前言
在我们开始之前，希望您能最好已经满足以下条件：
1、有一份编译后的Android源码（亲自动手实践才会有更深入的理解）
2、对Binder机制有一定的了解
本文启动流程分析基于Android 5.1的源码。为什么是5.1的源码呢？因为手边编译完的代码只有这个版本…另外，用什么版本的源码并不重要，大体的流程并无本质上的区别，仅仅是实现细节的调整，找一个你熟悉的版本就好。
1、启动时序图
作为一个轻微强迫症的人，整理的时序图，相信大家按图索骥，一定能搞明白整个启动流程：
说明：为了让大家更清楚的理解整个过程，将时序图中划分为三个部分：Launcher进程、System进程、App进程，其中有涉及共用的类以L / A进行区分表示跟哪个进程有关，便于理解。
2、关键类说明
整个启动流程因为会涉及到多次Binder通信，这里先简要说明一下几个类的用途，方便大家理解整个交互流程：
1、ActivityManagerService：AMS是Android中最核心的服务之一，主要负责系统中四大组件的启动、切换、调度及应用进程的管理和调度等工作，其职责与操作系统中的进程管理和调度模块相类似，因此它在Android中非常重要，它本身也是一个Binder的实现类。
2、Instrumentation：顾名思义，它用来监控应用程序和系统的交互。
3、ActivityThread：应用的入口类，系统通过调用main函数，开启消息循环队列。ActivityThread所在线程被称为应用的主线程（UI线程）。
4、ApplicationThread：ApplicationThread提供Binder通讯接口，AMS则通过代理调用此App进程的本地方法。
5、ActivityManagerProxy：AMS服务在当前进程的代理类，负责与AMS通信。
6、ApplicationThreadProxy：ApplicationThread在AMS服务中的代理类，负责与ApplicationThread通信。
3、流程分析
首先交代下整个流程分析的场景：用户点击Launcher上的应用图标到该应用主界面启动展示在用户眼前。
这整个过程涉及到跨进程通信，所以我们将其划分为时序图中所展示三个进程：Launcher进程、System进程、App进程。为了不贴过长的代码又能说清楚进程间交互的流程，这里简述几个重要的交互点。
从时序图上大家也可以看到调用链相当长，对应的代码量也比较大，而且时序图只是分析了这个一个场景下的流程。道阻且长，行则将至！
3.1 Launcher响应用户点击，通知AMS
Launcher做为应用的入口，还是有必要交代一下的，我们来看看Launcher的代码片段，Launcher使用的是packages/apps/Launcher3的的源码。
public class Launcher extends Activity implements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, View.OnTouchListener, PageSwitchListener, LauncherProviderChangeListener { ... /** * Launches the intent referred by the clicked shortcut. * * @param v The view representing the clicked shortcut. */ public void onClick(View v) { // Make sure that rogue clicks don't get through while allapps is launching, or after the // view has detached (it's possible for this to happen if the view is removed mid touch). if (v.getWindowToken() == null) { return; } ... Object tag = v.getTag(); if (tag instanceof ShortcutInfo) { onClickAppShortcut(v); } else if (tag instanceof FolderInfo) { ... } else if (v == mAllAppsButton) { onClickAllAppsButton(v); } else if (tag instanceof AppInfo) { startAppShortcutOrInfoActivity(v); } else if (tag instanceof LauncherAppWidgetInfo) { ... } } private void startAppShortcutOrInfoActivity(View v) { ... boolean success = startActivitySafely(v, intent, tag); ... } boolean startActivitySafely(View v, Intent intent, Object tag) { ... try { success = startActivity(v, intent, tag); } catch (ActivityNotFoundException e) { ... } return success; } boolean startActivity(View v, Intent intent, Object tag) { intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); try { ... if (user == null || user.equals(UserHandleCompat.myUserHandle())) { // Could be launching some bookkeeping activity startActivity(intent, optsBundle); } else { ... } return true; } catch (SecurityException e) { ... } return false; } }
通过starActicity辗转调用到Activity:startActivityForResult而后则调用至Instrumentation:execStartActivity，代码片段如下：
public class Instrumentation { ... public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token, Activity target, Intent intent, int requestCode, Bundle options) { IApplicationThread whoThread = (IApplicationThread) contextThread; ... try { ... int result = ActivityManagerNative.getDefault() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options); ... } catch (RemoteException e) { } return null; } ... }
这里的ActivityManagerNative.getDefault返回ActivityManagerService的远程接口，即ActivityManagerProxy接口，有人可能会问了为什么会是ActivityManagerProxy，这就涉及到Binder通信了，这里不再展开。通过Binder驱动程序，ActivityManagerProxy与AMS服务通信，则实现了跨进程到System进程。
3.2 AMS响应Launcher进程请求
从上面的流程我们知道，此时AMS应该处理Launcher进程发来的请求，请参看时序图及源码，此时我们来看ActivityStackSupervisor:startActivityUncheckedLocked方法，目测这个方法已经超过600行代码，来看一些关键代码片段：
public final class ActivityStackSupervisor implements DisplayListener { ... final int startActivityUncheckedLocked(ActivityRecord r, ActivityRecord sourceRecord, IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor, int startFlags, boolean doResume, Bundle options, TaskRecord inTask) { final Intent intent = r.intent; final int callingUid = r.launchedFromUid; ... final boolean launchSingleTop = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP; final boolean launchSingleInstance = r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE; final boolean launchSingleTask = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK; int launchFlags = intent.getFlags(); ... // We'll invoke onUserLeaving before onPause only if the launching // activity did not explicitly state that this is an automated launch. mUserLeaving = (launchFlags & Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0; ... ActivityRecord notTop = (launchFlags & Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP) != 