Activity启动流程浅析

在api26中，ActivityManagerNative类已经被弃用，同时代理类ActivityManagerProxy也被删除了。

Instrumentation类的execStartActivity方法如下,注意注释1处。

public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, String resultWho,
            Intent intent, int requestCode, Bundle options, UserHandle user) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        result = am.onStartActivity(intent);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            int result = ActivityManager.getService() //1
                .startActivityAsUser(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, resultWho,
                        requestCode, 0, null, options, user.getIdentifier());
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
}

api 25中Instrumentation类的的execStartActivity方法如下，注意注释2处。

public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final 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(who);
            int result = ActivityManagerNative.getDefault()//2
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
}

我们看到，在注释1处和注释2处代码不一样，接下来继续看不同部分的代码，api26：

public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

api25:

static public IActivityManager getDefault() {
        return gDefault.get();
}

private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
            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;
        }
};

这两个地方的代码返回的都是IActivityManager对象。但是不同的是：api25中使用的是静态方法asInterface，该方法属于ActivityManagerNative。asInterface的源码如下：

static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }

        return new ActivityManagerProxy(obj);
}

参考文章：Activity启动过程详解 说明了ActivityManagerNative类，还有ActivityManagerProxy类(已被移除)，是如何实现和AMS进程通信的，最终使用了ActivityManagerProxy代理类实现了startActivity。
参考文章：ActivityManagerProxy已经被删除

api26使用的是：

IActivityManager.Stub.asInterface(b);

再看api26中的ActivityManagerNative源码：

/**
 * {@hide}
 * @deprecated will be removed soon. See individual methods for alternatives. //将会被移除
 */
@Deprecated
public abstract class ActivityManagerNative {
    /**
     * Cast a Binder object into an activity manager interface, generating
     * a proxy if needed.
     *
     * @deprecated use IActivityManager.Stub.asInterface instead.
     */ //这里说可以用IActivityManager.Stub.asInterface来代替，这个方法已不被鼓励使用
    static public IActivityManager asInterface(IBinder obj) {
        return IActivityManager.Stub.asInterface(obj);
    }

    /**
     * Retrieve the system's default/global activity manager.
     *
     * @deprecated use ActivityManager.getService instead.
     */
    static public IActivityManager getDefault() {
        return ActivityManager.getService();
    }

    /**
     * Convenience for checking whether the system is ready.  For internal use only.
     *
     * @deprecated use ActivityManagerInternal.isSystemReady instead.
     */
    static public boolean isSystemReady() {
        return ActivityManager.isSystemReady();
    }

    /**
     * @deprecated use ActivityManager.broadcastStickyIntent instead.
     */
    static public void broadcastStickyIntent(Intent intent, String permission, int userId) {
        broadcastStickyIntent(intent, permission, AppOpsManager.OP_NONE, userId);
    }

    /**
     * Convenience for sending a sticky broadcast.  For internal use only.
     * If you don't care about permission, use null.
     *
     * @deprecated use ActivityManager.broadcastStickyIntent instead.
     */
    static public void broadcastStickyIntent(Intent intent, String permission, int appOp,
            int userId) {
        ActivityManager.broadcastStickyIntent(intent, appOp, userId);
    }

    /**
     * @deprecated use ActivityManager.noteWakeupAlarm instead.
     */
    static public void noteWakeupAlarm(PendingIntent ps, int sourceUid, String sourcePkg,
            String tag) {
        ActivityManager.noteWakeupAlarm(ps, sourceUid, sourcePkg, tag);
    }

    /**
     * @deprecated use ActivityManager.noteAlarmStart instead.
     */
    static public void noteAlarmStart(PendingIntent ps, int sourceUid, String tag) {
        ActivityManager.noteAlarmStart(ps, sourceUid, tag);
    }

    /**
     * @deprecated use ActivityManager.noteAlarmFinish instead.
     */
    static public void noteAlarmFinish(PendingIntent ps, int sourceUid, String tag) {
        ActivityManager.noteAlarmFinish(ps, sourceUid, tag);
    }
}

“@deprecated will be removed soon. See individual methods for alternatives. “上面注释说明这个类将会被弃用，我们可以用ActivityManager来替代，而且ActivityManagerProxy已经被删除了。

api26及以后启动activity时候，和AMS通信已经采用了AIDL方式。实际上api25和api26及之后，启动Activity启动Activity的时候，和AMS通信调用的也是queryLocalInterface，他们的实质还是一样的。

api26及之后只有ServiceManagerNative还没有被弃用，api25的：ApplicationThreadNative、ActivityManagerNative、BulkCursorNative、ContentProviderNative都已经不再使用了。

这两句代码：mInstrumentation.execStartActivity，mMainThread.sendActivityResult。是activity启动过程中很重要的核心逻辑。

在ActivityThread中一个Activity被启动时相关方法是这样的：schedule–>handle–>perform–>activity/Insturmation。