前言
在使用Android手机时，我们总会启动各种各样的App以满足生活的各种需求，你是否想过，我们是怎样启动这些APP的？今天我将带着读者一起探索在Android系统中，一个App是如何被启动的。
在开始分析之前，我们先回想下启动一个App的流程：
Android系统桌面->点击应用图标->启动App
从这个过程来看，只要弄明白：
1. Android系统桌面是什么
2. 点击应用图标后Android系统执行了什么操作
就可以解决我们“App如何被启动”的疑问。
Android系统桌面是什么
如何分析Android系统桌面是什么
在Android系统中，Activity是视图存在的根本，那么我们可以通过命令adb shell dumpsys activity activities判断是哪个Activity为我们呈现桌面视图的。
以我的小米5为例，通过USB连上电脑后，输入命令adb shell dumpsys activity activities得到结果如下：
可以看到，显示桌面视图的Activity是com.miui.home包下的名为Launcher的Activity。
因为虚拟机编译AOSP实在是太慢了，所以我没有编译AOSP得到系统镜像，然后运行模拟器跑AOSP，再通过Ubuntu的Shell跑命令。国内手机厂商虽然会对Android系统进行定制，但是命名和包名都会和原生尽可能保持一致的。
那么我们在IDE中搜索Launcher，看看这个Activity是什么。结果如下：
这里摘选的是Launcher2的Launcher进行分析，虽然新版本Android已经使用Launcher3作为桌面App了，但是我进入源码看了看发现核心的逻辑是没有变化的，所以选取了代码更短的Launcher2的Launcher进行分析。
点击应用图标后Android系统执行了什么操作
既然Launcher是Activity，那就意味着我们点击桌面的事件可以表达为：
呈现Android桌面视图（View）->点击View上某个应用图标->产生点击事件->点击事件被响应->通知Android系统的某个/某些进程->Android系统执行某些操作->启动App
Launcher如何响应由我们产生的点击事件
/**
* Launches the intent referred by the clicked shortcut.
*
* @param v The view representing the clicked shortcut.
*/
public void onClick(View v) {
// Make sure that rogue clicks don't get through while allapps is launching, or after the
// view has detached (it's possible for this to happen if the view is removed mid touch).
if (v.getWindowToken() == null) {
return;
}
if (!mWorkspace.isFinishedSwitchingState()) {
return;
}
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);
if (success && v instanceof BubbleTextView) {
mWaitingForResume = (BubbleTextView) v;
mWaitingForResume.setStayPressed(true);
}
} else if (tag instanceof FolderInfo) {
if (v instanceof FolderIcon) {
FolderIcon fi = (FolderIcon) v;
handleFolderClick(fi);
}
} else if (v == mAllAppsButton) {
if (isAllAppsVisible()) {
showWorkspace(true);
} else {
onClickAllAppsButton(v);
}
}
}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;
}从代码来看，产生点击事件后，如果产生点击事件的View的Tag是ShortcutInfo（即启动应用的快捷方式），就会取得ShortcutInfo中保存的Intent（这个Intent指向我们要启动的App），然后执行startActivitySafely(v, intent, tag)方法，而startActivitySafely方法只是对startActivity方法的简单封装。
所以，Launcher响应我们产生的点击事件后，实际上就是启动一个新的Activity。
我们现在回想下App开发时，每个App都需要有一个“MainActivity”，这个Activity必须在AndroidManifest.xml文件中有以下配置：
<intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter>
在配置AndroidManifest.xml文件时，将Activity的Action指定为android.intent.action.MAIN，会使Activity在一个新的Task中启动（Task是一个Activity栈）。将category指定为android.intent.category.LAUNCHER，表示通过Intent启动此Activity时，只接受category为LAUNCHER的Intent。
所以，Launcher将会通过App的快捷方式（ShortcutInfo）得到应用的Intent，并通过这个Intent启动应用的“MainActivity”，从而启动应用。
所以我们研究的问题就从“App启动流程”变为“Activity启动流程”。
Launcher通过Binder通知ActivityManagerService启动Activity
现在我们就进入Launcher的startActivity方法里面探索“Activity启动流程”吧：
boolean startActivity(View v, Intent intent, Object tag) {
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
try {
// Only launch using the new animation if the shortcut has not opted out (this is a
// private contract between launcher and may be ignored in the future).
