【 Android '四大組件' 】篇 -- Activity 的 "啓動流程"

核心源碼（ Android 9.0 ）

Source	Path（/packages/apps/Launcher3/）
BaseDraggingActivity.java	src/com/android/launcher3/BaseDraggingActivity.java
ItemClickHandler.java	src/com/android/launcher3/touch/ItemClickHandler.java
Launcher.java	src/com/android/launcher3/Launcher.java

Source	Path（/packages/apps/Launcher3/）
Activity.java	core/java/android/app/Activity.java
ActivityManager.java	core/java/android/app/ActivityManager.java
ActivityThread.java	core/java/android/app/ActivityThread.java
ClientTransaction.java	core/java/android/app/servertransaction/ClientTransaction.java
ClientTransactionHandler.java	core/java/android/app/ClientTransactionHandler.java
Instrumentation.java	core/java/android/app/Instrumentation.java
LaunchActivityItem.java	core/java/android/app/servertransaction/LaunchActivityItem.java
PauseActivityItem.java	core/java/android/app/servertransaction/PauseActivityItem.java
Process.java	core/java/android/os/Process.java
TransactionExecutor.java	core/java/android/app/servertransaction/TransactionExecutor.java
ActivityManagerService.java	services/core/java/com/android/server/am/ActivityManagerService.java
ActivityStack.java	services/core/java/com/android/server/am/ActivityStack.java
ActivityStarter.java	services/core/java/com/android/server/am/ActivityStarter.java
ActivityStartController.java	services/core/java/com/android/server/am/ActivityStartController.java
ActivityStackSupervisor.java	services/core/java/com/android/server/am/ActivityStackSupervisor.java
ClientLifecycleManager.java	services/core/java/com/android/server/am/ClientLifecycleManager.java

一、前言

點擊 Launcher 界面的應用圖標，運行各種眼花繚亂，功能強大的應用，是我們的日常行爲，作爲一名系統工程師，我們有必要一起深入研究一下，當我們點擊 APK 圖標，直到應用的主界面最後顯示出來，這期間從源碼角度到底發生了什麼，處理邏輯是什麼樣的？

這個流程個人覺得是非常必要深刻理解的，比如有時當我們點擊應用出現了 卡頓、黑白屏 現象，這些是應用本身的問題，還是系統的問題？當你分析此類問題的時候，如果對 Activity 的啓動流程不熟悉，分析起來會狠吃力，並且未必能深入分析得到最根本的原因。

廢話不多說，爲了更方便並且細緻的分析源碼流程，我們選取 Launcher 界面點擊應用 爲切入點進行分析（通過一個特定場景/操作是研究源碼最好的做法），接下來直接進入正題。（各位獅子們，這也是很多面試官喜歡提問的問題）

二、Launcher 的 onPause 流程

我們既然要啓動新的 APK 置於前臺，那麼當前活動必然要被切換到後臺，所以首先涉及到的就是 Launcher 的 onPause 流程。Activity 的生命週期可以閱讀 【 Android '四大組件' 】篇 -- Activity 的 "生命週期" 一文。

2.1 ItemClickHandler.onClick

當點擊應用程序 Launcher 的某個圖標時，回調接口 ItemClickHandler 會執行 onClick() 方法中的 startAppShortcutOrInfoActivity。

// packages/apps/Launcher3/src/com/android/launcher3/Launcher.java

public View createShortcut(ViewGroup parent, ShortcutInfo info) {
    BubbleTextView favorite = (BubbleTextView) LayoutInflater.from(parent.getContext())
            .inflate(R.layout.app_icon, parent, false);
    favorite.applyFromShortcutInfo(info);
    favorite.setOnClickListener(ItemClickHandler.INSTANCE);
    favorite.setOnFocusChangeListener(mFocusHandler);
    return favorite;
}


// packages/apps/Launcher3/src/com/android/launcher3/touch/ItemClickHandler.java

public class ItemClickHandler {
    public static final OnClickListener INSTANCE = ItemClickHandler::onClick;

    private static void onClick(View v) {
        ... ...
        if (tag instanceof ShortcutInfo) {
            onClickAppShortcut(v, (ShortcutInfo) tag, launcher);
        } else if (tag instanceof FolderInfo) {
            if (v instanceof FolderIcon) {
                onClickFolderIcon(v);
            }
        } else if (tag instanceof AppInfo) {
            // 執行 startAppShortcutOrInfoActivity 方法
            startAppShortcutOrInfoActivity(v, (AppInfo) tag, launcher);
        } else if (tag instanceof LauncherAppWidgetInfo) {
            if (v instanceof PendingAppWidgetHostView) {
                onClickPendingWidget((PendingAppWidgetHostView) v, launcher);
            }
        }
    }
    ... ...
}

2.2 ItemClickHandler.startAppShortcutOrInfoActivity

// packages/apps/Launcher3/src/com/android/launcher3/touch/ItemClickHandler.java

private static void startAppShortcutOrInfoActivity(View v, ItemInfo item, Launcher launcher) {
    Intent intent;
    if (item instanceof PromiseAppInfo) {
        PromiseAppInfo promiseAppInfo = (PromiseAppInfo) item;
        intent = promiseAppInfo.getMarketIntent(launcher);
    } else {
        intent = item.getIntent();
    }
    if (intent == null) {
        throw new IllegalArgumentException("Input must have a valid intent");
    }
    if (item instanceof ShortcutInfo) {
        ShortcutInfo si = (ShortcutInfo) item;
        if (si.hasStatusFlag(ShortcutInfo.FLAG_SUPPORTS_WEB_UI)
                && intent.getAction() == Intent.ACTION_VIEW) {
            intent = new Intent(intent);
            intent.setPackage(null);
        }
    }
    // 執行 startActivitySafely 方法
    launcher.startActivitySafely(v, intent, item);
}

2.3 Launcher.startActivitySafely

代碼最終走到 Launcher 的 startActivitySafely() 方法。

// packages/apps/Launcher3/src/com/android/launcher3/Launcher.java

public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
    // 調用父類 BaseDraggingActivity 的 startActivitySafely 方法
    boolean success = super.startActivitySafely(v, intent, item);
    if (success && v instanceof BubbleTextView) {
        BubbleTextView btv = (BubbleTextView) v;
        btv.setStayPressed(true);
        setOnResumeCallback(btv);
    }
    return success;    // 表示啓動新應用程序的 Activity 成功
}

