退出Android程序时清除所有activity的实现方法

import java.util.Stack;
public class ActivityManager {
  private static Stack<Activity> activityStack;
  private static ActivityManager instance;
  private ActivityManager() {
  }
  public static ActivityManager getScreenManager() {
    if (instance == null) {
      instance = new ActivityManager();
    }
    return instance;
  }
  //退出栈顶Activity
  public void popActivity(Activity activity) {
    if (activity != null) {
      //在从自定义集合中取出当前Activity时，也进行了Activity的关闭操作
      activity.finish();
      activityStack.remove(activity);
      activity = null;
    }
  }
  //获得当前栈顶Activity
  public Activity currentActivity() {
    Activity activity = null;
    if(!activityStack.empty())
     activity= activityStack.lastElement();
    return activity;
  }
  //将当前Activity推入栈中
  public void pushActivity(Activity activity) {
    if (activityStack == null) {
      activityStack = new Stack<Activity>();
    }
    activityStack.add(activity);
  }
  //退出栈中所有Activity
  public void popAllActivityExceptOne(Class cls) {
    while (true) {
      Activity activity = currentActivity();
      if (activity == null) {
        break;
      }
      if (activity.getClass().equals(cls)) {
        break;
      }
      popActivity(activity);
    }
  }
}

public class ApplicationEx extends Application {
  private static final String TAG = "ApplicationEx";
  private HttpClient httpClient; //采用apache网络连接组件
  private ActivityManager activityManager = null;
  public ApplicationEx() {
  }
  public ActivityManager getActivityManager() {
    return activityManager;
  }
  public void setActivityManager(ActivityManager activityManager) {
    this.activityManager = activityManager;
  }
  @Override
  public void onCreate() {
    super.onCreate();
    httpClient = createHttpClient();
   //初始化自定义Activity管理器
    activityManager = ActivityManager.getScreenManager();
  }
  @Override
  public void onLowMemory() {
    super.onLowMemory();
    shutdownHttpClient();
  }
  @Override
  public void onTerminate() {
    super.onTerminate();
    shutdownHttpClient();
  }
  private void shutdownHttpClient() {
    if (httpClient != null && httpClient.getConnectionManager() != null) {
      httpClient.getConnectionManager().shutdown();
    }
  }
  private HttpClient createHttpClient() {
    Log.d(TAG, "createHttpClient()...");
    HttpParams params = new BasicHttpParams();
    HttpProtocolParams.setVersion(params, HttpVersion.HTTP_1_1);
    HttpProtocolParams.setContentCharset(params, HTTP.UTF_8);
    HttpProtocolParams.setUseExpectContinue(params, true);
    SchemeRegistry schReg = new SchemeRegistry();
    schReg.register(new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
    schReg.register(new Scheme("https", SSLSocketFactory.getSocketFactory(), 443));
    //解决多线程访问安全问题
    ClientConnectionManager connectionManager = new ThreadSafeClientConnManager(params, schReg);
    return new DefaultHttpClient(connectionManager, params);
  }
  public HttpClient getHttpClient() {
    if (httpClient != null) {
      return httpClient;
    } else {
      return createHttpClient();
    }
  }
}

public abstract class AbstractTemplateActivity extends Activity {
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ApplicationEx application = (ApplicationEx) this.getApplication();
    application.getActivityManager().pushActivity(this);
  }
  @Override
  public void onBackPressed() {
    super.onBackPressed();
    ApplicationEx application = (ApplicationEx) getApplication();
    application.getActivityManager().popActivity(this);
  }
}