Android插件化架构 - Activity的启动流程分析

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我是靠谱客的博主伶俐萝莉，最近开发中收集的这篇文章主要介绍Android插件化架构 - Activity的启动流程分析，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

###1. 概述

Android插件化架构，目前第三方的框架比较多，早几年自己用的是DL框架，这个框架的源码比较简单主要用的是静态代理。如果我们自己要去写一个插件化架构框架那要解决的问题会分为几个方面，类的加载，资源和布局的加载，广播的管理方式，Activity的加载和生命周期管理，Service的插件化，ContentProvider的插件化等等等等，反正加载一个没有运行的app到主程序，需要解决的问题基本就这么多，如果能够一一解决那么就可以实现插件化了。
　　内涵段子项目部分我们实现几个，然后介绍一个360开源框架DroidPlugin原理一致，后面我们再一一实现，那么这一期实现什么呢？我们需要启动插件APP那么就需要启动里面的Activity,这些Activity事先是不会在主工程的AndroidManifest.xml中配置，启动一个没有注册的Activity肯定会报错，我们是否可以想个办法去绕过系统的检测，让没有在AndroidManifest.xml中配置的Activity照样可以启动呢？
　　看源码的时候我们其实时常强调一定要带着思想，要解决这么个问题我们肯定需要清楚的知道系统启动Activity的具体流程，当然可以直接去了解为什么报错，这里我们还是把启动流程全部走一遍，也方便以后开发中再遇到什么问题。
　　
###2. Activity启动流程源码分析


@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1);
}
}
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, mEmbeddedID, requestCode, ar.getResultCode(),
ar.getResultData());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received.
Setting
// this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}
cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}

来到Instrumentation中的execStartActivity方法

public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
Uri referrer = target != null ? target.onProvideReferrer() : null;
if (referrer != null) {
intent.putExtra(Intent.EXTRA_REFERRER, referrer);
}
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;
}
// 检测结果也就是否正常
public static void checkStartActivityResult(int res, Object intent) {
if (res >= ActivityManager.START_SUCCESS) {
return;
}
switch (res) {
case ActivityManager.START_INTENT_NOT_RESOLVED:
case ActivityManager.START_CLASS_NOT_FOUND:
// 在这里就会报各种错，AndroidManifest.xml中没有配置的错就出现在这
if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
throw new ActivityNotFoundException(
"Unable to find explicit activity class "
+ ((Intent)intent).getComponent().toShortString()
+ "; have you declared this activity in your AndroidManifest.xml?");
throw new ActivityNotFoundException(
"No Activity found to handle " + intent);
case ActivityManager.START_PERMISSION_DENIED:
throw new SecurityException("Not allowed to start activity "
+ intent);
case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
throw new AndroidRuntimeException(
"FORWARD_RESULT_FLAG used while also requesting a result");
case ActivityManager.START_NOT_ACTIVITY:
throw new IllegalArgumentException(
"PendingIntent is not an activity");
case ActivityManager.START_NOT_VOICE_COMPATIBLE:
throw new SecurityException(
"Starting under voice control not allowed for: " + intent);
case ActivityManager.START_VOICE_NOT_ACTIVE_SESSION:
throw new IllegalStateException(
"Session calling startVoiceActivity does not match active session");
case ActivityManager.START_VOICE_HIDDEN_SESSION:
throw new IllegalStateException(
"Cannot start voice activity on a hidden session");
case ActivityManager.START_CANCELED:
throw new AndroidRuntimeException("Activity could not be started for "
+ intent);
default:
throw new AndroidRuntimeException("Unknown error code "
+ res + " when starting " + intent);
}
}

ActivityManagerNative.getDefault().startActivity又是一个进程间的通信，如果不了解IPC的请移步Android进程间的通信 - IPC(机制)Binder的原理和源码阅读，来到ActivityManagerService中的startActivity方法：


