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Android app启动流程(Activity启动过程)【基于Android 8.1】_android startactivity

作者：花生_TL007 | 2024-06-10 09:01:25

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android startactivity

从startActivity一直到可见

从Launcher启动app的LauncherActivity

  1. Launcher.startActivity或者Context.startActivity【ContextImpl.startActivity 】

  2. 都会调用到Instrumentation.execStartActivity

    方法定义在frameworks/base/core/java/android/app/Instrumentation.java文件中：

    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        ......
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        ......
        if (mActivityMonitors != null) {
            ......
        }
        try {
            ......
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
            ......
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

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    ActivityManager.getService()返回ActivityManagerService的远程接口，通过Binder调用，进入AMS中。

    以上是在Launcher进程中

  3. ActivityManagerService.startActivity

    方法在\frameworks\base\services\core\java\com\android\server\am\ActivityManagerService.java文件中：

    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());
    }

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    简单调用ActivityManagerService.startActivityAsUser

  4. ActivityManagerService.startActivityAsUser

    方法在\frameworks\base\services\core\java\com\android\server\am\ActivityManagerService.java文件中

    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
                userId, false, ALLOW_FULL_ONLY, "startActivity", null);
        // TODO: Switch to user app stacks here.
        return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
                resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
                profilerInfo, null, null, bOptions, false, userId, null, null,
                "startActivityAsUser");
    }

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    简单地将操作转发给成员变量mActivityStarter的startActivityMayWait方法，这里的mActivityStarter的类型为com.android.server.am.ActivityStarter

  5. ActivityStarter.startActivityMayWait

    \frameworks\base\services\core\java\com\android\server\am\ActivityStarter.java
    mSupervisor.resolveActivity解析出ActivityInfo aInfo后，调用startActivityLocked

  6. ActivityStarter.startActivityLocked

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

    简单调用ActivityStarter.startActivity

  7. ActivityStarter.startActivity

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

    一堆检查操作后，直接调用ActivityStarter.startActivity

  8. ActivityStarter.startActivity

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

    直接调用ActivityStarter.startActivityUnchecked

  9. ActivityStarter.startActivityUnchecked

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

    这里检查了权限、任务栈、启动模式
    (如果是从桌面冷启动APP，那么会因为getReusableIntentActivity方法返回的reusedActivity是null，则不执行10和11。调用 ActivityStack 的 startActivityLocked 方法 mTargetStack.startActivityLocked，此处将启动的 Activity 插入到栈顶部并showStartingWindow显示一个starting window；继续调用mSupervisor.resumeFocusedStackTopActivityLocked)显示栈顶元素（参考自：https://blog.csdn.net/shulianghan/article/details/120249208）

  10. ActivityStarter.setTargetStackAndMoveToFrontIfNeeded
    \frameworks\base\services\core\java\com\android\server\am\ActivityStarter.java

    /**
     * Figure out which task and activity to bring to front when we have found an existing matching
     * activity record in history. May also clear the task if needed.
     * @param intentActivity Existing matching activity.
     * @return {@link ActivityRecord} brought to front.
     */

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    注释翻译：在我们找到历史记录中的现有匹配活动记录时，确定要将哪些任务和活动带到前面。 如果需要，也可以清除任务 。
    intentActivity.showStartingWindow显示一个starting window

11. ActivityStack.moveTaskToFrontLocked
\frameworks\base\services\core\java\com\android\server\am\ActivityStack.java

  1. ActivityStackSupervisor.resumeFocusedStackTopActivityLocked

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

  2. ActivityStackSupervisor.resumeFocusedStackTopActivityLocked

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

  3. ActivityStack.resumeTopActivityUncheckedLocked

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

  4. ActivityStack.resumeTopActivityInnerLocked

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

    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        ......
        final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
        ......
        final boolean userLeaving = mStackSupervisor.mUserLeaving;
        ......
        final boolean resumeWhilePausing = (next.info.flags & FLAG_RESUME_WHILE_PAUSING) != 0 && !lastResumedCanPip;

        boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        if (pausing && !resumeWhilePausing) {
            ......
            return true;
        } else if (mResumedActivity == next && next.state == ActivityState.RESUMED &&
                mStackSupervisor.allResumedActivitiesComplete()) {
            ......
            return true;
        }

        ......
        if (prev != null && prev != next) {
            ......
        }

        ......
        boolean anim = true;
        if (prev != null) {
            ......
        } else {
           ......
        }

