Android应用程序启动过程源代码分析（2）

Step 8. ActivityStack.startActivityLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

......

final int startActivityLocked(IApplicationThread caller,
Intent intent, String resolvedType,
Uri[] grantedUriPermissions,
int grantedMode, ActivityInfo aInfo, IBinder resultTo,
String resultWho, int requestCode,
int callingPid, int callingUid, boolean onlyIfNeeded,
boolean componentSpecified) {
int err = START_SUCCESS;

ProcessRecord callerApp = null;
if (caller != null) {
callerApp = mService.getRecordForAppLocked(caller);
if (callerApp != null) {
callingPid = callerApp.pid;
callingUid = callerApp.info.uid;
} else {
......
}
}

......

ActivityRecord sourceRecord = null;
ActivityRecord resultRecord = null;
if (resultTo != null) {
int index = indexOfTokenLocked(resultTo);

......

if (index >= 0) {
sourceRecord = (ActivityRecord)mHistory.get(index);
if (requestCode >= 0 && !sourceRecord.finishing) {
......
}
}
}

int launchFlags = intent.getFlags();

if ((launchFlags&Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0
&& sourceRecord != null) {
......
}

if (err == START_SUCCESS && intent.getComponent() == null) {
......
}

if (err == START_SUCCESS && aInfo == null) {
......
}

if (err != START_SUCCESS) {
......
}

......

ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,
intent, resolvedType, aInfo, mService.mConfiguration,
resultRecord, resultWho, requestCode, componentSpecified);

......

return startActivityUncheckedLocked(r, sourceRecord,
grantedUriPermissions, grantedMode, onlyIfNeeded, true);
}

......

}

从传进来的参数caller得到调用者的进程信息，并保存在callerApp变量中，这里就是Launcher应用程序的进程信息了。

前面说过，参数resultTo是Launcher这个Activity里面的一个Binder对象，通过它可以获得Launcher这个Activity的相关信息，保存在sourceRecord变量中。

再接下来，创建即将要启动的Activity的相关信息，并保存在r变量中：

ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,
intent, resolvedType, aInfo, mService.mConfiguration,
resultRecord, resultWho, requestCode, componentSpecified);

接着调用startActivityUncheckedLocked函数进行下一步操作。

Step 9. ActivityStack.startActivityUncheckedLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

......

final int startActivityUncheckedLocked(ActivityRecord r,
ActivityRecord sourceRecord, Uri[] grantedUriPermissions,
int grantedMode, boolean onlyIfNeeded, boolean doResume) {
final Intent intent = r.intent;
final int callingUid = r.launchedFromUid;

int launchFlags = intent.getFlags();

// We'll invoke onUserLeaving before onPause only if the launching
// activity did not explicitly state that this is an automated launch.
mUserLeaving = (launchFlags&Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;

......

ActivityRecord notTop = (launchFlags&Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP)
!= 0 ? r : null;

// If the onlyIfNeeded flag is set, then we can do this if the activity
// being launched is the same as the one making the call...  or, as
// a special case, if we do not know the caller then we count the
// current top activity as the caller.
if (onlyIfNeeded) {
......
}

if (sourceRecord == null) {
......
} else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
......
} else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE
|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {
......
}

if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
......
}

boolean addingToTask = false;
if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK
|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
// If bring to front is requested, and no result is requested, and
// we can find a task that was started with this same
// component, then instead of launching bring that one to the front.
if (r.resultTo == null) {
// See if there is a task to bring to the front.  If this is
// a SINGLE_INSTANCE activity, there can be one and only one
// instance of it in the history, and it is always in its own
// unique task, so we do a special search.
ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE
? findTaskLocked(intent, r.info)
: findActivityLocked(intent, r.info);
if (taskTop != null) {
......
}
}
}

......

if (r.packageName != null) {
// If the activity being launched is the same as the one currently
// at the top, then we need to check if it should only be launched
// once.
ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);
if (top != null && r.resultTo == null) {
if (top.realActivity.equals(r.realActivity)) {
......
}
}