boolean useLaunchAnimation = (v != null) &&
!intent.hasExtra(INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION);
UserHandle user = (UserHandle) intent.getParcelableExtra(ApplicationInfo.EXTRA_PROFILE);
LauncherApps launcherApps = (LauncherApps)
this.getSystemService(Context.LAUNCHER_APPS_SERVICE);
if (useLaunchAnimation) {
ActivityOptions opts = ActivityOptions.makeScaleUpAnimation(v, 0, 0,
v.getMeasuredWidth(), v.getMeasuredHeight());
if (user == null || user.equals(android.os.Process.myUserHandle())) {
// Could be launching some bookkeeping activity
startActivity(intent, opts.toBundle());
} else {
launcherApps.startMainActivity(intent.getComponent(), user,
intent.getSourceBounds(),
opts.toBundle());
}
} else {
if (user == null || user.equals(android.os.Process.myUserHandle())) {
startActivity(intent);
} else {
launcherApps.startMainActivity(intent.getComponent(), user,
intent.getSourceBounds(), null);
}
}
return true;
} catch (SecurityException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Launcher does not have the permission to launch " + intent +
". Make sure to create a MAIN intent-filter for the corresponding activity " +
"or use the exported attribute for this activity. "
+ "tag="+ tag + " intent=" + intent, e);
}
return false;
}在这个方法中，首先，将Intent的Flag设为Intent.FLAG_ACTIVITY_NEW_TASK，使得Android系统将创建一个新的Task来放置即将被打开的新Activity（应用的“MainActivity）。然后获取一个布尔值以用于后续判断是否显示启动App的动画。
然后获取Intent中是否传输了Parcelable格式的用户句柄，并通过Context.LAUNCHER_APPS_SERVICE获取用于在多用户情境下启动App的系统服务。
不管是否显示启动App的动画，最终都会执行startActivity(intent)或launcherApps.startMainActivity方法以启动应用的“MainActivity”。而launcherApps.startMainActivity只在用户句柄不为空且用户句柄不等于当前进程句柄时（其他用户的句柄）调用。
为什么用户句柄会影响Activity的启动方式
这一点和Android的多用户安全机制有关。
假设我们有用户A和用户B在使用同一台手机，用户A是无法访问到用户B的文件或者和用户B的App通信的。所以假如我们现在是用户A，但我们想启动用户B的App，是无法直接实现的，因为用户A没有权限访问到用户B的数据，即使我们在代码中强行把user id设为用户B的user id，交给内核执行时也会抛出SecurityException。因此我们需要取得用户A的句柄（和用户A相关的数据），将我们想启动的用户B的App的Intent、用户A的句柄交给内核，让拥有权限的Android系统服务（内核态进程）去访问用户B的数据并执行相关的操作。
假如是单用户情境，就会相对简单了。因为此时只有一个用户，而该用户始终有权限直接访问自己的数据。
startActivity(intent)如何启动Activity
进入Activity类后层层深入就可以看到最终调用的是startActivityForResult方法：
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());
}
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;
}
cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} 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);
}
}
}从代码上看，如果Launcher有mParent Activity，就会执行mParent.startActivityFromChild；如果没有，就会执行mInstrumentation.execStartActivity。进入mParent.startActivityFromChild方法会看到最终也是执行了mInstrumentation.execStartActivity。执行完成后，会取得一个ActivityResult对象，用于给调用者Activity传递一些数据，最后在Activity切换时显示Transition动画。
这里有一点需要指出的是：这里的ParentActivity指的是类似TabActivity、ActivityGroup关系的嵌套Activity。之所以要强调parent和child，是要避免混乱的Activity嵌套关系。
我们进入Instrumentation类看看execStartActivity方法吧：
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();
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;
}首先，我们通过参数IBinder contextThread取得一个IApplicationThread类型的对象whoThread，而contextThread是由mMainThread.getApplicationThread()取得的ApplicationThread对象，此时mMainThread指的就是Launcher应用的主线程，所以whoThread指代的自然是Launcher的ApplicationThread。
因为Activity的onProvideReferrer()方法默认返回null，除非该方法被重写，而我们使用的Launcher并没有重写该方法，所以不用管referrer。
然后判断是否有ActivityMonitor，如果有，则即将要打开的Activity是否和ActivityMonitor中保存的IntentFilter匹配，如果匹配则增加ActivityMonitor的计数。大致是用于监控符合匹配规则的Activity的数量的。
最后调用ActivityManagerNative.getDefault().startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options);启动Activity，并检查启动是否成功。换句话说，最终负责启动Activity的是ActivityManager，前面得到的ApplicationThread也是在这里使用的。
那么ActivityManager、ApplicationThread、ActivityThread都是什么呢？
ActivityManagerService通过Binder将Launcher切换到pause状态
首先，调用ActivityManagerNative.getDefault()方法实际调用的是asInterface(IBinder obj)方法，也就意味着我们使用的其实是ActivityManagerProxy，而ActivityManagerProxy则是ActivityManagerService的代理，详见下面的代码：
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;
}
};
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);
}那么进入ActivityManagerProxy：
class ActivityManagerProxy implements IActivityManager
{
public ActivityManagerProxy(IBinder remote)
{
mRemote = remote;
}
public IBinder asBinder()
{
return mRemote;
}
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 {
// 创建两个Parcel对象，data用于传输启动Activity需要的数据，reply用于获取
// 启动Activity操作执行后系统返回的响应
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
// caller 就是Launcher提供的ApplicationThread（也就是前面提到的whoThread）
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
// 记录启动新Activity的应用的包名，也就是Launcher的包名
data.writeString(callingPackage);
intent.writeToParcel(data, 0);
data.writeString(resolvedType);
// 将resultTo这个IBinder对象写入data，实际写入的就是前面的参数——IBinder token
// 而这个token是什么，我们暂时不管，后面会给出解释
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;
}
……省略余下代码……
}将数据都写入后，就通过mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0)传输数据并得到响应（写入reply）。
前面已经提到了，ActivityManagerProxy是ActivityManagerService的代理，那么调用mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0)实际上就是通过Binder建立Launcher所在的进程与system_server进程（Android Framework层的服务几乎都由system_server进程管理，因此ActivityManagerService运行在system_server进程中）的通信，并把我们写入data的数据通过Binder传递给ActivityManagerService。
ActivityManagerService得到我们用Parcelable封装的data后就会调用startActivity方法为Launcher启动Activity：
@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) {
enforceNotIsolatedCaller("startActivity");
userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
false, ALLOW_FULL_ONLY, "startActivity", null);
// TODO: Switch to user app stacks here.
return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, options, false, userId, null, null);
}
void enforceNotIsolatedCaller(String caller) {
if (UserHandle.isIsolated(Binder.getCallingUid())) {
throw new SecurityException("Isolated process not allowed to call " + caller);
}
}enforceNotIsolatedCaller("startActivity");作安全性检查，判断当前用户是否允许启动Activity，然后对之前传入的userId进行转换和安全性检查。最后调用mStackSupervisor.startActivityMayWait。这里的mStackSupervisor是ActivityStackSupervisor对象，前面提到过，Task是以堆栈形式组织Activity的集合，而Task又由ActivityStack管理，ActivityStackSupervisor则是管理ActivityStack的类。
由于代码太长，下面只截取部分关键代码讲解：
首先，通过下面代码段调用PackageManagerService解析Intent（我们想要打开的App的用于启动MainActivity的Intent），将解析的结果保存到ActivityInfo类型的对象里：
// Collect information about the target of the Intent.