2.4 BaseDraggingActivity.startActivitySafely

// packages/apps/Launcher3/src/com/android/launcher3/BaseDraggingActivity.java

public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
    ... ...
    // 添加標誌，表示要在一個新的 Task 中啓動這個 Activity
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    if (v != null) {
        intent.setSourceBounds(getViewBounds(v));
    }
    try {
        boolean isShortcut = Utilities.ATLEAST_MARSHMALLOW
                                && (item instanceof ShortcutInfo)
                                && (item.itemType == Favorites.ITEM_TYPE_SHORTCUT
                                || item.itemType == Favorites.ITEM_TYPE_DEEP_SHORTCUT)
                                && !((ShortcutInfo) item).isPromise();
        if (isShortcut) {
            startShortcutIntentSafely(intent, optsBundle, item);
        } else if (user == null || user.equals(Process.myUserHandle())) {
            startActivity(intent, optsBundle);    // 調用 Activity.startActivity 方法
        } else {
            LauncherAppsCompat.getInstance(this).startActivityForProfile(
                    intent.getComponent(), user, intent.getSourceBounds(), optsBundle);
        }
        getUserEventDispatcher().logAppLaunch(v, intent);
        return true;
    } catch (ActivityNotFoundException|SecurityException e) {
        Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
        Log.e(TAG, "Unable to launch. tag=" + item + " intent=" + intent, e);
    }
    return false;
}

2.5 Activity.startActivity

// framework/base/core/java/android/app/Activity.java

public void startActivity(Intent intent, @Nullable Bundle options) {
    if (options != null) {
        startActivityForResult(intent, -1, options);    // options 表示 Launcher 啓動 Activity 時的動畫效果
    } else {
        startActivityForResult(intent, -1);
    }
}

2.6 Activity.startActivityForResult

// framework/base/core/java/android/app/Activity.java

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
        @Nullable Bundle options) {
    // mParent 代表目前 Launcher Activity 的狀態
    if (mParent == null) {    // 第一次啓動 Activity，走此條分支
        options = transferSpringboardActivityOptions(options);
        // 執行 mInstrumentation.execStartActivity() 方法
        Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(this, 
                        mMainThread.getApplicationThread(), mToken,
                        this, intent, requestCode, options);
        ... ...
    } else {
        if (options != null) {
            // 最終會調用 mInstrumentation.execStartActivity() 方法
            mParent.startActivityFromChild(this, intent, requestCode, options);
        } else {
            mParent.startActivityFromChild(this, intent, requestCode);
        }
    }
}

mMainThread 是 Activity 類的 成員變量，它的類型是 ActivityThread，它代表的是應用程序的 主線程。

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {
    ... ...
    final ApplicationThread mAppThread = new ApplicationThread();
    ... ...
    private class ApplicationThread extends IApplicationThread.Stub {...}
    ... ...
    public ApplicationThread getApplicationThread()
    {
        return mAppThread;
    }
    ... ...
}

mMainThread.getApplicationThread() 獲得的是 ApplicationThread 內部成員變量，它是一個 Binder 對象。這個對象是幹嘛用的，我們後面會講解到。

2.7 Instrumentation.execStartActivity

Instrumentation 是 Android 系統中啓動 Activity 的一個實際操作類，它用來監控應用程序和系統的交互。

// frameworks/base/core/java/android/app/Instrumentation.java

public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token,
                        Activity target, Intent intent, int requestCode, Bundle options) {
    ... ...
    try {
        intent.migrateExtraStreamToClipData();
        intent.prepareToLeaveProcess(who);
        // 通過 ActivityManager.getService() 方法可以獲得 ActivityManagerService 提供的服務
        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;
}

我們研究下 ActivityManager.getService() 方法：

// frameworks/base/core/java/android/app/ActivityManager.java

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

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
          
public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

我們發現 ActivityManagerService 與 ActivityManger 的 Binder 通信採用了 AIDL 的方式。

ActivityManager.getService().startActivity 方法實際上調用到了 ActivityManagerService 的 startActivity() 方法。

如果對於 AIDL 不瞭解，你可以查看 【 Android '核心機制' 】篇 -- 無處不在的 "AIDL" 一文。

2.8 ActivityManagerService.startActivity

下面看看 ActivityManagerService 的 startActivity 方法：

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@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) {
    // 調轉到 startActivityAsUser 方法
    return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
            resultWho, requestCode, startFlags, profilerInfo, bOptions,
            UserHandle.getCallingUserId());
}

2.9 ActivityManagerService.startActivityAsUser

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

// Android P 新增，這邊的流程與以前的就完全不一樣了，但最終調取的方法還是一樣的，細心跟一下
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) {
    // 繼續調 startActivityAsUser 方法
    return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
            resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
            true /*validateIncomingUser*/);
}

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,
        boolean validateIncomingUser) {
    enforceNotIsolatedCaller("startActivity");

    userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,
                      Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

    // 在 Android P 之前調用的是 ActivityStartController.startActivityMayWait 方法
    return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
            .setCaller(caller)
            .setCallingPackage(callingPackage)
            .setResolvedType(resolvedType)
            .setResultTo(resultTo)
            .setResultWho(resultWho)
            .setRequestCode(requestCode)            
            .setStartFlags(startFlags)
            .setProfilerInfo(profilerInfo)
            .setActivityOptions(bOptions)
            .setMayWait(userId)      // 設置了 mRequest.mayWait = true，後面會用到
            .execute();
}

2.10 ActivityStartController.obtainStarter

// frameworks/base/services/core/java/com/android/server/am/ActivityStartController.java

import com.android.server.am.ActivityStarter.Factory;
private final Factory mFactory;

ActivityStarter obtainStarter(Intent intent, String reason) {
    // obtain()、setIntent()、setReason() 分別幹了什麼，下面一一說明
    return mFactory.obtain().setIntent(intent).setReason(reason);
}

這邊用到了工廠模式，我們跟一下具體的處理流程。

2.10.1 DefaultFactory.obtain

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

interface Factory {
    void setController(ActivityStartController controller);
    ActivityStarter obtain();
    void recycle(ActivityStarter starter);
}

// DefaultFactory 爲實現類
static class DefaultFactory implements Factory {
    ... ...
    @Override
    public ActivityStarter obtain() {
        ActivityStarter starter = mStarterPool.acquire();

        // obtain() 方法最終返回的是 ActivityStarter 的實例
        if (starter == null) {
            starter = new ActivityStarter(mController, mService, mSupervisor, mInterceptor);
        }
        return starter;
    }
    ... ...
}

2.10.2 ActivityStarter.setIntent

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

ActivityStarter setIntent(Intent intent) {
    mRequest.intent = intent;        // 爲 mRequest 賦值 
    return this;
}

2.10.3 ActivityStarter.setReason

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

ActivityStarter setReason(String reason) {
    mRequest.reason = reason;        // 爲 mRequest 賦值
    return this;
}

mActivityStartController.obtainStarter() 方法最終獲得一個包含所有啓動信息的 ActivityStarter 對象，然後作了各種賦值處理，最後執行 execute 方法。