} else {
......
}

boolean newTask = false;

// Should this be considered a new task?
if (r.resultTo == null && !addingToTask
&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
// todo: should do better management of integers.
mService.mCurTask++;
if (mService.mCurTask <= 0) {
mService.mCurTask = 1;
}
r.task = new TaskRecord(mService.mCurTask, r.info, intent,
(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);
......
newTask = true;
if (mMainStack) {
mService.addRecentTaskLocked(r.task);
}

} else if (sourceRecord != null) {
......
} else {
......
}

......

startActivityLocked(r, newTask, doResume);
return START_SUCCESS;
}

......

}

函数首先获得intent的标志值，保存在launchFlags变量中。

这个intent的标志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION没有置位，因此 ，成员变量mUserLeaving的值为true。
这个intent的标志值的位Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP也没有置位，因此，变量notTop的值为null。
由于在这个例子的AndroidManifest.xml文件中，MainActivity没有配置launchMode属值，因此，这里的r.launchMode为默认值0，表示以标准（Standard，或者称为ActivityInfo.LAUNCH_MULTIPLE）的方式来启动这个Activity。Activity的启动方式有四种，其余三种分别是ActivityInfo.LAUNCH_SINGLE_INSTANCE、ActivityInfo.LAUNCH_SINGLE_TASK和ActivityInfo.LAUNCH_SINGLE_TOP，具体可以参考官方网站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。
传进来的参数r.resultTo为null，表示Launcher不需要等这个即将要启动的MainActivity的执行结果。
由于这个intent的标志值的位Intent.FLAG_ACTIVITY_NEW_TASK被置位，而且Intent.FLAG_ACTIVITY_MULTIPLE_TASK没有置位，因此，下面的if语句会被执行：

if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK
|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
// If bring to front is requested, and no result is requested, and
// we can find a task that was started with this same
// component, then instead of launching bring that one to the front.
if (r.resultTo == null) {
// See if there is a task to bring to the front.  If this is
// a SINGLE_INSTANCE activity, there can be one and only one
// instance of it in the history, and it is always in its own
// unique task, so we do a special search.
ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE
? findTaskLocked(intent, r.info)
: findActivityLocked(intent, r.info);
if (taskTop != null) {
......
}
}
}

这段代码的逻辑是查看一下，当前有没有Task可以用来执行这个Activity。由于r.launchMode的值不为ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通过findTaskLocked函数来查找存不存这样的Task，这里返回的结果是null，即taskTop为null，因此，需要创建一个新的Task来启动这个Activity。

接着往下看：

if (r.packageName != null) {
// If the activity being launched is the same as the one currently
// at the top, then we need to check if it should only be launched
// once.
ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);
if (top != null && r.resultTo == null) {
if (top.realActivity.equals(r.realActivity)) {
......
}
}

}

这段代码的逻辑是看一下，当前在堆栈顶端的Activity是否就是即将要启动的Activity，有些情况下，如果即将要启动的Activity就在堆栈的顶端，那么，就不会重新启动这个Activity的别一个实例了，具体可以参考官方网站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。现在处理堆栈顶端的Activity是Launcher，与我们即将要启动的MainActivity不是同一个Activity，因此，这里不用进一步处理上述介绍的情况。

执行到这里，我们知道，要在一个新的Task里面来启动这个Activity了，于是新创建一个Task：

if (r.resultTo == null && !addingToTask
&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
// todo: should do better management of integers.
mService.mCurTask++;
if (mService.mCurTask <= 0) {
mService.mCurTask = 1;
}
r.task = new TaskRecord(mService.mCurTask, r.info, intent,
(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);
......
newTask = true;
if (mMainStack) {
mService.addRecentTaskLocked(r.task);
}

}

新建的Task保存在r.task域中，同时，添加到mService中去，这里的mService就是ActivityManagerService了。

最后就进入startActivityLocked(r, newTask, doResume)进一步处理了。这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

......