        ......
        ActivityStack lastStack = mStackSupervisor.getLastStack();
        if (next.app != null && next.app.thread != null) {
            ......
            setResumedActivityLocked(next, "resumeTopActivityInnerLocked");
            ......
            boolean notUpdated = true;
            if (mStackSupervisor.isFocusedStack(this)) {
                ......
                notUpdated = !mService.updateDisplayOverrideConfigurationLocked(config, next, false /* deferResume */, mDisplayId);
            }

            if (notUpdated) {
                .....
                ActivityRecord nextNext = topRunningActivityLocked();
                ......
                if (nextNext != next) {
                    // Do over!
                    mStackSupervisor.scheduleResumeTopActivities();
                }
                if (!next.visible || next.stopped) {
                    next.setVisibility(true);
                }
                next.completeResumeLocked();
                if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                return true;
            }

            try {
                ......
                if (next.newIntents != null) {
                    next.app.thread.scheduleNewIntent(
                            next.newIntents, next.appToken, false /* andPause */);
                }
                ......
                next.app.thread.scheduleResumeActivity(next.appToken, next.app.repProcState,
                        mService.isNextTransitionForward(), resumeAnimOptions);
                ......
            } catch (Exception e) {
                ......
                return true;
            }

            // From this point on, if something goes wrong there is no way
            // to recover the activity.
            try {
                next.completeResumeLocked();
            } catch (Exception e) {
                ......
                return true;
            }
        } else {
            // Whoops, need to restart this activity!
            ......
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }
        return true;
    }

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    此处调用mStackSupervisor.pauseBackStacks一直等着结果回来以后，继续向下执行，这个结果是pausing =true。ActivityStackSupervisor.pauseBackStacks又调用ActivityStack.startPausingLocked里面会将mResumedActivity置为null，所以pausing |= startPausingLocked(userLeaving, false, next, false)不会执行到。resumeTopActivityInnerLocked第一次调用的时候，pausing && !resumeWhilePausing = true，所以，当在if (pausing && !resumeWhilePausing)代码块中直接return true了，后面代码不会执行。

    startPausingLocked最终会调用到completePauseLocked，而completePauseLocked最终又会间接调用到resumeTopActivityInnerLocked，这一次调用到resumeTopActivityInnerLocked的时候，会调用到mStackSupervisor.startSpecificActivityLocked(next, true, true）。也就是这个过程中，此时resumeTopActivityInnerLocked已经调用了两次了.

  5. ActivityStackSupervisor.pauseBackStacks

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

    boolean pauseBackStacks(boolean userLeaving, ActivityRecord resuming, boolean dontWait) {
        boolean someActivityPaused = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
            for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
                final ActivityStack stack = stacks.get(stackNdx);
                if (!isFocusedStack(stack) && stack.mResumedActivity != null) {
                    if (DEBUG_STATES) Slog.d(TAG_STATES, "pauseBackStacks: stack=" + stack +
                            " mResumedActivity=" + stack.mResumedActivity);
                    someActivityPaused |= stack.startPausingLocked(userLeaving, false, resuming,
                            dontWait);
                }
            }
        }
        return someActivityPaused;
    }

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    此处调用stack.startPausingLocked，返回someActivityPaused=true

  6. ActivityStack.startPausingLocked

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

    final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
            ActivityRecord resuming, boolean pauseImmediately) {
        ......
        ActivityRecord prev = mResumedActivity;

        ......
        mResumedActivity = null;
        mPausingActivity = prev;
        mLastPausedActivity = prev;
        mLastNoHistoryActivity = (prev.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_HISTORY) != 0
                || (prev.info.flags & ActivityInfo.FLAG_NO_HISTORY) != 0 ? prev : null;
        prev.state = ActivityState.PAUSING;
        prev.getTask().touchActiveTime();
        clearLaunchTime(prev);
        ......
        if (prev.app != null && prev.app.thread != null) {
            if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Enqueueing pending pause: " + prev);
            try {
                ......
                prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing,
                        userLeaving, prev.configChangeFlags, pauseImmediately);
            } catch (Exception e) {
                ......
            }
        } else {
            ......
        }
        ......
        if (mPausingActivity != null) {
            ......
            if (pauseImmediately) {
                // If the caller said they don't want to wait for the pause, then complete
                // the pause now.
                completePauseLocked(false, resuming);
                return false;

            } else {
                schedulePauseTimeout(prev);
                return true;
            }

        } else {
            // This activity failed to schedule the
            // pause, so just treat it as being paused now.
            if (resuming == null) {
                mStackSupervisor.resumeFocusedStackTopActivityLocked();
            }
            return false;
        }
    }

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    prev.app.thread.schedulePauseActivity通过ApplicationThread调用android.app.ActivityThread.H发送Handler消息在Launcher进程的主线程中调用handlePauseActivity。

    从上面调用得知pauseImmediately=false，会调用schedulePauseTimeout(prev)，

    pause超时后，发送handler消息立刻执行ActivityStack.activityPausedLocked。

  7. ActivityThread.handlePauseActivity

    private void handlePauseActivity(IBinder token, boolean finished,
            boolean userLeaving, int configChanges, boolean dontReport, int seq) {
        ......
        if (r != null) {
            if (userLeaving) {
                performUserLeavingActivity(r);
            }

            r.activity.mConfigChangeFlags |= configChanges;
            performPauseActivity(token, finished, r.isPreHoneycomb(), handlePauseActivity");
            ......
            // Tell the activity manager we have paused.
            if (!dontReport) {
                try {
                    ActivityManager.getService().activityPaused(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }
            mSomeActivitiesChanged = true;
        }
    }

