1.前言

• 最近一直在看 《Android进阶解密》 的一本书，这本书编写逻辑、流程都非常好，而且很容易看懂，非常推荐大家去看看（没有收广告费，单纯觉得作者写的很好）。
• 上一篇简单的介绍了Android进阶（二）： 应用进程启动过程，最终知道了ActivityThread就是代表应用进程。
• 今天就介绍ActivityThread启动之后，是如何启动 Application （基于Android 8.0 系统）。
• 文章中实例 linhaojian的Github

2.Application启动过程的时序图

Application启动流程.png

3.源码分析

3.1 ActivityThread初始化：

public static void main(String[] args) {
        //...
        //创建ActivityThread对象 & 调用attach()
        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);
        //创建主线程的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);
        //创建主线程的Looper
        Looper.loop(); 
}

• 主要做了2件事，实例化ActivityThread & 创建主线程Handler与Looper接收信息；

3.2 ActivityThread的attach函数：

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

• 注释1：获取ActivityManagerService的代理对象；
• 注释2：通过代理对象调用attachApplication()，获取启动application所需信息(应用进程相关数据)；

3.3 ActivityManagerService的attachApplication函数：

    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);
        }
    }

• 注释1：调用attachApplicationLocked()；

3.4 ActivityManagerService的attachApplicationLocked函数：

    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid, int callingUid, long startSeq) {
        // 根据pid获取存储在AMS中，对应进程的相关信息
        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);
         // ...
    }

• 注释1：根据pid获取存储在AMS中，对应应用进程的相关信息；
• 注释2：通知ActivityThread启动application（IApplicationThread是ActivityThread的内部类，负责与ActivityManagerService通讯）；

3.5 ActivityThread的handleBindApplication函数：

• AMS中调用了ActivityThread的bindApplication函数，其内部其实是完成了Handler切换到主线程，并且最后活调用handleBindApplication(),下面我们看看其内部源码;

    private void handleBindApplication(AppBindData data) {
        // 将UI线程注册为运行时的虚拟机.
        VMRuntime.registerSensitiveThread();
        if (data.trackAllocation) {
            DdmVmInternal.enableRecentAllocations(true);
        }
        // ...
        // 创建上下文对象
        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类
            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 {
                //调用Applcation的OnCreate函数
                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();
            }
        }
    }

• 注释1：创建Instrumentation，负责跟踪Application还在Activity的生命周期；
• 注释2：创建Application对象 & 调用其attach()；
• 注释3：调用Instrumentation的onCreate()，内部是空实现；
• 注释4：调用Instrumentation的callApplicationOnCreate()，内部是调用application的onCreate，如下代码；

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

4.类关系

Application启动类结构.png

• ActivityThread ：通过IActivityManager类，通知AMS准备application启动所需进程数据 ；
• ActivityManagerService ：获取application启动所需进程数据 ；
• Instrumentation ：创建&启动Application；跟踪Application的生命周期；

5.总结

• 到此，Application启动过程介绍完毕。
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