Calling startActivity() from outside of an Activity

android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity

Caused by: android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?

Context中有一个startActivity方法，Activity继承自Context，重载了startActivity方法。如果使用 Activity的startActivity方法，不会有任何限制，而如果使用Context的startActivity方法的话，就需要开启一个新的task，遇到上面那个异常的，都是因为使用了Context的startActivity方法。解决办法是，加一个flag。

intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

以下为Context源码中的startActivity：

/**
 * Launch a new activity.  You will not receive any information about when
 * the activity exits.
 *
 * 
Note that if this method is being called from outside of an
 * {@link android.app.Activity} Context, then the Intent must include
 * the {@link Intent#FLAG_ACTIVITY_NEW_TASK} launch flag.  This is because,
 * without being started from an existing Activity, there is no existing
 * task in which to place the new activity and thus it needs to be placed
 * in its own separate task.
 *
 * 
This method throws {@link ActivityNotFoundException}
 * if there was no Activity found to run the given Intent.
 *
 * @param intent The description of the activity to start.
 *
 * @throws ActivityNotFoundException
 *
 * @see PackageManager#resolveActivity
 */
public abstract void startActivity(Intent intent);

以上解释了为什么使用Context的startActivity方法（比如在Service中或者BroadcastReceiver中启动Activity）为什么需要添加flag：FLAG_ACTIVITY_NEW_TASK

以上转自：http://www.cnblogs.com/rioder/archive/2011/11/02/2233584.html