一张图搞定，Activity的启动流程

【齐天的博客】转载请注明出处（万分感谢！）：
https://blog.csdn.net/qijinglai/article/details/81163574
一口吃个胖子肯定是不可能的，就拿考试或找工作来说，突击的结局大多是被虐的很惨，本着一个突击不如多花一阵时间沉淀自己的原则继续我的读源码的路。

接着上次聊聊Activity的生命周期，onSaveInstanceState/onRestoreInstanceState源码分析，继续聊Activity，Activity的所有知识中，我认为最难的就是启动流程了，源码有一大大串，所以老习惯先来张图，只想大概了解流程看这张图足够了，分析实在太长了，所以还是推荐看着图自己点源码浏览

虽然过程复杂，但总归有入口就一定有出口，通常我们跳转页面使用的是startActivity(Intent intent)，那就从这里开始对照流程图一步一步看看他到底是如何实现的。

一、从startActivity开始

    @Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }
    @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);
            }
        }
    }

一个参数的startActivity调用两个参数的，两个参数的调用startActivityForResult。可以发现由于我们可能是首次启动Activity，所以这里的mParent为空，所以会执行if那里，所以这里面我们只需要关心Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options);就够了。就是说startActivity实际上是调用了Instrumentation的execStartActivity方法

（这里说一下Instrumentation，Instrumentation是启动Activity的一个实际操作类，负责应用进程端的启动，除了这一端还有SystemServer服务进程端的启动，二者配合完成启动）

    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; ifinal ActivityMonitor am = mActivityMonitors.get(i);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        result = am.onStartActivity(intent);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else 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 = ActivityManager.getService()
                .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;
    }

这里执行的是ActivityManager.getService().startActivity

    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton IActivityManagerSingleton =
            new Singleton() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

到了这里就引出了一个在Android系统中非常重要的概念：Binder机制。
因为ActivityManagerService(AMS)负责系统中四大组件的启动、切换、调度及应用程序的管理和调度等工作，Android中最核心的服务。所以Activity启动一定是要经他手的，但是问题来了，AMS是SystemServer服务进程端，属于跨进程，如何跨进程通信呢？是不是发现找来找去都找不到IActivityManager？那当然了，这里使用的是AIDL跨进程通信方式，他被写到了aidl文件里。
上面代码中IActivityManager.Stub.asInterface(b)返回了一个IActivityManager的代理对象，基于Binder机制，通过调用代理对象的方法，使得AMS对应的方法被调用。所以可以猜出AMS肯定继承了IActivityManager.Stub，事实也确实如此。

二、AMS接收startActivity的请求

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
    ...

再稍微看一下aidl文件，他在\core\java\android\app\IActivityManager.aidl

interface IActivityManager {
   ...
   ...
    int startActivity(in IApplicationThread caller, in String callingPackage, in Intent intent,
            in String resolvedType, in IBinder resultTo, in String resultWho, int requestCode,
            int flags, in ProfilerInfo profilerInfo, in Bundle options);
   ...

连接了两端的startActivity方法，所以下一步我们来看AMS中的startActivity

    @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, "startActivityAsUser");
    }

跟进去他调用的是startActivityAsUser，前面是调用者的权限检查，然后核心代码调用ActivityStarter的startActivityMayWait很长的一段代码

    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, WaitResult outResult,
            Configuration globalConfig, Bundle bOptions, boolean ignoreTargetSecurity, int userId,
            TaskRecord inTask, String reason) {
        // Refuse possible leaked file descriptors
        if (intent != null && intent.hasFileDescriptors()) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }
        mSupervisor.mActivityMetricsLogger.notifyActivityLaunching();
        boolean componentSpecified = intent.getComponent() != null;

        // Save a copy in case ephemeral needs it
        final Intent ephemeralIntent = new Intent(intent);
        // Don't modify the client's object!
        intent = new Intent(intent);
        if (componentSpecified
                && intent.getData() != null
                && Intent.ACTION_VIEW.equals(intent.getAction())
                && mService.getPackageManagerInternalLocked()
                        .isInstantAppInstallerComponent(intent.getComponent())) {
            // intercept intents targeted directly to the ephemeral installer the
            // ephemeral installer should never be started with a raw URL; instead
            // adjust the intent so it looks like a "normal" instant app launch
            intent.setComponent(null /*component*/);
            componentSpecified = false;
        }

        ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId);
        if (rInfo == null) {
            UserInfo userInfo = mSupervisor.getUserInfo(userId);
            if (userInfo != null && userInfo.isManagedProfile()) {
                // Special case for managed profiles, if attempting to launch non-cryto aware
                // app in a locked managed profile from an unlocked parent allow it to resolve
                // as user will be sent via confirm credentials to unlock the profile.
                UserManager userManager = UserManager.get(mService.mContext);
                boolean profileLockedAndParentUnlockingOrUnlocked = false;
                long token = Binder.clearCallingIdentity();
                try {
                    UserInfo parent = userManager.getProfileParent(userId);
                    profileLockedAndParentUnlockingOrUnlocked = (parent != null)
                            && userManager.isUserUnlockingOrUnlocked(parent.id)
                            && !userManager.isUserUnlockingOrUnlocked(userId);
                } finally {
                    Binder.restoreCallingIdentity(token);
                }
                if (profileLockedAndParentUnlockingOrUnlocked) {
                    rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId,
                            PackageManager.MATCH_DIRECT_BOOT_AWARE
                                    | PackageManager.MATCH_DIRECT_BOOT_UNAWARE);
                }
            }
        }
        // Collect information about the target of the Intent.
        ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);

        ActivityOptions options = ActivityOptions.fromBundle(bOptions);
        synchronized (mService) {
            final int realCallingPid = Binder.getCallingPid();
            final int realCallingUid = Binder.getCallingUid();
            int callingPid;
            if (callingUid >= 0) {
                callingPid = -1;
            } else if (caller == null) {
                callingPid = realCallingPid;
                callingUid = realCallingUid;
            } else {
                callingPid = callingUid = -1;
            }

            final ActivityStack stack = mSupervisor.mFocusedStack;
            stack.mConfigWillChange = globalConfig != null
                    && mService.getGlobalConfiguration().diff(globalConfig) != 0;
            if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                    "Starting activity when config will change = " + stack.mConfigWillChange);

            final long origId = Binder.clearCallingIdentity();

            if (aInfo != null &&
                    (aInfo.applicationInfo.privateFlags
                            & ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE) != 0) {
                // This may be a heavy-weight process!  Check to see if we already
                // have another, different heavy-weight process running.
                if (aInfo.processName.equals(aInfo.applicationInfo.packageName)) {
                    final ProcessRecord heavy = mService.mHeavyWeightProcess;
                    if (heavy != null && (heavy.info.uid != aInfo.applicationInfo.uid
                            || !heavy.processName.equals(aInfo.processName))) {
                        int appCallingUid = callingUid;
                        if (caller != null) {
                            ProcessRecord callerApp = mService.getRecordForAppLocked(caller);
                            if (callerApp != null) {
                                appCallingUid = callerApp.info.uid;
                            } else {
                                Slog.w(TAG, "Unable to find app for caller " + caller
                                        + " (pid=" + callingPid + ") when starting: "
                                        + intent.toString());
                                ActivityOptions.abort(options);
                                return ActivityManager.START_PERMISSION_DENIED;
                            }
                        }

                        IIntentSender target = mService.getIntentSenderLocked(
                                ActivityManager.INTENT_SENDER_ACTIVITY, "android",
                                appCallingUid, userId, null, null, 0, new Intent[] { intent },
                                new String[] { resolvedType }, PendingIntent.FLAG_CANCEL_CURRENT
                                        | PendingIntent.FLAG_ONE_SHOT, null);

