Activity启动流程
文章摘抄自：http://liuwangshu.cn/framework/component/1-activity-start-1.html
一.Launcher点击桌面按钮启动应用
我们知道Android系统每个应用都是一个单独的进程，所以我们在点击桌面图标时ActivityManagerService会检查当前应用的进程是否存在，如果不存在会就会请求Zygote进程将需要的应用程序进程启动。在系统启动时在Zygote的Java框架层中，会创建一个Server端的Socket，这个Socket用来等待ActivityManagerService来请求Zygote来创建新的应用程序进程的。我们知道Zygote进程通过fock自身创建的应用程序进程，这样应用程序程序进程就会获得Zygote进程在启动时创建的虚拟机实例。当然，在应用程序创建过程中除了获取虚拟机实例，还可以获得Binder线程池和消息循环，这样运行在应用进程中应用程序就可以方便的使用Binder进行进程间通信以及消息处理机制了。
关于应用程序的进程启动在后面的文章进行分析，今天讲一下应用程序启动的流程。
1.Launcher请求ActivityManagerService过程
packages/apps/Launcher3/src/com/android/launcher3/Launcher.java
点击按钮后会执行按钮的点击事件，触发startActivitySafely方法
/**
* Launches the intent referred by the clicked shortcut.
*
* @param v The view representing the clicked shortcut.
*/
public void onClick(View v) {
。。。。。省略。。。。。
Object tag = v.getTag();
if (tag instanceof ShortcutInfo) {
// Open shortcut
final Intent intent = ((ShortcutInfo) tag).intent;
int[] pos = new int[2];
v.getLocationOnScreen(pos);
intent.setSourceBounds(new Rect(pos[0], pos[1],
pos[0] + v.getWidth(), pos[1] + v.getHeight()));
boolean success = startActivitySafely(v, intent, tag);
。。。。。省略。。。。。
}
boolean startActivitySafely(View v, Intent intent, Object tag) {
boolean success = false;
try {
success = startActivity(v, intent, tag);
} catch (ActivityNotFoundException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Unable to launch. tag=" + tag + " intent=" + intent, e);
}
return success;
}
startActivitySafely函数中主要就是调用了startActivity函数：
packages/apps/Launcher3/src/com/android/launcher3/Launcher.java
private boolean startActivity(View v, Intent intent, Object tag) {
设置Flag为Intent.FLAG_ACTIVITY_NEW_TASK，这样根Activity会在新的任务栈中启动。
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
try {
...
if (user == null || user.equals(UserHandleCompat.myUserHandle())) {
StrictMode.VmPolicy oldPolicy = StrictMode.getVmPolicy();
try {
StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder().detectAll()
.penaltyLog().build());
startActivity(intent, optsBundle);
} finally {
StrictMode.setVmPolicy(oldPolicy);
}
} else {
launcherApps.startActivityForProfile(intent.getComponent(), user,
intent.getSourceBounds(), optsBundle);
}
return true;
} catch (SecurityException e) {
...
}
return false;
}
**
startActivity(intent, optsBundle)处调用了startActivity函数：
frameworks/base/core/java/android/app/Activity.java
Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
startActivityForResult(intent, -1);
}
}
**
startActivity会调用startActivityForResult函数，其中第二个参数为-1，表示Launcher不需要知道Activity启动的结果，startActivityForResult函数的代码如下所示。
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
...
} else {
...