@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
enforceNotIsolatedCaller("startActivity");
userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
userId, false, ALLOW_FULL_ONLY, "startActivity", null);
// TODO: Switch to user app stacks here.
return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, bOptions, false, userId, null, null);
}
final int startActivityMayWait(IApplicationThread caller, int callingUid,
String callingPackage, Intent intent, String resolvedType,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int startFlags,
ProfilerInfo profilerInfo, IActivityManager.WaitResult outResult, Configuration config,
Bundle bOptions, boolean ignoreTargetSecurity, int userId,
IActivityContainer iContainer, TaskRecord inTask) {
// PackageManagerService-----> 扫描app，注册组件
ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId);
// Collect information about the target of the Intent.
ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);
rInfo = mSupervisor.resolveIntent(intent, null /*resolvedType*/, userId);
int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
aInfo, rInfo, voiceSession, voiceInteractor,
resultTo, resultWho, requestCode, callingPid,
callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
options, ignoreTargetSecurity, componentSpecified, outRecord, container,
inTask);
}
final int startActivityLocked(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
ActivityRecord[] outActivity, ActivityStackSupervisor.ActivityContainer container,
TaskRecord inTask) {
// 验证intent、Class、Permission等
// 保存将要启动的Activity的Record
err = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
true, options, inTask);
return err;
}
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {
// 检查将要启动的Activity的launchMode和启动Flag
// 根据launcheMode和Flag配置task
final boolean dontStart = top != null && mStartActivity.resultTo == null
&& top.realActivity.equals(mStartActivity.realActivity)
&& top.userId == mStartActivity.userId
&& top.app != null && top.app.thread != null
&& ((mLaunchFlags & FLAG_ACTIVITY_SINGLE_TOP) != 0
|| mLaunchSingleTop || mLaunchSingleTask);
// 举一个例子 SingleTop
if (dontStart) {
top.deliverNewIntentLocked(
mCallingUid, mStartActivity.intent, mStartActivity.launchedFromPackage);
// Don't use mStartActivity.task to show the toast. We're not starting a new activity
// but reusing 'top'. Fields in mStartActivity may not be fully initialized.
mSupervisor.handleNonResizableTaskIfNeeded(
top.task, preferredLaunchStackId, topStack.mStackId);
return START_DELIVERED_TO_TOP;
}
mTargetStack.startActivityLocked(mStartActivity, newTask, mKeepCurTransition, mOptions);
if (mDoResume) {
mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
mOptions);
}
}

进入ActvityStack中的startActivityLocked()方法


// 任务栈历史栈配置
final void startActivityLocked(ActivityRecord r, boolean newTask, boolean keepCurTransition,
ActivityOptions options) {
if (!r.mLaunchTaskBehind && (taskForIdLocked(taskId) == null || newTask)) {
// 加入栈顶 管理栈
insertTaskAtTop(rTask, r);
// 管显示
mWindowManager.moveTaskToTop(taskId);
}
if (!newTask) {
// 不是一个新的Task
task.addActivityToTop(r);
r.putInHistory();
addConfigOverride(r, task);
}
}

进入ActivityStack的resumeTopActivityInnerLocked()方法


private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
// Find the first activity that is not finishing.
final ActivityRecord next = topRunningActivityLocked();
// The activity may be waiting for stop, but that is no longer
// appropriate for it.
mStackSupervisor.mStoppingActivities.remove(next);
mStackSupervisor.mGoingToSleepActivities.remove(next);
next.sleeping = false;
mStackSupervisor.mWaitingVisibleActivities.remove(next);
if (mResumedActivity != null) {
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Pausing " + mResumedActivity);
pausing |= startPausingLocked(userLeaving, false, next, dontWaitForPause);
}
}
final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
ActivityRecord resuming, boolean dontWait) {
if (prev.app != null && prev.app.thread != null) {
if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Enqueueing pending pause: " + prev);
try {
EventLog.writeEvent(EventLogTags.AM_PAUSE_ACTIVITY,
prev.userId, System.identityHashCode(prev),
prev.shortComponentName);
mService.updateUsageStats(prev, false);
// 暂停Activity
prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing,
userLeaving, prev.configChangeFlags, dontWait);
}
completePauseLocked(false, resuming);
}

