一、启动模式
1、 standard模式。
默认模式，可以不用写配置。在这个模式下，都会默认创建一个新的实例。因此，在这种模式下，可以有多个相同的实例，也允许多个相同Activity叠加。应用场景：绝大多数Activity。
2、singleTop模式
栈顶复用模式，如果要开启的activity在任务栈的顶部已经存在，就不会创建新的实例，而是调用 onNewIntent() 方法来将 intent 转送给该实例，避免栈顶的activity被重复的创建。
3、singleTask模式
栈内复用模式，activity只会在任务栈里面存在一个实例。如果要激活的activity，在任务栈里面已经存在，就不会创建新的activity，而是复用这个已经存在的activity，调用onNewIntent() 方法，并且清空这个activity任务栈上面所有的activity, 如果实例不存在，就创建实例并放入栈中。
4、singleInstance模式
与 "singleTask" 相似，唯一不同的是系统不会将任何其他 Activity 启动到包含该实例的任务中。该 Activity 始终是其任务唯一的成员；由该 Activity 启动的任何 Activity 都会在其他的任务中打开。
onNewIntent() 使用注意
方法体中需手动调用 setIntent(intent)，否则之后的 getIntent() 获取的都是旧的 intent 对象。
二、常用的 Intent Flag
1、 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
* <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and Back
* Stack</a> for more information about tasks.
*
* <p>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.
*
* <p>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 {@link #FLAG_ACTIVITY_MULTIPLE_TASK} for a flag
* to disable this behavior.
*
* <p>This flag can not be used when the caller is requesting a result from
* the activity being launched.
*/
public static final int FLAG_ACTIVITY_NEW_TASK = 0x10000000;
如果设置了，此活动将成为此历史堆栈上新任务的开始。当使用这个 flag 时,如果 task 中已经有了你要启动的 Activity ，就不再启动一个新的 Activity；当前 task 会被带到前台。可以用 FLAG_ACTIVITY_MULTIPLE_TASK 使这种行为无效。当调用者 startActivityForResult() 时，不能使用此标志。
比如栈中情况是 A,B,C，在 C 中启动 D，如果在 Manifest.xml 文件中给 D 添加了 Affinity(默认是包名) 的值和 C 所在的 Task 中的不一样，则会在新标记的 Affinity 所存在的 Task 中看 D 是否已经启动，如果已经启动直接将 D 所在的 task 带入到前台，否则直接将 activity 启动；如果是默认的或者指定的 Affinity 和 Task 一样，就和标准模式一样启动一个新的 Activity。此 flag 与启动模式 singleTask 效果不太一样，对于非 Activity 启动的 Activity（比如Service或者通知中启动的Activity）需要显示的设置 Intent.FLAG_ACTIVITY_NEW_TASK。
2、 FLAG_ACTIVITY_CLEAR_TOP
/**
* If set, and the activity being launched is already running in the
* current task, then instead of launching a new instance of that activity,
* all of the other activities on top of it will be closed and this Intent
* will be delivered to the (now on top) old activity as a new Intent.
*
* <p>For example, consider a task consisting of the activities: A, B, C, D.
* If D calls startActivity() with an Intent that resolves to the component
* of activity B, then C and D will be finished and B receive the given
* Intent, resulting in the stack now being: A, B.
*
* <p>The currently running instance of activity B in the above example will
* either receive the new intent you are starting here in its
* onNewIntent() method, or be itself finished and restarted with the
* new intent. If it has declared its launch mode to be "multiple" (the
* default) and you have not set {@link #FLAG_ACTIVITY_SINGLE_TOP} in
* the same intent, then it will be finished and re-created; for all other
* launch modes or if {@link #FLAG_ACTIVITY_SINGLE_TOP} is set then this
* Intent will be delivered to the current instance's onNewIntent().
*
* <p>This launch mode can also be used to good effect in conjunction with
* {@link #FLAG_ACTIVITY_NEW_TASK}: if used to start the root activity
* of a task, it will bring any currently running instance of that task
* to the foreground, and then clear it to its root state. This is
* especially useful, for example, when launching an activity from the
* notification manager.
*
* <p>See
* <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and Back
* Stack</a> for more information about tasks.