                        Intent newIntent = new Intent();
                        if (requestCode >= 0) {
                            // Caller is requesting a result.
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_HAS_RESULT, true);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_INTENT,
                                new IntentSender(target));
                        if (heavy.activities.size() > 0) {
                            ActivityRecord hist = heavy.activities.get(0);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_APP,
                                    hist.packageName);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_TASK,
                                    hist.getTask().taskId);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_NEW_APP,
                                aInfo.packageName);
                        newIntent.setFlags(intent.getFlags());
                        newIntent.setClassName("android",
                                HeavyWeightSwitcherActivity.class.getName());
                        intent = newIntent;
                        resolvedType = null;
                        caller = null;
                        callingUid = Binder.getCallingUid();
                        callingPid = Binder.getCallingPid();
                        componentSpecified = true;
                        rInfo = mSupervisor.resolveIntent(intent, null /*resolvedType*/, userId);
                        aInfo = rInfo != null ? rInfo.activityInfo : null;
                        if (aInfo != null) {
                            aInfo = mService.getActivityInfoForUser(aInfo, userId);
                        }
                    }
                }
            }

            final ActivityRecord[] outRecord = new ActivityRecord[1];
            int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
                    aInfo, rInfo, voiceSession, voiceInteractor,
                    resultTo, resultWho, requestCode, callingPid,
                    callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                    options, ignoreTargetSecurity, componentSpecified, outRecord, inTask,
                    reason);

            Binder.restoreCallingIdentity(origId);

            if (stack.mConfigWillChange) {
                // If the caller also wants to switch to a new configuration,
                // do so now.  This allows a clean switch, as we are waiting
                // for the current activity to pause (so we will not destroy
                // it), and have not yet started the next activity.
                mService.enforceCallingPermission(android.Manifest.permission.CHANGE_CONFIGURATION,
                        "updateConfiguration()");
                stack.mConfigWillChange = false;
                if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                        "Updating to new configuration after starting activity.");
                mService.updateConfigurationLocked(globalConfig, null, false);
            }

            if (outResult != null) {
                outResult.result = res;
                if (res == ActivityManager.START_SUCCESS) {
                    mSupervisor.mWaitingActivityLaunched.add(outResult);
                    do {
                        try {
                            mService.wait();
                        } catch (InterruptedException e) {
                        }
                    } while (outResult.result != START_TASK_TO_FRONT
                            && !outResult.timeout && outResult.who == null);
                    if (outResult.result == START_TASK_TO_FRONT) {
                        res = START_TASK_TO_FRONT;
                    }
                }
                if (res == START_TASK_TO_FRONT) {
                    final ActivityRecord r = outRecord[0];

                    // ActivityRecord may represent a different activity, but it should not be in
                    // the resumed state.
                    if (r.nowVisible && r.state == RESUMED) {
                        outResult.timeout = false;
                        outResult.who = r.realActivity;
                        outResult.totalTime = 0;
                        outResult.thisTime = 0;
                    } else {
                        outResult.thisTime = SystemClock.uptimeMillis();
                        mSupervisor.waitActivityVisible(r.realActivity, outResult);
                        // Note: the timeout variable is not currently not ever set.
                        do {
                            try {
                                mService.wait();
                            } catch (InterruptedException e) {
                            }
                        } while (!outResult.timeout && outResult.who == null);
                    }
                }
            }

            mSupervisor.mActivityMetricsLogger.notifyActivityLaunched(res, outRecord[0]);
            return res;
        }
    }

很长的代码一句一句看吗？说实话如果一个字一个字的看确实能体会更多的设计理念，但是更容易产生钻牛角尖的心里。所以我一般会从返回值(res)和出现次数最多的变量(mSupervisor)入手，然后大概解释一下这段代码，首先判断了下是显式跳转还是隐式跳转，之后在调用resolveIntent、resolveActivity进行匹配，然后进行核心代码：

            int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
                    aInfo, rInfo, voiceSession, voiceInteractor,
                    resultTo, resultWho, requestCode, callingPid,
                    callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                    options, ignoreTargetSecurity, componentSpecified, outRecord, inTask,
                    reason);

所以实际上调用的是startActivityLocked

    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, TaskRecord inTask, String reason) {

        if (TextUtils.isEmpty(reason)) {
            throw new IllegalArgumentException("Need to specify a reason.");
        }
        mLastStartReason = reason;
        mLastStartActivityTimeMs = System.currentTimeMillis();
        mLastStartActivityRecord[0] = null;

        mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
                aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
                callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
                inTask);

        if (outActivity != null) {
            // mLastStartActivityRecord[0] is set in the call to startActivity above.
            outActivity[0] = mLastStartActivityRecord[0];
        }