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    ActivityManager.getService().activityPaused(token)，此处调用AMS的activityPaused

  8. ActivityManagerService.activityPaused

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

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    此处调用ActivityStack.activityPausedLocked

  9. ActivityStack.activityPausedLocked

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

    final void activityPausedLocked(IBinder token, boolean timeout) {
        if (DEBUG_PAUSE) Slog.v(TAG_PAUSE,
            "Activity paused: token=" + token + ", timeout=" + timeout);

        final ActivityRecord r = isInStackLocked(token);
        if (r != null) {
            mHandler.removeMessages(PAUSE_TIMEOUT_MSG, r);
            if (mPausingActivity == r) {
                ......
                try {
                    completePauseLocked(true /* resumeNext */, null /* resumingActivity */);
                } finally {
                    mService.mWindowManager.continueSurfaceLayout();
                }
                return;
            } else {
                ......
            }
        }
        ......
    }

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    此处调用ActivityStack.completePauseLocked

  10. ActivityStack.completePauseLocked

    private void completePauseLocked(boolean resumeNext, ActivityRecord resuming) {
        ActivityRecord prev = mPausingActivity;
        if (prev != null) {
            ......
            mPausingActivity = null;
        }

        if (resumeNext) {
            final ActivityStack topStack = mStackSupervisor.getFocusedStack();
            if (!mService.isSleepingOrShuttingDownLocked()) {
                mStackSupervisor.resumeFocusedStackTopActivityLocked(topStack, prev, null);
            } else {
                mStackSupervisor.checkReadyForSleepLocked();
                ActivityRecord top = topStack.topRunningActivityLocked();
                if (top == null || (prev != null && top != prev)) {
                    // If there are no more activities available to run, do resume anyway to start
                    // something. Also if the top activity on the stack is not the just paused
                    // activity, we need to go ahead and resume it to ensure we complete an
                    // in-flight app switch.
                    mStackSupervisor.resumeFocusedStackTopActivityLocked();
                }
            }
        }

        if (prev != null) {
            ......
        }
        ......
    }

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    这里调用了mStackSupervisor.resumeFocusedStackTopActivityLocked(topStack, prev, null);最终由调用到ActivityStack.resumeTopActivityInnerLocked，然后继而调用mStackSupervisor.startSpecificActivityLocked(next, true, true)；**这一轮调用不是刚开始15那轮调用，此时15那轮调用还没结束呢！**一直等pause的全部逻辑结束后，ActivityStack.resumeTopActivityInnerLocked的第一轮调用中才到if (pausing && !resumeWhilePausing)中return true;

    ActivityStack.resumeTopActivityInnerLocked两次调用的调用栈如下:
    在这里插入图片描述

在这里插入图片描述
22. 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);

    r.getStack().setLaunchTime(r);

    if (app != null && app.thread != null) {
        try {
            if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
                    || !"android".equals(r.info.packageName)) {
                // Don't add this if it is a platform component that is marked
                // to run in multiple processes, because this is actually
                // part of the framework so doesn't make sense to track as a
                // separate apk in the process.
                app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
                        mService.mProcessStats);
            }
            realStartActivityLocked(r, app, andResume, checkConfig);
            return;
        } catch (RemoteException e) {
            Slog.w(TAG, "Exception when starting activity "
                    + r.intent.getComponent().flattenToShortString(), e);
        }

        // If a dead object exception was thrown -- fall through to
        // restart the application.
    }

    mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
            "activity", r.intent.getComponent(), false, false, true);
}

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app启动，所以if代码块不会执行到。

  1. ActivityManagerService.startProcessLocked

    方法在\frameworks\base\services\core\java\com\android\server\am\startProcessLocked.java文件中：

    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 */);
    }

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    此处hostingType = “activity”

  2. ActivityManagerService.startProcessLocked

    方法在\frameworks\base\services\core\java\com\android\server\am\startProcessLocked.java文件中：

    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) {
        ......
        startProcessLocked(
                app, hostingType, hostingNameStr, abiOverride, entryPoint, entryPointArgs);
        ......
        return (app.pid != 0) ? app : null;
    }

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    此处hostingType = “activity”

  3. ActivityManagerService.startProcessLocked

    方法在\frameworks\base\services\core\java\com\android\server\am\startProcessLocked.java文件中：

    private final void startProcessLocked(ProcessRecord app, String hostingType,
            String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
        ......

