每一个App在启动时，都会由Zygote fork出一个进程，进程具有独立的资源空间，用于承载App上运行的各种Activity/Service等组件。

当进程创建后就要真正去启动一个App了，那么App的入口函数是什么呢？答案是ActivityThread的main函数。（进程创建后反射调用ActivityThread main函数）

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

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public final class ActivityThread {}

可能有人会认为ActivityThread就是我们常说的主线程或UI线程，但是上面可以看到ActivityThread类是一个final类，也没有继承Thread。因为ActivityThread的Main函数是App的入口函数，那么调用它的线程就是UI线程。

OK，接下来分析一下ActivityThread具体做了些什么

main()函数

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public static void main(String[] args) {
  
        ......
        //1.创建主线程looper,主线程初始化
        Looper.prepareMainLooper();

        //2.主线程Handler初始化
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }
  
        Looper.loop();
    }

1.主线程初始化

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

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//主线程Looper的初始化
public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

//普通线程初始化
public static void prepare() {
        prepare(true);
    }

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

从上面的代码中可以看出，主线程初始化时，prepare传入false，而普通线程初始化时传入true，这个参数为quitAllowed，表示线程是否可以退出，主线程无法退出。prepare函数中创建一个Looper对象，并将对象保存在ThreadLocal中。

在prepare之后，又将主线程Looper赋值给了成员变量sMainLooper，这个成员变量的作用是向其他线程提供主线程Looper对象。

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public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

这样当我们调用Looper.getMainLooper()时可以获取到主线程的Looper对象了。

2.主线程Handler初始化

在注释2处创建了ActivityThread对象，并获取了主线程的Handler。(attach稍等)

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final H mH = new H();
final Handler getHandler() {
     return mH;
}

由此可见主线程的Handler作为ActivityThread的成员变量，是在ActivityThread的main方法被执行，ActivityThread被创建时而初始化。

3.ActivityThread.attch()

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

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final ApplicationThread mAppThread = new ApplicationThread();

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

ApplicationThread是ActivityThread内部类，继承自IApplicationThread.Stub，作为服务端接受AMS发出的请求并执行，ApplicationThread是ActivityThread与AMS连接的桥梁。

attch方法实际调用了AMS的attachApplication方法，去看下AMS里的实现

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

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

AMS中的attachApplicationLocked方法有些复杂，这个流程可以理解为ActivityThread创建时（App启动时）需要向AMS注册自己，用于AMS管理ActivityThread中的所有四大组件的生命周期。

我们看下attachApplicationLocked中的关键部分

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

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private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {
    ...
    //1.创建Application
  thread.bindApplication(...);
  ...
  //2.创建Activity
    if (mStackSupervisor.attachApplicationLocked(app)) {
        ...
    }
}

3.1 thread.bindApplication

AMS通过远程调用，最终又会调用到ActivityThread中ApplicationThread内部类方法。

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

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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<String, IBinder> services, Bundle coreSettings) {

            if (services != null) {
                // Setup the service cache in the ServiceManager
                ServiceManager.initServiceCache(services);
            }

            setCoreSettings(coreSettings);

            AppBindData data = new AppBindData();
            data.processName = processName;
            data.appInfo = appInfo;
            data.providers = providers;
            data.instrumentationName = instrumentationName;
            data.instrumentationArgs = instrumentationArgs;
            data.instrumentationWatcher = instrumentationWatcher;
            data.instrumentationUiAutomationConnection = instrumentationUiConnection;
            data.debugMode = debugMode;
            data.enableBinderTracking = enableBinderTracking;
            data.trackAllocation = trackAllocation;
            data.restrictedBackupMode = isRestrictedBackupMode;
            data.persistent = persistent;
            data.config = config;
            data.compatInfo = compatInfo;
            data.initProfilerInfo = profilerInfo;
            sendMessage(H.BIND_APPLICATION, data);
        }

在方法中将AMS传递回的数据又发送给主线程中

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

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case BIND_APPLICATION:
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
    AppBindData data = (AppBindData)msg.obj;
    handleBindApplication(data);
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    break;

在主线程中调用handleBindApplication处理

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

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private void handleBindApplication(AppBindData data) {
        ......

