文章目录
- 1 闲话
- 2 从ActivityThread到Activity的onCreate调用链
1 闲话
喜欢读源码的同学应该早就注意到了，android api28以后，Activity的启动流程改变了许多，如android 28以后，ActivityThread中已经删除掉了LAUNCH_ACTIVITY的标签，而且也将一些逻辑单独提出来放在了一些新的类里面。由于我只是关注了它从ActivityThread的main以及startActivity走到最终调用Activity的生命周期，没有仔细阅读其细节，所以本文主要讲这个流程，细节先不写。
2 从ActivityThread到Activity的onCreate调用链
先给出流程描述：
- 我们启动应用程序时，首先会启动的是zygote进程，然后由该进程
启动系统服务以及其它服务等。当一个应用进程启动后，就会加载
在AndroidManifest.xml里面设置的默认加载的Activity。此时的
加载入口是ActivityThread里面的main方法。该方法是整个程序的
入口。在该方法里，首先会创造一个主线程相关的消息机制，即调用
Looper.prepareMainLooper方法，然后创建一个ActivityThread对象，
并调用ActivityThread里的attach方法。最后调用Looper.prepare来
开启消息循环。 - 在attach方法中，首先判断是否是系统应用，如果是系统应用会走
另外一套逻辑，我们关注的是不是系统应用这一部分。在不是系统应用
这一部分的代码逻辑中，首先会通过AIDL来获取ActivityManagerService
的对象，然后调用ActivityManagerService里面的attachApplication. - 在ActivityManagerService的attachApplication中，会调用ActivityManagerService
下的attachApplicationLocked. - attachApplicationLocked方法比较长，主要做看以下几件事：
（1）调用ActivityThread的bindApplication方法启动Application
（2）调用ActivityTaskManagerService的内部类LocalService的 attachApplicationLocked方法，该方法会调用RootActivityContainer的attachApplication方法，然后调用ActivityStackSupervisor的realStartActivityLocked方法去启动ActivityStack的栈顶的Activity
（3）调用ActivityService的attachApplicationLocked方法启动当前进程的Service
（4）检查是否有广播broadcast到这个application，如果有则广播
- 主要关注第2件事。最终会调用ActivityStackSupervisor的realStartActivityLocked方法，该方法会创建一个ClientTransaction对象，然后调用addCallback方法加入一个LaunchActivityItem对象(这个地方后面会用到)，然后一系列的调用，最终会调用ApplicationThread的scheduleTransaction方法，该方法会调用ActivityThread的scheduleTransaction方法，在该方法中会调用sendMessage发送msg.what为EXCUTE_TRANSACTION的消息到ActivityThread的H中进行处理。在H中，会调用TransactionExecutor中的execute方法，该方法会调用executeCallbacks方法，而该方法中会调用LaunchActivityItem中的execute方法。然后该方法中会继续调用ClientTransactionHandler的handleLaunchActivity方法，该方法是ActivityThread的父类，最终调用的是ActivityThread的handleLaunchActivity方法，然后调用performLaunchActivity方法，最终会调用Instrumentation的callActivityOnCreate方法，最后会调用Activity的onCreate方法，到此Activity的启动大致结束。
流程图
我们都知道，ActivityThread的main方法是android程序的入口，先看看该源码：
//ActivityThread.java
public static void main(String[] args) {
//省略
//创建主线程的Looper
Looper.prepareMainLooper();
//省略
//创建ActivityThread对象
ActivityThread thread = new ActivityThread();
//attach是核心
thread.attach(false, startSeq);
//省略
//启动消息循环
Looper.loop();
}在ActivityThread的main方法里，会调用Looper.prepareMainLooper()来创建Looper及消息队列。然后创建ActivityThread对象，调用ActivityThread的attach方法，然后开启消息循环。这里主要关注点是attach方法，我们看看attach源码：
//ActivityThread.java
final ApplicationThread mAppThread = new ApplicationThread();
private void attach(boolean system, long startSeq) {
//省略
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {//如果不是系统级应用就走这里，一般app都不是系统级的
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
//通过AIDL获取ActivityManagerService
final IActivityManager mgr = ActivityManager.getService();
try {
//重点
mgr.attachApplication(mAppThread, startSeq);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
//省略
}
}对于我们自己的应用，一般都是非系统应用，所以会走上述if语句。在这里，它会创建一个ActivityManagerService对象，并调用ActivityManagerService的attachApplication方法，这是一个重要的方法，我们跟进去看看源码：
//ActivityManagerService
@Override
public final void attachApplication(IApplicationThread thread, long startSeq) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
//重点
attachApplicationLocked(thread, callingPid, callingUid, startSeq);
Binder.restoreCallingIdentity(origId);
}
}可以看到上述代码，它会继续调用ActivityManagerService的attachApplicationLocked方法，源码如下：
//ActivityManagerService
@GuardedBy("this")
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid, int callingUid, long startSeq) {
ProcessRecord app;//app的声明
//省略
//对app的初始化，可以看出app实际就是ProcessRecord类型
if (app == null && startSeq > 0) {
final ProcessRecord pending = mProcessList.mPendingStarts.get(startSeq);
if (pending != null && pending.startUid == callingUid && pending.startSeq == startSeq
&& mProcessList.handleProcessStartedLocked(pending, pid, pending
.isUsingWrapper(),
startSeq, true)) {
app = pending;
}
}
//省略
app.makeActive(thread, mProcessStats);
//省略
// See if the top visible activity is waiting to run in this process...
