼，FragmentActivity#onSaveInstanceState()中，this.mFragments.saveAllState會保存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);
}
...
}
}
最終通過new FragmentState()將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;
...
而序列化時，只保存了Fragment的類名，ClassLoader信息就丟失了。
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;
}
當後臺App由於內存不足被kill後，從最近使用的App列表中返回App，系統會恢復切後臺時所處的Activity，在Activity#onCreate()中通過this.mFragments.restoreAllState恢復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類，通過FragmentState#instantiate()方法，將Context, ClassName傳遞給Fragment#instantiate()方法，最終通過Context的ClassLoader加載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);
}
...
這個Context是Activity#mFragments中的HostCallbacks通過onGetHost獲取的，就是Activity自身。
final FragmentController mFragments = FragmentController.createController(new HostCallbacks());
class HostCallbacks extends FragmentHostCallback<Activity> {
@Override
public Activity onGetHost() {
return Activity.this;
}
}
下面瞭解一下Replugin的ClassLoader機制，Replugin中有宿主和插件之分，通過RePluginClassLoader加載宿主的類，通過PluginDexClassLoader加載插件的類，插件中的Activity就是PluginDexClassLoader加載的。如上所述，在內存不足的情況下，Activity中的ReportFragment是在狀態恢復時，由PluginDexClassLoader加載的。
再回到出問題的代碼，此時findfragmentByTag得到的ReportFragment就是由PluginDexClassLoader加載的，而轉型後的ReportFragment是由系統的ClassLoader加載的，ClassLoader不同，自然就不能轉型。
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
解決方案
知道了問題的來龍去脈，就好找解決方案了，只要保證LifeCycle使用的ReportFragment都是由LifeCycle組件加載，就不會有轉型問題，但是Activity的狀態保存步驟又不能去除，綜上得到的方案就是：在Activity創建了ReportFragment之後，LifeCycle插入ReportFragment之前，移除Activity創建的ReportFragment，這樣LifeCycle就會自行創建ReportFragment，就能保證LifeCycle使用的ReportFragment都是自己加載的。
方案一
在所有Activity的公共父類的onCreate中執行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();
}
}
這種方法的缺點是，第三方的Activity不會繼承我們的公共父類，無法執行這段代碼，那就看方案二。
方案二
在Application#onCreate中執行HookActivityLifecycle#hook方法，通過反射在Activity生命週期的監聽器list的第一位中插入一個監聽器，在監聽器中的onActivityCreated回調中移除ReportFragment。這個方案的缺點是，Activity中的FragmentManager中的commitNow方法在api 24及以上版本才實現，如果不用commitNow方法，在LifeCycle判斷是否插入ReportFragment時，ReportFragment還沒來得及被移除。
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方法沒有版本限制。
*/
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();
}
}
}
測試驗證
這個問題的復現場景就是內存不足，那麼如果模擬內存不足的情況呢，參考模擬內存不足