        // send up app name; do this *before* waiting for debugger
        //设置进程名, 也就是说进程名是在进程真正创建以后的BIND_APPLICATION过程中才取名
        Process.setArgV0(data.processName);
        android.ddm.DdmHandleAppName.setAppName(data.processName,
                                                UserHandle.myUserId());
        //重置时区
        TimeZone.setDefault(null);
        LocaleList.setDefault(data.config.getLocales());
  
        //获取LoadedApk对象
        // (public final LoadedApk getPackageInfoNoCheck())
        data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);
  			......
          
        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);
            mInitialApplication = app;

            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);
                }
            }

          
            mInstrumentation.onCreate(data.instrumentationArgs);
            //调用Application.onCreate()回调方法
            mInstrumentation.callApplicationOnCreate(app);
        } finally {
            StrictMode.setThreadPolicy(savedPolicy);
        }
    }

handleBindApplication中调用LoadedApk类中makeApplication方法

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

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    public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
        // 表示Application是个单例
        if (mApplication != null) {
            return mApplication;
        }

        Application app = null;

        // 通过反射初始化Application
        String appClass = mApplicationInfo.className;
        if (forceDefaultAppClass || (appClass == null)) {
            appClass = "android.app.Application";
        }

        try {
            java.lang.ClassLoader cl = getClassLoader();
            if (!mPackageName.equals("android")) {
                Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
                        "initializeJavaContextClassLoader");
                initializeJavaContextClassLoader();
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            }
            ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
            appContext.setOuterContext(app);
        } catch (Exception e) {
            if (!mActivityThread.mInstrumentation.onException(app, e)) {
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                throw new RuntimeException(
                    "Unable to instantiate application " + appClass
                    + ": " + e.toString(), e);
            }
        }
        mActivityThread.mAllApplications.add(app);
        mApplication = app;
   ......
       return app;
}

到这里应用的Application 就创建出来了，创建Application后再调用callApplicationOnCreate，回调Application的onCreate方法。

3.2 mStackSupervisor.attachApplicationLocked(app)

mStackSupervisor是AMS的成员变量，是Activity堆栈管理辅助类实例

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

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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) {
                            Slog.w(TAG, "Exception in new application when starting activity "
                                  + hr.intent.getComponent().flattenToShortString(), e);
                            throw e;
                        }
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
        }
        return didSomething;
    }

attachApplicationLocked中获取App要启动的top Activity，然后realStartActivityLocked去启动。

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final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
      
      ......
      app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                    System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
                    new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,
                    task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
                    newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);
     ......       
            
}

调用ApplicationThread的scheduleLaunchActivity方法

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

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@Override
    public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
            ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
            CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
            int procState, Bundle state, PersistableBundle persistentState,
            List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
            boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) 
		.......
        sendMessage(H.LAUNCH_ACTIVITY, r);
    }
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case LAUNCH_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
            final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

            r.packageInfo = getPackageInfoNoCheck(
            r.activityInfo.applicationInfo, r.compatInfo);
            handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

最终调用handleLaunchActivity，最终启动一个Activity

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

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private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    
        //通过反射去创建一个Activity，然后会调用Activity的各个生命周期方法
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            Bundle oldState = r.state;
            // onResume
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

            if (!r.activity.mFinished && r.startsNotResumed) {
               
                performPauseActivityIfNeeded(r, reason);

                if (r.isPreHoneycomb()) {
                    r.state = oldState;
                }
            }
        } else {
            // If there was an error, for any reason, tell the activity manager to stop us.
            try {
                ActivityManagerNative.getDefault()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

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

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private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {

        // 从ActivityClientRecord中获取待启动的Activity的组件信息
        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);
        }

        Activity activity = null;
        try {
            //反射创建Activity
            java.lang.ClassLoader cl = r.packageInfo.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) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
            //再次调用Application的创建方法，Application是个单例。但是如果这个Activity在另一个进程中，就需要再次创建Application
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);

            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                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);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }

                activity.mCalled = false;
                // 调用Activity的onCreate方法回调，终于看到熟悉的Activity onCreate啦
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    // onStart回调
                    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) {
                        throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                            " did not call through to super.onPostCreate()");
                    }
                }
            }
            r.paused = true;

            mActivities.put(r.token, r);

        } catch (SuperNotCalledException e) {
            throw e;

        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to start activity " + component
                    + ": " + e.toString(), e);
            }
        }

        return activity;
    }

后面就交给App里面处理逻辑了。

使用gityuan一张图总结一下