Intent intent = new Intent(ActivityA.this,ActivityB.class);
startActivity(intent);

@Override
public void startActivity(Intent intent, Bundle options) {
    warnIfCallingFromSystemProcess();
    if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
        throw new AndroidRuntimeException(
                "Calling startActivity() from outside of an Activity "
                + " context requires the FLAG_ACTIVITY_NEW_TASK flag."
                + " Is this really what you want?");
    }
    mMainThread.getInstrumentation().execStartActivity(
        getOuterContext(), mMainThread.getApplicationThread(), null,
        (Activity)null, intent, -1, options);
}

public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target,
        Intent intent, int requestCode, Bundle options) {
    IApplicationThread whoThread = (IApplicationThread) contextThread;
    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) {
    }
    return null;
}
 

public abstract class ActivityManagerNative extends Binder implements IActivityManager

@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
        Intent intent, String resolvedType, IBinder resultTo,
        String resultWho, int requestCode, int startFlags,
        String profileFile, ParcelFileDescriptor profileFd, Bundle options) {
    return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
            resultWho, requestCode,
            startFlags, profileFile, profileFd, options, UserHandle.getCallingUserId());
}
 

final int startActivityMayWait(IApplicationThread caller, int callingUid,String callingPackage, Intent intent, String resolvedType, IBinder resultTo,String resultWho, int requestCode, int startFlags, String profileFile,
ParcelFileDescriptor profileFd, WaitResult outResult, Configuration config,Bundle options, int userId) {

     ..........

     int res = startActivityLocked(caller, intent, resolvedType,aInfo, resultTo, resultWho, requestCode, callingPid, callingUid,callingPackage, startFlags, options, componentSpecified, null);

    .........
  }
}
 

    final boolean realStartActivityLocked(ActivityRecord r,ProcessRecord app, boolean andResume, boolean checkConfig)
        throws RemoteException {

    r.startFreezingScreenLocked(app, 0);
    if (false) Slog.d(TAG, "realStartActivity: setting app visibility true");
    mWindowManager.setAppVisibility(r.appToken, true);

    // schedule launch ticks to collect information about slow apps.
    r.startLaunchTickingLocked();

    // Have the window manager re-evaluate the orientation of
    // the screen based on the new activity order.  Note that
    // as a result of this, it can call back into the activity
    // manager with a new orientation.  We don't care about that,
    // because the activity is not currently running so we are
    // just restarting it anyway.
    if (checkConfig) {
        Configuration config = mWindowManager.updateOrientationFromAppTokens(
                mService.mConfiguration,
                r.mayFreezeScreenLocked(app) ? r.appToken : null);
        mService.updateConfigurationLocked(config, r, false, false);
    }

    r.app = app;
    app.waitingToKill = null;
    r.launchCount++;
    r.lastLaunchTime = SystemClock.uptimeMillis();

    if (localLOGV) Slog.v(TAG, "Launching: " + r);

    int idx = app.activities.indexOf(r);
    if (idx < 0) {
        app.activities.add(r);
    }
    mService.updateLruProcessLocked(app, true, true);

    final ActivityStack stack = r.task.stack;
    try {
        if (app.thread == null) {
            throw new RemoteException();
        }
        List<ResultInfo> results = null;
        List<Intent> newIntents = null;
        if (andResume) {
            results = r.results;
            newIntents = r.newIntents;
        }
        if (DEBUG_SWITCH) Slog.v(TAG, "Launching: " + r
                + " icicle=" + r.icicle
                + " with results=" + results + " newIntents=" + newIntents
                + " andResume=" + andResume);
        if (andResume) {
            EventLog.writeEvent(EventLogTags.AM_RESTART_ACTIVITY,
                    r.userId, System.identityHashCode(r),
                    r.task.taskId, r.shortComponentName);
        }
        if (r.isHomeActivity() && r.isNotResolverActivity()) {
            // Home process is the root process of the task.
            mService.mHomeProcess = r.task.mActivities.get(0).app;
        }
        mService.ensurePackageDexOpt(r.intent.getComponent().getPackageName());
        r.sleeping = false;
        r.forceNewConfig = false;
        mService.showAskCompatModeDialogLocked(r);
        r.compat = mService.compatibilityInfoForPackageLocked(r.info.applicationInfo);
        String profileFile = null;
        ParcelFileDescriptor profileFd = null;
        boolean profileAutoStop = false;
        if (mService.mProfileApp != null && mService.mProfileApp.equals(app.processName)) {
            if (mService.mProfileProc == null || mService.mProfileProc == app) {
                mService.mProfileProc = app;
                profileFile = mService.mProfileFile;
                profileFd = mService.mProfileFd;
                profileAutoStop = mService.mAutoStopProfiler;
            }
        }
        app.hasShownUi = true;
        app.pendingUiClean = true;
        if (profileFd != null) {
            try {
                profileFd = profileFd.dup();
            } catch (IOException e) {
                if (profileFd != null) {
                    try {
                        profileFd.close();
                    } catch (IOException o) {
                    }
                    profileFd = null;
                }
            }
        }
        app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_TOP);
        app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                System.identityHashCode(r), r.info,
                new Configuration(mService.mConfiguration), r.compat,
                app.repProcState, r.icicle, results, newIntents, !andResume,
                mService.isNextTransitionForward(), profileFile, profileFd,
                profileAutoStop);