if (normalMode) {//普通模式
try {
//重点,app.getWindowProcessController()获得的是WindowProcessController对象，后面
//会用到，需要找出它的初始化，app指的是ProcessRecord,在ProcessRecord里面可以对mWindowProcessController
//赋值的地方只有ProcessRecord构造函数，所以查看当前方法里的ProcessRecord对象构造的地方，
//我们主要找到mWindowProcessController设置Thread的地方，即mWindowProcessController.setThread地方，
//在ProcessRecord里调用mWindowProcessController.setThread地方有：makeActive，makeInactive两个
//方法。往上追述，可以发现这个thread是ApplicationThread
//mAtmInternal初始化的地方分别是：ActivityManagerService构造函数，最终调用的是
didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
}
}
}在上述代码中，最终会走普通模式的if语句，最主要的是下面一行代码：
didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());这里的mAtmInternal指的是LocalService，它继承自ActivityTaskManagerInternal，所以mAtmInternal.attachApplication最终调用的是LocalService的attachApplication，在LocalService的attachApplication里会调用RootActivityContainer.attachApplication。这里的app指的是ProcessRecord，app.getWindowProcessController()得到的值是WindowProcessController对象，这里主要关注的是WindowProcessController设置Thread的地方，往上追述发现该Thread其实是ApplicationThread对象。我们看一下LocalService的attachApplication源码：
//ActivityTaskManagerService内部类
final class LocalService extends ActivityTaskManagerInternal {
@HotPath(caller = HotPath.PROCESS_CHANGE)
@Override
public boolean attachApplication(WindowProcessController wpc) throws RemoteException {
synchronized (mGlobalLockWithoutBoost) {
//重点，mRootActivityContainer是RootActivityContainer类型
return mRootActivityContainer.attachApplication(wpc);
}
}
}看看RootActivityContainer.attachApplication的源码
//RootActivityContainer
boolean attachApplication(WindowProcessController app) throws RemoteException {
final String processName = app.mName;
boolean didSomething = false;
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
final ActivityDisplay display = mActivityDisplays.get(displayNdx);
final ActivityStack stack = display.getFocusedStack();
if (stack != null) {
stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
final ActivityRecord top = stack.topRunningActivityLocked();
final int size = mTmpActivityList.size();
for (int i = 0; i < size; i++) {
final ActivityRecord activity = mTmpActivityList.get(i);
if (activity.app == null && app.mUid == activity.info.applicationInfo.uid
&& processName.equals(activity.processName)) {
try {
//重点
if (mStackSupervisor.realStartActivityLocked(activity, app,
top == activity /* andResume */, true /* checkConfig */)) {
didSomething = true;
}
} catch (RemoteException e) {
Slog.w(TAG, "Exception in new application when starting activity "
+ top.intent.getComponent().flattenToShortString(), e);
throw e;
}
}
}
}
}
if (!didSomething) {
ensureActivitiesVisible(null, 0, false /* preserve_windows */);
}
return didSomething;
}mStackSupervisor.realStartActivityLocked这个方法是重点，我们进去看看
//ActivityStackSupervisor
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
//省略
// 往上追述可得，proc.getThread()得到的是ApplicationThread
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
//省略
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
r.icicle, r.persistentState, results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
//这个地方对mLifecycleStateRequest进行设置，后面会用到
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// 重要的是这里
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
//省略
}我们看看 mService.getLifecycleManager().scheduleTransaction(clientTransaction);的源码，这句最后调用了ClientLifecycleManager里面的scheduleTransaction方法，如下：
//ClientLifecycleManager.java里面
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
//重点
transaction.schedule();
if (!(client instanceof Binder)) {
// If client is not an instance of Binder - it's a remote call and at this point it is
// safe to recycle the object. All objects used for local calls will be recycled after
// the transaction is executed on client in ActivityThread.