2.11 ActivityStarter.execute

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

// 上面執行了 setMayWait(userId) 方法
ActivityStarter setMayWait(int userId) {
    mRequest.mayWait = true;
    mRequest.userId = userId;
    return this;
}
    
int execute() {
    try {
        if (mRequest.mayWait) {    // setMayWait() 方法中置爲 true
            return startActivityMayWait(mRequest.caller, mRequest.callingUid,
                    mRequest.callingPackage, mRequest.intent, mRequest.resolvedType,
                    mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                    mRequest.resultWho, mRequest.requestCode, mRequest.startFlags,
                    mRequest.profilerInfo, mRequest.waitResult, mRequest.globalConfig,
                    mRequest.activityOptions, mRequest.ignoreTargetSecurity, mRequest.userId,
                    mRequest.inTask, mRequest.reason,
                    mRequest.allowPendingRemoteAnimationRegistryLookup,
                    mRequest.originatingPendingIntent);
        } else {
            return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,
                    mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,
                    mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                    mRequest.resultWho, mRequest.requestCode, mRequest.callingPid,
                    mRequest.callingUid, mRequest.callingPackage, mRequest.realCallingPid,
                    mRequest.realCallingUid, mRequest.startFlags, mRequest.activityOptions,
                    mRequest.ignoreTargetSecurity, mRequest.componentSpecified,
                    mRequest.outActivity, mRequest.inTask, mRequest.reason,
                    mRequest.allowPendingRemoteAnimationRegistryLookup,
                    mRequest.originatingPendingIntent);
        }
    } finally {
        onExecutionComplete();
    }
}

繞了一圈還，最終還是走到了 startActivityMayWait() 方法！

2.12 流程小結

2.13 ActivityStarter.startActivityMayWait

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private 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, SafeActivityOptions options, boolean ignoreTargetSecurity,
        int userId, TaskRecord inTask, String reason,
        boolean allowPendingRemoteAnimationRegistryLookup,
        PendingIntentRecord originatingPendingIntent) {
    // Refuse possible leaked file descriptors
    if (intent != null && intent.hasFileDescriptors()) {
        throw new IllegalArgumentException("File descriptors passed in Intent");
    }
    mSupervisor.getActivityMetricsLogger().notifyActivityLaunching();
    boolean componentSpecified = intent.getComponent() != null;

    final int realCallingPid = Binder.getCallingPid();
    final int realCallingUid = Binder.getCallingUid();
    ... ...
    // 解析 Intent 的數據
    ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId,
            0 /* matchFlags */,
                    computeResolveFilterUid(
                            callingUid, realCallingUid, mRequest.filterCallingUid));
    ... ...
    // 收集 intent 中的相關信息
    ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);

    synchronized (mService) {
        ... ...
        // 用於記錄 Activity 的數據
        final ActivityRecord[] outRecord = new ActivityRecord[1];
        // 啓動 Activity
        int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,
                voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,
                callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,
                ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,
                allowPendingRemoteAnimationRegistryLookup, originatingPendingIntent);

        Binder.restoreCallingIdentity(origId);
        ... ...
        mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(res, outRecord[0]);
        return res;
    }
}

2.14 ActivityStarter.startActivity -01

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivity(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,
        SafeActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
        ActivityRecord[] outActivity, TaskRecord inTask, String reason,
        boolean allowPendingRemoteAnimationRegistryLookup,
        PendingIntentRecord originatingPendingIntent) {
    ... ...
    // 跳轉 startActivity
    mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
            aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
            callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
            options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
            inTask, allowPendingRemoteAnimationRegistryLookup, originatingPendingIntent);
    ... ...
    return getExternalResult(mLastStartActivityResult);
}

2.15 ActivityStarter.startActivity -02

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivity(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,
        SafeActivityOptions options,
        boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,
        TaskRecord inTask, boolean allowPendingRemoteAnimationRegistryLookup,
        PendingIntentRecord originatingPendingIntent) {
    ... ...
    // 跳轉 startActivity
    return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
            true /* doResume */, checkedOptions, inTask, outActivity);
}

2.16 ActivityStarter.startActivity -03

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

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 {
        // 暫停佈局
        mService.mWindowManager.deferSurfaceLayout();
        // 調用 startActivityUnchecked 方法
        result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                startFlags, doResume, options, inTask, outActivity);
    } finally {
        final ActivityStack stack = mStartActivity.getStack();
        if (!ActivityManager.isStartResultSuccessful(result) && stack != null) {
            stack.finishActivityLocked(mStartActivity, RESULT_CANCELED,
                    null /* intentResultData */, "startActivity", true /* oomAdj */);
        }
        // 繼續佈局
        mService.mWindowManager.continueSurfaceLayout();
    }
    postStartActivityProcessing(r, result, mTargetStack);
    return result;
}

調用了 3 次 startActivity() ，最終執行了 startActivityUnchecked() 方法，這個方法裏會根據 啓動標誌位 和 Activity 啓動模式來決定如何啓動一個 Activity。

2.17 ActivityStarter.startActivityUnchecked

// frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
        ActivityRecord[] outActivity) {
            
    // 初始化啓動 Activity 的各種配置，會先重置再重新配置，其中包括 ActivityRecord、Intent 等
    setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
            voiceInteractor);

    // 計算出啓動的 FLAG，並將計算的值賦值給 mLaunchFlags
    computeLaunchingTaskFlags();
    computeSourceStack();
    // 將 mLaunchFlags 設置給 Intent，達到設定 Activity 的啓動方式的目的
    mIntent.setFlags(mLaunchFlags);
    // 獲取是否有可以複用的 activity
    ActivityRecord reusedActivity = getReusableIntentActivity();
    ... ...
    // 可複用 Activity 不爲空
    if (reusedActivity != null) {
        ... ...
    }
    ... ...
    mTargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
            mOptions);
    if (mDoResume) {
        final ActivityRecord topTaskActivity =
                mStartActivity.getTask().topRunningActivityLocked();
        if (!mTargetStack.isFocusable()
                || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                && mStartActivity != topTaskActivity)) {
            ... ...
        } else {
            if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
                mTargetStack.moveToFront("startActivityUnchecked");
            }
            // 調用 resumeFocusedStackTopActivityLocked() 方法
            mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
                    mOptions);
        }
    } else if (mStartActivity != null) {
        mSupervisor.mRecentTasks.add(mStartActivity.getTask());
    }
    ... ...
    return START_SUCCESS;
}

2.18 ActivityStackSupervisor.resumeFocusedStackTopActivityLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

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

    if (!readyToResume()) {
        return false;
    }

    if (targetStack != null && isFocusedStack(targetStack)) {
        // 調用 resumeTopActivityUncheckedLocked 方法
        return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
    }

    final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
    if (r == null || !r.isState(RESUMED)) {
        mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
    } else if (r.isState(RESUMED)) {
        mFocusedStack.executeAppTransition(targetOptions);
    }

    return false;
}

2.19 ActivityStack.resumeTopActivityUncheckedLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
    if (mStackSupervisor.inResumeTopActivity) {
        // Don't even start recursing.
        return false;
    }

    boolean result = false;
    try {
        // Protect against recursion.
        mStackSupervisor.inResumeTopActivity = true;
        result = resumeTopActivityInnerLocked(prev, options);    // 關鍵方法

        final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
        if (next == null || !next.canTurnScreenOn()) {
            checkReadyForSleep();
        }
    } finally {
        mStackSupervisor.inResumeTopActivity = false;
    }