ActivityInfo aInfo =
resolveActivity(intent, resolvedType, startFlags, profilerInfo, userId);
// Method - resolveActivity
ActivityInfo resolveActivity(Intent intent, String resolvedType, int startFlags,
ProfilerInfo profilerInfo, int userId) {
// Collect information about the target of the Intent.
ActivityInfo aInfo;
try {
ResolveInfo rInfo =
AppGlobals.getPackageManager().resolveIntent(
intent, resolvedType,
PackageManager.MATCH_DEFAULT_ONLY
| ActivityManagerService.STOCK_PM_FLAGS, userId);
aInfo = rInfo != null ? rInfo.activityInfo : null;
} catch (RemoteException e) {
aInfo = null;
}
……省略，大致是做一些安全性检查和相关信息的设置……
return aInfo;
}然后互斥锁锁住ActivityManagerService的实例mService，如果解析的ActivityInfo不为空，且ApplicationInfo有ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE标记，意味着调用者App是属于heavy-weight process，如果现在有另一个heavy-weight process正在运行，则需要进行一些额外的处理。然后进入到startActivityLocked方法。
这里通过注释我们可以发现，若App有
ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE标记，App就可视为heavy-weight process，该标记可以在AndroidManifest.xml中设置，它是用于声明App是否享受系统提供的Activity状态保存/恢复功能的。但是似乎没有App能成为heavy-weight process，因为PackageParser的parseApplication方法并不会解析该标签。
在startActivityLocked方法中，得到Launcher（Activity）的ActivityRecord（Activity相关的信息），并创建我们要启动的Activity的ActivityRecord，最终执行startActivityUncheckedLocked继续启动Activity：
ActivityRecord sourceRecord = null;
ActivityRecord resultRecord = null;
if (resultTo != null) {
sourceRecord = isInAnyStackLocked(resultTo);
if (DEBUG_RESULTS) Slog.v(TAG_RESULTS,
"Will send result to " + resultTo + " " + sourceRecord);
if (sourceRecord != null) {
if (requestCode >= 0 && !sourceRecord.finishing) {
resultRecord = sourceRecord;
}
}
}
…………
ActivityRecord r = new ActivityRecord(mService, callerApp, callingUid, callingPackage,
intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho,
requestCode, componentSpecified, voiceSession != null, this, container, options);
……
err = startActivityUncheckedLocked(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, true, options, inTask);进入startActivityUncheckedLocked方法，完成一些简单的初始化后，向下执行到这段代码：如果Intent里有Intent.FLAG_ACTIVITY_NEW_DOCUMENT标记（在AndroidManifest.xml中声明），且即将要打开的Activity的启动模式又被声明为SingleInstance或SingleTask，那么Intent中携带的标记和AndroidManifest中声明的标记出现冲突，而AndroidManifest的优先级是高于Intent的，因此将launchFlags的对应位置为0。
然后是对launchFlags一系列的置位，目的是设置启动模式。
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 &&
(launchSingleInstance || launchSingleTask)) {
// We have a conflict between the Intent and the Activity manifest, manifest wins.
Slog.i(TAG, "Ignoring FLAG_ACTIVITY_NEW_DOCUMENT, launchMode is " +
"\"singleInstance\" or \"singleTask\"");
launchFlags &=
~(Intent.FLAG_ACTIVITY_NEW_DOCUMENT | Intent.FLAG_ACTIVITY_MULTIPLE_TASK);
} else {
switch (r.info.documentLaunchMode) {
case ActivityInfo.DOCUMENT_LAUNCH_NONE:
break;
case ActivityInfo.DOCUMENT_LAUNCH_INTO_EXISTING:
launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
break;
case ActivityInfo.DOCUMENT_LAUNCH_ALWAYS:
launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
break;
case ActivityInfo.DOCUMENT_LAUNCH_NEVER:
launchFlags &= ~Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
break;
}
}
}
……
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 && r.resultTo == null) {
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
}
// If we are actually going to launch in to a new task, there are some cases where
// we further want to do multiple task.
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
if (launchTaskBehind
|| r.info.documentLaunchMode == ActivityInfo.DOCUMENT_LAUNCH_ALWAYS) {
launchFlags |= Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
}
}
……
if (inTask == null) {
if (sourceRecord == null) {
// This activity is not being started from another... in this
// case we -always- start a new task.
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0 && inTask == null) {
Slog.w(TAG, "startActivity called from non-Activity context; forcing " +
"Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
}
} else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
// The original activity who is starting us is running as a single
// instance... this new activity it is starting must go on its
// own task.
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
} else if (launchSingleInstance || launchSingleTask) {
// The activity being started is a single instance... it always
// gets launched into its own task.
