这里分析的是app第一次启动时的第一个Activity的onCreate()方法的启动流程。

图

先贴一下大致的流程图吧，不会画，很丑

ActivityThread.main()

APP的启动入口应该都知道是ActivityThread的main方法了，所以整个APP的启动就从这里开始，直到我们看到首页。代码不多：

package android.app;
public final class ActivityThread {
    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"));
        }
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }
}

其实通过这个代码我们可以发现一个有趣的现象,APP可以一直运行不会退出就因为这个main方法最后的Looper.loop()是个死循环。

当一个APP启动的时候，系统会从Zygone线程中为应用fork一个新的线程，然后在线程中执行ActivityThread的main方法。
main方法中主要流程就是：

• 准备一个主线程的Looper
• 新建一个ActivityThread，并执行他的attach方法
• 开启Looper的loop方法开始无限循环的读取消息队列的消息 并处理。

ActivityThread.attach(false)

下面看一下ActivityThread.attach()，在main()中传入的参数是FALSE，说明这个APP不是系统APP。所以我们主要就看不是系统APP的处理。

private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
//不是系统APP
            ViewRootImpl.addFirstDrawHandler(new Runnable() {
                @Override
                public void run() {
//确定虚拟机正常
                    ensureJitEnabled();
                }
            });
          //.....略
//获取一个ActivityManagerService  AMS
            final IActivityManager mgr = ActivityManagerNative.getDefault();
            try {
//mAppThread = new ApplicationThread(),绑定到AMS上
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
           //......略
        } else {
           //.....略
        }

ActivityManagerNative.getDefault()就是获取一个AMS,从下面的代码可以看到用的是单例模式从SystemManager中获取到。

package android.app;
public abstract class ActivityManagerNative extends Binder implements IActivityManager{
    static public IActivityManager getDefault() {
        return gDefault.get();
    }
    private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
//获取AMS
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };
}

(AMS)mgr.attachApplication()

package com.android.server.am;
public final class ActivityManagerService extends ActivityManagerNative
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
     @Override
    public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
//执行AMS的这个方法
            attachApplicationLocked(thread, callingPid);
            Binder.restoreCallingIdentity(origId);
        }
    }
    
    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid) {
        //......略
    
  
        try {
                      ProfilerInfo profilerInfo = profileFile == null ? null
                    : new ProfilerInfo(profileFile, profileFd, samplingInterval, profileAutoStop);
//执行了ApplicationThread的bindApplication方法
            thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
                    profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
                    app.instrumentationUiAutomationConnection, testMode,
                    mBinderTransactionTrackingEnabled, enableTrackAllocation,
                    isRestrictedBackupMode || !normalMode, app.persistent,
                    new Configuration(mConfiguration), app.compat,
                    getCommonServicesLocked(app.isolated),
                    mCoreSettingsObserver.getCoreSettingsLocked());
            //.....
        } catch (Exception e) {
           //......
        } 
        // See if the top visible activity is waiting to run in this process...
        if (normalMode) {
            try {
                if (mStackSupervisor.attachApplicationLocked(app)) {
                    didSomething = true;
                }
            } catch (Exception e) {
      
            }
        }
        return true;
    }
}

这里 主要看两个方法，thread.bindApplication(......)方法和mStackSupervisor.attachApplicationLocked(app)。
ApplicationThread是ActivityThread的内部类，里面主要还是把这个ApplicationThread绑定到AMS中。
mStackSupervisor是一个ActivityStackSupervisor类，在AMS创建的时候初始化。

mStackSupervisor.attachApplicationLocked(app)

下面看他的方法

package com.android.server.am;
public final class ActivityStackSupervisor implements DisplayListener {
    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 {
//看这个方法就知道是真正启动一个Activity
                            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;
    }

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

//这个传进来是true,所以一定会进去
        if (andResume) {
//冻结尚未启动的其他APP
            r.startFreezingScreenLocked(app, 0);
            mWindowManager.setAppVisibility(r.appToken, true);
            // 搜集启动比较慢的APP的信息
            r.startLaunchTickingLocked();
        }


        final ActivityStack stack = task.stack;
        try {
            if (app.thread == null) {
                throw new RemoteException();
            }
            List<ResultInfo> results = null;
            List<ReferrerIntent> newIntents = null;
            if (andResume) {
                results = r.results;
                newIntents = r.newIntents;
            }
           //...

            if (r.isHomeActivity()){
//如果是桌面的activity，就加到栈底
                mService.mHomeProcess = task.mActivities.get(0).app;
            }
           
        //。。。。
            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);
        } catch (RemoteException e) {          
            }
        }

        return true;
    }
}

上面的方法里一大堆代码，主要就是检查准备要启动的Activity的信息，信息正常的话就执行 app.thread.scheduleLaunchActivity(.....)方法。
然后就又回到了ApplicationThread：

ApplicationThread.scheduleLaunchActivity(.....)

package android.app;
public final class ActivityThread {
    private class ApplicationThread extends ApplicationThreadNative {
        @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) {
            updateProcessState(procState, false);
            ActivityClientRecord r = new ActivityClientRecord();
            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.referrer = referrer;
            r.voiceInteractor = voiceInteractor;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;
            r.persistentState = persistentState;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profilerInfo = profilerInfo;

            r.overrideConfig = overrideConfig;
            updatePendingConfiguration(curConfig);

            sendMessage(H.LAUNCH_ACTIVITY, r);
        } 
   }
}

里面创建了一个ActivityClientRecord来保存activity的各种信息，然后放在消息里发送给H，消息的标识是H.LAUNCH_ACTIVITY，消息的内容就是这个ActivityClientRecord。
H也是ActivityThread的一个内部类：

ActivityThread.H

private class H extends Handler {
    public static final int LAUNCH_ACTIVITY         = 100;
    public void handleMessage(Message msg) {
        switch (msg.what) {
                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);
                } break;
              }
    }
}

这个handler接收到LAUNCH_ACTIVITY的信息后，会调用handleLaunchActivity(...)方法

ActivityThread.handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");

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

ActivityThread.performLaunchActivity(r, customIntent)

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
   //获取activity的信息
        ActivityInfo aInfo = r.activityInfo;
        //...对信息进行一些判断 略
        Activity activity = null;
        try {
//通过反射获取到activity类
            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)) {
//如果没在清单里注册就会出现这个错误，应该都遇见过吧。。
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
//获取application
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            if (activity != null) {
//生成activity的Context
                Context appContext = createBaseContextForActivity(r, activity);
               //....略
//将activity绑定到application
                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);
             //...略
//调用callActivityOnCreate方法
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }       
            }
        } catch (SuperNotCalledException e) {         
        }
        return activity;
    }

继续

Instrumentation.callActivityOnCreate

package android.app;
public class Instrumentation {
    public void callActivityOnCreate(Activity activity, Bundle icicle) {
        prePerformCreate(activity);
        activity.performCreate(icicle);
        postPerformCreate(activity);
    }
}

是不是终于看到了activity的方法，马上就到了

activity.performCreate(icicle);

package android.app;
public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback {
    
    final void performCreate(Bundle icicle) {
        restoreHasCurrentPermissionRequest(icicle);
        onCreate(icicle);
        mActivityTransitionState.readState(icicle);
        performCreateCommon();
    }
}

看到没，执行到了onCreate方法，还有一些生命周期的方法也是这样的逻辑，包括service和广播。