        try {
            ......
            String invokeWith = null;
            if ((app.info.flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
                // Debuggable apps may include a wrapper script with their library directory.
                String wrapperFileName = app.info.nativeLibraryDir + "/wrap.sh";
                StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
                try {
                    if (new File(wrapperFileName).exists()) {
                        invokeWith = "/system/bin/logwrapper " + wrapperFileName;
                    }
                } finally {
                    StrictMode.setThreadPolicy(oldPolicy);
                }
            }    
            ......
            if (entryPoint == null) entryPoint = "android.app.ActivityThread";
            ......
            ProcessStartResult startResult;
            if (hostingType.equals("webview_service")) {
                ......
            } else {
                startResult = Process.start(entryPoint,
                        app.processName, uid, uid, gids, debugFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, invokeWith, entryPointArgs);
            }
            ......
        } catch (RuntimeException e) {
            ......
        }
    }

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    此处hostingType = “activity”。invokeWith = null。

  4. Process.start

    \frameworks\base\core\java\android\os\Process.java

    public static final ProcessStartResult start(final String processClass,
                                  final String niceName,
                                  int uid, int gid, int[] gids,
                                  int debugFlags, int mountExternal,
                                  int targetSdkVersion,
                                  String seInfo,
                                  String abi,
                                  String instructionSet,
                                  String appDataDir,
                                  String invokeWith,
                                  String[] zygoteArgs) {
        return zygoteProcess.start(processClass, niceName, uid, gid, gids,
                    debugFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, zygoteArgs);
    }

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    这里的processClass就是要启动的app的ActivityThread，最终会找到ActivityThread.main；

    invokeWith = null.

  5. ZygoteProcess.start

    \frameworks\base\core\java\android\os\ZygoteProcess.java

    public final Process.ProcessStartResult start(final String processClass,
                                                  final String niceName,
                                                  int uid, int gid, int[] gids,
                                                  int debugFlags, int mountExternal,
                                                  int targetSdkVersion,
                                                  String seInfo,
                                                  String abi,
                                                  String instructionSet,
                                                  String appDataDir,
                                                  String invokeWith,
                                                  String[] zygoteArgs) {
        try {
            return startViaZygote(processClass, niceName, uid, gid, gids,
                    debugFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, zygoteArgs);
        } catch (ZygoteStartFailedEx ex) {
            ......
        }
    }

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    这里的processClass就是要启动的app的ActivityThread，最终会找到ActivityThread.main

    invokeWith = null.

  6. ZygoteProcess.startViaZygote

    \frameworks\base\core\java\android\os\ZygoteProcess.java

    private Process.ProcessStartResult startViaZygote(final String processClass,
                                                      final String niceName,
                                                      final int uid, final int gid,
                                                      final int[] gids,
                                                      int debugFlags, int mountExternal,
                                                      int targetSdkVersion,
                                                      String seInfo,
                                                      String abi,
                                                      String instructionSet,
                                                      String appDataDir,
                                                      String invokeWith,
                                                      String[] extraArgs)
                                                      throws ZygoteStartFailedEx {
        ArrayList<String> argsForZygote = new ArrayList<String>();
        ............
        synchronized(mLock) {
            return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
        }
    }

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    把zygote进程fork子进程所需要的参数放入argsForZygote

  7. ZygoteProcess.zygoteSendArgsAndGetResult

    \frameworks\base\core\java\android\os\ZygoteProcess.java

    private static Process.ProcessStartResult zygoteSendArgsAndGetResult(
            ZygoteState zygoteState, ArrayList<String> args)
            throws ZygoteStartFailedEx {
        try {
            ......
            final BufferedWriter writer = zygoteState.writer;
            final DataInputStream inputStream = zygoteState.inputStream;

            writer.write(Integer.toString(args.size()));
            writer.newLine();

            for (int i = 0; i < sz; i++) {
                String arg = args.get(i);
                writer.write(arg);
                writer.newLine();
            }

            writer.flush();

            // Should there be a timeout on this?
            Process.ProcessStartResult result = new Process.ProcessStartResult();

            // Always read the entire result from the input stream to avoid leaving
            // bytes in the stream for future process starts to accidentally stumble
            // upon.
            result.pid = inputStream.readInt();
            result.usingWrapper = inputStream.readBoolean();

            if (result.pid < 0) {
                throw new ZygoteStartFailedEx("fork() failed");
            }
            return result;
        } catch (IOException ex) {
            zygoteState.close();
            throw new ZygoteStartFailedEx(ex);
        }
    }

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    通过socket和zygote进程通信，用BufferedWriter把参数写入socket传递给zygote进程，最后通过DataInputStream把zygote进程fork子进程的结果读出来.