        // Aborted results are treated as successes externally, but we must track them internally.
        return mLastStartActivityResult != START_ABORTED ? mLastStartActivityResult : START_SUCCESS;
    }

实际调用了ActivityStarter的startActivity

    private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity) {
        int result = START_CANCELED;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
        } finally {
            // If we are not able to proceed, disassociate the activity from the task. Leaving an
            // activity in an incomplete state can lead to issues, such as performing operations
            // without a window container.
            if (!ActivityManager.isStartResultSuccessful(result)
                    && mStartActivity.getTask() != null) {
                mStartActivity.getTask().removeActivity(mStartActivity);
            }
            mService.mWindowManager.continueSurfaceLayout();
        }

        postStartActivityProcessing(r, result, mSupervisor.getLastStack().mStackId,  mSourceRecord,
                mTargetStack);

        return result;
    }

处理了WindowManagerService之间的交互，目的是保证Activity对应的UI能正常显示，其中又主要掉用startActivityUnchecked

    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity) {

        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
                voiceInteractor);

        computeLaunchingTaskFlags();

        computeSourceStack();

        mIntent.setFlags(mLaunchFlags);

        ActivityRecord reusedActivity = getReusableIntentActivity();

        final int preferredLaunchStackId =
                (mOptions != null) ? mOptions.getLaunchStackId() : INVALID_STACK_ID;
        final int preferredLaunchDisplayId =
                (mOptions != null) ? mOptions.getLaunchDisplayId() : DEFAULT_DISPLAY;

        if (reusedActivity != null) {
            // When the flags NEW_TASK and CLEAR_TASK are set, then the task gets reused but
            // still needs to be a lock task mode violation since the task gets cleared out and
            // the device would otherwise leave the locked task.
            if (mSupervisor.isLockTaskModeViolation(reusedActivity.getTask(),
                    (mLaunchFlags & (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))
                            == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))) {
                mSupervisor.showLockTaskToast();
                Slog.e(TAG, "startActivityUnchecked: Attempt to violate Lock Task Mode");
                return START_RETURN_LOCK_TASK_MODE_VIOLATION;
            }

            if (mStartActivity.getTask() == null) {
                mStartActivity.setTask(reusedActivity.getTask());
            }
            if (reusedActivity.getTask().intent == null) {
                // This task was started because of movement of the activity based on affinity...
                // Now that we are actually launching it, we can assign the base intent.
                reusedActivity.getTask().setIntent(mStartActivity);
            }

            // This code path leads to delivering a new intent, we want to make sure we schedule it
            // as the first operation, in case the activity will be resumed as a result of later
            // operations.
            if ((mLaunchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
                    || isDocumentLaunchesIntoExisting(mLaunchFlags)
                    || mLaunchSingleInstance || mLaunchSingleTask) {
                final TaskRecord task = reusedActivity.getTask();

                // In this situation we want to remove all activities from the task up to the one
                // being started. In most cases this means we are resetting the task to its initial
                // state.
                final ActivityRecord top = task.performClearTaskForReuseLocked(mStartActivity,
                        mLaunchFlags);

                // The above code can remove {@code reusedActivity} from the task, leading to the
                // the {@code ActivityRecord} removing its reference to the {@code TaskRecord}. The
                // task reference is needed in the call below to
                // {@link setTargetStackAndMoveToFrontIfNeeded}.
                if (reusedActivity.getTask() == null) {
                    reusedActivity.setTask(task);
                }

                if (top != null) {
                    if (top.frontOfTask) {
                        // Activity aliases may mean we use different intents for the top activity,
                        // so make sure the task now has the identity of the new intent.
                        top.getTask().setIntent(mStartActivity);
                    }
                    deliverNewIntent(top);
                }
            }

            sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, reusedActivity);

            reusedActivity = setTargetStackAndMoveToFrontIfNeeded(reusedActivity);

            final ActivityRecord outResult =
                    outActivity != null && outActivity.length > 0 ? outActivity[0] : null;

