上文介绍了Android应用程序的启动过程，即应用程序默认Activity的启动过程，一般来说，这种默认Activity是在新的进程和任务中启动的；本文将继续分析在应用程序内部启动非默认Activity的过程的源代码，这种非默认Activity一般是在原来的进程和任务中启动的。
这篇博客和上一篇很相似，这里主要讲下和之前要启动一个进程来启动Activity那篇博客的区别。
我们首先看下在一个进程内部启动另一个Activity时，可以使用如下这种方式先新建一个Intent，然后调用Activity的startActivity方法。
Intent intent = new Intent("包名.subactivity");
startActivity(intent); 我们先来回顾下Activity的startActivity函数：
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1);
}
}最后调用了startActivityForResult函数，然后是调用了Instrumentation的execStartActivity函数。这里我们要注意mToken这个参数
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.
}最后在Instrumentation的execStartActivity函数里面调用了AMS的startActivity方法，我们注意这个token参数。
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target, requestCode, 0, null, options);然后在AMS，最后调用了ActivityStackSupervisor的startActivityMayWait，其中之前的token就是resultTo参数。
@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);
}我们再来看ActivityStackSupervisor的startActivityLocked函数有下面这段代码。
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;
}
}
}这里的resultTo就是每个Activity在调用startActivity传过来的，就是调用那个Activity的token。而isInAnyStackLocked函数就是通过token来获取ActivityRecord对象。
与之前相同的步骤省略，我们直接看startActivityUncheckedLocked函数，后面的流程和上篇博客一样。之前在Launcher中起的Activity的是FLAG_ACTIVITY_NEW_TASK的所以会重新新建一个TaskRecord对象。而我们现在不会，因为在上个函数，我们获取了sourceRecord，而且我们的flag没有FLAG_ACTIVITY_NEW_TASK，因此最后把sourceRecord中的task的stack变成targetStack。而且如果这个task不是在最前面就把它放在最前面，最后把TaskRecord放在ActivityRecord的task变量。
else if (sourceRecord != null) {
final TaskRecord sourceTask = sourceRecord.task;
if (isLockTaskModeViolation(sourceTask)) {
Slog.e(TAG, "Attempted Lock Task Mode violation r=" + r);
return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
}
targetStack = sourceTask.stack;
targetStack.moveToFront("sourceStackToFront");
final TaskRecord topTask = targetStack.topTask();
if (topTask != sourceTask) {
targetStack.moveTaskToFrontLocked(sourceTask, noAnimation, options,
r.appTimeTracker, "sourceTaskToFront");
}
......
r.setTask(sourceTask, null);
我们继续看startActivityLocked函数，这里我们传入的newTask为false。
TaskRecord task = null;
if (!newTask) {
// If starting in an existing task, find where that is...
boolean startIt = true;
for (int taskNdx = mTaskHistory.size() - 1; taskNdx >= 0; --taskNdx) {
task = mTaskHistory.get(taskNdx);
if (task.getTopActivity() == null) {
// All activities in task are finishing.
continue;
}
if (task == r.task) {
// Here it is! Now, if this is not yet visible to the
// user, then just add it without starting; it will
// get started when the user navigates back to it.
if (!startIt) {
if (DEBUG_ADD_REMOVE) Slog.i(TAG, "Adding activity " + r + " to task "
+ task, new RuntimeException("here").fillInStackTrace());
task.addActivityToTop(r);
r.putInHistory();
mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken,
r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,
(r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0,
r.userId, r.info.configChanges, task.voiceSession != null,
r.mLaunchTaskBehind);
if (VALIDATE_TOKENS) {
validateAppTokensLocked();
}
ActivityOptions.abort(options);
return;
}
break;
} else if (task.numFullscreen > 0) {
startIt = false;
}
}
} 这里传进来的参数newTask为false，doResume为true。当newTask为false，表示即将要启动的Activity是在原有的Task运行时，如果这个原有的Task当前对用户不可见时，这时候就不需要继续执行下去了，因为即使把这个Activity启动起来，用户也看不到，还不如先把它保存起来，等到下次这个Task对用户可见的时候，再启动不迟。这里，这个原有的Task，即运行MainActivity的Task当前对用户是可见的，因此，会继续往下执行。
接下去执行就会把我们要启动的Activity通过mHistroy.add(addPos, r)添加到堆栈顶端去，然后调用resumeTopActivityLocked进一步操作。
后面的话和前面的博客一致，也是先处理Activity的pause，然后最后再启动Activity。最后是调用了startSpecificActivityLocked函数。
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);
}
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}之前的博客调用mService.getProcessRecordLocked，没有获取到相关进程，而现在因为已经有进程了，所以就直接调用realStartActivityLocked函数了。而在这个函数后面就直接调用ActivityThread的handleLaunchActivity了。
所以这里是通过ActivityRecord的processName和info.applicationInfo.uid来获取是否有这个进程的。我们来看看这个函数：
final ProcessRecord getProcessRecordLocked(String processName, int uid, boolean keepIfLarge) {
if (uid == Process.SYSTEM_UID) {//系统进程
// The system gets to run in any process. If there are multiple
// processes with the same uid, just pick the first (this
// should never happen).
SparseArray<ProcessRecord> procs = mProcessNames.getMap().get(processName);
if (procs == null) return null;
final int procCount = procs.size();
for (int i = 0; i < procCount; i++) {
final int procUid = procs.keyAt(i);
if (UserHandle.isApp(procUid) || !UserHandle.isSameUser(procUid, uid)) {
// Don't use an app process or different user process for system component.
continue;
}
return procs.valueAt(i);
}
}
ProcessRecord proc = mProcessNames.get(processName, uid);
if (false && proc != null && !keepIfLarge//这个参数为true，不会执行
&& proc.setProcState >= ActivityManager.PROCESS_STATE_CACHED_EMPTY
&& proc.lastCachedPss >= 4000) {
// Turn this condition on to cause killing to happen regularly, for testing.
if (proc.baseProcessTracker != null) {
proc.baseProcessTracker.reportCachedKill(proc.pkgList, proc.lastCachedPss);
}
proc.kill(Long.toString(proc.lastCachedPss) + "k from cached", true);
} else if (proc != null && !keepIfLarge//这个参数为true，不会执行
&& mLastMemoryLevel > ProcessStats.ADJ_MEM_FACTOR_NORMAL
&& proc.setProcState >= ActivityManager.PROCESS_STATE_CACHED_EMPTY) {
if (DEBUG_PSS) Slog.d(TAG_PSS, "May not keep " + proc + ": pss=" + proc.lastCachedPss);
if (proc.lastCachedPss >= mProcessList.getCachedRestoreThresholdKb()) {
if (proc.baseProcessTracker != null) {
proc.baseProcessTracker.reportCachedKill(proc.pkgList, proc.lastCachedPss);
}
proc.kill(Long.toString(proc.lastCachedPss) + "k from cached", true);
}
}
return proc;
}所以我们只需要看下mProcessNames.get方法，mProcessMap是一个ProcessMap对象，我们来看下其get方法，先从一个map中key为processName获取其SparseArray，然后根据uid再后去ProcessRecord对象。
public class ProcessMap<E> {
final ArrayMap<String, SparseArray<E>> mMap
= new ArrayMap<String, SparseArray<E>>();
public E get(String name, int uid) {
SparseArray<E> uids = mMap.get(name);
if (uids == null) return null;
return uids.get(uid);
}因此最后得到ProcessRecord必须uid和processName满足才行。
