Activity组件的主要作用是展示一个界面并和用户交互，它扮演的是一种前台界面的角色，在显示调用的情形下，只需要通过如下代码即可完成。
Intent intent=new Intent(this,TestActivity.class);
startActivity(intent);
通过上面代码即可启动一个具体的Activity，然后这个Activity就会被系统启动并展示在用户的眼前，但是系统内部是如何启动一个Activity的呢？比如新Activity的对象是何时创建的，onCreate方法又是在何时回调的等等，本文将针对这些问题对Activity的整体流程进行讲解。
我们从Activity的startActivity方法开始分析，startActivity的方法有好几种重载方式，它们最终会调用startActivityForResult，它的实现如下所示。
public void startActivityForResult(Intent intent, int requestCode, @Nullable 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());
}
...ignore some code...
} else {
if (options != null) {
//当现在的Activity有父Activity的时候会调用，但是在startActivityFromChild()内部实际还是调用的mInstrumentation.execStartActivity()
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
mParent.startActivityFromChild(this, intent, requestCode);
}
}
...ignore some code...
}
上面代码我们只关注mParent=null这部分逻辑就行，需注意mMainThread.getApplicationThread()这个参数，它的类型是ApplicationThread，ApplicationThread是ActivityThread的一个内部类，ApplicationThread和ActivityThread在Activity的启动过程中发挥着很重要的作用。接着看Instrumentation的execStartActivity方法，代码如下。
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
...ignore some code...
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, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
}
return null;
}
从上面代码可以看出，启动Activity真正的实现由ActivityManagerNative.getDefault()的startActivity方法来完成，
这里的ActivityManagerNative.getDefault返回的就是ActivityManagerService的远程接口，即ActivityManagerProxy。
public abstract class ActivityManagerNative extends Binder implements IActivityManager
{
//从类声明上，我们可以看到ActivityManagerNative是Binder的一个子类，而且实现了IActivityManager接口
static public IActivityManager getDefault() {
return gDefault.get();
}
//通过单例模式获取一个IActivityManager对象，这个对象通过asInterface(b)获得
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;
}
};
}
//最终返回的还是一个ActivityManagerProxy对象
static public IActivityManager asInterface(IBinder obj) {
if (obj == null) {
return null;
}
IActivityManager in =
(IActivityManager)obj.queryLocalInterface(descriptor);
if (in != null) {
return in;
}
//这里面的Binder类型的obj参数会作为ActivityManagerProxy的成员变量保存为mRemote成员变量，负责进行IPC通信
return new ActivityManagerProxy(obj);
}
}
再看ActivityManagerProxy.startActivity()，在这里面做的事情就是IPC通信，利用Binder对象，调用transact()，把所有需要的参数封装成Parcel对象，向AMS发送数据进行通信。
public int startActivity(IApplicationThread caller, String callingPackage, Intent intent,
String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
data.writeString(callingPackage);
intent.writeToParcel(data, 0);
data.writeString(resolvedType);
data.writeStrongBinder(resultTo);
data.writeString(resultWho);
data.writeInt(requestCode);
data.writeInt(startFlags);
if (profilerInfo != null) {
data.writeInt(1);
profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
} else {
data.writeInt(0);
}
if (options != null) {
data.writeInt(1);
options.writeToParcel(data, 0);
} else {
data.writeInt(0);
}
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
reply.readException();
int result = reply.readInt();
reply.recycle();
data.recycle();
return result;
}
ActivityManagerProxy只是通过这种方式对要传输给服务器的数据进行打包，真正实现的是ActivityManagerService。
但是这个地方并不是直接由客户端传递给服务器，而是通过Binder驱动进行中转。Binder驱动原理暂时忽略，我们把他当做一个中转站就OK，客户端调用ActivityManagerProxy接口里面的方法，把数据传送给Binder驱动，然后Binder驱动就会把这些东西转发给服务器的ActivityManagerServices，由ActivityManagerServices去真正的实施具体的操作。
客户端：ActivityManagerProxy =====>Binder驱动=====> ActivityManagerService：服务端
从上面的分析看，，ActivityManagerNative.getDefault()实际上是AMS，因此Activity的启动过程转移到了AMS中，接着我们分析AMS的startActivity方法。
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, options,
UserHandle.getCallingUserId());
}
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) {
...ignore some code...
return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, options, userId, null, null);
}
可以看出，Activity的启动过程又转移到了ActivityStackSupervisor的startActivityMayWait方法中了，在startActivityMayWait中又调用了startActivityLocked方法，然后startActivityLocked方法又调用了startActivityUncheckedLocked()，此时要启动的Activity已经通过检验，被认为是一个正当的启动请求。终于，在这里调用到了ActivityStack的startActivityLocked(ActivityRecord r, boolean newTask,boolean doResume, boolean keepCurTransition, Bundle options)。
ActivityRecord代表的就是要开启的Activity对象，里面分装了很多信息，比如所在的ActivityTask等，如果这是首次打开应用，那么这个Activity会被放到ActivityTask的栈顶。
ActivityStack.startActivityLocked()方法如下：
final void startActivityLocked(ActivityRecord r, boolean newTask,
boolean doResume, boolean keepCurTransition, Bundle options) {
//ActivityRecord中存储的TaskRecord信息
TaskRecord rTask = r.task;
...ignore some code...