进入ApplicationThread的schedulePauseActivity()方法


public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges, boolean dontReport) {
sendMessage(
finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
token,
(userLeaving ? USER_LEAVING : 0) | (dontReport ? DONT_REPORT : 0),
configChanges,
seq);
}
public void handleMessage(Message msg) {
switch (msg.what) {
...
case PAUSE_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
SomeArgs args = (SomeArgs) msg.obj;
handlePauseActivity((IBinder) args.arg1, false,
(args.argi1 & USER_LEAVING) != 0, args.argi2,
(args.argi1 & DONT_REPORT) != 0, args.argi3);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
}
private void handlePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges, boolean dontReport, int seq) {
//...
performPauseActivity(token, finished, r.isPreHoneycomb(), "handlePauseActivity");
//...
// Tell the activity manager we have paused.
if (!dontReport) {
try {
ActivityManagerNative.getDefault().activityPaused(token);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
final Bundle performPauseActivity(IBinder token, boolean finished,
boolean saveState, String reason) {
ActivityClientRecord r = mActivities.get(token);
return r != null ? performPauseActivity(r, finished, saveState, reason) : null;
}
final Bundle performPauseActivity(ActivityClientRecord r, boolean finished,
boolean saveState, String reason) {
// ...
performPauseActivityIfNeeded(r, reason);
}
private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
// ...
mInstrumentation.callActivityOnPause(r.activity);
}

进入Instrumentation的callActivityOnPause()方法


public void callActivityOnPause(Activity activity) {
activity.performPause();
}

进入Activity的performPause()方法

final void performPause() {
mDoReportFullyDrawn = false;
mFragments.dispatchPause();
mCalled = false;
// 调用onPause()暂停方法
onPause();
mResumed = false;
if (!mCalled && getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.GINGERBREAD) {
throw new SuperNotCalledException(
"Activity " + mComponent.toShortString() +
" did not call through to super.onPause()");
}
mResumed = false;
}

总算是回调到了Activity的onPause方法，哈哈还是挺简单的，Activity生命周期中的onPause方法终于被我们找到了。也就是说我们在启动一个Activity的时候最先被执行的是栈顶的Activity的onPause方法。我们对Activity这些生命周期早已背得滚瓜烂熟。接着往下看然后回到我们ApplicationThread的handlePauseActivity()方法中的ActivityManagerNative.getDefault().activityPaused(token);进入ActivityManagerService中的activityPaused()方法


@Override
public final void activityPaused(IBinder token) {
final long origId = Binder.clearCallingIdentity();
synchronized(this) {
ActivityStack stack = ActivityRecord.getStackLocked(token);
if (stack != null) {
stack.activityPausedLocked(token, false);
}
}
Binder.restoreCallingIdentity(origId);
}

进入ActivityStack中的activityPausedLocked()方法


final void activityPausedLocked(IBinder token, boolean timeout){
completePauseLocked(true, null);
}
private void completePauseLocked(boolean resumeNext, ActivityRecord resuming) {
ActivityRecord prev = mPausingActivity;
if (resumeNext) {
final ActivityStack topStack = mStackSupervisor.getFocusedStack();
mStackSupervisor.resumeFocusedStackTopActivityLocked(topStack, prev, null);
}
}

进入ActivityStackSupervisor中的resumeFocusedStackTopActivityLocked()方法


boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}

进入ActivityStack中的resumeTopActivityUncheckedLocked()方法


boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
result = resumeTopActivityInnerLocked(prev, options);
}
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
// 这个方法我们已经很熟悉了
mStackSupervisor.startSpecificActivityLocked(next, true, true);
}