0 ? r : null; // If the onlyIfNeeded flag is set, then we can do this if the activity // being launched is the same as the one making the call... or, as // a special case, if we do not know the caller then we count the // current top activity as the caller. if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) { ... } ... // If the caller is not coming from another activity, but has given us an // explicit task into which they would like us to launch the new activity, // then let's see about doing that. if (sourceRecord == null && inTask != null && inTask.stack != null) { final Intent baseIntent = inTask.getBaseIntent(); final ActivityRecord root = inTask.getRootActivity(); ... // If this task is empty, then we are adding the first activity -- it // determines the root, and must be launching as a NEW_TASK. if (launchSingleInstance || launchSingleTask) { ... } ... } ... if (inTask == null) { if (sourceRecord == null) { // This activity is not being started from another... in this // case we -always- start a new task. if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0 && inTask == null) { Slog.w(TAG, "startActivity called from non-Activity context; forcing " + "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent); launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } } else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) { // The original activity who is starting us is running as a single // instance... this new activity it is starting must go on its // own task. launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } else if (launchSingleInstance || launchSingleTask) { // The activity being started is a single instance... it always // gets launched into its own task. launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } } ... // We may want to try to place the new activity in to an existing task. We always // do this if the target activity is singleTask or singleInstance; we will also do // this if NEW_TASK has been requested, and there is not an additional qualifier telling // us to still place it in a new task: multi task, always doc mode, or being asked to // launch this as a new task behind the current one. if (((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0 && (launchFlags & Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0) || launchSingleInstance || launchSingleTask) { // If bring to front is requested, and no result is requested and we have not // been given an explicit task to launch in to, and // we can find a task that was started with this same // component, then instead of launching bring that one to the front. if (inTask == null && r.resultTo == null) { // See if there is a task to bring to the front. If this is // a SINGLE_INSTANCE activity, there can be one and only one // instance of it in the history, and it is always in its own // unique task, so we do a special search. ActivityRecord intentActivity = !launchSingleInstance ? findTaskLocked(r) : findActivityLocked(intent, r.info); if (intentActivity != null) { ... } } } ... if (r.packageName != null) { // If the activity being launched is the same as the one currently // at the top, then we need to check if it should only be launched // once. ActivityStack topStack = getFocusedStack(); ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(notTop); if (top != null && r.resultTo == null) { if (top.realActivity.equals(r.realActivity) && top.userId == r.userId) { ... } } } else{ ... } boolean newTask = false; boolean keepCurTransition = false; TaskRecord taskToAffiliate = launchTaskBehind && sourceRecord != null ? sourceRecord.task : null; // Should this be considered a new task? if (r.resultTo == null && inTask == null && !addingToTask && (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) { ... if (reuseTask == null) { r.setTask(targetStack.createTaskRecord(getNextTaskId(), newTaskInfo != null ? newTaskInfo : r.info, newTaskIntent != null ? newTaskIntent : intent, voiceSession, voiceInteractor, !launchTaskBehind /* toTop */), taskToAffiliate); ... } else { r.setTask(reuseTask, taskToAffiliate); } ... } else if (sourceRecord != null) { } else if (!addingToTask && (launchFlags&Intent.FLAG_ACTIVITY_REORDER_TO_FRONT) != 0) { } else if (inTask != null){ } else { } ... targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options); ... return ActivityManager.START_SUCCESS; } ... }
函数经过intent的标志值设置，通过findTaskLocked函数来查找存不存这样的Task，这里返回的结果是null，即intentActivity为null，因此，需要创建一个新的Task来启动这个Activity。现在处理堆栈顶端的Activity是Launcher，与我们即将要启动的MainActivity不是同一个Activity，创建了一个新的Task里面来启动这个Activity。