    3~~29是在AMS即system_server进程

  8. ZygoteInit.main

    \frameworks\base\core\java\com\android\internal\os\ZygoteInit.java

    public static void main(String argv[]) {
        ZygoteServer zygoteServer = new ZygoteServer();
        ......
        final Runnable caller;
        try {
            ......
            caller = zygoteServer.runSelectLoop(abiList);
        } catch (Throwable ex) {
            Log.e(TAG, "System zygote died with exception", ex);
            throw ex;
        } finally {
            zygoteServer.closeServerSocket();
        }
        // We're in the child process and have exited the select loop. Proceed to execute the
        // command.
        if (caller != null) {
            caller.run();
        }
    }

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    Zygote进程是在系统启动后就被启动的进程，初始化工作是在ZygoteInit.main中完成，zygoteServer.runSelectLoop启动一个死循环，就是为了fork进程，这里会取出ams发送来的参数，fork一个子进程，此处即app进程。
    如果是子进程，会执行caller.run()。原因后续分析。

  9. ZygoteServer.runSelectLoop

    \frameworks\base\core\java\com\android\internal\os\ZygoteServer.java

    Runnable runSelectLoop(String abiList) {
        ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
        ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

        fds.add(mServerSocket.getFileDescriptor());
        peers.add(null);

        while (true) {
            StructPollfd[] pollFds = new StructPollfd[fds.size()];
            for (int i = 0; i < pollFds.length; ++i) {
                pollFds[i] = new StructPollfd();
                pollFds[i].fd = fds.get(i);
                pollFds[i].events = (short) POLLIN;
            }
            try {
                Os.poll(pollFds, -1);
            } catch (ErrnoException ex) {
                throw new RuntimeException("poll failed", ex);
            }
            for (int i = pollFds.length - 1; i >= 0; --i) {
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }

                if (i == 0) {
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    try {
                        ZygoteConnection connection = peers.get(i);
                        final Runnable command = connection.processOneCommand(this);

                        if (mIsForkChild) {
                            // We're in the child. We should always have a command to run at this
                            // stage if processOneCommand hasn't called "exec".
                            if (command == null) {
                                throw new IllegalStateException("command == null");
                            }

                            return command;
                        } else {
                            // We're in the server - we should never have any commands to run.
                            if (command != null) {
                                throw new IllegalStateException("command != null");
                            }

                            // We don't know whether the remote side of the socket was closed or
                            // not until we attempt to read from it from processOneCommand. This shows up as
                            // a regular POLLIN event in our regular processing loop.
                            if (connection.isClosedByPeer()) {
                                connection.closeSocket();
                                peers.remove(i);
                                fds.remove(i);
                            }
                        }
                    } catch (Exception e) {
                       ......
                        } else {
                            // We're in the child so any exception caught here has happened post
                            // fork and before we execute ActivityThread.main (or any other main()
                            // method). Log the details of the exception and bring down the process.
                            Log.e(TAG, "Caught post-fork exception in child process.", e);
                            throw e;
                        }
                    }
                }
            }
        }
    }

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    zygote进程通过ZygoteServer.runSelectLoop启动一个死循环，就是为了fork进程，

    划重点：final Runnable command = connection.processOneCommand(this);

  10. ZygoteConnection.processOneCommand

    \frameworks\base\core\java\com\android\internal\os\ZygoteConnection.java

    Runnable processOneCommand(ZygoteServer zygoteServer) {
        ......

        pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
                parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
                parsedArgs.niceName, fdsToClose, fdsToIgnore, parsedArgs.instructionSet,
                parsedArgs.appDataDir);

        try {
            if (pid == 0) {
                // in child
                zygoteServer.setForkChild();
                //void setForkChild() {
        	   //	mIsForkChild = true;
    		   //}
                zygoteServer.closeServerSocket();
                IoUtils.closeQuietly(serverPipeFd);
                serverPipeFd = null;

                return handleChildProc(parsedArgs, descriptors, childPipeFd);
            } else {
                // In the parent. A pid < 0 indicates a failure and will be handled in
                // handleParentProc.
                IoUtils.closeQuietly(childPipeFd);
                childPipeFd = null;
                handleParentProc(pid, descriptors, serverPipeFd);
                return null;
            }
        } finally {
            IoUtils.closeQuietly(childPipeFd);
            IoUtils.closeQuietly(serverPipeFd);
        }
    }

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    Zygote.forkAndSpecialize刚刚fork出来的子进程会完全继承父进程的所有状态，包括运行时、文件、资源等，所以子进程也有while循环，也会在main方法里没出来；

    pid == 0是子进程，zygoteServer.setForkChild()标记为子进程，即把当前进程的mIsForkChild设置为true；由Zygote进程创建的子进程会继承Zygote进程中创建的Socket文件描述符，而这里的子进程又不会用到它，因此，这里就调用closeServerSocket方法来关闭它，再通过ZygoteConnection.handleChildProc返回一个Runnable，因为mIsForkChild为true，这样新的子进程就跳出while循环，并且把刚刚的runnable return出去，在ZygoteInit.main中赋给caller，caller调用run方法，最终调用到ActivityThread.main