进入ActivityStackSupervisor中的startSpecificActivityLocked()方法


void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
r.task.stack.setLaunchTime(r);
if (app != null && app.thread != null) {
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {
// Don't add this if it is a platform component that is marked
// to run in multiple processes, because this is actually
// part of the framework so doesn't make sense to track as a
// separate apk in the process.
app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
// 真的要启动Activity了
realStartActivityLocked(r, app, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
// If a dead object exception was thrown -- fall through to
// restart the application.
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
// scheduleLaunchActivity 启动
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);
}

进入ApplicationThread中的scheduleLaunchActivity()方法


public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
}
}
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}
// Make sure we are running with the most recent config.
handleConfigurationChanged(null, null);
if (localLOGV) Slog.v(
TAG, "Handling launch of " + r);
// Initialize before creating the activity
WindowManagerGlobal.initialize();
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
if (!r.activity.mFinished && r.startsNotResumed) {
// The activity manager actually wants this one to start out paused, because it
// needs to be visible but isn't in the foreground. We accomplish this by going
// through the normal startup (because activities expect to go through onResume()
// the first time they run, before their window is displayed), and then pausing it.
// However, in this case we do -not- need to do the full pause cycle (of freezing
// and such) because the activity manager assumes it can just retain the current
// state it has.
performPauseActivityIfNeeded(r, reason);
// We need to keep around the original state, in case we need to be created again.
// But we only do this for pre-Honeycomb apps, which always save their state when
// pausing, so we can not have them save their state when restarting from a paused
// state. For HC and later, we want to (and can) let the state be saved as the
// normal part of stopping the activity.
if (r.isPreHoneycomb()) {
r.state = oldState;
}
}
} else {
// If there was an error, for any reason, tell the activity manager to stop us.
try {
ActivityManagerNative.getDefault()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}

###3. Activity启动流程总结

刚开始去看估计要看个一两天才会有所收货，其实看源码要么是为了解决问题要么是为了学习，否则还是不要轻易的进入这趟浑水里面，代码太多了没有思想会蒙的。首先可以大致先走一遍流程，然后再去抓细节，比如怎么分配栈的，怎么处理启动模式的，怎么创建Activity的，怎么调用生命周期的，怎么传递序列化对象参数的等等…
　　看了源码之后我们不得不佩服Google的工程师，写的代码可扩展真的很不错，要是换做一般人版本迭代几次代码就改不动了，得要重新架构重新写，这也是我们学习的方式，源码其实是最好的书籍。这里有好几个思想，C/S架构思想就是服务思想，模块化思想，分层次思想等等…

最后我们看一下在启动流程中出现的几个主要的类都分别有什么功能：

** ActivityManagerService** 组件通信系统核心管理类（ActivityManagerNative）IPC通信
** ActivityStackSupervisor** 管理整个手机的Activity任务栈
ActivityStack Activity栈（任务栈）
** PackageManagerService** 主要负责对系统的apk进行管理，不管是系统apk(/system/app)，还是我们手工安装上去的，系统所有的apk都是由其管理的。
** ActivityThread** Activity的入口是onCreate方法，Android上一个应用的入口是ActivityThread。和普通的Java类一样有一个main方法。用于控制与管理一个应用进程的主线程的操作，包括管理与处理activity manager发送过来的关于activities、广播以及其他的操作请求

ActivityManagerService和ActivityStack位于同一个进程中，而ApplicationThread和ActivityThread位于另一个进程中。其中，ActivityManagerService是负责管理Activity的生命周期的，ActivityManagerService还借助ActivityStack是来把所有的Activity按照后进先出的顺序放在一个堆栈中；对于每一个应用程序来说，都有一个ActivityThread来表示应用程序的主进程，而每一个ActivityThread都包含有一个ApplicationThread实例，它是一个Binder对象，负责和其它进程进行通信。

这么痛苦的去读源码到底有什么用呢？那么结合动态代码设计模式以及Activity的启动流程，我们就可以尝试去拦截Activity的启动流程，让没有在AndroidMainfest.xml中注册的Activity也能启动并且不报错，这样我们就算是跨出了插件化架构的第一步了，如果你不去了解Activity启动流程那么就是然并卵。

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