transaction.recycle();
}
}上述代码最后调用的是ClientTransaction的schedule，我们来看看：
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this);
}最后调用的是mClient的scheduleTransaction，这个mClient是哪来的呢？我们往上级返回查看ClientTransaction的创建地方，首先，它是上级方法scheduleTransaction传进来的（transaction），我们继续往上看，transaction是它所在方法的上级方法，即ActivityStackSupervisor的realStartActivityLocked里通过下面的方法得到的
ClientTransaction.obtain(proc.getThread(), r.appToken);我们看看ClientTransaction的obtain方法：
/** Obtain an instance initialized with provided params. */
public static ClientTransaction obtain(IApplicationThread client, IBinder activityToken) {
ClientTransaction instance = ObjectPool.obtain(ClientTransaction.class);
if (instance == null) {
instance = new ClientTransaction();
}
//找到了，这里的mClient是被传进来的client初始化的。而client是proc.getThread(),我们上述
//分析得到proc.getThread()方法获得的是ApplicationThread的方法
instance.mClient = client;
instance.mActivityToken = activityToken;
return instance;
}所以回到上面，最后调用的是ApplicationThread的scheduleTransaction方法。我们来看看该方法：
private class ApplicationThread extends IApplicationThread.Stub{
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
}如上所示，最后调用的是ActivityThread的scheduleTransaction方法，来看看该方法：
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}如上所示，最终发送消息给ActivityThread.H，将transaction交给H的handleMessage方法进行处理：
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
if (isSystem()) {
transaction.recycle();
}
break;msg.obj就是sendMessage里的transaction，来看看mTransactionExecutor的初始化
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);所以最后调用的是TransactionExecutor里的execute方法，进去看看
//TransactionExecutor.java
public void execute(ClientTransaction transaction) {
if (token != null) {
final Map<IBinder, ClientTransactionItem> activitiesToBeDestroyed =
mTransactionHandler.getActivitiesToBeDestroyed();
final ClientTransactionItem destroyItem = activitiesToBeDestroyed.get(token);
if (destroyItem != null) {
if (transaction.getLifecycleStateRequest() == destroyItem) {
// It is going to execute the transaction that will destroy activity with the
// token, so the corresponding to-be-destroyed record can be removed.
activitiesToBeDestroyed.remove(token);
}
}
}
if (DEBUG_RESOLVER) Slog.d(TAG, transactionToString(transaction, mTransactionHandler));
executeCallbacks(transaction);
executeLifecycleState(transaction);
mPendingActions.clear();
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "End resolving transaction");
}在executeCallbacks中
//TransactionExecutor.java
@VisibleForTesting
public void executeCallbacks(ClientTransaction transaction) {
final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
if (callbacks == null || callbacks.isEmpty()) {
// No callbacks to execute, return early.
return;
}
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callbacks in transaction");
final IBinder token = transaction.getActivityToken();
ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
// In case when post-execution state of the last callback matches the final state requested
// for the activity in this transaction, we won't do the last transition here and do it when
// moving to final state instead (because it may contain additional parameters from server).
final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
: UNDEFINED;
// Index of the last callback that requests some post-execution state.
final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);
final int size = callbacks.size();
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callback: " + item);
final int postExecutionState = item.getPostExecutionState();
final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,
item.getPostExecutionState());
if (closestPreExecutionState != UNDEFINED) {
cycleToPath(r, closestPreExecutionState, transaction);
}
//重点是这里
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
if (r == null) {
// Launch activity request will create an activity record.
r = mTransactionHandler.getActivityClient(token);
}
if (postExecutionState != UNDEFINED && r != null) {
// Skip the very last transition and perform it by explicit state request instead.
final boolean shouldExcludeLastTransition =
i == lastCallbackRequestingState && finalState == postExecutionState;
cycleToPath(r, postExecutionState, shouldExcludeLastTransition, transaction);
}
}
}主要看item.execute，这个item是从callback中获取出来的，如下所示：
final ClientTransactionItem item = callbacks.get(i);这个callbacks又是transaction.getCallbacks()得到，而这getCallbacks得到的值是通过addCallback添加的，在ActivityStackSupervisor的realStartActivityLocked中添加进去的，如下所示：
//ActivityStackSupervisor
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
//省略
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
r.icicle, r.persistentState, results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken));
//省略
}而LaunchActivityItem.obtain得到的是一个LaunchActivityItem对象，所以item是LaunchActivityItem类型。那么我们看看LaunchActivityItem里的execute方法
//LaunchActivityItem.java
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
mPendingResults, mPendingNewIntents, mIsForward,
mProfilerInfo, client, mAssistToken);
//重点
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}我们看一下execute的第一个参数client，这个方法调用如下：
//TransactionExecutor.java
item.execute(mTransactionHandler, token, mPendingActions);mTransactionHandler是TransactionExecutor的成员变量，而在TransactionExecutor中mTransactionHandler被初始化的地方只有它的构造函数，那么我们看看它构造函数是在哪里被调用的。追溯后发现，是在ActivityThread里被构造的：
//TransactionExecutor.java
public TransactionExecutor(ClientTransactionHandler clientTransactionHandler) {
mTransactionHandler = clientTransactionHandler;
}
//ActivityThread.java
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);所以，mTransactionHandler是ActivityThread类型。所以最终调用的是ActivityThread的handleLaunchActivity方法，我们看看该方法：
@Override
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}
// Make sure we are running with the most recent config.