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
}
}
……
// 因为我们是从Launcher启动目的Activity，所以sourceRecord不为null，值为Launcher的ActivityRecord
if (sourceRecord != null) {
if (sourceRecord.finishing) {
// 如果sourceRecord表示的Activity正在结束/被销毁，那么我们不能把该Activity看作启动目的
// Activity的源Activity，因为和源Activity关联的Task现在可能是空的（没有Activity）或者
// 也在结束/被销毁的过程中，所以我们不能盲目地把目的Activity放到该Task中。取而代之的是，
// 我们会为它找到一个可用的Task，但我们要先保存源Activity的Task的信息，使得我们在创建新
// 的可用的Task时能用到里面的一些信息。
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
Slog.w(TAG, "startActivity called from finishing " + sourceRecord
+ "; forcing " + "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
newTaskInfo = sourceRecord.info;
newTaskIntent = sourceRecord.task.intent;
}
sourceRecord = null;
sourceStack = null;
} else {
sourceStack = sourceRecord.task.stack;
}
} else {
sourceStack = null;
}
……
// 为目的Activity创建新的Task
if (r.resultTo == null && inTask == null && !addingToTask
&& (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
newTask = true;
targetStack = computeStackFocus(r, newTask);
targetStack.moveToFront("startingNewTask");
if (reuseTask == null) {
r.setTask(targetStack.createTaskRecord(getNextTaskId(),
newTaskInfo != null ? newTaskInfo : r.info,
newTaskIntent != null ? newTaskIntent : intent,
voiceSession, voiceInteractor, !launchTaskBehind /* toTop */),
taskToAffiliate);
if (DEBUG_TASKS) Slog.v(TAG_TASKS,
"Starting new activity " + r + " in new task " + r.task);
} else {
r.setTask(reuseTask, taskToAffiliate);
}
if (isLockTaskModeViolation(r.task)) {
Slog.e(TAG, "Attempted Lock Task Mode violation r=" + r);
return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
}
if (!movedHome) {
if ((launchFlags &
(FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME))
== (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME)) {
// Caller wants to appear on home activity, so before starting
// their own activity we will bring home to the front.
r.task.setTaskToReturnTo(HOME_ACTIVITY_TYPE);
}
}
}完成上面一系列的处理后，调用ActivityStack的startActivityLocked方法继续执行启动Activity需要的操作，targetStack是通过这行代码targetStack = computeStackFocus(r, newTask)为用户新建的ActivityStack：
mService.grantUriPermissionFromIntentLocked(callingUid, r.packageName,
intent, r.getUriPermissionsLocked(), r.userId);
if (sourceRecord != null && sourceRecord.isRecentsActivity()) {
r.task.setTaskToReturnTo(RECENTS_ACTIVITY_TYPE);
}
if (newTask) {
EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, r.userId, r.task.taskId);
}
ActivityStack.logStartActivity(EventLogTags.AM_CREATE_ACTIVITY, r, r.task);
targetStack.mLastPausedActivity = null;
targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options);
if (!launchTaskBehind) {
// Don't set focus on an activity that's going to the back.
mService.setFocusedActivityLocked(r, "startedActivity");
}
return ActivityManager.START_SUCCESS;进入到ActivityStack的startActivityLocked方法，首先为目的Activity创建ProcessRecord，然后用WindowManager进行一些切换窗口的操作，最后调用mStackSupervisor.resumeTopActivitiesLocked(this, r, options)。
……
if (!isHomeStack() || numActivities() > 0) {
// We want to show the starting preview window if we are
// switching to a new task, or the next activity's process is
// not currently running.
boolean showStartingIcon = newTask;
ProcessRecord proc = r.app;
if (proc == null) {
proc = mService.mProcessNames.get(r.processName, r.info.applicationInfo.uid);
}
if (proc == null || proc.thread == null) {
showStartingIcon = true;
}
……调用WindowManager切换窗口……
}
……
if (doResume) {
mStackSupervisor.resumeTopActivitiesLocked(this, r, options);
}进入到resumeTopActivitiesLocked方法，调用resumeTopActivityLocked方法将所有ActivityStack（多个显示设备，每个设备对应一个ActivityStack）栈顶的Activity切换到resume状态（生命周期的onResume），而resumeTopActivityLocked方法先避免递归调用，然后调用ActivityStack的resumeTopActivityInnerLocked方法。
boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
Bundle targetOptions) {
if (targetStack == null) {
targetStack = mFocusedStack;
}
// Do targetStack first.
boolean result = false;
if (isFrontStack(targetStack)) {
result = targetStack.resumeTopActivityLocked(target, targetOptions);
}
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
final ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
final ActivityStack stack = stacks.get(stackNdx);
if (stack == targetStack) {
// Already started above.
continue;
}
if (isFrontStack(stack)) {
stack.resumeTopActivityLocked(null);
}
}
}
return result;
}下面这段代码主要就是做一些前期的检查，避免做多余的工作浪费时间，并且确保目标Activity处于正确的“状态”，使得我们后面能把它切换到resume状态并显示。
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, Bundle options) {
// 判断ActivityManagerService是否已经启动完毕
if (!mService.mBooting && !mService.mBooted) {
// Not ready yet!
return false;
}
// 获取parentActivity，如果parentActivity还未处于resume状态，则不能将stack栈顶的Activity切换为resume状态（Activity的嵌套关系不能弄乱）
ActivityRecord parent = mActivityContainer.mParentActivity;
if ((parent != null && parent.state != ActivityState.RESUMED) ||
!mActivityContainer.isAttachedLocked()) {
// Do not resume this stack if its parent is not resumed.