    return result;
}

2.20 ActivityStack.resumeTopActivityInnerLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
    if (!mService.mBooting && !mService.mBooted) {
        // Not ready yet!
        return false;
    }
    ... ...
    boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
    if (mResumedActivity != null) {
        if (DEBUG_STATES) Slog.d(TAG_STATES,
                "resumeTopActivityLocked: Pausing " + mResumedActivity);
        // 先執行 startPausingLocked() 方法
        pausing |= startPausingLocked(userLeaving, false, next, false);
    }
    ... ...
    ActivityStack lastStack = mStackSupervisor.getLastStack();
    if (next.app != null && next.app.thread != null) {
        ... ...
        synchronized(mWindowManager.getWindowManagerLock()) {
            ... ...
            try {
                ... ...    
            } catch (Exception e) {
                ... ...
                // 執行 startSpecificActivityLocked() 方法
                mStackSupervisor.startSpecificActivityLocked(next, true, false);
                return true;
            }
        }

        try {
            next.completeResumeLocked();
        } catch (Exception e) {
            requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED, null,
                    "resume-exception", true);
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return true;
        }
    } else {
        ... ...
        // 執行 startSpecificActivityLocked() 方法
        mStackSupervisor.startSpecificActivityLocked(next, true, true);
    }

    if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
    return true;
}

在 resumeTopActivityInnerLocked 方法中會去判斷是否有 Activity 處於 Resume 狀態。

如果有的話會先讓這個 Activity 執行 Pausing 過程（其實也就是本案例中 Launcher 的 onPause 流程），然後再執行 startSpecificActivityLocked 方法來啓動需要啓動的 Activity。

2.21 ActivityStack.startPausingLocked

我們先分析 startPausingLocked() 源碼，應用的啓動流程 startSpecificActivityLocked() 放在第三節討論！

// frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

// 棧頂 Activity 執行 onPause 方法退出
final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
        ActivityRecord resuming, boolean pauseImmediately) {
    ... ...
    ActivityRecord prev = mResumedActivity;
    ... ...
    // 把 prev 賦值給 mPausingActivity，這個值在 Activity 啓動過程中會用到，
    // 這邊其實就是將 mResumedActivity 賦值給 mPausingActivity，就是我們的 Launcher。
    mPausingActivity = prev;
    ... ...
    if (prev.app != null && prev.app.thread != null) {
        try {
            EventLogTags.writeAmPauseActivity(prev.userId, System.identityHashCode(prev),
                    prev.shortComponentName, "userLeaving=" + userLeaving);
            mService.updateUsageStats(prev, false);
            /*
             * Android P 在這裏引入了 ClientLifecycleManager 和 ClientTransactionHandler 來輔助
             * 管理 Activity 生命週期，它會發送 EXECUTE_TRANSACTION 消息
             * 到 ActivityThread.H 裏面繼續處理。
             
             * 通過 ClientLifecycleManager 的 scheduleTransaction 方法把 PauseActivityItem
             * 事件加入到執行計劃中，開始棧頂的 pausing 過程。
             */
            mService.getLifecycleManager().scheduleTransaction(prev.app.thread, prev.appToken,
                    PauseActivityItem.obtain(prev.finishing, userLeaving,
                                    prev.configChangeFlags, pauseImmediately));
        } catch (Exception e) {
            // Ignore exception, if process died other code will cleanup.
            Slog.w(TAG, "Exception thrown during pause", e);
            mPausingActivity = null;
            mLastPausedActivity = null;
            mLastNoHistoryActivity = null;
        }
    } else {
        mPausingActivity = null;
        mLastPausedActivity = null;
        mLastNoHistoryActivity = null;
    }
    ... ...
}

2.22 ClientLifecycleManager.scheduleTransaction

// frameworks/base/services/core/java/com/android/server/am/ClientLifecycleManager.java

class ClientLifecycleManager {
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) {
            transaction.recycle();
        }
    }
    ... ...
}

// frameworks/base/core/java/android/app/servertransaction/ClientTransaction.java

public class ClientTransaction implements Parcelable, ObjectPoolItem {

    /** Target client. */
    private IApplicationThread mClient;
    
    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }
}

ClientTransaction.schedule 方法的 mClient 是一個 IApplicationThread 類型，ActivityThread 的內部類 ApplicationThread 派生這個接口類並實現了對應的方法。所以直接跳轉到 ApplicationThread 中的 scheduleTransaction 方法。

2.23 ActivityThread.scheduleTransaction

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {

    private class ApplicationThread extends IApplicationThread.Stub {
        ... ...    
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);
        }
    }
}

2.24 流程小結

2.25 ClientTransactionHandler.scheduleTransaction

ActivityThread 類中並沒有定義 scheduleTransaction 方法，所以調用的是他父類 ClientTransactionHandler 的 scheduleTransaction 方法。

// frameworks/base/core/java/android/app/ClientTransactionHandler.java

public abstract class ClientTransactionHandler {

    abstract void sendMessage(int what, Object obj);
    
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        // 調用了 sendMessage 方法，這個方法是一個抽象方法，
        // 它的實現是在 ClientTransactionHandler 派生類的 ActivityThread 中。
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }
}

2.26 ActivityThread.sendMessage

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {

    private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);

    void sendMessage(int what, Object obj) {
        sendMessage(what, obj, 0, 0, false);
    }

    private void sendMessage(int what, Object obj, int arg1) {
        sendMessage(what, obj, arg1, 0, false);
    }

    private void sendMessage(int what, Object obj, int arg1, int arg2) {
        sendMessage(what, obj, arg1, arg2, false);
    }

    private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        // ActivityThread.sendMessage 方法會把消息發送給內部名字叫 H 的 Handler
        mH.sendMessage(msg);
    }
    
    class H extends Handler {
        public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    // Handler H 的實例接收到 EXECUTE_TRANSACTION 消息後調用 
                    // TransactionExecutor.execute 方法切換 Activity 狀態。
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) {
                        transaction.recycle();
                    }
                    // TODO(lifecycler): Recycle locally scheduled transactions.
                    break;
            }
        }
    }
}

2.27 TransactionExecutor.execute

// frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java

public class TransactionExecutor {

    public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);

        executeCallbacks(transaction);    // 重要方法 1
        executeLifecycleState(transaction);    // 重要方法 2
        
        mPendingActions.clear();
        log("End resolving transaction");
    }

    // 重要方法 1
    public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null) {
            // No callbacks to execute, return early.
            return;
        }

        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            ... ...
            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            ... ...
        }
    }

    // 重要方法 2
    /** Transition to the final state if requested by the transaction. */
    private void executeLifecycleState(ClientTransaction transaction) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
        ... ...
        // 執行當前生命週期狀態之前的狀態
        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);

        // Execute the final transition with proper parameters.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }
    
}

前面在 ActivityStack.startPausingLocked 方法裏面 scheduleTransaction 傳遞的是 PauseActivityItem 對象，所以 executeLifecycleState 方法裏調用的 execute 方法其實是 PauseActivityItem.execute方法。