handleConfigurationChanged(null, null);
if (localLOGV) Slog.v(
TAG, "Handling launch of " + r);
// Initialize before creating the activity
if (!ThreadedRenderer.sRendererDisabled
&& (r.activityInfo.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
HardwareRenderer.preload();
}
WindowManagerGlobal.initialize();
// Hint the GraphicsEnvironment that an activity is launching on the process.
GraphicsEnvironment.hintActivityLaunch();
//重点这里
final Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
if (!r.activity.mFinished && pendingActions != null) {
pendingActions.setOldState(r.state);
pendingActions.setRestoreInstanceState(true);
pendingActions.setCallOnPostCreate(true);
}
} else {
// If there was an error, for any reason, tell the activity manager to stop us.
try {
ActivityTaskManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
return a;
}看看performLaunchActivity方法
/** Core implementation of activity launch. */
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
ActivityInfo aInfo = r.activityInfo;
//省略
//重要的是这里
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
//省略
return activity;
}看看callActivityOnCreate方法
public void callActivityOnCreate(Activity activity, Bundle icicle) {
prePerformCreate(activity);
activity.performCreate(icicle);
postPerformCreate(activity);
}到此，通过ActivityThread的main方法启动Activity流程已经结束，我们来总结一下。
总结：
- 我们启动应用程序时，首先会启动的是zygote进程，然后由该进程
启动系统服务以及其它服务等。当一个应用进程启动后，就会加载
在AndroidManifest.xml里面设置的默认加载的Activity。此时的
加载入口是ActivityThread里面的main方法。该方法是整个程序的
入口。在该方法里，首先会创造一个主线程相关的消息机制，即调用
Looper.prepareMainLooper方法，然后创建一个ActivityThread对象，
并调用ActivityThread里的attach方法。最后调用Looper.prepare来
开启消息循环。 - 在attach方法中，首先判断是否是系统应用，如果是系统应用会走
另外一套逻辑，我们关注的是不是系统应用这一部分。在不是系统应用
这一部分的代码逻辑中，首先会通过AIDL来获取ActivityManagerService
的对象，然后调用ActivityManagerService里面的attachApplication. - 在ActivityManagerService的attachApplication中，会调用ActivityManagerService
下的attachApplicationLocked. - attachApplicationLocked方法比较长，主要做看以下几件事：
（1）调用ActivityThread的bindApplication方法启动Application
（2）调用ActivityTaskManagerService的内部类LocalService的 attachApplicationLocked方法，该方法会调用RootActivityContainer的attachApplication方法，然后调用ActivityStackSupervisor的realStartActivityLocked方法去启动ActivityStack的栈顶的Activity
（3）调用ActivityService的attachApplicationLocked方法启动当前进程的Service
（4）检查是否有广播broadcast到这个application，如果有则广播
- 主要关注第2件事。最终会调用ActivityStackSupervisor的realStartActivityLocked方法，该方法会创建一个ClientTransaction对象，然后调用addCallback方法加入一个LaunchActivityItem对象(这个地方后面会用到)，然后一系列的调用，最终会调用ApplicationThread的scheduleTransaction方法，该方法会调用ActivityThread的scheduleTransaction方法，在该方法中会调用sendMessage发送msg.what为EXCUTE_TRANSACTION的消息到ActivityThread的H中进行处理。在H中，会调用TransactionExecutor中的execute方法，该方法会调用executeCallbacks方法，而该方法中会调用LaunchActivityItem中的execute方法。然后该方法中会继续调用ClientTransactionHandler的handleLaunchActivity方法，该方法是ActivityThread的父类，最终调用的是ActivityThread的handleLaunchActivity方法，然后调用performLaunchActivity方法，最终会调用Instrumentation的callActivityOnCreate方法，最后会调用Activity的onCreate方法，到此Activity的启动大致结束。
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