private final void startActivityLocked(ActivityRecord r, boolean newTask,
boolean doResume) {
final int NH = mHistory.size();

int addPos = -1;

if (!newTask) {
......
}

// Place a new activity at top of stack, so it is next to interact
// with the user.
if (addPos < 0) {
addPos = NH;
}

// If we are not placing the new activity frontmost, we do not want
// to deliver the onUserLeaving callback to the actual frontmost
// activity
if (addPos < NH) {
......
}

// Slot the activity into the history stack and proceed
mHistory.add(addPos, r);
r.inHistory = true;
r.frontOfTask = newTask;
r.task.numActivities++;
if (NH > 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.
......
} else {
// If this is the first activity, don't do any fancy animations,
// because there is nothing for it to animate on top of.
......
}

......

if (doResume) {
resumeTopActivityLocked(null);
}
}

......

}

这里的NH表示当前系统中历史任务的个数，这里肯定是大于0，因为Launcher已经跑起来了。当NH>0时，并且现在要切换新任务时，要做一些任务切的界面操作，这段代码我们就不看了，这里不会影响到下面启Activity的过程，有兴趣的读取可以自己研究一下。

这里传进来的参数doResume为true，于是调用resumeTopActivityLocked进一步操作。
Step 10. Activity.resumeTopActivityLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

......

/**
* Ensure that the top activity in the stack is resumed.
*
* @param prev The previously resumed activity, for when in the process
* of pausing; can be null to call from elsewhere.
*
* @return Returns true if something is being resumed, or false if
* nothing happened.
*/
final boolean resumeTopActivityLocked(ActivityRecord prev) {
// Find the first activity that is not finishing.
ActivityRecord next = topRunningActivityLocked(null);

// Remember how we'll process this pause/resume situation, and ensure
// that the state is reset however we wind up proceeding.
final boolean userLeaving = mUserLeaving;
mUserLeaving = false;

if (next == null) {
......
}

next.delayedResume = false;

// If the top activity is the resumed one, nothing to do.
if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
......
}

// If we are sleeping, and there is no resumed activity, and the top
// activity is paused, well that is the state we want.
if ((mService.mSleeping || mService.mShuttingDown)
&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
......
}

......

// If we are currently pausing an activity, then don't do anything
// until that is done.
if (mPausingActivity != null) {
......
}

......

// We need to start pausing the current activity so the top one
// can be resumed...
if (mResumedActivity != null) {
......
startPausingLocked(userLeaving, false);
return true;
}

......
}

......

}

函数先通过调用topRunningActivityLocked函数获得堆栈顶端的Activity，这里就是MainActivity了，这是在上面的Step 9设置好的，保存在next变量中。

接下来把mUserLeaving的保存在本地变量userLeaving中，然后重新设置为false，在上面的Step 9中，mUserLeaving的值为true，因此，这里的userLeaving为true。
这里的mResumedActivity为Launcher，因为Launcher是当前正被执行的Activity。
当我们处理休眠状态时，mLastPausedActivity保存堆栈顶端的Activity，因为当前不是休眠状态，所以mLastPausedActivity为null。
有了这些信息之后，下面的语句就容易理解了：

// If the top activity is the resumed one, nothing to do.
if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
......
}

// If we are sleeping, and there is no resumed activity, and the top
// activity is paused, well that is the state we want.
if ((mService.mSleeping || mService.mShuttingDown)
&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
......
}

它首先看要启动的Activity是否就是当前处理Resumed状态的Activity，如果是的话，那就什么都不用做，直接返回就可以了；否则再看一下系统当前是否休眠状态，如果是的话，再看看要启动的Activity是否就是当前处于堆栈顶端的Activity，如果是的话，也是什么都不用做。