2.28 PauseActivityItem.execute

// frameworks/base/core/java/android/app/servertransaction/PauseActivityItem.java

public class PauseActivityItem extends ActivityLifecycleItem {

    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        // 跳轉到 ActivityThread.handlePauseActivity 方法
        client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,
                "PAUSE_ACTIVITY_ITEM");
    }
}

2.29 ActivityThread.handlePauseActivity

// frameworks/base/core/java/android/app/ActivityThread.java

@Override
public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,
        int configChanges, PendingTransactionActions pendingActions, String reason) {
    ActivityClientRecord r = mActivities.get(token);
    if (r != null) {
        ... ...
        performPauseActivity(r, finished, reason, pendingActions);
        ... ...
    }
}

2.30 ActivityThread.performPauseActivity

// frameworks/base/core/java/android/app/ActivityThread.java

final Bundle performPauseActivity(IBinder token, boolean finished, String reason,
        PendingTransactionActions pendingActions) {
    ActivityClientRecord r = mActivities.get(token);
    return r != null ? performPauseActivity(r, finished, reason, pendingActions) : null;
}

private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
        PendingTransactionActions pendingActions) {
    ... ...
    performPauseActivityIfNeeded(r, reason);
    ... ...
    return shouldSaveState ? r.state : null;
}

2.31 ActivityThread.performPauseActivityIfNeeded

// frameworks/base/core/java/android/app/ActivityThread.java

private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
    if (r.paused) {
        // You are already paused silly...
        return;
    }

    try {
        r.activity.mCalled = false;
        mInstrumentation.callActivityOnPause(r.activity);    // 關鍵方法
        if (!r.activity.mCalled) {
            throw new SuperNotCalledException("Activity " + safeToComponentShortString(r.intent)
                    + " did not call through to super.onPause()");
        }
    } catch (SuperNotCalledException e) {
        throw e;
    } catch (Exception e) {
        if (!mInstrumentation.onException(r.activity, e)) {
            throw new RuntimeException("Unable to pause activity "
                    + safeToComponentShortString(r.intent) + ": " + e.toString(), e);
        }
    }
    r.setState(ON_PAUSE);
}

2.32 Instrumentation.callActivityOnPause

// frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnPause(Activity activity) {
    activity.performPause();      // 直接調用 Activity.performPause 方法
}

2.33 Activity.performPause

// frameworks/base/core/java/android/app/Activity.java

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback,
        AutofillManager.AutofillClient {

    final void performPause() {
        ... ...
        onPause();
        ... ...
    }
}

2.34 流程小結

至此，Launcher 的 onPause() 流程分析結束！同時，我們需要明白一點：在啓動一個 Activity 的時候最先被執行的是棧頂的 Activity 的 onPause 方法。

三、啓動進程

Launcher 的 onPause() 流程分析完，接下來我們看看新進程的啓動流程：startSpecificActivityLocked() 方法。在這個方法中會去根據 進程 和 線程 是否存在判斷 App 是否已經啓動。

3.1 ActivityStackSupervisor.startSpecificActivityLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

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);

    getLaunchTimeTracker().setLaunchTime(r);

    // 如果 app 存在，並且已經啓動
    if (app != null && app.thread != null) {
        try {
            if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
                    || !"android".equals(r.info.packageName)) {
                app.addPackage(r.info.packageName, r.info.applicationInfo.longVersionCode,
                        mService.mProcessStats);
            }
            realStartActivityLocked(r, app, andResume, checkConfig);
            return;
        } catch (RemoteException e) {
            Slog.w(TAG, "Exception when starting activity "
                    + r.intent.getComponent().flattenToShortString(), e);
        }
    }

    // 啓動新的進程，mService 爲 ActivityManagerService
    mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
            "activity", r.intent.getComponent(), false, false, true);
}

如果已經啓動，就會調用 ActivityManagerService.realStartActivityLocked 方法繼續處理。

如果沒有啓動，則會調用 ActivityManagerService.startProcessLocked 方法創建新的進程處理。

3.2 ActivityManagerService.startProcessLocked -01

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

final ProcessRecord startProcessLocked(String processName,
        ApplicationInfo info, boolean knownToBeDead, int intentFlags,
        String hostingType, ComponentName hostingName, boolean allowWhileBooting,
        boolean isolated, boolean keepIfLarge) {
    return startProcessLocked(processName, info, knownToBeDead, intentFlags, hostingType,
            hostingName, allowWhileBooting, isolated, 0 /* isolatedUid */, keepIfLarge,
            null /* ABI override */, null /* entryPoint */, null /* entryPointArgs */,
            null /* crashHandler */);
}

3.3 ActivityManagerService.startProcessLocked -02

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
        boolean knownToBeDead, int intentFlags, String hostingType, ComponentName hostingName,
        boolean allowWhileBooting, boolean isolated, int isolatedUid, boolean keepIfLarge,
        String abiOverride, String entryPoint, String[] entryPointArgs, Runnable crashHandler) {
    ... ...
    
    final boolean success = startProcessLocked(app, hostingType, hostingNameStr, abiOverride);
}

3.3 ActivityManagerService.startProcessLocked -03

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private final boolean startProcessLocked(ProcessRecord app,
        String hostingType, String hostingNameStr, String abiOverride) {
    return startProcessLocked(app, hostingType, hostingNameStr,
            false /* disableHiddenApiChecks */, abiOverride);
}

3.4 ActivityManagerService.startProcessLocked -04

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private final boolean startProcessLocked(ProcessRecord app, String hostingType,
        String hostingNameStr, boolean disableHiddenApiChecks, String abiOverride) {

    try {
        ... ...
        return startProcessLocked(hostingType, hostingNameStr, entryPoint, app, uid, gids,
                runtimeFlags, mountExternal, seInfo, requiredAbi, instructionSet, invokeWith,
                startTime);
    } catch (RuntimeException e) {
        forceStopPackageLocked(app.info.packageName, UserHandle.getAppId(app.uid), false,
                false, true, false, false, UserHandle.getUserId(app.userId), "start failure");
        return false;
    }
}

3.5 ActivityManagerService.startProcessLocked -05

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private boolean startProcessLocked(String hostingType, String hostingNameStr, String entryPoint,
        ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
        String seInfo, String requiredAbi, String instructionSet, String invokeWith,
        long startTime) {
    ... ...
    final ProcessStartResult startResult = startProcess(hostingType, entryPoint, app,
                    uid, gids, runtimeFlags, mountExternal, seInfo, requiredAbi, instructionSet,
                    invokeWith, startTime);
    ... ...
}

3.6 ActivityManagerService.startProcess

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private ProcessStartResult startProcess(String hostingType, String entryPoint,
        ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
        String seInfo, String requiredAbi, String instructionSet, String invokeWith,
            long startTime) {
    try {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
                app.processName);
        checkTime(startTime, "startProcess: asking zygote to start proc");
        final ProcessStartResult startResult;
        if (hostingType.equals("webview_service")) {
            startResult = startWebView(entryPoint,
                    app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                    app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                    app.info.dataDir, null,
                    new String[] {PROC_START_SEQ_IDENT + app.startSeq});
        } else {
            // 執行 Process.start 方法
            startResult = Process.start(entryPoint,
                    app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                    app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                    app.info.dataDir, invokeWith,
                    new String[] {PROC_START_SEQ_IDENT + app.startSeq});
        }
        checkTime(startTime, "startProcess: returned from zygote!");
        return startResult;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    }
}

