我们在ActivityStack类中可以看到某个方法用于返回当前任务栈顶端的ActivityRecord对象，如果栈是空的，就返回null，ActivityRecord用于代表在历史任务栈中的一个Activity对象的信息。

    /*** Returns the top activity in any existing task matching the given* Intent.  Returns null if no such task is found.*/private ActivityRecord findTaskLocked(Intent intent, ActivityInfo info) {ComponentName cls = intent.getComponent();if (info.targetActivity != null) {cls = new ComponentName(info.packageName, info.targetActivity);}TaskRecord cp = null;final int N = mHistory.size();for (int i=(N-1); i>=0; i--) {ActivityRecord r = (ActivityRecord)mHistory.get(i);if (!r.finishing && r.task != cp&& r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE) {cp = r.task;//Slog.i(TAG, "Comparing existing cls=" + r.task.intent.getComponent().flattenToShortString()//        + "/aff=" + r.task.affinity + " to new cls="//        + intent.getComponent().flattenToShortString() + "/aff=" + taskAffinity);if (r.task.affinity != null) {if (r.task.affinity.equals(info.taskAffinity)) {//Slog.i(TAG, "Found matching affinity!");return r;}} else if (r.task.intent != null&& r.task.intent.getComponent().equals(cls)) {//Slog.i(TAG, "Found matching class!");//dump();//Slog.i(TAG, "For Intent " + intent + " bringing to top: " + r.intent);return r;} else if (r.task.affinityIntent != null&& r.task.affinityIntent.getComponent().equals(cls)) {//Slog.i(TAG, "Found matching class!");//dump();//Slog.i(TAG, "For Intent " + intent + " bringing to top: " + r.intent);return r;}}}return null;

贴一下ActivityRecord类的信息：

/*** An entry in the history stack, representing an activity.*/
class ActivityRecord extends IApplicationToken.Stub {final ActivityManagerService service; // ownerfinal ActivityStack stack; // ownerfinal ActivityInfo info; // all about mefinal int launchedFromUid; // always the uid who started the activity.final Intent intent;    // the original intent that generated usfinal ComponentName realActivity;  // the intent component, or target of an alias.final String shortComponentName; // the short component name of the intentfinal String resolvedType; // as per original caller;final String packageName; // the package implementing intent's componentfinal String processName; // process where this component wants to runfinal String taskAffinity; // as per ActivityInfo.taskAffinityfinal boolean stateNotNeeded; // As per ActivityInfo.flagsfinal boolean fullscreen;     // covers the full screen?final boolean componentSpecified;  // did caller specifiy an explicit component?final boolean isHomeActivity; // do we consider this to be a home activity?final String baseDir;   // where activity source (resources etc) locatedfinal String resDir;   // where public activity source (public resources etc) locatedfinal String dataDir;   // where activity data should goCharSequence nonLocalizedLabel;  // the label information from the package mgr.int labelRes;           // the label information from the package mgr.int icon;               // resource identifier of activity's icon.int theme;              // resource identifier of activity's theme.TaskRecord task;        // the task this is in.long launchTime;        // when we starting launching this activitylong startTime;         // last time this activity was startedlong cpuTimeAtResume;   // the cpu time of host process at the time of resuming activityConfiguration configuration; // configuration activity was last running inActivityRecord resultTo; // who started this entry, so will get our replyfinal String resultWho; // additional identifier for use by resultTo.final int requestCode;  // code given by requester (resultTo)ArrayList results;      // pending ActivityResult objs we have receivedHashSet<WeakReference<PendingIntentRecord>> pendingResults; // all pending intents for this actArrayList newIntents;   // any pending new intents for single-top modeHashSet<ConnectionRecord> connections; // All ConnectionRecord we holdUriPermissionOwner uriPermissions; // current special URI access perms.ProcessRecord app;  // if non-null, hosting applicationBitmap thumbnail;       // icon representation of paused screenCharSequence description; // textual description of paused screenActivityState state;    // current state we are inBundle  icicle;         // last saved activity stateboolean frontOfTask;    // is this the root activity of its task?boolean launchFailed;   // set if a launched failed, to abort on 2nd tryboolean haveState;      // have we gotten the last activity state?boolean stopped;        // is activity pause finished?boolean delayedResume;  // not yet resumed because of stopped app switches?boolean finishing;      // activity in pending finish list?boolean configDestroy;  // need to destroy due to config change?int configChangeFlags;  // which config values have changedboolean keysPaused;     // has key dispatching been paused for it?boolean inHistory;      // are we in the history stack?int launchMode;         // the launch mode activity attribute.boolean visible;        // does this activity's window need to be shown?boolean waitingVisible; // true if waiting for a new act to become visboolean nowVisible;     // is this activity's window visible?boolean thumbnailNeeded;// has someone requested a thumbnail?boolean idle;           // has the activity gone idle?boolean hasBeenLaunched;// has this activity ever been launched?boolean frozenBeforeDestroy;// has been frozen but not yet destroyed.String stringName;      // for caching of toString().