}
}
**mParent是Activity类型的，表示当前Activity的父类。因为目前根Activity还没有创建出来，因此，mParent == null成立。接着调用Instrumentation的execStartActivity方法，Instrumentation主要用来监控应用程序和系统的交互，execStartActivity方法的代码如下所示。
frameworks/base/core/java/android/app/Instrumentation.java
**
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
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) {
throw new RuntimeException("Failure from system", e);
}
return null;
}
**
调用ActivityManagerService的startActivity方法，这里是用ActivityManagerNative.getDefault()来获取ActivityManagerService（以后简称为AMS)的代理对象,我们来看一下是怎么获取的。
frameworks/base/core/java/android/app/ActivityManagerNative.java
static public IActivityManager getDefault() {
return gDefault.get();
}
private static final Singleton gDefault = new Singleton() {
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;
}
};
}
**
getDefault()方法调用了gDefault的get()方法，gDefault是一个Singleton类，IBinder b = ServiceManager.getService("activity")获取到“activity”的IBinder引用,就是IBinder类型的ActivityManagerService引用，然后用asInterface方法封装成ActivityManagerProxy(以后简称为AMP)类型对象，并将它保存到gDefault中，此后调用ActivityManagerNative的getDefault方法就会直接获得AMS的代理AMP对象。回到Instrumentation类的execStartActivity方法中，从上面得知就是调用AMP的startActivity，其中AMP是ActivityManagerNative的内部类，代码如下所示。
frameworks/base/core/java/android/app/ActivityManagerNative.java
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);//1
reply.readException();
int result = reply.readInt();
reply.recycle();
data.recycle();
return result;
}
**
首先会将传入的参数写入到Parcel类型的data中。在注释1处通过IBinder类型对象mRemote向AMS发送一个START_ACTIVITY_TRANSACTION类型的进程间通信请求。那么服务端AMS就会从Binder线程池中读取我们客户端发来的数据，最终会调用ActivityManagerNative的onTransact方法中执行，如下所示。
frameworks/base/core/java/android/app/ActivityManagerNative.java
@Override
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
switch (code) {
case START_ACTIVITY_TRANSACTION:
{
...
int result = startActivity(app, callingPackage, intent, resolvedType,
resultTo, resultWho, requestCode, startFlags, profilerInfo, options);
reply.writeNoException();
reply.writeInt(result);
return true;
}
}
**
onTransact中会调用AMS的startActivity方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
**
**
**
2.ActivityManageService到ApplicationThread的调用流程
**AMS的startActivity方法中return了startActivityAsUser方法：
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
enforceNotIsolatedCaller("startActivity");
userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
userId, false, ALLOW_FULL_ONLY, "startActivity", null);
return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, bOptions, false, userId, null, null);
startActivityAsUser方法中又return了mActivityStarter的startActivityMayWait方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
final int startActivityMayWait(IApplicationThread caller, int callingUid,
String callingPackage, Intent intent, String resolvedType,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int startFlags,
ProfilerInfo profilerInfo, IActivityManager.WaitResult outResult, Configuration config,
Bundle bOptions, boolean ignoreTargetSecurity, int userId,
IActivityContainer iContainer, TaskRecord inTask) {
...
int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
aInfo, rInfo, voiceSession, voiceInteractor,
resultTo, resultWho, requestCode, callingPid,
callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
options, ignoreTargetSecurity, componentSpecified, outRecord, container,
inTask);
...
return res;
}
}
又调用了startActivityLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
final int startActivityLocked(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
ActivityRecord[] outActivity, ActivityStackSupervisor.ActivityContainer container,
TaskRecord inTask) {
...
doPendingActivityLaunchesLocked(false);
...
return err;
}
startActivityLocked函数代码非常多，我们只需要关注doPendingActivityLaunchesLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
final void doPendingActivityLaunchesLocked(boolean doResume) {
while (!mPendingActivityLaunches.isEmpty()) {
final PendingActivityLaunch pal = mPendingActivityLaunches.remove(0);
final boolean resume = doResume && mPendingActivityLaunches.isEmpty();
try {
final int result = startActivityUnchecked(
pal.r, pal.sourceRecord, null, null, pal.startFlags, resume, null, null);
postStartActivityUncheckedProcessing(
pal.r, result, mSupervisor.mFocusedStack.mStackId, mSourceRecord,
mTargetStack);
} catch (Exception e) {
Slog.e(TAG, "Exception during pending activity launch pal=" + pal, e);
pal.sendErrorResult(e.getMessage());
}
}
}
接着又调用startActivityUnchecked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {
...
mSupervisor.resumeFocusedStackTopActivityLocked();
...
return START_SUCCESS;
}
startActivityUnchecked方法中调用了ActivityStackSupervisor类型的mSupervisor的resumeFocusedStackTopActivityLocked方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
if (targetStack != null && isFocusedStack(targetStack)) {
return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
if (r == null || r.state != RESUMED) {
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);//1
}
return false;
}
在注释1处又调用了ActivityStack类型mFocusedStack的resumeTopActivityUncheckedLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
...
try {
...