到這裏，我們就明白了：ActivityManagerService.startProcessLocked 方法經過多次跳轉最終會通過 Process.start 方法來爲應用創建進程。

進程的創建流程和源碼分析，這邊我們就不再討論了！在 【 Android '進程線程' 】篇 -- 聊聊 "進程" 如何 "創建" 一文中，我們作了詳細的分析！

四、Activity 進程

我們在進程創建的博文中，討論過源碼最後會執行到 ActivityThread.main 方法，執行主線程的初始化工作。

4.1 初始化主線程

// frameworks/base/core/java/android/app/ActivityThread.java

public static void main(String[] args) {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");

    CloseGuard.setEnabled(false);

    Environment.initForCurrentUser();

    EventLogger.setReporter(new EventLoggingReporter());

    final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
    TrustedCertificateStore.setDefaultUserDirectory(configDir);

    Process.setArgV0("<pre-initialized>");

    Looper.prepareMainLooper();    // 創建主線程 looper

    long startSeq = 0;
    if (args != null) {
        for (int i = args.length - 1; i >= 0; --i) {
            if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
                startSeq = Long.parseLong(
                        args[i].substring(PROC_START_SEQ_IDENT.length()));
            }
        }
    }
    ActivityThread thread = new ActivityThread();    // 初始化 ActivityThread
    thread.attach(false, startSeq);    // attach 到系統進程

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

    if (false) {
        Looper.myLooper().setMessageLogging(new
                LogPrinter(Log.DEBUG, "ActivityThread"));
    }

    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    Looper.loop();    // 主線程啓動，進入循環狀態

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

4.2 啓動 Activity

// frameworks/base/core/java/android/app/ActivityThread.java

private void attach(boolean system, long startSeq) {
    sCurrentActivityThread = this;
    mSystemThread = system;
    if (!system) {
        ... ...
        final IActivityManager mgr = ActivityManager.getService();
        try {
            // 通過 ActivityManagerService 爲這個應用綁定一個 Application
            mgr.attachApplication(mAppThread, startSeq);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
        // 添加一個垃圾回收觀察者，每當系統觸發垃圾回收的時候就會在 run 方法裏面
        // 去計算應用使用了多少內存，如果超過總量的四分之三就會嘗試釋放內存
        BinderInternal.addGcWatcher(new Runnable() {
            @Override public void run() {
                if (!mSomeActivitiesChanged) {
                    return;
                }
                Runtime runtime = Runtime.getRuntime();
                long dalvikMax = runtime.maxMemory();
                long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                if (dalvikUsed > ((3*dalvikMax)/4)) {
                    if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                            + " total=" + (runtime.totalMemory()/1024)
                            + " used=" + (dalvikUsed/1024));
                    mSomeActivitiesChanged = false;
                    try {
                        mgr.releaseSomeActivities(mAppThread);
                    } catch (RemoteException e) {
                        throw e.rethrowFromSystemServer();
                    }
                }
            }
        });
    } else {
        ... ...
    }
    ... ...
    // 爲根 View 添加 config 回調接收 config 變化相關的信息
    ViewRootImpl.addConfigCallback(configChangedCallback);
}

4.2.1 ActivityManagerService.attachApplication

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@Override
public final void attachApplication(IApplicationThread thread, long startSeq) {
    synchronized (this) {
        int callingPid = Binder.getCallingPid();
        final int callingUid = Binder.getCallingUid();
        final long origId = Binder.clearCallingIdentity();
        attachApplicationLocked(thread, callingPid, callingUid, startSeq);
        Binder.restoreCallingIdentity(origId);
    }
}

4.2.2 ActivityManagerService.attachApplicationLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

private final boolean attachApplicationLocked(IApplicationThread thread,
        int pid, int callingUid, long startSeq) {
    ... ... 
    // See if the top visible activity is waiting to run in this process...
    if (normalMode) {
        try {
            if (mStackSupervisor.attachApplicationLocked(app)) {
                didSomething = true;
            }
        } catch (Exception e) {
            Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
            badApp = true;
        }
    }
    ... ...
    return true;
}

4.3 ActivityStackSupervisor.attachApplicationLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
    final String processName = app.processName;
    boolean didSomething = false;
    for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
        final ActivityDisplay display = mActivityDisplays.valueAt(displayNdx);
        for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
            final ActivityStack stack = display.getChildAt(stackNdx);
            if (!isFocusedStack(stack)) {
                continue;
            }
            stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
            final ActivityRecord top = stack.topRunningActivityLocked();
            final int size = mTmpActivityList.size();
            for (int i = 0; i < size; i++) {
                final ActivityRecord activity = mTmpActivityList.get(i);
                if (activity.app == null && app.uid == activity.info.applicationInfo.uid
                        && processName.equals(activity.processName)) {
                    try {
                        // 執行 realStartActivityLocked 方法
                        if (realStartActivityLocked(activity, app,
                                top == activity /* andResume */, true /* checkConfig */)) {
                            didSomething = true;
                        }
                    } catch (RemoteException e) {
                        Slog.w(TAG, "Exception in new application when starting activity "
                                + top.intent.getComponent().flattenToShortString(), e);
                        throw e;
                    }
                }
            }
        }
    }
    if (!didSomething) {
        ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
    }
    return didSomething;
}

4.4 ActivityStackSupervisor.realStartActivityLocked

realStartActivityLocked 就是應用進程已經啓動的情況下去啓動 Activity 所調用的方法。

// frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

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

    try {
        ... ...
        try {
            ... ...
            final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                    r.appToken);
            // 爲 ClientTransaction 對象添加 LaunchActivityItem 的 callback
            clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                    System.identityHashCode(r), r.info,
                    mergedConfiguration.getGlobalConfiguration(),
                    mergedConfiguration.getOverrideConfiguration(), r.compat,
                    r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
                    r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                    profilerInfo));

            final ActivityLifecycleItem lifecycleItem;
            // 設置當前的生命週期狀態
            if (andResume) {
                lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward());
            } else {
                lifecycleItem = PauseActivityItem.obtain();
            }
            clientTransaction.setLifecycleStateRequest(lifecycleItem);

            // 調用 ClientLifecycleManager.scheduleTransaction 方法
            mService.getLifecycleManager().scheduleTransaction(clientTransaction);
    ... ...
}

調用 ClientLifecycleManager.scheduleTransaction 方法之後具體是如何執行的前面已經分析過了，這裏就不再分析了。

我們這裏看一下執行 callback 後跳轉到 LaunchActivityItem.execute方法。

4.5 LaunchActivityItem.execute

// frameworks/base/core/java/android/app/servertransaction/LaunchActivityItem.java