// TODO: If in a loop, make sure that parent stack resumeTopActivity is called 1st.
return false;
}
// 如果有正在初始化的Activity没有位于ActivityStack的栈顶，且正在执行window的启动和显示，
// 则要将window相关的操作取消。因为这类Activity的窗口有可能被孤立，那么它们有可能永远也不会进入resume状态
cancelInitializingActivities();
// 取得当前ActivityStack栈顶Activity的ActivityRecord
final ActivityRecord next = topRunningActivityLocked(null);
// 记住我们怎样处理pause/resume状态切换，并确保无论何时结束处理都会重置状态
final boolean userLeaving = mStackSupervisor.mUserLeaving;
mStackSupervisor.mUserLeaving = false;
final TaskRecord prevTask = prev != null ? prev.task : null;
if (next == null) {
// next为null表示当前ActivityStack没有要显示的Activity
final String reason = "noMoreActivities";
if (!mFullscreen) {
// 如果当前ActivityStack不是全屏的，将焦点切换到下一个拥有Activity的可见ActivityStack中
final ActivityStack stack = getNextVisibleStackLocked();
if (adjustFocusToNextVisibleStackLocked(stack, reason)) {
return mStackSupervisor.resumeTopActivitiesLocked(stack, prev, null);
}
}
// 如果ActivityStack是全屏的，却没有可以显示的Activity，那么就显示桌面（Launcher）
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: No more activities go home");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ?
HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo();
return isOnHomeDisplay() &&
mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, reason);
}
next.delayedResume = false;
// 如果当前栈顶Activity处于resume状态，且就是我们要打开的Activity，则直接结束
if (mResumedActivity == next && next.state == ActivityState.RESUMED &&
mStackSupervisor.allResumedActivitiesComplete()) {
// Make sure we have executed any pending transitions, since there
// should be nothing left to do at this point.
mWindowManager.executeAppTransition();
mNoAnimActivities.clear();
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Top activity resumed " + next);
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return false;
}
// 对prevActivity（Launcher）所在的Task进行一些判断，如果prevTask和nextTask相同，那么直接将
// prevTask直接设为栈顶Task；如果prevTask不是当前ActivityStack栈顶的Task，那么它后面的Task
// 都应该放到Launcher的Task后面；后面则是有关是否为桌面的判断和处理了。
final TaskRecord nextTask = next.task;
if (prevTask != null && prevTask.stack == this &&
prevTask.isOverHomeStack() && prev.finishing && prev.frontOfTask) {
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
if (prevTask == nextTask) {
prevTask.setFrontOfTask();
} else if (prevTask != topTask()) {
// This task is going away but it was supposed to return to the home stack.
// Now the task above it has to return to the home task instead.
final int taskNdx = mTaskHistory.indexOf(prevTask) + 1;
mTaskHistory.get(taskNdx).setTaskToReturnTo(HOME_ACTIVITY_TYPE);
} else if (!isOnHomeDisplay()) {
return false;
} else if (!isHomeStack()){
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Launching home next");
final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ?
HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo();
return isOnHomeDisplay() &&
mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, "prevFinished");
}
}
// 如果ActivityManagerService处于休眠状态，而且此时没有Activity处于resume状态
// 且栈顶Activity处于pause状态，则中断调度
if (mService.isSleepingOrShuttingDown()
&& mLastPausedActivity == next
&& mStackSupervisor.allPausedActivitiesComplete()) {
// Make sure we have executed any pending transitions, since there
// should be nothing left to do at this point.
mWindowManager.executeAppTransition();
mNoAnimActivities.clear();
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Going to sleep and all paused");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return false;
}
// Make sure that the user who owns this activity is started. If not,
// we will just leave it as is because someone should be bringing
// another user's activities to the top of the stack.
if (mService.mStartedUsers.get(next.userId) == null) {
Slog.w(TAG, "Skipping resume of top activity " + next
+ ": user " + next.userId + " is stopped");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return false;
}
// 确保我们要启动的Activity没有处于stop队列、休眠队列、等待变为可见队列中
mStackSupervisor.mStoppingActivities.remove(next);
mStackSupervisor.mGoingToSleepActivities.remove(next);
next.sleeping = false;
mStackSupervisor.mWaitingVisibleActivities.remove(next);
if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resuming " + next);
// If we are currently pausing an activity, then don't do anything
// until that is done.
if (!mStackSupervisor.allPausedActivitiesComplete()) {
if (DEBUG_SWITCH || DEBUG_PAUSE || DEBUG_STATES) Slog.v(TAG_PAUSE,
"resumeTopActivityLocked: Skip resume: some activity pausing.");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return false;
}
……待续……
}后面做的工作就是：将Launcher切换到pause状态，用WindowManager将Launcher的窗口隐藏。现在只完成了Activity相关的预处理工作，目标应用的进程和主线程还没有创建，因此后面会进入if的false分支调用mStackSupervisor.startSpecificActivityLocked方法创建应用进程；如果目标Activity的进程和主线程已经创建，则进入if语句的true分支直接将目标Activity切换到resume状态，并显示目标Activity的窗口。
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, Bundle options) {
……续上……
// 步入setLaunchSource方法后可以知道，该方法实际是通过PowerManager的setWorkSource方法
// 设置WakeLock，使得在执行后面的工作时系统不会进入休眠状态
mStackSupervisor.setLaunchSource(next.info.applicationInfo.uid);
// 现在开始将当前Activity切换到pause状态，使得栈顶Activity可以切换到resume状态
boolean dontWaitForPause = (next.info.flags&ActivityInfo.FLAG_RESUME_WHILE_PAUSING) != 0;
// 将后台ActivityStack的Activity切换到pause状态
boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, true, dontWaitForPause);
// 将当前ActivityStack中正在显示Activity切换到pause状态
if (mResumedActivity != null) {
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Pausing " + mResumedActivity);
pausing |= startPausingLocked(userLeaving, false, true, dontWaitForPause);
}
if (pausing) {
if (DEBUG_SWITCH || DEBUG_STATES) Slog.v(TAG_STATES,
"resumeTopActivityLocked: Skip resume: need to start pausing");
// At this point we want to put the upcoming activity's process
// at the top of the LRU list, since we know we will be needing it
// very soon and it would be a waste to let it get killed if it
// happens to be sitting towards the end.
