在非Activity（比如Service，BroadcastReceiver）中startActivity需要添加flag Intent.FLAG_ACTIVITY_NEW_TASK。否则会报Crash：android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag.
以下Service为前提说明
简单来说：
每个Activity启动都需要依赖一个Task栈，在Service中启动Activity时一般使用ApplicationContext，它没自己的Task栈，（Activity有自己的栈，但Service中保存Activity实例容易造成内存泄漏）。
补充说明：
在MainActivity启动时，系统自动创建了App的Task栈，并且taskAffinity默认为包名：com.test.haha。
以下为三种情况：
-
情况1：只在Activity添加
FLAG_ACTIVITY_NEW_TASK
使用Task栈和MainActivity的Task栈是一样的，因为没有在AndroidMainfest添加taskAffinity，那么taskAffinity的默认值为包名即com.test.haha。 -
情况2：只在AndroidMainfest添加taskAffinity 不添加flag，无效
-
情况3：同时在Activity添加
FLAG_ACTIVITY_NEW_TASK和 在AndroidMainfest添加taskAffinity(若指定为包名，则转到情况1)则创建新的Task栈，行为同启动模式singleTask。
可以使用adb shell dumpsys activity activities 或者 打印getTaskId()来验证。
请参考Google文档：任务和返回栈
注意： 当调用者需要从启动的Activity获取返回结果时，不能使用此标志。即使用startActivityForResult()要注意，没有返回值的。（见官网注释最后一句）
官网注释：
FLAG_ACTIVITY_NEW_TASK
If set, this activity will become the start of a new task on this history stack. A task (from the activity that started it to the next task activity) defines an atomic group of activities that the user can move to. Tasks can be moved to the foreground and background; all of the activities inside of a particular task always remain in the same order. See Tasks and Back Stack for more information about tasks.
This flag is generally used by activities that want to present a “launcher” style behavior: they give the user a list of separate things that can be done, which otherwise run completely independently of the activity launching them.
When using this flag, if a task is already running for the activity you are now starting, then a new activity will not be started; instead, the current task will simply be brought to the front of the screen with the state it was last in. See FLAG_ACTIVITY_MULTIPLE_TASK for a flag to disable this behavior.
This flag can not be used when the caller is requesting a result from the activity being launched.