    ,这个runnable由ZygoteInit.zygoteInit调用RuntimeInit.applicationInit创建

    补充:

    linux创建子进程–fork()方法

    https://www.cnblogs.com/xingzc/p/5988069.html

    (1)fork()的定义
    fork()函数是Unix中派生新进程的唯一方法，声明如下：
    我们需要理解的是，调用一次fork()方法，该方法会返回两次。一次是在调用进程(也就是派生出的子进程的父进程)中返回一次，返回值是新派生的进程的进程ID。一次是在子进程中返回，返回值是0，代表当前进程为子进程。如果返回值为-1的话，则代表在派生新进程的过程中出错。
    那么在程序中，我们就可以根据此返回值来判断当前进程是父进程还是子进程，来实现一些具体的操作。
    (2)fork()的实质过程
    父进程中在调用fork()派生新进程，实际上相当于创建了进程的一个拷贝；即在fork()之前的进程拥有的资源会被复制到新的进程中去。网络服务器在处理并发请求时，也可以采取这种派生新进程的方式: 父进程调用accept()后调用fork()来处理每一个连接。那么，所接受的已连接的套接口随后就在父子进程中共享。通常来说，子进程会在这连接套接口中读和写操作，父进程则关闭这个已连的套接口(可以参考:http://blog.csdn.net/moxiaomomo/article/details/6791763)
    (3)fork()的用法
    fork()有两个典型用法：(1)一个进程进行自身的复制，这样每个副本可以独立的完成具体的操作，在多核处理器中可以并行处理数据。这也是网络服务器的其中一个典型用途，多进程处理多连接请求。 (2)一个进程想执行另一个程序。比如一个软件包含了两个程序，主程序想调起另一个程序的话，它就可以先调用fork来创建一个自身的拷贝，然后通过exec函数来替换成将要运行的新程序。

  11. ZygoteConnection.handleChildProc

    private Runnable handleChildProc(Arguments parsedArgs, FileDescriptor[] descriptors,
            FileDescriptor pipeFd) {
        .......
        if (parsedArgs.invokeWith != null) {
            ......
        } else {
            return ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, null /* classLoader */);
        }
    }

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    从20,21可知，parsedArgs.invokeWith = null

  12. ZygoteInit.zygoteInit

    \frameworks\base\core\java\com\android\internal\os\ZygoteInit.java

    public static final Runnable zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) {
        ......
        RuntimeInit.commonInit();
        ZygoteInit.nativeZygoteInit();
        return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
    }

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调用zygoteInitNative方法来执行一个Binder进程间通信机制的初始化工作：打开Binder驱动，mmap映射，启动Binder线程池(此时启动一个Binder主线程)。这个过程参考进程的Binder线程池工作过程
String[] argv里面就是ActivityThread.main相关信息，将这些信息参数封装进一个Runnable里。

  1. RuntimeInit.applicationInit

    \frameworks\base\core\java\com\android\internal\os\RuntimeInit.java

    protected static Runnable applicationInit(int targetSdkVersion, String[] argv,
            ClassLoader classLoader) {
        ......
        // Remaining arguments are passed to the start class's static main
        return findStaticMain(args.startClass, args.startArgs, classLoader);
    }

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    方法名很直观，应用初始化

  2. RuntimeInit.findStaticMain

    \frameworks\base\core\java\com\android\internal\os\RuntimeInit.java

    private static Runnable findStaticMain(String className, String[] argv,
            ClassLoader classLoader) {
        Class<?> cl;

        try {
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }

        Method m;
        try {
            m = cl.getMethod("main", new Class[] { String[].class });
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException(
                    "Missing static main on " + className, ex);
        } catch (SecurityException ex) {
            throw new RuntimeException(
                    "Problem getting static main on " + className, ex);
        }

        int modifiers = m.getModifiers();
        if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
            throw new RuntimeException(
                    "Main method is not public and static on " + className);
        }

        /*
         * This throw gets caught in ZygoteInit.main(), which responds
         * by invoking the exception's run() method. This arrangement
         * clears up all the stack frames that were required in setting
         * up the process.
         */
        return new MethodAndArgsCaller(m, argv);
    }
    
    static class MethodAndArgsCaller implements Runnable {
        /** method to call */
        private final Method mMethod;

        /** argument array */
        private final String[] mArgs;

        public MethodAndArgsCaller(Method method, String[] args) {
            mMethod = method;
            mArgs = args;
        }

        public void run() {
            try {
                mMethod.invoke(null, new Object[] { mArgs });
            } catch (IllegalAccessException ex) {
                throw new RuntimeException(ex);
            } catch (InvocationTargetException ex) {
                Throwable cause = ex.getCause();
                if (cause instanceof RuntimeException) {
                    throw (RuntimeException) cause;
                } else if (cause instanceof Error) {
                    throw (Error) cause;
                }
                throw new RuntimeException(ex);
            }
        }
    }

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    这里就找到了app进程的ActivityThread.main，最终在ZygoteInit.main中调用.