if (next.app != null && next.app.thread != null) {
mService.updateLruProcessLocked(next.app, true, null);
}
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
return true;
}
……
ActivityStack lastStack = mStackSupervisor.getLastStack();
if (next.app != null && next.app.thread != null) {
if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resume running: " + next);
// 目标Activity已经可见
mWindowManager.setAppVisibility(next.appToken, true);
next.startLaunchTickingLocked();
ActivityRecord lastResumedActivity =
lastStack == null ? null :lastStack.mResumedActivity;
ActivityState lastState = next.state;
mService.updateCpuStats();
// 目标Activity切换到resume状态
if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to RESUMED: " + next + " (in existing)");
next.state = ActivityState.RESUMED;
mResumedActivity = next;
next.task.touchActiveTime();
mRecentTasks.addLocked(next.task);
mService.updateLruProcessLocked(next.app, true, null);
updateLRUListLocked(next);
mService.updateOomAdjLocked();
……
mStackSupervisor.startSpecificActivityLocked(next, true, true);
}
……
}ActivityManagerService为即将打开的应用创建进程
进入ActivityStackSupervisor类的startSpecificActivityLocked方法，首先通过应用的包名和uid取得ProcessRecord，判断ProcessRecord是否被创建，若创建，则直接启动Activity；否则调用ActivityManagerService的startProcessLocked方法创建应用进程。
void 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);
}进入到ActivityManagerService的startProcessLocked方法，首先判断要创建的进程是否为隔离进程（isolated），由于不是隔离进程，则直接进入true分支，然后再次获取ProcessRecord。如果Intent有FLAG_FROM_BACKGROUND标记，则在后台启动badProcess；否则清空进程的崩溃次数，并将进程移出badProcess集合（如果进程存在的话）。然后调用newProcessRecordLocked方法创建ProcessRecord，最后再调用另一个重载的startProcessLocked方法创建进程。
final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
boolean knownToBeDead, int intentFlags, String hostingType, ComponentName hostingName,
boolean allowWhileBooting, boolean isolated, int isolatedUid, boolean keepIfLarge,
String abiOverride, String entryPoint, String[] entryPointArgs, Runnable crashHandler) {
long startTime = SystemClock.elapsedRealtime();
ProcessRecord app;
if (!isolated) {
app = getProcessRecordLocked(processName, info.uid, keepIfLarge);
checkTime(startTime, "startProcess: after getProcessRecord");
if ((intentFlags & Intent.FLAG_FROM_BACKGROUND) != 0) {
// If we are in the background, then check to see if this process
// is bad. If so, we will just silently fail.
if (mBadProcesses.get(info.processName, info.uid) != null) {
if (DEBUG_PROCESSES) Slog.v(TAG, "Bad process: " + info.uid
+ "/" + info.processName);
return null;
}
} else {
// When the user is explicitly starting a process, then clear its
// crash count so that we won't make it bad until they see at
// least one crash dialog again, and make the process good again
// if it had been bad.
if (DEBUG_PROCESSES) Slog.v(TAG, "Clearing bad process: " + info.uid
+ "/" + info.processName);
mProcessCrashTimes.remove(info.processName, info.uid);
if (mBadProcesses.get(info.processName, info.uid) != null) {
EventLog.writeEvent(EventLogTags.AM_PROC_GOOD,
UserHandle.getUserId(info.uid), info.uid,
info.processName);
mBadProcesses.remove(info.processName, info.uid);
if (app != null) {
app.bad = false;
}
}
}
} else {
// If this is an isolated process, it can't re-use an existing process.
app = null;
}
……
String hostingNameStr = hostingName != null
? hostingName.flattenToShortString() : null;
if (app == null) {
checkTime(startTime, "startProcess: creating new process record");
app = newProcessRecordLocked(info, processName, isolated, isolatedUid);
if (app == null) {
Slog.w(TAG, "Failed making new process record for "
+ processName + "/" + info.uid + " isolated=" + isolated);
return null;
}
app.crashHandler = crashHandler;
checkTime(startTime, "startProcess: done creating new process record");
} else {
// If this is a new package in the process, add the package to the list
app.addPackage(info.packageName, info.versionCode, mProcessStats);
checkTime(startTime, "startProcess: added package to existing proc");
}
// 如果系统还没启动完毕，则等待系统启动完毕后再启动进程
if (!mProcessesReady
&& !isAllowedWhileBooting(info)
&& !allowWhileBooting) {
if (!mProcessesOnHold.contains(app)) {
mProcessesOnHold.add(app);
}
if (DEBUG_PROCESSES) Slog.v(TAG_PROCESSES,
"System not ready, putting on hold: " + app);
checkTime(startTime, "startProcess: returning with proc on hold");
return app;
}
checkTime(startTime, "startProcess: stepping in to startProcess");
startProcessLocked(
app, hostingType, hostingNameStr, abiOverride, entryPoint, entryPointArgs);
checkTime(startTime, "startProcess: done starting proc!");
return (app.pid != 0) ? app : null;
}调用newProcessRecordLocked方法根据ApplicationInfo创建ProcessRecord，并让ActivityManagerService管理该ProcessRecord，过程比较简单就不贴代码了，直接看startProcessLocked(app, hostingType, hostingNameStr, abiOverride, entryPoint, entryPointArgs)方法吧。
进入startProcessLocked方法，首先将app的pid初始化，若进程已经存在（pid不等于0），则先清除超时信息，再讲pid置为0，然后确保app不在mProcessesOnHold列表中。
mProcessesOnHold代表在系统启动完毕前尝试启动的进程，这部分进程会先在该列表中待着，等到系统启动完毕再启动。
完成一系列的初始化操作后，调用Process.start方法创建应用进程，然后以进程pid为key，app（ProcessRecord）为value存储到ActivityManagerService的mPidsSelfLocked中。
Process.start方法创建应用进程是通过Zygote进程完成的，设置好参数和创建选项后通过zygoteState.writer将数据交给Zygote进程，它会调用fork()创建进程。在这里要注意一个地方，我们通过if (entryPoint == null) entryPoint = "android.app.ActivityThread"这行代码设置了进程创建完成后的入口点（Process.start的参数注释），因此Zygote进程完成了进程创建的操作后就会执行ActivityThread的main()方法。
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
long startTime = SystemClock.elapsedRealtime();
if (app.pid > 0 && app.pid != MY_PID) {
checkTime(startTime, "startProcess: removing from pids map");
synchronized (mPidsSelfLocked) {
mPidsSelfLocked.remove(app.pid);
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
}
checkTime(startTime, "startProcess: done removing from pids map");
app.setPid(0);
}
if (DEBUG_PROCESSES && mProcessesOnHold.contains(app)) Slog.v(TAG_PROCESSES,
"startProcessLocked removing on hold: " + app);
mProcessesOnHold.remove(app);
……
// Start the process. It will either succeed and return a result containing
// the PID of the new process, or else throw a RuntimeException.
