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; // owner
    final ActivityStack stack; // owner
    final ActivityInfo info; // all about me
    final int launchedFromUid; // always the uid who started the activity.
    final Intent intent;    // the original intent that generated us
    final ComponentName realActivity;  // the intent component, or target of an alias.
    final String shortComponentName; // the short component name of the intent
    final String resolvedType; // as per original caller;
    final String packageName; // the package implementing intent's component
    final String processName; // process where this component wants to run
    final String taskAffinity; // as per ActivityInfo.taskAffinity
    final boolean stateNotNeeded; // As per ActivityInfo.flags
    final 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) located
    final String resDir;   // where public activity source (public resources etc) located
    final String dataDir;   // where activity data should go
    CharSequence 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 activity
    long startTime;         // last time this activity was started
    long cpuTimeAtResume;   // the cpu time of host process at the time of resuming activity
    Configuration configuration; // configuration activity was last running in
    ActivityRecord resultTo; // who started this entry, so will get our reply
    final String resultWho; // additional identifier for use by resultTo.
    final int requestCode;  // code given by requester (resultTo)
    ArrayList results;      // pending ActivityResult objs we have received
    HashSet<WeakReference<PendingIntentRecord>> pendingResults; // all pending intents for this act
    ArrayList newIntents;   // any pending new intents for single-top mode
    HashSet<ConnectionRecord> connections; // All ConnectionRecord we hold
    UriPermissionOwner uriPermissions; // current special URI access perms.
    ProcessRecord app;  // if non-null, hosting application
    Bitmap thumbnail;       // icon representation of paused screen
    CharSequence description; // textual description of paused screen
    ActivityState state;    // current state we are in
    Bundle  icicle;         // last saved activity state
    boolean frontOfTask;    // is this the root activity of its task?
    boolean launchFailed;   // set if a launched failed, to abort on 2nd try
    boolean 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 changed
    boolean 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 vis
    boolean 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().