            // When there is a reused activity and the current result is a trampoline activity,
            // set the reused activity as the result.
            if (outResult != null && (outResult.finishing || outResult.noDisplay)) {
                outActivity[0] = reusedActivity;
            }

            if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
                // We don't need to start a new activity, and the client said not to do anything
                // if that is the case, so this is it!  And for paranoia, make sure we have
                // correctly resumed the top activity.
                resumeTargetStackIfNeeded();
                return START_RETURN_INTENT_TO_CALLER;
            }
            setTaskFromIntentActivity(reusedActivity);

            if (!mAddingToTask && mReuseTask == null) {
                // We didn't do anything...  but it was needed (a.k.a., client don't use that
                // intent!)  And for paranoia, make sure we have correctly resumed the top activity.
                resumeTargetStackIfNeeded();
                if (outActivity != null && outActivity.length > 0) {
                    outActivity[0] = reusedActivity;
                }

                return START_TASK_TO_FRONT;
            }
        }

        if (mStartActivity.packageName == null) {
            final ActivityStack sourceStack = mStartActivity.resultTo != null
                    ? mStartActivity.resultTo.getStack() : null;
            if (sourceStack != null) {
                sourceStack.sendActivityResultLocked(-1 /* callingUid */, mStartActivity.resultTo,
                        mStartActivity.resultWho, mStartActivity.requestCode, RESULT_CANCELED,
                        null /* data */);
            }
            ActivityOptions.abort(mOptions);
            return START_CLASS_NOT_FOUND;
        }

        // 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.
        final ActivityStack topStack = mSupervisor.mFocusedStack;
        final ActivityRecord topFocused = topStack.topActivity();
        final ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(mNotTop);
        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);
        if (dontStart) {
            // For paranoia, make sure we have correctly resumed the top activity.
            topStack.mLastPausedActivity = null;
            if (mDoResume) {
                mSupervisor.resumeFocusedStackTopActivityLocked();
            }
            ActivityOptions.abort(mOptions);
            if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
                // We don't need to start a new activity, and the client said not to do
                // anything if that is the case, so this is it!
                return START_RETURN_INTENT_TO_CALLER;
            }

            deliverNewIntent(top);

            // 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.getTask(), preferredLaunchStackId,
                    preferredLaunchDisplayId, topStack.mStackId);

            return START_DELIVERED_TO_TOP;
        }

        boolean newTask = false;
        final TaskRecord taskToAffiliate = (mLaunchTaskBehind && mSourceRecord != null)
                ? mSourceRecord.getTask() : null;

        // Should this be considered a new task?
        int result = START_SUCCESS;
        if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
                && (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
            newTask = true;
            result = setTaskFromReuseOrCreateNewTask(
                    taskToAffiliate, preferredLaunchStackId, topStack);
        } else if (mSourceRecord != null) {
            result = setTaskFromSourceRecord();
        } else if (mInTask != null) {
            result = setTaskFromInTask();
        } else {
            // This not being started from an existing activity, and not part of a new task...
            // just put it in the top task, though these days this case should never happen.
            setTaskToCurrentTopOrCreateNewTask();
        }
        if (result != START_SUCCESS) {
            return result;
        }

        mService.grantUriPermissionFromIntentLocked(mCallingUid, mStartActivity.packageName,
                mIntent, mStartActivity.getUriPermissionsLocked(), mStartActivity.userId);
        mService.grantEphemeralAccessLocked(mStartActivity.userId, mIntent,
                mStartActivity.appInfo.uid, UserHandle.getAppId(mCallingUid));
        if (mSourceRecord != null) {
            mStartActivity.getTask().setTaskToReturnTo(mSourceRecord);
        }
        if (newTask) {
            EventLog.writeEvent(
                    EventLogTags.AM_CREATE_TASK, mStartActivity.userId,
                    mStartActivity.getTask().taskId);
        }
        ActivityStack.logStartActivity(
                EventLogTags.AM_CREATE_ACTIVITY, mStartActivity, mStartActivity.getTask());
        mTargetStack.mLastPausedActivity = null;

        sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, mStartActivity);

        mTargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
                mOptions);
        if (mDoResume) {
            final ActivityRecord topTaskActivity =
                    mStartActivity.getTask().topRunningActivityLocked();
            if (!mTargetStack.isFocusable()
                    || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                    && mStartActivity != topTaskActivity)) {
                // If the activity is not focusable, we can't resume it, but still would like to
                // make sure it becomes visible as it starts (this will also trigger entry
                // animation). An example of this are PIP activities.
                // Also, we don't want to resume activities in a task that currently has an overlay
                // as the starting activity just needs to be in the visible paused state until the
                // over is removed.
                mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
                // Go ahead and tell window manager to execute app transition for this activity
                // since the app transition will not be triggered through the resume channel.
                mWindowManager.executeAppTransition();
            } else {
                // If the target stack was not previously focusable (previous top running activity
                // on that stack was not visible) then any prior calls to move the stack to the
                // will not update the focused stack.  If starting the new activity now allows the
                // task stack to be focusable, then ensure that we now update the focused stack
                // accordingly.
                if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
                    mTargetStack.moveToFront("startActivityUnchecked");
                }
                mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
                        mOptions);
            }
        } else {
            mTargetStack.addRecentActivityLocked(mStartActivity);
        }
        mSupervisor.updateUserStackLocked(mStartActivity.userId, mTargetStack);

        mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTask(), preferredLaunchStackId,
                preferredLaunchDisplayId, mTargetStack.mStackId);

        return START_SUCCESS;
    }

首先setInitialState()、computeLaunchingTaskFlags()、computeSourceStack()验证是否是特定的FLAG,后面执行了不同启动模式下的不同栈的处理。这些处理中关键的方法是mSupervisor.resumeFocusedStackTopActivityLocked()

    boolean resumeFocusedStackTopActivityLocked() {
        return resumeFocusedStackTopActivityLocked(null, null, null);
    }

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

            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        ...
        return false;
    }

    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        ...
            result = resumeTopActivityInnerLocked(prev, options);
        ...
        return result;
    }

    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        ...

        mStackSupervisor.startSpecificActivityLocked(next, true, true);

        ...

    }

    void startSpecificActivityLocked(ActivityRecord r,
                boolean andResume, boolean checkConfig) {

        ...

        realStartActivityLocked(r, app, andResume, checkConfig);

        ...

    }

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
                boolean andResume, boolean checkConfig) throws RemoteException {
        ...

        app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                        System.identityHashCode(r), r.info,
                        // TODO: Have this take the merged configuration instead of separate global and
                        // override configs.
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, !andResume,
                        mService.isNextTransitionForward(), profilerInfo);

        ...    

变量app是一个ProcessRecord对象，它的成员变量thread是IApplicationThread类型，ApplicationThread继承了IApplicationThread.Stub，因此app.thread是一个IApplicationThread的代理对象。和IActivityManager一样，app.thread调用scheduleLaunchActivity方法，通过Binder机制，会使ApplicationThread的scheduleLaunchActivity方法被调用。由此实现了又一次进程间的通信，将启动Activity的操作交给了ApplicationThread类。
上面这一段可以重点看一下resumeTopActivityInnerLocked就不贴所有代码了，这里遇到了生命周期方法，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，后当前ActivityResume

通过IActivityManager→ActivityManagerService实现了应用进程与SystemServer进程的通讯 通过AppicationThread ← IApplicationThread实现了SystemServer进程与应用进程的通讯

三、接收AMS消息

ApplicationThread是ActivityThread的内部类，因此ApplicationThread可以调用外部类ActivityThread的方法，也就是说，启动Activity的操作交给了ActivityThread来处理。

下面继续看ApplicationThread$scheduleLaunchActivity

        @Override
            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 pendingResults, List 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);
            }

传递的就是启动需要的参数，看一下sendMessage

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        if (DEBUG_MESSAGES) Slog.v(
            TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
            + ": " + arg1 + " / " + obj);
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        mH.sendMessage(msg);
}

最终是mH.sendMessage(msg)，也就是说H这个类接收了消息
Android消息通信机制，之前详细写过Handler源码分析了，

    private class H extends Handler {

        ...
        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;

        ...