上面两个条件都不满足，因此，在继续往下执行之前，首先要把当处于Resumed状态的Activity推入Paused状态，然后才可以启动新的Activity。但是在将当前这个Resumed状态的Activity推入Paused状态之前，首先要看一下当前是否有Activity正在进入Pausing状态，如果有的话，当前这个Resumed状态的Activity就要稍后才能进入Paused状态了，这样就保证了所有需要进入Paused状态的Activity串行处理。
这里没有处于Pausing状态的Activity，即mPausingActivity为null，而且mResumedActivity也不为null，于是就调用startPausingLocked函数把Launcher推入Paused状态去了。
Step 11. ActivityStack.startPausingLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

......

private final void startPausingLocked(boolean userLeaving, boolean uiSleeping) {
if (mPausingActivity != null) {
......
}
ActivityRecord prev = mResumedActivity;
if (prev == null) {
......
}
......
mResumedActivity = null;
mPausingActivity = prev;
mLastPausedActivity = prev;
prev.state = ActivityState.PAUSING;
......

if (prev.app != null && prev.app.thread != null) {
......
try {
......
prev.app.thread.schedulePauseActivity(prev, prev.finishing, userLeaving,
prev.configChangeFlags);
......
} catch (Exception e) {
......
}
} else {
......
}

......

}

......

}

函数首先把mResumedActivity保存在本地变量prev中。在上一步Step 10中，说到mResumedActivity就是Launcher，因此，这里把Launcher进程中的ApplicationThread对象取出来，通过它来通知Launcher这个Activity它要进入Paused状态了。当然，这里的prev.app.thread是一个ApplicationThread对象的远程接口，通过调用这个远程接口的schedulePauseActivity来通知Launcher进入Paused状态。
参数prev.finishing表示prev所代表的Activity是否正在等待结束的Activity列表中，由于Laucher这个Activity还没结束，所以这里为false；参数prev.configChangeFlags表示哪些config发生了变化，这里我们不关心它的值。
Step 12. ApplicationThreadProxy.schedulePauseActivity
这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中：

class ApplicationThreadProxy implements IApplicationThread {

......

public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges) throws RemoteException {
Parcel data = Parcel.obtain();
data.writeInterfaceToken(IApplicationThread.descriptor);
data.writeStrongBinder(token);
data.writeInt(finished ? 1 : 0);
data.writeInt(userLeaving ? 1 :0);
data.writeInt(configChanges);
mRemote.transact(SCHEDULE_PAUSE_ACTIVITY_TRANSACTION, data, null,
IBinder.FLAG_ONEWAY);
data.recycle();
}

......

}

这个函数通过Binder进程间通信机制进入到ApplicationThread.schedulePauseActivity函数中。
Step 13. ApplicationThread.schedulePauseActivity
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中，它是ActivityThread的内部类：

public final class ActivityThread {

......

private final class ApplicationThread extends ApplicationThreadNative {

......

public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges) {
queueOrSendMessage(
finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
token,
(userLeaving ? 1 : 0),
configChanges);
}

......

}

......

}

这里调用的函数queueOrSendMessage是ActivityThread类的成员函数。

上面说到，这里的finished值为false，因此，queueOrSendMessage的第一个参数值为H.PAUSE_ACTIVITY，表示要暂停token所代表的Activity，即Launcher。
Step 14. ActivityThread.queueOrSendMessage
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

......

private final void queueOrSendMessage(int what, Object obj, int arg1) {
queueOrSendMessage(what, obj, arg1, 0);
}

private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {
synchronized (this) {
......
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
mH.sendMessage(msg);
}
}

......

}

这里首先将相关信息组装成一个msg，然后通过mH成员变量发送出去，mH的类型是H，继承于Handler类，是ActivityThread的内部类，因此，这个消息最后由H.handleMessage来处理。

Step 15. H.handleMessage
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

......

private final class H extends Handler {

......

public void handleMessage(Message msg) {
......
switch (msg.what) {

......

case PAUSE_ACTIVITY:
handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
maybeSnapshot();
break;

......

}
......

}

......

}

这里调用ActivityThread.handlePauseActivity进一步操作，msg.obj是一个ActivityRecord对象的引用，它代表的是Launcher这个Activity。