@Override
public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
    Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
    ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
            mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
            mPendingResults, mPendingNewIntents, mIsForward,
            mProfilerInfo, client);
    // 調用 ActivityThread.handleLaunchActivity 方法
    client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
    Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}

4.6 ActivityThread.handleLaunchActivity

// frameworks/base/core/java/android/app/ActivityThread.java

public Activity handleLaunchActivity(ActivityClientRecord r,
        PendingTransactionActions pendingActions, Intent customIntent) {
    ... ...
    final Activity a = performLaunchActivity(r, customIntent);
    ... ...
}

4.7 ActivityThread.performLaunchActivity

// frameworks/base/core/java/android/app/ActivityThread.java

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ActivityInfo aInfo = r.activityInfo;
    if (r.packageInfo == null) {
        r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                Context.CONTEXT_INCLUDE_CODE);
    }
    
    // 初始化 ComponentName
    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);
    }
    
    // 初始化 ContextImpl 和 Activity
    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) {
        ... ...
    }

    try {
        // 初始化Application
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);

        if (activity != null) {
            CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
            Configuration config = new Configuration(mCompatConfiguration);
            if (r.overrideConfig != null) {
                config.updateFrom(r.overrideConfig);
            }
            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;
            // 設置 Activity 的 Theme
            int theme = r.activityInfo.getThemeResource();
            if (theme != 0) {
                activity.setTheme(theme);
            }

            activity.mCalled = false;
            // 調用 Instrumentation.callActivityOnCreate 方法
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
            if (!activity.mCalled) {
            ... ...
            r.activity = activity;
        }
        r.setState(ON_CREATE);

        mActivities.put(r.token, r);

    } catch (SuperNotCalledException e) {
        ... ...
    }

    return activity;
}

4.8 Instrumentation.callActivityOnCreate

// frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnCreate(Activity activity, Bundle icicle,
        PersistableBundle persistentState) {
    prePerformCreate(activity);
    activity.performCreate(icicle, persistentState);  // 執行此方法
    postPerformCreate(activity);
}

4.9 Activity.performCreate

// frameworks/base/core/java/android/app/Activity.java

final void performCreate(Bundle icicle, PersistableBundle persistentState) {
    ... ...
    if (persistentState != null) {
        onCreate(icicle, persistentState);
    } else {
        onCreate(icicle);
    }
    ... ...
}

至此 executeCallbacks() 執行完畢，開始執行 executeLifecycleState 方法，會先執行 cycleToPath 方法。

4.10 TransactionExecutor.cycleToPath

// frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java

private void cycleToPath(ActivityClientRecord r, int finish,
            boolean excludeLastState) {
    final int start = r.getLifecycleState();
    log("Cycle from: " + start + " to: " + finish + " excludeLastState:" + excludeLastState);
    final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
    performLifecycleSequence(r, path);
}

4.11 TransactionExecutor.performLifecycleSequence

// frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java

private void performLifecycleSequence(ActivityClientRecord r, IntArray path) {
        final int size = path.size();
    for (int i = 0, state; i < size; i++) {
        state = path.get(i);
        log("Transitioning to state: " + state);
        // 生命週期狀態是從 ON_CREATE 狀態到 ON_RESUME 狀態，中間有一個 ON_START 狀態，
        // 所以會執行 ActivityThread.handleStartActivity 方法。
        switch (state) {
            case ON_CREATE:
                mTransactionHandler.handleLaunchActivity(r, mPendingActions,
                        null /* customIntent */);
                break;
            case ON_START:
                mTransactionHandler.handleStartActivity(r, mPendingActions);
                break;
            case ON_RESUME:
                mTransactionHandler.handleResumeActivity(r.token, false /* finalStateRequest */,
                        r.isForward, "LIFECYCLER_RESUME_ACTIVITY");
                break;
            case ON_PAUSE:
                mTransactionHandler.handlePauseActivity(r.token, false /* finished */,
                        false /* userLeaving */, 0 /* configChanges */, mPendingActions,
                        "LIFECYCLER_PAUSE_ACTIVITY");
                break;
            case ON_STOP:
                mTransactionHandler.handleStopActivity(r.token, false /* show */,
                        0 /* configChanges */, mPendingActions, false /* finalStateRequest */,
                        "LIFECYCLER_STOP_ACTIVITY");
                break;
            case ON_DESTROY:
                mTransactionHandler.handleDestroyActivity(r.token, false /* finishing */,
                        0 /* configChanges */, false /* getNonConfigInstance */,
                        "performLifecycleSequence. cycling to:" + path.get(size - 1));
                break;
            case ON_RESTART:
                mTransactionHandler.performRestartActivity(r.token, false /* start */);
                break;
            default:
                throw new IllegalArgumentException("Unexpected lifecycle state: " + state);
        }
    }
}

4.12 ActivityThread.handleStartActivity

// frameworks/base/core/java/android/app/ActivityThread.java

public void handleStartActivity(ActivityClientRecord r,
        PendingTransactionActions pendingActions) {
    ... ...
    // Start
    activity.performStart("handleStartActivity");
    r.setState(ON_START);
    ... ...
}

4.13 Activity.performStart

// frameworks/base/core/java/android/app/Activity.java

final void performStart(String reason) {
    ... ...
    mInstrumentation.callActivityOnStart(this);
    ... ...
}

4.14 Instrumentation.callActivityOnStart

// frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnStart(Activity activity) {
    // 經過多次跳轉最後會調用 Activity.onStart 方法
    activity.onStart();
}

執行完畢 cycleToPath，開始執行 ResumeActivityItem.execute 方法。

4.15 ResumeActivityItem.execute

// frameworks/base/core/java/android/app/servertransaction/ResumeActivityItem.java

public void execute(ClientTransactionHandler client, IBinder token,
        PendingTransactionActions pendingActions) {
    Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
    client.handleResumeActivity(token, true /* finalStateRequest */, mIsForward,
            "RESUME_ACTIVITY");
    Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}

4.16 ActivityThread.handleResumeActivity

// frameworks/base/core/java/android/app/ActivityThread.java

public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
        String reason) {
    ... ...
    // TODO Push resumeArgs into the activity for consideration
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    if (r == null) {
        // We didn't actually resume the activity, so skipping any follow-up actions.
        return;
    }
    ... ...
    if (localLOGV) Slog.v(TAG, "Scheduling idle handler for " + r);
    
    // 這邊的方法我們在最後會說明，其中涉及到 onStop() 方法
    Looper.myQueue().addIdleHandler(new Idler());
}

4.17 ActivityThread.performResumeActivity

// frameworks/base/core/java/android/app/ActivityThread.java

public ActivityClientRecord performResumeActivity(IBinder token, boolean finalStateRequest,
        String reason) {
    ... ...
    try {
        r.activity.onStateNotSaved();
        r.activity.mFragments.noteStateNotSaved();
        checkAndBlockForNetworkAccess();
        if (r.pendingIntents != null) {
            deliverNewIntents(r, r.pendingIntents);
            r.pendingIntents = null;
        }
        if (r.pendingResults != null) {
            deliverResults(r, r.pendingResults, reason);
            r.pendingResults = null;
        }
        // 執行 performResume 方法
        r.activity.performResume(r.startsNotResumed, reason);

        r.state = null;
        r.persistentState = null;
        r.setState(ON_RESUME);
    } catch (Exception e) {
        if (!mInstrumentation.onException(r.activity, e)) {
            throw new RuntimeException("Unable to resume activity "
                    + r.intent.getComponent().toShortString() + ": " + e.toString(), e);
        }
    }
    return r;
}