boolean isActivityProcess = (entryPoint == null);
if (entryPoint == null) entryPoint = "android.app.ActivityThread";
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
app.processName);
checkTime(startTime, "startProcess: asking zygote to start proc");
Process.ProcessStartResult startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
app.info.dataDir, entryPointArgs);
……
}进入到ActivityThread的main方法，首先进行一些初始化（包括参数设置、性能安全监控之类的），然后初始化Looper（Looper、Handler消息机制），创建ActivityThread，存储线程的Handler，最后启动Looper监听消息。
ActivityThread通过Binder将ApplicationThread对象传递给ActivityManagerService，并完成启动Activity的后续工作
到这里ActivityThread的初始化就完成了，但是回想一下前面的工作，我们现在将Launcher切换到了pause状态，但由于目标应用进程和线程还没有创建，所以我们还没有把目标应用的MainActivity切换到resume状态。所以就意味着，我们还需要进行应用进程和ActivityManagerService所在的system_server进程的通信，告诉ActivityManagerService我们已经创建好了进程和线程，接下来把MainActivity状态切换到resume中，就能打开应用了。
这一步工作在哪里完成的呢？
thread.attach(false)
final IActivityManager mgr = ActivityManagerNative.getDefault();看到这行代码有没有熟悉的感觉？前面我们就通过ActivityManagerNative.getDefault()取得ActivityManagerService的代理对象，完成了启动MainActivity的前期工作。这里再次取得代理对象，并调用了ActivityManagerService的attachApplication方法。
public static void main(String[] args) {
……
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
// Ignore
}
……
} else {
……
}
……
}进入到ActivityManagerService的attachApplication方法，前面我们已经存储过目标应用的pid-ProcessRecord键值对了，因此这里的app不为null。然后向下执行，激活ProcessRecord并将ProcessRecord绑定到应用进程。然后通过Binder（thread.bindApplication）将各种应用相关信息传递给应用进程，进行应用进程一些必要的设置。最后调用mStackSupervisor.attachApplicationLocked(app)方法将ApplicationThread对象传递给ActivityManagerService方便后续应用进程与ActivityManagerService的通信（如：将MainActivity切换到resume状态），并完成启动应用的所有工作。
@Override
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
// Find the application record that is being attached... either via
// the pid if we are running in multiple processes, or just pull the
// next app record if we are emulating process with anonymous threads.
ProcessRecord app;
if (pid != MY_PID && pid >= 0) {
synchronized (mPidsSelfLocked) {
app = mPidsSelfLocked.get(pid);
}
} else {
app = null;
}
// 此时app不为null，跳过
if (app == null) {
……
}
// 清除ProcessRecord中的信息，以确保没有不相关进程的信息
if (app.thread != null) {
handleAppDiedLocked(app, true, true);
}
// Tell the process all about itself.
if (DEBUG_ALL) Slog.v(
TAG, "Binding process pid " + pid + " to record " + app);
// 注册DeathRecipient，确保应用意外关闭时系统进程能收到通知
final String processName = app.processName;
try {
AppDeathRecipient adr = new AppDeathRecipient(
app, pid, thread);
thread.asBinder().linkToDeath(adr, 0);
app.deathRecipient = adr;
} catch (RemoteException e) {
app.resetPackageList(mProcessStats);
startProcessLocked(app, "link fail", processName);
return false;
}
EventLog.writeEvent(EventLogTags.AM_PROC_BOUND, app.userId, app.pid, app.processName);
app.makeActive(thread, mProcessStats);
app.curAdj = app.setAdj = -100;
app.curSchedGroup = app.setSchedGroup = Process.THREAD_GROUP_DEFAULT;
app.forcingToForeground = null;
updateProcessForegroundLocked(app, false, false);
app.hasShownUi = false;
app.debugging = false;
app.cached = false;
app.killedByAm = false;
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
……
// 调用ActivityThread的bindApplication方法
try {
……
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
updateLruProcessLocked(app, false, null);
app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();
} catch (Exception e) {
……
}
……
// See if the top visible activity is waiting to run in this process...
if (normalMode) {
try {
if (mStackSupervisor.attachApplicationLocked(app)) {
didSomething = true;
}
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
}
}
// Find any services that should be running in this process...
if (!badApp) {
……
}
// Check if a next-broadcast receiver is in this process...
if (!badApp && isPendingBroadcastProcessLocked(pid)) {
……
}
// Check whether the next backup agent is in this process...