  3. ActivityThread.main

    public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();
        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();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }
        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }
        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();

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

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    ActivityThread.main中在启动主消息循环之前，创建了一个ActivityThread对象并调用attach方法，ActivityThread对象创建的时候，ActivityThread对象的成员变量ApplicationThread 对象立刻创建final ApplicationThread mAppThread = new ApplicationThread();。

  4. ActivityThread.attach

    private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            ......
            final IActivityManager mgr = ActivityManager.getService();
            try {
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
            ......
        } else {
            ......
        }

        ......
    }

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    将app进程的ApplicationThread对象mAppThread绑定到AMS

  5. ActivityManagerService.attachApplication

    @Override
    public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            ......
            attachApplicationLocked(thread, callingPid);
            ......
        }
    }

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  6. ActivityManagerService.attachApplicationLocked

    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid) {

        ......
        try {
            ......
            if (app.instr != null) {
                thread.bindApplication(processName, appInfo, providers,
                        app.instr.mClass,
                        profilerInfo, app.instr.mArguments,
                        app.instr.mWatcher,
                        app.instr.mUiAutomationConnection, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.persistent,
                        new Configuration(getGlobalConfiguration()), app.compat,
                        getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial);
            } else {
                thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
                        null, null, null, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.persistent,
                        new Configuration(getGlobalConfiguration()), app.compat,
                        getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial);
            }
            ......
        } catch (Exception e) {
            ......
            return false;
        }
        ......
        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;
    }

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    AMS通过Binder对象即ApplicationThread对象thread，调用.ActivityThread.ApplicationThread.bindApplication，紧接着调用mStackSupervisor.attachApplicationLocked(app)，ActivityStackSupervisor.attachApplicationLocked里面调用了ActivityStackSupervisor.realStartActivityLocked

  7. ActivityThread.ApplicationThread.bindApplication

    public final void bindApplication(String processName, ApplicationInfo appInfo,
        List<ProviderInfo> providers, ComponentName instrumentationName,
        ProfilerInfo profilerInfo, Bundle instrumentationArgs,
        IInstrumentationWatcher instrumentationWatcher,
        IUiAutomationConnection instrumentationUiConnection, int debugMode,
        boolean enableBinderTracking, boolean trackAllocation,
        boolean isRestrictedBackupMode, boolean persistent, Configuration config,
        CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
        String buildSerial) {

    ......
    sendMessage(H.BIND_APPLICATION, data);
}

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    case BIND_APPLICATION:里调用handleBindApplication

  8. ActivityThread.handleBindApplication

    private void handleBindApplication(AppBindData data) {
    ......
    final InstrumentationInfo ii;
    ......
    // Continue loading instrumentation.
        if (ii != null) {
            ......

            try {
                final ClassLoader cl = instrContext.getClassLoader();
                mInstrumentation = (Instrumentation)
                    cl.loadClass(data.instrumentationName.getClassName()).newInstance();
            } catch (Exception e) {
               ......
            }
            ......
        } else {
            mInstrumentation = new Instrumentation();
        }
    ......
    try {
        // If the app is being launched for full backup or restore, bring it up in
        // a restricted environment with the base application class.
        Application app = data.info.makeApplication(data.restrictedBackupMode, null);
        ......
        try {
            mInstrumentation.onCreate(data.instrumentationArgs);
        }
        catch (Exception e) {
            ......
        }

        try {
            mInstrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            if (!mInstrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to create application " + app.getClass().getName()
                    + ": " + e.toString(), e);
            }
        }
    } finally {
        StrictMode.setThreadPolicy(savedPolicy);
    }

    ......
}

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    初始化ActivityThread成员变量mInstrumentation；通过LoadedApk.makeApplication创建Application对象，然后回调Application的onCreate，注意：Application的attachBaseContext方法在makeAppliaction就调用了.

  9. LoadedApk.makeApplication

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

    String appClass = mApplicationInfo.className;
    if (forceDefaultAppClass || (appClass == null)) {
        appClass = "android.app.Application";
    }

    try {
        java.lang.ClassLoader cl = getClassLoader();
        ......
        ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
        app = mActivityThread.mInstrumentation.newApplication(
                cl, appClass, appContext);
        appContext.setOuterContext(app);
    } catch (Exception e) {
        .......
    }
    m......

    if (instrumentation != null) {
        try {
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            if (!instrumentation.onException(app, e)) {
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                throw new RuntimeException(
                    "Unable to create application " + app.getClass().getName()
                    + ": " + e.toString(), e);
            }
        }
    }
    ......
    return app;
}

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    如果Application对象已经创建，则直接返回；如果没有创建，则交由ActivityThread的成员变量mInstrumentation的方法newApplication创建.