        if ((app.info.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0) {
            // This may be a heavy-weight process!  Note that the package
            // manager will ensure that only activity can run in the main
            // process of the .apk, which is the only thing that will be
            // considered heavy-weight.
            if (app.processName.equals(app.info.packageName)) {
                if (mService.mHeavyWeightProcess != null
                        && mService.mHeavyWeightProcess != app) {
                    Slog.w(TAG, "Starting new heavy weight process " + app
                            + " when already running "
                            + mService.mHeavyWeightProcess);
                }
                mService.mHeavyWeightProcess = app;
                Message msg = mService.mHandler.obtainMessage(
                        ActivityManagerService.POST_HEAVY_NOTIFICATION_MSG);
                msg.obj = r;
                mService.mHandler.sendMessage(msg);
            }
        }

    } catch (RemoteException e) {
        if (r.launchFailed) {
            // This is the second time we failed -- finish activity
            // and give up.
            Slog.e(TAG, "Second failure launching "
                  + r.intent.getComponent().flattenToShortString()
                  + ", giving up", e);
            mService.appDiedLocked(app, app.pid, app.thread);
            stack.requestFinishActivityLocked(r.appToken, Activity.RESULT_CANCELED, null,
                    "2nd-crash", false);
            return false;
        }

        // This is the first time we failed -- restart process and
        // retry.
        app.activities.remove(r);
        throw e;
    }

    r.launchFailed = false;
    if (stack.updateLRUListLocked(r)) {
        Slog.w(TAG, "Activity " + r
              + " being launched, but already in LRU list");
    }

    if (andResume) {
        // As part of the process of launching, ActivityThread also performs
        // a resume.
        stack.minimalResumeActivityLocked(r);
    } else {
        // This activity is not starting in the resumed state... which
        // should look like we asked it to pause+stop (but remain visible),
        // and it has done so and reported back the current icicle and
        // other state.
        if (DEBUG_STATES) Slog.v(TAG, "Moving to STOPPED: " + r
                + " (starting in stopped state)");
        r.state = ActivityState.STOPPED;
        r.stopped = true;
    }

    // Launch the new version setup screen if needed.  We do this -after-
    // launching the initial activity (that is, home), so that it can have
    // a chance to initialize itself while in the background, making the
    // switch back to it faster and look better.
    if (isFrontStack(stack)) {
        mService.startSetupActivityLocked();
    }

    return true;
}
 

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r), r.info,new Configuration(mService.mConfiguration), r.compat,
app.repProcState, r.icicle, results, newIntents, !andResume,
mService.isNextTransitionForward(), profileFile, profileFd,
profileAutoStop);
 

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,int procState, Bundle state, List<ResultInfo> pendingResults,List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {

        updateProcessState(procState, false);

        ActivityClientRecord r = new ActivityClientRecord();

        r.token = token;
        r.ident = ident;
        r.intent = intent;
        r.activityInfo = info;
        r.compatInfo = compatInfo;
        r.state = state;

        r.pendingResults = pendingResults;
        r.pendingIntents = pendingNewIntents;

        r.startsNotResumed = notResumed;
        r.isForward = isForward;

        r.profileFile = profileName;
        r.profileFd = profileFd;
        r.autoStopProfiler = autoStopProfiler;

        updatePendingConfiguration(curConfig);

        queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
    }
 

public void handleMessage(Message msg) {    
    if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
        switch (msg.what) {
            case LAUNCH_ACTIVITY: {
                Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                ActivityClientRecord r = (ActivityClientRecord)msg.obj;

                r.packageInfo = getPackageInfoNoCheck(
                        r.activityInfo.applicationInfo, r.compatInfo);
                handleLaunchActivity(r, null);
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            } break;

     ..........
}