if (!badApp && mBackupTarget != null && mBackupTarget.appInfo.uid == app.uid) {
……
}
……
return true;
}ActivityManagerService通知ActivityThread启动Activity
回到ActivityThread，先看bindApplication方法，就是将上面传的数据存储在AppBindData中，然后通过Message、Handler发送出去，我们再看看Handler是怎么处理H.BIND_APPLICATION类型的Message的。
public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode,
boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent,
Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services,
Bundle coreSettings) {
……
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
data.providers = providers;
data.instrumentationName = instrumentationName;
data.instrumentationArgs = instrumentationArgs;
data.instrumentationWatcher = instrumentationWatcher;
data.instrumentationUiAutomationConnection = instrumentationUiConnection;
data.debugMode = debugMode;
data.enableOpenGlTrace = enableOpenGlTrace;
data.restrictedBackupMode = isRestrictedBackupMode;
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfilerInfo = profilerInfo;
sendMessage(H.BIND_APPLICATION, data);
}这里我就直接截出代码段了，最终调用了handleBindApplication方法。
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;
进入到handleBindApplication方法，首先进行一些初始化操作，然后取出data中存储的进程名，为应用进程设置进程名。然后创建应用的Context，也就是应用的运行上下文，通过Context我们可以访问到应用相关的各种资源文件（图片、布局文件等等）。然后创建进程的Instrumentation对象、Application对象，装载Provider，最终调用mInstrumentation.callApplicationOnCreate(app)方法，也就是调用我们开发App时，Application类（或子类）的onCreate()方法。
至此应用进程相关的初始化和相关的设置就完成了，接下来只要切换MainActivity的状态就大功告成了。
private void handleBindApplication(AppBindData data) {
……
// send up app name; do this *before* waiting for debugger
Process.setArgV0(data.processName);
android.ddm.DdmHandleAppName.setAppName(data.processName,
UserHandle.myUserId());
……设置进程运行信息……
final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);
……继续进程的初始化……
if (data.instrumentationName != null) {
……
} else {
mInstrumentation = new Instrumentation();
}
……
try {
// If the app is being launched for full backup or restore, bring it up in
// a restricted environment with the base application class.
Application app = data.info.makeApplication(data.restrictedBackupMode, null);
mInitialApplication = app;
// don't bring up providers in restricted mode; they may depend on the
// app's custom Application class
if (!data.restrictedBackupMode) {
List<ProviderInfo> providers = data.providers;
if (providers != null) {
installContentProviders(app, providers);
// For process that contains content providers, we want to
// ensure that the JIT is enabled "at some point".
mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
}
}
……
try {
mInstrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
} finally {
StrictMode.setThreadPolicy(savedPolicy);
}
}进入到ActivityStackSupervisor类的attachApplicationLocked方法，该方法遍历mActivityDisplays列表得到当前所有ActivityStack，然后取得前台ActivityStack栈顶的ActivityRecord，不为空则启动该对该ActivityRecord调用realStartActivityLocked方法。
boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
final String processName = app.processName;
boolean didSomething = false;
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
final ActivityStack stack = stacks.get(stackNdx);
if (!isFrontStack(stack)) {
continue;
}
ActivityRecord hr = stack.topRunningActivityLocked(null);
if (hr != null) {
if (hr.app == null && app.uid == hr.info.applicationInfo.uid
&& processName.equals(hr.processName)) {
try {
if (realStartActivityLocked(hr, app, true, true)) {
didSomething = true;
}
} catch (RemoteException e) {
Slog.w(TAG, "Exception in new application when starting activity "
+ hr.intent.getComponent().flattenToShortString(), e);
throw e;
}
}
}
}
}
if (!didSomething) {
ensureActivitiesVisibleLocked(null, 0);
}
return didSomething;
}ActivityThread调度执行Activity生命周期方法，完成Activity的启动
进入到realStartActivityLocked方法，进行一些前期处理后调用ActivityThread的scheduleLaunchActivity方法，将创建ActivityClientRecord存储我们传入的各种应用相关的数据，通过Handler机制发送。当Handler接收到LAUNCH_ACTIVITY类型的消息时，执行handleLaunchActivity方法。
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
new Configuration(stack.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);
@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<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;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
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;进入到handleLaunchActivity方法，首先进行参数设置，然后调用performLaunchActivity方法得到目标应用的MainActivity并使其分别调用onCreate、onStart方法，然后调用handleResumeActivity方法让MainActivity进入resume状态，完成启动。
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
……
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);
……
} else {
……
}
}
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 初始化设置
……
// 通过反射获得MainActivity
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);
}
}
try {
……
if (activity != null) {
// 为MainActivity创建运行的上下文环境Context，并与Activity绑定
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);
……
// 回调MainActivity生命周期的onCreate方法
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
……
// 回调MainActivity生命周期的onStart方法
if (!r.activity.mFinished) {
activity.performStart();
r.stopped = false;
}
……
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// TODO Push resumeArgs into the activity for consideration
ActivityClientRecord r = performResumeActivity(token, clearHide);
……
}总结
用文字总结App启动流程可以分为以下步骤：
1. Launcher通过Binder建立Launcher所在进程与system_server进程（ActivityManagerService所在进程）的通信，通知ActivityManagerService即将要启动一个Activity
2. ActivityManagerService通过Binder让Launcher进入pause状态
3. Launcher进入pause状态后，通过Binder告知ActivityManagerService，随后ActivityManagerService创建一个进程（将要打开的应用进程）并启动ActivityThread（应用的UI线程）
4. ActivityThread通过Binder将ApplicationThread类型的Binder对象传递给ActivityManagerService，方便ActivityManagerService后续与其的通信
5. 准备工作完成后，ActivityManagerService通知ActivityThread启动Activity
6. ActivityThread调度执行Activity的生命周期方法，完成启动Activity的工作
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