  10. Instrumentation.newApplication

    static public Application newApplication(Class<?> clazz, Context context)
        throws InstantiationException, IllegalAccessException, 
        ClassNotFoundException {
    Application app = (Application)clazz.newInstance();
    app.attach(context);
    return app;
}

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  11. Application.attach

    final void attach(Context context) {
    attachBaseContext(context);
    mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
}

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    Application.attach里调用attachBaseContext(context);所以看出比onCreate执行的时机早。这些都是在ActivityThread的主消息循环中执行的。

    接下来回到AMS中的attachApplicationLocked中调用的mStackSupervisor.attachApplicationLocked(app)

  12. ActivityStackSupervisor.attachApplicationLocked

    boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
    final String processName = app.processName;
    boolean didSomething = false;
    for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
        ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
        for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
            final ActivityStack stack = stacks.get(stackNdx);
            if (!isFocusedStack(stack)) {
                continue;
            }
            ActivityRecord hr = stack.topRunningActivityLocked();
            if (hr != null) {
                if (hr.app == null && app.uid == hr.info.applicationInfo.uid
                        && processName.equals(hr.processName)) {
                    try {
                        if (realStartActivityLocked(hr, app, true, true)) {
                            didSomething = true;
                        }
                    } catch (RemoteException e) {
                        ......
                        throw e;
                    }
                }
            }
        }
    }
    ......
    return didSomething;
}

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    这里调用了realStartActivityLocked

  13. ActivityStackSupervisor.realStartActivityLocked

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

    ......
    final ActivityStack stack = task.getStack();
    try {
        if (app.thread == null) {
            throw new 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);

        ......
    } catch (RemoteException e) {
        ......
        throw e;
    }
    ......
    return true;
}

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    这里通过AppliactionThread调用ActivityThread.ApplicationThread.scheduleLaunchActivity，通过ActivityThread.H调用到handleLaunchActivity

  14. ActivityThread.handleLaunchActivity

    private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    .......
    Activity a = performLaunchActivity(r, customIntent);

    if (a != null) {
        ......
        handleResumeActivity(r.token, false, r.isForward,
                !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

        ......
    } else {
        ......
    }
}

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  15. ActivityThread.performLaunchActivity

    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);
    }

    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) {
        ......
    }

    try {
        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;
            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) {
                ......
            }
            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) {
                    ......
                }
            }
        }
        r.paused = true;

        mActivities.put(r.token, r);

    } catch (SuperNotCalledException e) {
        throw e;

    } catch (Exception e) {
        ......
    }

    return activity;
}

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    通过反射创建Activity对象，创建Activity的上下文信息，并通过attach方法将这些上下文信息设置到Activity中去，通过mInstrumentation的callActivityOnCreate调用Activity.onCreate，LauncherActivity就启动起来了，整个应用程序也启动起来了，但是此时还不可见。ActivityThread.handleLaunchActivity中还调用了handleResumeActivity

  16. ActivityThread.handleResumeActivity

    final void handleResumeActivity(IBinder token,
        boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
    ......
    r = performResumeActivity(token, clearHide, reason);

    if (r != null) {
        final Activity a = r.activity;
        ......
        if (r.window == null && !a.mFinished && willBeVisible) {
            r.window = r.activity.getWindow();
            View decor = r.window.getDecorView();
            decor.setVisibility(View.INVISIBLE);
            ViewManager wm = a.getWindowManager();
            WindowManager.LayoutParams l = r.window.getAttributes();
            a.mDecor = decor;
            l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
            l.softInputMode |= forwardBit;
            if (r.mPreserveWindow) {
                a.mWindowAdded = true;
                r.mPreserveWindow = false;
                // Normally the ViewRoot sets up callbacks with the Activity
                // in addView->ViewRootImpl#setView. If we are instead reusing
                // the decor view we have to notify the view root that the
                // callbacks may have changed.
                ViewRootImpl impl = decor.getViewRootImpl();
                if (impl != null) {
                    impl.notifyChildRebuilt();
                }
            }
            if (a.mVisibleFromClient) {
                if (!a.mWindowAdded) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                } else {
                    ......
                }
            }

       ......
        if (!r.activity.mFinished && willBeVisible
                && r.activity.mDecor != null && !r.hideForNow) {
            ......
            if (r.activity.mVisibleFromClient) {
                r.activity.makeVisible();
            }
        }

        ......

    } else {
       ......
    }
}

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    这里调用了performResumeActivity 和activity.makeVisible()

    makeVisible之后，Activity才真正可见并且可交互。

  17. Activity.makeVisible

    void makeVisible() {
    if (!mWindowAdded) {
        ViewManager wm = getWindowManager();
        wm.addView(mDecor, getWindow().getAttributes());
        mWindowAdded = true;
    }
    mDecor.setVisibility(View.VISIBLE);
}

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PS：感谢老罗的Android之旅

Android应用程序启动过程源代码分析
Android系统进程Zygote启动过程的源代码分析

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