以Activity A 啟動(dòng) Activity B，Activity B所在進(jìn)程未啟動(dòng)為例
大體經(jīng)過(guò)已經(jīng)幾個(gè)階段：

startActivity.png

第一步 Activity A 調(diào)用AMS 啟動(dòng)接口

這是一個(gè)binder通訊勋桶，正常啟動(dòng)Activity的方式脱衙，一般我們都會(huì)通過(guò)以下的方式啟動(dòng)一個(gè)新的Activity

startActivity(new Intent(A_Activity.this,B_Activity.class));

這是Activity類的接口侥猬，最終會(huì)調(diào)用到

    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }

            cancelInputsAndStartExitTransition(options);
            // TODO Consider clearing/flushing other event sources and events for child windows.
        } else {
          
        }
    }

這里關(guān)鍵點(diǎn)會(huì)調(diào)用Instrumentation 的execStartActivity方法。
frameworks/base/core/java/android/app/Instrumentation.java
源碼注釋大體意思是Instrumentation類會(huì)在應(yīng)用的任何代碼執(zhí)行前被實(shí)列化捐韩，用來(lái)監(jiān)控系統(tǒng)與應(yīng)用的交互退唠。可在以應(yīng)用的AndroidManifest.xml中<instrumentation>標(biāo)簽來(lái)注冊(cè)一個(gè)Instrumentation的實(shí)現(xiàn)荤胁。
基本上在application和activity的所有生命周期調(diào)用中瞧预，都會(huì)先調(diào)用instrumentation的相應(yīng)方法。Instrumentation另一個(gè)重要作用是提供Android組件單元測(cè)試仅政。這里不做深究垢油。

 public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
       ...
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            //核心在這一句
            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;
    }

ActivityManager.getService().startActivity 這一步會(huì)通過(guò)binder的方式調(diào)用到AMS的接口

第二步 收集應(yīng)用信息，pause ActivityA

startActivity --> startActivityAsUser -- > mActivityStarter.startActivityMayWait

這里會(huì)調(diào)用到類 ActivityStarter

/**
 * Controller for interpreting how and then launching activities.
 *
 * This class collects all the logic for determining how an intent and flags should be turned into
 * an activity and associated task and stack.
 */
class ActivityStarter

從注釋上看圆丹，我們了解了ActivityStarter的作用是收集所有的邏輯滩愁，用來(lái)決定如何啟動(dòng)activity， ActivityStarter又通過(guò)ActivityStackSupervisor來(lái)管理ActivityStack

略過(guò)中間環(huán)節(jié)辫封，會(huì)調(diào)用到ActivityStack的resumeTopActivityInnerLocked

boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
    // ......
    boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
    if (mResumedActivity != null) {
        if (DEBUG_STATES) Slog.d(TAG_STATES,
                "resumeTopActivityLocked: Pausing " + mResumedActivity);
        pausing |= startPausingLocked(userLeaving, false, next, false);
    }
    // ......
    if (next.attachedToProcess()) {
        try {
        // ......
            transaction.setLifecycleStateRequest(
                    ResumeActivityItem.obtain(next.app.getReportedProcState(),
                            getDisplay().mDisplayContent.isNextTransitionForward()));
            mService.getLifecycleManager().scheduleTransaction(transaction);
        } catch (Exception e) {
            // ......
            mStackSupervisor.startSpecificActivityLocked(next, true, false);
            return true;
        }
        // ......
    } else {
        // Whoops, need to restart this activity!
        if (!next.hasBeenLaunched) {
            next.hasBeenLaunched = true;
        } else {
            if (SHOW_APP_STARTING_PREVIEW) {
                next.showStartingWindow(null /* prev */, false /* newTask */,
                        false /* taskSwich */);
            }
            if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Restarting: " + next);
        }
        if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Restarting " + next);
        mStackSupervisor.startSpecificActivityLocked(next, true, true);
    }

    return true;
}

此處先通過(guò)startPausingLocked流程使mResumedActivity pause硝枉。

第三步 通過(guò)Zygote創(chuàng)建應(yīng)用進(jìn)程

下一步則通過(guò) ActivityStackSupervisor 中的startSpecificActivityLocked方法來(lái)執(zhí)行啟動(dòng)ActivityB流程

    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);


        if (app != null && app.thread != null) {
            try {
                realStartActivityLocked(r, app, andResume, checkConfig);
                return;
            } 