result = resumeTopActivityInnerLocked(prev, options);
} finally {
mStackSupervisor.inResumeTopActivity = false;
}
return result;
}
紧接着查看ActivityStack的resumeTopActivityInnerLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
mStackSupervisor.startSpecificActivityLocked(next, true, true);
}
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return true;
resumeTopActivityInnerLocked方法代码非常多，我们只需要关注调用了ActivityStackSupervisor类型mStackSupervisor的startSpecificActivityLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
r.task.stack.setLaunchTime(r);
if (app != null && app.thread != null) {//1
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {
app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
realStartActivityLocked(r, app, andResume, checkConfig);//2
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
在注释1处如果当前Activity所在的Application运行的话，会执行注释2处的代码。realStartActivityLocked方法的代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
...
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);
...
return true;
}
这里的 app.thread指的是IApplicationThread，它的实现是ActivityThread的内部类ApplicationThread，其中ApplicationThread继承了ApplicationThreadNative，而ApplicationThreadNative继承了Binder并实现了IApplicationThread接口。
3.ActivityThread启动Activity
**在应用程序进程启动时会创建ActivityThread实例。ActivityThread作为应用程序进程的核心类，它是如何启动应用程序（Activity）的呢？我们接着往下看。
根据上文接着查看ApplicationThread的scheduleLaunchActivity方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
@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 pendingResults, List 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);
}
scheduleLaunchActivity方法会将启动Activity的参数封装成ActivityClientRecord ，sendMessage方法向H类发送类型为LAUNCH_ACTIVITY的消息，并将ActivityClientRecord 传递过去，sendMessage方法的代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
...
mH.sendMessage(msg);
}
这里mH指的是H，它是ActivityThread的内部类并继承Handler，H的代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
...
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");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;//1
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);//2
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");//3
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;
...
}
查看H的handleMessage方法中对LAUNCH_ACTIVITY的处理，在注释1处将传过来的msg的成员变量obj转换为ActivityClientRecord。
在注释2处通过getPackageInfoNoCheck方法获得LoadedApk类型的对象并赋值给ActivityClientRecord 的成员变量packageInfo 。应用程序进程要启动Activity时需要将该Activity所属的APK加载进来，而LoadedApk就是用来描述已加载的APK文件。
在注释3处调用handleLaunchActivity方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
...
Activity a = performLaunchActivity(r, customIntent);//1
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);//2
if (!r.activity.mFinished && r.startsNotResumed) {
performPauseActivityIfNeeded(r, reason);
if (r.isPreHoneycomb()) {
r.state = oldState;
}
}
} else {
try {
ActivityManagerNative.getDefault()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
注释1处的performLaunchActivity方法用来启动Activity ，注释2处的代码用来将Activity 的状态置为Resume。如果该Activity为null则会通知ActivityManager停止启动Activity。来查看performLaunchActivity方法做了什么：
frameworks/base/core/java/android/app/ActivityThread.java
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
ActivityInfo aInfo = r.activityInfo;//1
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);//2
}
ComponentName component = r.intent.getComponent();//3
...
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);//4
...
}
} catch (Exception e) {
...
}
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);//5
...
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);//6
...
}
/**
*7
*/
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);
...
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);//8
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
}
return activity;
}
注释1处用来获取ActivityInfo，在注释2处获取APK文件的描述类LoadedApk。注释3处获取要启动的Activity的ComponentName类，ComponentName类中保存了该Activity的包名和类名。注释4处根据ComponentName中存储的Activity类名，用类加载器来创建该Activity的实例。注释5处用来创建Application，makeApplication方法内部会调用Application的onCreate方法。注释6处用来创建要启动Activity的上下文环境。注释7处调用Activity的attach方法初始化Activity，attach方法中会创建Window对象（PhoneWindow）并与Activity自身进行关联。注释8处会调用Instrumentation的callActivityOnCreate方法来启动Activity：
frameworks/base/core/java/android/app/Instrumentation.java
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);//1
postPerformCreate(activity);
}
注释1处调用了Activity的performCreate方法，代码如下所示。
frameworks/base/core/java/android/app/Activity.java
final void performCreate(Bundle icicle) {
restoreHasCurrentPermissionRequest(icicle);
onCreate(icicle);
mActivityTransitionState.readState(icicle);
performCreateCommon();
}
performCreate方法中会调用Activity的onCreate方法，这样Activity就启动了，即应用程序就启动了。