//如果不是在新的ActivityTask(也就是TaskRecord)中的话，就找出要运行在的TaskRecord对象
TaskRecord task = null;
if (!newTask) {
boolean startIt = true;
for (int taskNdx = mTaskHistory.size() - 1; taskNdx >= 0; --taskNdx) {
task = mTaskHistory.get(taskNdx);
if (task.getTopActivity() == null) {
// task中的所有Activity都结束了
continue;
}
if (task == r.task) {
// 找到了
if (!startIt) {
task.addActivityToTop(r);
r.putInHistory();
mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken,
r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,
(r.info.flags & ActivityInfo.FLAG_SHOW_ON_LOCK_SCREEN) != 0,
r.userId, r.info.configChanges, task.voiceSession != null,
r.mLaunchTaskBehind);
if (VALIDATE_TOKENS) {
validateAppTokensLocked();
}
ActivityOptions.abort(options);
return;
}
break;
} else if (task.numFullscreen > 0) {
startIt = false;
}
}
}
...ignore some code...
// Place a new activity at top of stack, so it is next to interact
// with the user.
task = r.task;
task.addActivityToTop(r);
task.setFrontOfTask();
...ignore some code...
if (doResume) {
mStackSupervisor.resumeTopActivitiesLocked(this, r, options);
}
}
从ActivityStackSupervisor到ActivityStack，又调回ActivityStackSupervisor, 看下StackSupervisor.resumeTopActivitiesLocked(this, r, options)
boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
Bundle targetOptions) {
if (targetStack == null) {
targetStack = getFocusedStack();
}
// Do targetStack first.
boolean result = false;
if (isFrontStack(targetStack)) {
result = targetStack.resumeTopActivityLocked(target, targetOptions);
}
...ignore some code...
return result;
}
又调回ActivityStack的resumeTopActivityLocked()方法
final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
if (inResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
// Protect against recursion.
inResumeTopActivity = true;
result = resumeTopActivityInnerLocked(prev, options);
} finally {
inResumeTopActivity = false;
}
return result;
}
然后调用ActivityStack的resumeTopActivityInnerLocked(prev, options)方法，resumeTopActivityInnerLocked方法又调用了ActivityStackSupervisor的startSpecificActivityLocked方法，源码如下：
startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
r.task.stack.setLaunchTime(r);
if (app != null && app.thread != null) {
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {
// Don't add this if it is a platform component that is marked
// to run in multiple processes, because this is actually
// part of the framework so doesn't make sense to track as a
// separate apk in the process.
app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
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.
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
从上面代码可以看出startSpecificActivityLocked 方法调用了realStartActivityLocked，为了更清晰地说明Activity的启动过程在ActivityStackSupervisor和ActivityStack之间传递顺序，下面给出一张图。
在ActivityStackSupervisor的realStartActivityLocked方法中有如下一段代码：
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
r.compat, r.launchedFromPackage, r.task.voiceInteractor, app.repProcState,
r.icicle, r.persistentState, results, newIntents, !andResume,
mService.isNextTransitionForward(), profilerInfo);
app.thread. scheduleLaunchActivity,实际是调用ApplicationThreadProxy.scheduleLaunchActivity()方法，ApplicationThreadProxy是ApplicationThreadNative的内部类
客户端：ApplicationThread <=====Binder驱动<===== ApplicationThreadProxy：服务端
这里和上面的逻辑一样。ApplicationThreadProxy把数据封装好，利用Binder对象，调用transact()，把所有需要的参数封装成Parcel对象，向发送ApplicationThread数据，绕了一大圈，Activity的启动过程最终回到了ApplicationThread，ApplicationThread通过scheduleLaunchActivity启动Activity，代码如下所示
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, 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;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
在ApplicationThread中，scheduleLaunchActivity实现很简单，就是发送一个启动的消息(H.LAUNCH_ACTIVITY)交给Handler处理，这个Handler就叫H，接着看一下Handler对消息的处理
private class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
//ignore some code
public vhandleMessage(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");
final 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: {
//ignore some code
}
从 case LAUNCH_ACTIVITY这可以知道，Activity的启动过程由ActivityThread的handleLaunchActivity实现，源码如下：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//ignore some code
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);
...
从上面源码看出，performLaunchActivity方法最终完成了Activity对象的创建和启动过程，并且ActivityThread通过handleResumeActivity方法来调用被启动Activity的onResume方法，performLaunchActivity这个方法主要完成了如下几件事
1.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);
}
2.通过Instrumentation的newActivity方法使用类加载器创建Activity对象
Activity activity = null;
try {
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);
}
}
Instrumentation的newActivity方法,实现比较简单，就是通过类加载器来创建Activity对象:
public Activity newActivity(ClassLoader cl, String className,
Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
return (Activity)cl.loadClass(className).newInstance();
}
3.通过LoadApk的makeApplication方法创建Application对象
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();
if (!mPackageName.equals("android")) {
initializeJavaContextClassLoader();
}
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app);
...ignore some code...
if (instrumentation != null) {
try {
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!instrumentation.onException(app, e)) {
...ignore some code...
从makeApplication的实现可以看出，如果Application创建过了，就不会再创建，这也意味着一个应用只有一个Application对象，Application对象也是通过Instrumentation(newApplication)完成的,和newActivity一样，都是通过类加载器实现。Application创建后，系统会通过Instrumentation的callApplicationOnCreate来调用Application的onCreate方法。
4.创建Context对象并通过Activity的attach方法完成一些重要数据的初始化
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
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);
Context是通过Activity的attach方法和Activity关联的，除此之后，在attach中Activity还会完成Window的创建并建立关联，这样当Window接到外部输入事件后就可以将事件传递给Activity。
5.调用Activity的onCreate方法
mInstrumentation.callActivityOnCreate(activity, r.state)，由于Activity的onCreate已经被调用，这也意味着Activity已经完成了整个启动过程。
参考文章
【凯子哥带你学Framework】Activity启动过程全解析