    }

将打开Activity的必要参数拿出来，调用ActivityThread$handleLaunchActivity方法启动Activity

    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
        if (!ThreadedRenderer.sRendererDisabled) {
            GraphicsEnvironment.earlyInitEGL();
        }
        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 {
                ActivityManager.getService()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

四、收到消息开始启动

performLaunchActivity方法启动Activity。若Activity不为空，则调用handleResumeActivity方法，内部会调用Activity$onResume方法。当Activity为空，也就是创建Activity的实例出错了，最终会调用ActivityManagerService$finishActivity方法。也就是说，当创建Activity实例出错后，停止启动Activity的具体操作交给ActivityManagerService来处理。

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        ActivityInfo aInfo = r.activityInfo;
        if (r.loadedApk == null) {
            r.loadedApk = getLoadedApk(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
            Application app = r.loadedApk.makeApplication(false, mInstrumentation);

            if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
            if (localLOGV) Slog.v(
                    TAG, r + ": app=" + app
                    + ", appName=" + app.getPackageName()
                    + ", pkg=" + r.loadedApk.getPackageName()
                    + ", comp=" + r.intent.getComponent().toShortString()
                    + ", dir=" + r.loadedApk.getAppDir());

            if (activity != null) {
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                appContext.setOuterContext(activity);
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                checkAndBlockForNetworkAccess();
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }

                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                if (!r.activity.mFinished) {
                    if (r.isPersistable()) {
                        if (r.state != null || r.persistentState != null) {
                            mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                                    r.persistentState);
                        }
                    } else if (r.state != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                    }
                }
                if (!r.activity.mFinished) {
                    activity.mCalled = false;
                    if (r.isPersistable()) {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state,
                                r.persistentState);
                    } else {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state);
                    }
                    if (!activity.mCalled) {
                        throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                            " did not call through to super.onPostCreate()");
                    }
                }
            }
            r.paused = true;

            mActivities.put(r.token, r);

        } catch (SuperNotCalledException e) {
            throw e;

        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to start activity " + component
                    + ": " + e.toString(), e);
            }
        }

        return activity;
    }

获取要启动的Activity的ComponentName对象：里面包含了包名，类名相关的信息；调用newActivity方法，attach方法后启动Activity（调用Instrumentation$callActivityOnCreate方法）

    public Activity newActivity(ClassLoader cl, String className,
                Intent intent)
                throws InstantiationException, IllegalAccessException,
                ClassNotFoundException {
            return (Activity)cl.loadClass(className).newInstance();
        }

public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
        if (mApplication != null) {
            return mApplication;
        }

    Application app = null;

    //...

    app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);

    //...

    mApplication = app;

    instrumentation.callApplicationOnCreate(app);
    //...

    return app;
}

   final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this, window);
        mWindow.setWindowControllerCallback(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        mUiThread = Thread.currentThread();

        mMainThread = aThread;
        mInstrumentation = instr;
        mToken = token;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mReferrer = referrer;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        mLastNonConfigurationInstances = lastNonConfigurationInstances;
        if (voiceInteractor != null) {
            if (lastNonConfigurationInstances != null) {
                mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
            } else {
                mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
                        Looper.myLooper());
            }
        }

        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
    }

创建Activity，创建Application，调用Attach，最后mInstrumentation.callActivityOnCreate(activity, r.state);

    public void callActivityOnCreate(Activity activity, Bundle icicle) {
            prePerformCreate(activity);
            activity.performCreate(icicle);
            postPerformCreate(activity);
    }
    final void performCreate(Bundle icicle) {
            restoreHasCurrentPermissionRequest(icicle);
            onCreate(icicle);
            mActivityTransitionState.readState(icicle);
            performCreateCommon();
    }

终于等到你onCreate，end

没打草稿直接一边看着源码一边敲的，看起来有点乱，不过对照着流程图还是很清晰的哈。

我觉得以后也不会有这么长的文了，真是太累了。。。最后还是主推我的流程图，简直画的好！