Activity启动流程源码解析（原创）

1、Activity启动流程

原帖地址
我的GitHub

Intent intent=new Intent(this,activity.class);
startActivity(intent);

1. 通过以上代码将会启动一个Activity，startActivity方法会有好几种重载方式，但是他们最终都会调用startActivityForResult方法，实现如下：

    public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
        if (mParent == null) {
            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());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

我们只需关注mParent==null这部分，mParent代表的是ActivityGroup，一开始被用来嵌入多个Activity，但是API13已经被废弃了，系统目前采用的都是Fragment代替ActivityGroup。然后调用mInstrumentation.execStartActivity方法，方法的参数中的mMainThread.getApplicationThread()类型是ApplicationThread,ApplicationThread是ActivityThread的一个内部类。
2. mInstrumentation.execStartActivity:

    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; ifinal ActivityMonitor am = mActivityMonitors.get(i);
                    if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess();
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
        }
        return null;

可以看到，启动Activity的真正实现是由ActivityManagerNative.getDefault()
.startActivity实现的，AMS继承自ActivityManagerNative，ActivityManagerNative.getDefault()是一个AMS，采用单例模式提供对象，第一次调用会通过creat方法初始化AMS，然后后续调用直接返回即可。
3. AMS的startActivity方法

    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo,
            String resultWho, int requestCode, int startFlags,
            String profileFile, ParcelFileDescriptor profileFd, Bundle options) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode,
                startFlags, profileFile, profileFd, options, UserHandle.getCallingUserId());
    }

1. startActivity转移到了startActivityAsUser中

    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo,
            String resultWho, int requestCode, int startFlags,
            String profileFile, ParcelFileDescriptor profileFd, Bundle options, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
                false, true, "startActivity", null);
        return mMainStack.startActivityMayWait(caller, -1, callingPackage, intent, resolvedType,
                resultTo, resultWho, requestCode, startFlags, profileFile, profileFd,
                null, null, options, userId);
    }

1. 转移到了ActivityStack的mMainStack.startActivityMayWait中，

 int res = startActivityLocked(caller, intent, resolvedType,
                    aInfo, resultTo, resultWho, requestCode, callingPid, callingUid,
                    callingPackage, startFlags, options, componentSpecified, null);

1. mMainStack.startActivityMayWait方法中调用了startActivityLocked方法。

    err = startActivityUncheckedLocked(r, sourceRecord,
                startFlags, true, options);

1. startActivityLocked方法调用startActivityUncheckedLocked方法

resumeTopActivityLocked(null, options);

1. 接着调用resumeTopActivityLocked

startSpecificActivityLocked(next, true, false);

1. 接着调用startSpecificActivityLocked

        if (app != null && app.thread != null) {
            try {
                app.addPackage(r.info.packageName);
                realStartActivityLocked(r, app, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }

            // If a dead object exception was thrown -- fall through to
            // restart the application.
        }

1. 接着调用realStartActivityLocked方法，

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                    System.identityHashCode(r), r.info,
                    new Configuration(mService.mConfiguration),
                    r.compat, r.icicle, results, newIntents, !andResume,
                    mService.isNextTransitionForward(), profileFile, profileFd,
                    profileAutoStop);

1. realStartActivityLocked中有一句：app.thread.scheduleLaunchActivity，app.thread类型为IApplicationThread，IApplicationThread的实现类为ApplicationThread，即调用的是ApplicationThread的scheduleLaunchActivity：

        public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
                ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
                Bundle state, List pendingResults,
                List pendingNewIntents, boolean notResumed, boolean isForward,
                String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {
            ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;

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

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

            r.profileFile = profileName;
            r.profileFd = profileFd;
            r.autoStopProfiler = autoStopProfiler;

            updatePendingConfiguration(curConfig);

            queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
        }

1. scheduleLaunchActivity发送一个H.LAUNCH_ACTIVITY的消息来交给Handler处理，Handler H的代码如下：

public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case RELAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;
                    handleRelaunchActivity(r);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case PAUSE_ACTIVITY_FINISHING:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    handlePauseActivity((IBinder)msg.obj, true, msg.arg1 != 0, msg.arg2);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_ACTIVITY_SHOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    handleStopActivity((IBinder)msg.obj, true, msg.arg2);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_ACTIVITY_HIDE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    handleStopActivity((IBinder)msg.obj, false, msg.arg2);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SHOW_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityShowWindow");
                    handleWindowVisibility((IBinder)msg.obj, true);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case HIDE_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityHideWindow");
                    handleWindowVisibility((IBinder)msg.obj, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case RESUME_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
                    handleResumeActivity((IBinder)msg.obj, true,
                            msg.arg1 != 0, true);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SEND_RESULT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDeliverResult");
                    handleSendResult((ResultData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case DESTROY_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDestroy");
                    handleDestroyActivity((IBinder)msg.obj, msg.arg1 != 0,
                            msg.arg2, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                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;
                case EXIT_APPLICATION:
                    if (mInitialApplication != null) {
                        mInitialApplication.onTerminate();
                    }
                    Looper.myLooper().quit();
                    break;
                case NEW_INTENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityNewIntent");
                    handleNewIntent((NewIntentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case RECEIVER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
                    handleReceiver((ReceiverData)msg.obj);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate");
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
                    handleBindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNBIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind");
                    handleUnbindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SERVICE_ARGS:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStart");
                    handleServiceArgs((ServiceArgsData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
                    handleStopService((IBinder)msg.obj);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case REQUEST_THUMBNAIL:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "requestThumbnail");
                    handleRequestThumbnail((IBinder)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CONFIGURATION_CHANGED:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "configChanged");
                    mCurDefaultDisplayDpi = ((Configuration)msg.obj).densityDpi;
                    handleConfigurationChanged((Configuration)msg.obj, null);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CLEAN_UP_CONTEXT:
                    ContextCleanupInfo cci = (ContextCleanupInfo)msg.obj;
                    cci.context.performFinalCleanup(cci.who, cci.what);
                    break;
                case GC_WHEN_IDLE:
                    scheduleGcIdler();
                    break;
                case DUMP_SERVICE:
                    handleDumpService((DumpComponentInfo)msg.obj);
                    break;
                case LOW_MEMORY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "lowMemory");
                    handleLowMemory();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case ACTIVITY_CONFIGURATION_CHANGED:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityConfigChanged");
                    handleActivityConfigurationChanged((IBinder)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case PROFILER_CONTROL:
                    handleProfilerControl(msg.arg1 != 0, (ProfilerControlData)msg.obj, msg.arg2);
                    break;
                case CREATE_BACKUP_AGENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupCreateAgent");
                    handleCreateBackupAgent((CreateBackupAgentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case DESTROY_BACKUP_AGENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupDestroyAgent");
                    handleDestroyBackupAgent((CreateBackupAgentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SUICIDE:
                    Process.killProcess(Process.myPid());
                    break;
                case REMOVE_PROVIDER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "providerRemove");
                    completeRemoveProvider((ProviderRefCount)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case ENABLE_JIT:
                    ensureJitEnabled();
                    break;
                case DISPATCH_PACKAGE_BROADCAST:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastPackage");
                    handleDispatchPackageBroadcast(msg.arg1, (String[])msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SCHEDULE_CRASH:
                    throw new RemoteServiceException((String)msg.obj);
                case DUMP_HEAP:
                    handleDumpHeap(msg.arg1 != 0, (DumpHeapData)msg.obj);
                    break;
                case DUMP_ACTIVITY:
                    handleDumpActivity((DumpComponentInfo)msg.obj);
                    break;
                case DUMP_PROVIDER:
                    handleDumpProvider((DumpComponentInfo)msg.obj);
                    break;
                case SLEEPING:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "sleeping");
                    handleSleeping((IBinder)msg.obj, msg.arg1 != 0);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SET_CORE_SETTINGS:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "setCoreSettings");
                    handleSetCoreSettings((Bundle) msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UPDATE_PACKAGE_COMPATIBILITY_INFO:
                    handleUpdatePackageCompatibilityInfo((UpdateCompatibilityData)msg.obj);
                    break;
                case TRIM_MEMORY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "trimMemory");
                    handleTrimMemory(msg.arg1);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNSTABLE_PROVIDER_DIED:
                    handleUnstableProviderDied((IBinder)msg.obj, false);
                    break;
                case REQUEST_ACTIVITY_EXTRAS:
                    handleRequestActivityExtras((RequestActivityExtras)msg.obj);
                    break;
            }
            if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
        }

1. 调用ActivityThread的handleLaunchActivity(r, null)方法

Activity a = performLaunchActivity(r, customIntent);

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

1. performLaunchActivity完成Activity对象的创建，handleResumeActivity完成Activity的onResume一系列方法。

2、performLaunchActivity的流程

1. 从ActiivtyClientRecord中获取Activity的组建信息

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        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);
        }

1. 调用mInstrumentation.newActivity创建Activity

activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);

    public Activity newActivity(ClassLoader cl, String className,
            Intent intent)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        return (Activity)cl.loadClass(className).newInstance();
    }

通过类加载器来创建Activity
3. 通过LoadedApk的makeApplication方法创建Application

    public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
        if (mApplication != null) {
            return mApplication;
        }

        Application app = null;

        String appClass = mApplicationInfo.className;
        if (forceDefaultAppClass || (appClass == null)) {
            appClass = "android.app.Application";
        }

        try {
            java.lang.ClassLoader cl = getClassLoader();
            ContextImpl appContext = new ContextImpl();
            appContext.init(this, null, mActivityThread);
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
            appContext.setOuterContext(app);
        } catch (Exception e) {
            if (!mActivityThread.mInstrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to instantiate application " + appClass
                    + ": " + e.toString(), e);
            }
        }
        mActivityThread.mAllApplications.add(app);
        mApplication = app;

        if (instrumentation != null) {
            try {
                instrumentation.callApplicationOnCreate(app);
            } catch (Exception e) {
                if (!instrumentation.onException(app, e)) {
                    throw new RuntimeException(
                        "Unable to create application " + app.getClass().getName()
                        + ": " + e.toString(), e);
                }
            }
        }

        return app;

里面有以下代码

app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);

    public Application newApplication(ClassLoader cl, String className, Context context)
            throws InstantiationException, IllegalAccessException, 
            ClassNotFoundException {
        return newApplication(cl.loadClass(className), context);
    }

可以看到也是通过类加载器来加载Application的，然后调用zai makeApplication方法中调用Application的onCreate方法了

instrumentation.callApplicationOnCreate(app);

1. 调用Activity的onCreate

mInstrumentation.callActivityOnCreate(activity, r.state);

完成！
原帖地址
我的GitHub