经过栈顶检测，则需要将Launcher推入Paused状态，才可以启动新的Activity。后续则调用至ActivityStack:startPausingLocked，我们来看一下这个函数：
final class ActivityStack { ... final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping, boolean resuming, boolean dontWait) { if (mPausingActivity != null) { ... } ActivityRecord prev = mResumedActivity; if (prev == null) { ... } ... mResumedActivity = null; mPausingActivity = prev; mLastPausedActivity = prev; mLastNoHistoryActivity = (prev.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_HISTORY) != 0 || (prev.info.flags & ActivityInfo.FLAG_NO_HISTORY) != 0 ? prev : null; prev.state = ActivityState.PAUSING; ... if (prev.app != null && prev.app.thread != null) { try { ... prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing, userLeaving, prev.configChangeFlags, dontWait); } catch (Exception e) { ... } } else { ... } ... } ... }
这里的prev.app.thread是一个ApplicationThread对象的远程接口，通过调用这个远程接口的schedulePauseActivity来通知Launcher进入Paused状态。至此，AMS对Launcher的请求已经响应，这是我们发现又通过Binder通信回调至Launcher进程。
3.3 Launcher进程挂起Launcher，再次通知AMS
这个流程相对会简单一些，我们来看ActivityThread:
public final class ActivityThread { ... private void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving, int configChanges, boolean dontReport) { ActivityClientRecord r = mActivities.get(token); if (r != null) { ... performPauseActivity(token, finished, r.isPreHoneycomb()); // Make sure any pending writes are now committed. if (r.isPreHoneycomb()) { QueuedWork.waitToFinish(); } // Tell the activity manager we have paused. if (!dontReport) { try { ActivityManagerNative.getDefault().activityPaused(token); } catch (RemoteException ex) { } } ... } } ... }
这部分Launcher的ActivityThread处理页面Paused并且再次通过ActivityManagerProxy通知AMS。
3.4 AMS创建新的进程
创建新进程的时候，AMS会保存一个ProcessRecord信息，如果应用程序中的AndroidManifest.xml配置文件中，我们没有指定Application标签的process属性，系统就会默认使用package的名称。每一个应用程序都有自己的uid，因此，这里uid + process的组合就可以为每一个应用程序创建一个ProcessRecord。
public final class ActivityManagerService extends ActivityManagerNative implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback { ... private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) { ... try { ... // Start the process. It will either succeed and return a result containing // the PID of the new process, or else throw a RuntimeException. boolean isActivityProcess = (entryPoint == null); if (entryPoint == null) entryPoint = "android.app.ActivityThread"; Process.ProcessStartResult startResult = Process.start(entryPoint, app.processName, uid, uid, gids, debugFlags, mountExternal, app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet, app.info.dataDir, entryPointArgs); ... } catch () { ... } } ... }
这里主要是调用Process:start接口来创建一个新的进程，新的进程会导入android.app.ActivityThread类，并且执行它的main函数，这就是每一个应用程序都有一个ActivityThread实例来对应的原因。
3.5 应用进程初始化
我们来看Activity的main函数，这里绑定了主线程的Looper，并进入消息循环，大家应该知道，整个Android系统是消息驱动的，这也是为什么主线程默认绑定Looper的原因：
public final class ActivityThread { ... public static void main(String[] args) { ... Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); ... Looper.loop(); ... } private void attach(boolean system) { ... if (!system) { ... final IActivityManager mgr = ActivityManagerNative.getDefault(); try { mgr.attachApplication(mAppThread); } catch (RemoteException ex) { // Ignore } } else { ... } ... } ... }
attach函数最终调用了ActivityManagerService的远程接口ActivityManagerProxy的attachApplication函数，传入的参数是mAppThread，这是一个ApplicationThread类型的Binder对象，它的作用是AMS与应用进程进行进程间通信的。
3.6 在AMS中注册应用进程，启动启动栈顶页面
前面我们提到了AMS负责系统中四大组件的启动、切换、调度及应用进程的管理和调度等工作，通过上一个流程我们知道应用进程创建后通过Binder驱动与AMS产生交互，此时AMS则将应用进程创建后的信息进行了一次注册，如果拿Windows系统程序注册到的注册表来理解这个过程，可能会更形象一些。
mMainStack.topRunningActivityLocked(null)从堆栈顶端取出要启动的Activity，并在realStartActivityLockedhan函数中通过ApplicationThreadProxy调回App进程启动页面。
public final class ActivityStackSupervisor implements DisplayListener { ... final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app, boolean andResume, boolean checkConfig) throws RemoteException { ... r.app = app; ... try { ... app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken, System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration), r.compat, r.launchedFromPackage, r.task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results, newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo); ... } catch (RemoteException e) { ... } ... } ... }
此时在App进程，我们可以看到，经过一些列的调用链最终调用至MainActivity:onCreate函数，之后会调用至onResume，而后会通知AMS该MainActivity已经处于resume状态。至此，整个启动流程告一段落。
4、总结
通过上述流程，相信大家可以有了一个基本的认知，这里我们忽略细节简化流程，单纯从进程角度来看下图： launch_app_sim
图上所画这里就不在赘述，Activity启动后至Resume状态，此时可交互。以上就是分析Android中应用启动流程的全部内容了，如何有疑问欢迎大家指正交流。