            // 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);
    }

這個(gè)函數(shù)，先判斷ActivityB所在的進(jìn)程是否啟動(dòng)倦微，如果已經(jīng)啟動(dòng)則執(zhí)行realStartActivityLocked妻味， 如果沒(méi)有則走mService.startProcessLocked流程去啟動(dòng)一個(gè)進(jìn)程. mService為ActivityManagerService

private final void startProcessLocked(ProcessRecord app,) {

       if (entryPoint == null) entryPoint = "android.app.ActivityThread";
       startResult = Process.start(entryPoint,);

       synchronized (mPidsSelfLocked) {
                this.mPidsSelfLocked.put(startResult.pid, app);
                if (isActivityProcess) {
                    Message msg = mHandler.obtainMessage(PROC_START_TIMEOUT_MSG);
                    msg.obj = app;
                    mHandler.sendMessageDelayed(msg, startResult.usingWrapper
                            ? PROC_START_TIMEOUT_WITH_WRAPPER : PROC_START_TIMEOUT);
                }
            }
}

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

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

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

 public final Process.ProcessStartResult start(final String processClass,) {
        try {
            return startViaZygote(processClass, );
        } 
    }

這里 entryPoint = "android.app.ActivityThread" 為啟動(dòng)的java類， 該變量會(huì)作為參數(shù)給到Process欣福。 Process.start(entryPoint,) 最終會(huì)通過(guò)socket跟Zygote通訊弧可，Zygote 會(huì)去fork一個(gè)進(jìn)程，入口就是entryPoint劣欢。

Zygote啟動(dòng)完進(jìn)程后會(huì)返回一個(gè)pid，這個(gè)pid包含在startResult里面裁良，接著會(huì)將pid和app 放入mPidsSelfLocked凿将， 并發(fā)送一個(gè)延時(shí)消息，如果ActivityThread在一定的TIMEOUT（10s）還沒(méi)向AMS報(bào)告价脾，則該消息會(huì)被執(zhí)行牧抵，AMS會(huì)去清除這個(gè)應(yīng)用的所有信息

第四步 應(yīng)用進(jìn)程啟動(dòng)及跟AMS交互

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

    public static void main(String[] args) {
        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        Looper.loop();
    }

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

這里的attach就是向AMS注冊(cè)

下面回到AMS中去執(zhí)行

private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {
        //首先根據(jù)pid取出 ProcessRecord
        app = mPidsSelfLocked.get(pid);

        //thread為應(yīng)用端的binder對(duì)象，會(huì)去通知應(yīng)用端做一些Application初始化
         thread.bindApplication(processName,...)
         ...
         //處理被掛起的應(yīng)用組件（activity侨把，service犀变，broadcast），之前應(yīng)用進(jìn)程還沒(méi)被創(chuàng)建
         mStackSupervisor.attachApplicationLocked(app)
         ...
         mServices.attachApplicationLocked()

         sendPendingBroadcastsLocked(app)
         
}

boolean attachApplicationLocked(ProcessRecord app)  {
      final ActivityRecord top = stack.topRunningActivityLocked();
      realStartActivityLocked(activity, app,
}

stack為mFocusStack秋柄， 這一部為取出棧頂?shù)腁ctivity获枝，即我們要啟動(dòng)的activity，雖然之前activity沒(méi)啟動(dòng)骇笔，但ams里面棧的信息在啟動(dòng)進(jìn)程前都已經(jīng)準(zhǔn)備好了省店。

final boolean realStartActivityLocked() {
    app.thread.scheduleLaunchActivity
}

app為ProcessRecord對(duì)象嚣崭，thread為應(yīng)用注冊(cè)到AMS的binder對(duì)象. 這里調(diào)用了應(yīng)用的scheduleLaunchActivity，從名字看表示可以加載activity了

第五步 Activity啟動(dòng)

下面就切回到ActivityThread中去執(zhí)行

public final void scheduleLaunchActivity() {
    ActivityClientRecord r = new ActivityClientRecord();
    sendMessage(H.LAUNCH_ACTIVITY, r);
}

通過(guò)發(fā)送消息給主線程去處理

 public void handleMessage(Message msg) {
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {

                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
                } break;

getPackageInfoNoCheck 會(huì)去loadApk加載apk的一些信息懦傍，后面啟動(dòng)activity需要調(diào)用到雹舀，下面會(huì)去執(zhí)行的activity的生命周期

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);
            }
}

private Activity performLaunchActivity {
      //創(chuàng)建一個(gè)Activity
      activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
       // 獲取之前創(chuàng)建的Application
       Application app = r.packageInfo.makeApplication(false, mInstrumentation);
        //創(chuàng)建Context, 并將Context賦值給activity
        ContextImpl appContext = createBaseContextForActivity(r);
       activity.attach(appContext, this, ...）

       //調(diào)用到Activity onCreate函數(shù) 
       mInstrumentation.callActivityOnCreate(activity, r.state);
       //調(diào)用到Activity onStart函數(shù) 
       activity.performStart();
}

大體流程可以用下圖表示

activity啟動(dòng).png

參考：
網(wǎng)絡(luò)課程 --剖析Framework面試 沖擊Android高級(jí)職位