1.前言

• 最近一直在看 《Android進階解密》 的一本書，這本書編寫邏輯、流程都非常好，而且很容易看懂，非常推薦大家去看看（沒有收廣告費，單純覺得作者寫的很好）。
• 上一篇簡單的介紹了Android進階（二）： 應用程序啟動過程，最終知道了ActivityThread就是代表應用程序。
• 今天就介紹ActivityThread啟動之後，是如何啟動 Application （基於Android 8.0 系統）。
• 文章中例項 linhaojian的Github

2.Application啟動過程的時序圖

3.原始碼分析

3.1 ActivityThread初始化：

public static void main(String[] args) {
        //...
        //建立ActivityThread物件 & 呼叫attach()
        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);
        //建立主執行緒的Handler
        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);
        //建立主執行緒的Looper
        Looper.
 
loop(); 
}

• 主要做了2件事，例項化ActivityThread & 建立主執行緒Handler與Looper接收資訊；

3.2 ActivityThread的attach函式：

    private void attach(boolean system, long startSeq) {
        //...
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            // 獲取ActivityManagerService的代理物件
            final IActivityManager mgr = ActivityManager.getService();// 1
            try {
                //通知AMS進行application的初始化
                mgr.attachApplication(mAppThread, startSeq);// 2
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
    }

• 註釋1：獲取ActivityManagerService的代理物件；
• 註釋2：通過代理物件呼叫attachApplication()，獲取啟動application所需資訊(應用程序相關資料)；

3.3 ActivityManagerService的attachApplication函式：

    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);// 1
            Binder.restoreCallingIdentity(origId);
        }
    }

• 註釋1：呼叫attachApplicationLocked()；

3.4 ActivityManagerService的attachApplicationLocked函式：

    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid, int callingUid, long startSeq) {
        // 根據pid獲取儲存在AMS中，對應程序的相關資訊
        ProcessRecord app;
        long startTime = SystemClock.uptimeMillis();
        if (pid != MY_PID && pid >= 0) {
            synchronized (mPidsSelfLocked) {
                app = mPidsSelfLocked.get(pid);// 1
            }
        } else {
            app = null;
        }
        // ...
        thread.bindApplication(processName, appInfo, providers, null, profilerInfo, // 2
            null, null, null, testMode,
            mBinderTransactionTrackingEnabled, enableTrackAllocation,
            isRestrictedBackupMode || !normalMode, app.persistent,
            new Configuration(getGlobalConfiguration()), app.compat,
            getCommonServicesLocked(app.isolated),
            mCoreSettingsObserver.getCoreSettingsLocked(),
            buildSerial, isAutofillCompatEnabled);
         // ...
    }

• 註釋1：根據pid獲取儲存在AMS中，對應應用程序的相關資訊；
• 註釋2：通知ActivityThread啟動application（IApplicationThread是ActivityThread的內部類，負責與ActivityManagerService通訊）；

3.5 ActivityThread的handleBindApplication函式：

• AMS中呼叫了ActivityThread的bindApplication函式，其內部其實是完成了Handler切換到主執行緒，並且最後活呼叫handleBindApplication(),下面我們看看其內部原始碼;

    private void handleBindApplication(AppBindData data) {
        // 將UI執行緒註冊為執行時的虛擬機器.
        VMRuntime.registerSensitiveThread();
        if (data.trackAllocation) {
            DdmVmInternal.enableRecentAllocations(true);
        }
        // ...
        // 建立上下文物件
        final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);
        updateLocaleListFromAppContext(appContext,
                mResourcesManager.getConfiguration().getLocales());
        if (!Process.isIsolated()) {
            final int oldMask = StrictMode.allowThreadDiskWritesMask();
            try {
                setupGraphicsSupport(appContext);
            } finally {
                StrictMode.setThreadPolicyMask(oldMask);
            }
        } else {
            ThreadedRenderer.setIsolatedProcess(true);
        }
        // Continue loading instrumentation.
        if (ii != null) {
            // ...
        } else {
            mInstrumentation = new Instrumentation();// 1
            mInstrumentation.basicInit(this);
        }
        Application app;
        final StrictMode.ThreadPolicy savedPolicy = StrictMode.allowThreadDiskWrites();
        final StrictMode.ThreadPolicy writesAllowedPolicy = StrictMode.getThreadPolicy();
        try {
            // If the app is being launched for full backup or restore, bring it up in
            // a restricted environment with the base application class.
            // 初始化Applcation類
            app = data.info.makeApplication(data.restrictedBackupMode, null); // 2
            // Propagate autofill compat state
            app.setAutofillCompatibilityEnabled(data.autofillCompatibilityEnabled);
            mInitialApplication = app;
            // don't bring up providers in restricted mode; they may depend on the
            // app's custom Application class
            if (!data.restrictedBackupMode) {
                if (!ArrayUtils.isEmpty(data.providers)) {
                    installContentProviders(app, data.providers);
                    // For process that contains content providers, we want to
                    // ensure that the JIT is enabled "at some point".
                    mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
                }
            }
            // Do this after providers, since instrumentation tests generally start their
            // test thread at this point, and we don't want that racing.
            try {
                mInstrumentation.onCreate(data.instrumentationArgs); // 3
            }
            catch (Exception e) {
                throw new RuntimeException(
                    "Exception thrown in onCreate() of "
                    + data.instrumentationName + ": " + e.toString(), e);
            }
            try {
                //呼叫Applcation的OnCreate函式
                mInstrumentation.callApplicationOnCreate(app); // 4
            } catch (Exception e) {
                if (!mInstrumentation.onException(app, e)) {
                    throw new RuntimeException(
                      "Unable to create application " + app.getClass().getName()
                      + ": " + e.toString(), e);
                }
            }
        } finally {
            // If the app targets < O-MR1, or doesn't change the thread policy
            // during startup, clobber the policy to maintain behavior of b/36951662
            if (data.appInfo.targetSdkVersion < Build.VERSION_CODES.O_MR1
                    || StrictMode.getThreadPolicy().equals(writesAllowedPolicy)) {
                StrictMode.setThreadPolicy(savedPolicy);
            }
        }
        // Preload fonts resources
        FontsContract.setApplicationContextForResources(appContext);
        if (!Process.isIsolated()) {
            try {
                final ApplicationInfo info =
                        getPackageManager().getApplicationInfo(
                                data.appInfo.packageName,
                                PackageManager.GET_META_DATA /*flags*/,
                                UserHandle.myUserId());
                if (info.metaData != null) {
                    final int preloadedFontsResource = info.metaData.getInt(
                            ApplicationInfo.METADATA_PRELOADED_FONTS, 0);
                    if (preloadedFontsResource != 0) {
                        data.info.getResources().preloadFonts(preloadedFontsResource);
                    }
                }
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        }
    }

• 註釋1：**建立Instrumentation，負責跟蹤Application還在Activity的生命週期；
• 註釋2：建立Application物件 & 呼叫其attach()；
• 註釋3：呼叫Instrumentation的onCreate()，內部是空實現；
• 註釋4：呼叫Instrumentation的callApplicationOnCreate()，內部是呼叫application的onCreate，如下程式碼；

public class Instrumentation {
    public void callApplicationOnCreate(Application app) {
        app.onCreate();
    }
}

4.類關係

• ActivityThread ：通過IActivityManager類，通知AMS準備application啟動所需程序資料 ；
• ActivityManagerService ：獲取application啟動所需程序資料 ；
• Instrumentation ：建立&啟動Application；跟蹤Application的生命週期；

5.總結

• 到此，**Application啟動過程**介紹完畢。
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