4.18 Activity.performResume

// frameworks/base/core/java/android/app/Activity.java

final void performResume(boolean followedByPause, String reason) {
    performRestart(true /* start */, reason);
    ... ...
    // mResumed is set by the instrumentation
    mInstrumentation.callActivityOnResume(this);
    ... ...
}

4.19 Instrumentation.callActivityOnResume

// frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnResume(Activity activity) {
    activity.mResumed = true;
    // 最終調用到 Activity.onResume 方法
    activity.onResume();
    ... ...
}

至此，Activity 啓動完畢。

五、棧頂活動 onStop

我們在討論 ActivityThread.handleResumeActivity 源碼的時候，最後一行代碼還未討論。

// frameworks/base/core/java/android/app/ActivityThread.java

public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
        String reason) {
    ... ...
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    ... ...
    // 當 MessageQueue 空閒的時候就會執行這個 Handler，即執行完當前 Activity 的 onResume() 方法後執行
    Looper.myQueue().addIdleHandler(new Idler());
}

爲什麼要單獨討論這行代碼，因爲上面分析了棧頂 Activity 的 onPause()、新 Activity 的 onCreate()、onStart()、onResume()，但缺失一個棧頂 Activity 的 onStop() 生命週期，此方法就是在這行代碼中處理的。

5.1 Idler

// frameworks/base/core/java/android/app/ActivityThread.java

private class Idler implements MessageQueue.IdleHandler {
    @Override
    public final boolean queueIdle() {
        ... ...
        if (a != null) {
            mNewActivities = null;
            IActivityManager am = ActivityManager.getService();
            ActivityClientRecord prev;
            do {
                if (a.activity != null && !a.activity.mFinished) {
                    try {
                        // 調用 ActivityManagerService.activityIdle 方法
                        am.activityIdle(a.token, a.createdConfig, stopProfiling);
                        a.createdConfig = null;
                    } catch (RemoteException ex) {
                        throw ex.rethrowFromSystemServer();
                    }
                }
                prev = a;
                a = a.nextIdle;
                prev.nextIdle = null;
            } while (a != null);
        }
        ... ...
        return false;
    }
}

5.2 ActivityManagerService.activityIdle

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public final void activityIdle(IBinder token, Configuration config, boolean stopProfiling) {
    final long origId = Binder.clearCallingIdentity();
    synchronized (this) {
        ActivityStack stack = ActivityRecord.getStackLocked(token);
        if (stack != null) {
            // 調用 ActivityStackSupervisor.activityIdleInternalLocked 方法
            ActivityRecord r =
                    mStackSupervisor.activityIdleInternalLocked(token, false /* fromTimeout */,
                            false /* processPausingActivities */, config);
            if (stopProfiling) {
                if ((mProfileProc == r.app) && mProfilerInfo != null) {
                    clearProfilerLocked();
                }
            }
        }
    }
    Binder.restoreCallingIdentity(origId);
}

5.3 ActivityStackSupervisor.activityIdleInternalLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

final ActivityRecord activityIdleInternalLocked(final IBinder token, boolean fromTimeout,
        boolean processPausingActivities, Configuration config) {

    for (int i = 0; i < NS; i++) {
        r = stops.get(i);
        final ActivityStack stack = r.getStack();
        if (stack != null) {
            if (r.finishing) {
                stack.finishCurrentActivityLocked(r, ActivityStack.FINISH_IMMEDIATELY, false,
                        "activityIdleInternalLocked");
            } else {
                stack.stopActivityLocked(r);
            }
        }
    }
    ... ...
    return r;
}

5.4 ActivityStack.stopActivityLocked

// frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

final void stopActivityLocked(ActivityRecord r) {
    ... ...
    if (r.app != null && r.app.thread != null) {
        adjustFocusedActivityStack(r, "stopActivity");
        r.resumeKeyDispatchingLocked();
        try {
            ... ...
            mService.getLifecycleManager().scheduleTransaction(r.app.thread, r.appToken,
                    StopActivityItem.obtain(r.visible, r.configChangeFlags));
            if (shouldSleepOrShutDownActivities()) {
                r.setSleeping(true);
            }
            Message msg = mHandler.obtainMessage(STOP_TIMEOUT_MSG, r);
            mHandler.sendMessageDelayed(msg, STOP_TIMEOUT);
        } catch (Exception e) {
            ... ...
        }
    }
}

又見到了 ClientLifecycleManager.scheduleTransaction 方法，前面已經分析過多次，會去執行 StopActivityItem.execute 方法，然後經過多次跳轉，最終執行了 Activity.onStop 方法。

5.5 StopActivityItem.execute

// frameworks/base/core/java/android/app/servertransaction/StopActivityItem.java

@Override
public void execute(ClientTransactionHandler client, IBinder token,
        PendingTransactionActions pendingActions) {
    Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
    client.handleStopActivity(token, mShowWindow, mConfigChanges, pendingActions,
            true /* finalStateRequest */, "STOP_ACTIVITY_ITEM");
    Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}

5.6 ActivityThread.handleStopActivity

// frameworks/base/core/java/android/app/ActivityThread.java

@Override
public void handleStopActivity(IBinder token, boolean show, int configChanges,
        PendingTransactionActions pendingActions, boolean finalStateRequest, String reason) {
    ... ...
    performStopActivityInner(r, stopInfo, show, true /* saveState */, finalStateRequest,
                reason);
    ... ...
}

5.7 ActivityThread.performStopActivityInner

// frameworks/base/core/java/android/app/ActivityThread.java

private void performStopActivityInner(ActivityClientRecord r, StopInfo info, boolean keepShown,
        boolean saveState, boolean finalStateRequest, String reason) {
    if (r != null) {
        ... ...
        if (!keepShown) {
            callActivityOnStop(r, saveState, reason);
        }
    }
}

5.8 ActivityThread.callActivityOnStop

// frameworks/base/core/java/android/app/ActivityThread.java

private void callActivityOnStop(ActivityClientRecord r, boolean saveState, String reason) {
    ... ...
    try {
        r.activity.performStop(false /*preserveWindow*/, reason);
    } catch (SuperNotCalledException e) {
        throw e;
    } catch (Exception e) {
        ... ...
    }
    ... ...
}

5.9 Activity.performStop

// frameworks/base/core/java/android/app/Activity.java

final void performStop(boolean preserveWindow, String reason) {
    ... ...
    if (!mStopped) {
        ... ...
        mInstrumentation.callActivityOnStop(this);
    }
}

5.10 Instrumentation.callActivityOnStop

// frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnStop(Activity activity) {
    activity.onStop();
}