*/
public static final int FLAG_ACTIVITY_CLEAR_TOP = 0x04000000;
如果申明了 launch mode 是 "multiple" (默认情况就是)且没有在同一个 Intent 中设置 FLAG_ACTIVITY_SINGLE_TOP，Activity 将会结束且重新创建（回调 onCreate 生命周期方法)；对于其他所有的 launch modes 或者设置了 FLAG_ACTIVITY_SINGLE_TOP，Intent 将被分发给实例的 onNewIntent() 方法。
比如栈中情况是 A,B,C,D，在 D 中启动 B(加入该flag)， 栈中的情况将为 A,B，B 会执行 onCreate() ...。如果希望与 launch mode 中 singleTask 效果相同执行 onNewIntent()，可以同时加上 FLAG_ACTIVITY_SINGLE_TOP。
3、 FLAG_ACTIVITY_SINGLE_TOP
/**
* If set, the activity will not be launched if it is already running
* at the top of the history stack.
*/
public static final int FLAG_ACTIVITY_SINGLE_TOP = 0x20000000;
如果设置，如果此 activity 已经在历史堆栈的顶部将不会被启动。
相当于 launch mode 的 singleTop，比如栈中情况是 A,B,C,D，在 D 中启动D(加入该flag)，栈中的情况还是 A,B,C,D。
4、 FLAG_ACTIVITY_CLEAR_TASK
/**
* If set in an Intent passed to {@link Context#startActivity Context.startActivity()},
* this flag will cause any existing task that would be associated with the
* activity to be cleared before the activity is started. That is, the activity
* becomes the new root of an otherwise empty task, and any old activities
* are finished. This can only be used in conjunction with {@link #FLAG_ACTIVITY_NEW_TASK}.
*/
public static final int FLAG_ACTIVITY_CLEAR_TASK = 0X00008000;
如果在一个 Intent 中设置，会导致在此 activity 开启之前，任何与该 activity 相关的 task 都会被清除。此 activity 将会是一个空 task 的最底部的 activity，之前所有的 activities 将被结束，此 flag 只能与 FLAG_ACTIVITY_NEW_TASK 配合使用。
5、 FLAG_ACTIVITY_REORDER_TO_FRONT
/**
* If set in an Intent passed to {@link Context#startActivity Context.startActivity()},
* this flag will cause the launched activity to be brought to the front of its
* task's history stack if it is already running.
*
* <p>For example, consider a task consisting of four activities: A, B, C, D.
* If D calls startActivity() with an Intent that resolves to the component
* of activity B, then B will be brought to the front of the history stack,
* with this resulting order: A, C, D, B.
*
* This flag will be ignored if {@link #FLAG_ACTIVITY_CLEAR_TOP} is also
* specified.
*/
public static final int FLAG_ACTIVITY_REORDER_TO_FRONT = 0X00020000;
如果在 Intent 中设置 ，如果待启动的 activity 已经开启在运行了，此 flag 将使其位于任务历史堆栈的前面。例如栈中情况是 A,B,C,D，如果 D 启动 B，栈中将变成A,C,D,B （B 将回调 onNewIntent() ）。如果 FLAG_ACTIVITY_CLEAR_TOP 也被指定，此标志将被忽略。
6、 FLAG_ACTIVITY_FORWARD_RESULT
/**
* If set and this intent is being used to launch a new activity from an
* existing one, then the reply target of the existing activity will be
* transfered to the new activity. This way the new activity can call
* {@link android.app.Activity#setResult} and have that result sent back to
* the reply target of the original activity.
*/
public static final int FLAG_ACTIVITY_FORWARD_RESULT = 0x02000000;
如果在 Intent 中设置此 flag 从现有的 activity 去开启一个新的 activity ，现有的 activty 将会把回复的目标转移给新 activity. 新 activity 可以调用 setResult() 将结果发送给现有 activity 的回复目标。
例如：A 通过 startActivityForResult 启动 B，B 启动 C，但 B 为过渡页可以 finish 了，A 在期望 C 把结果返回。这种情况，B 可以在启动 C 的时候加入该flag。
7、 FLAG_ACTIVITY_PREVIOUS_IS_TOP
/**
* If set and this intent is being used to launch a new activity from an
* existing one, the current activity will not be counted as the top
* activity for deciding whether the new intent should be delivered to
* the top instead of starting a new one. The previous activity will
* be used as the top, with the assumption being that the current activity
* will finish itself immediately.
*/
public static final int FLAG_ACTIVITY_PREVIOUS_IS_TOP = 0x01000000;
如果当前的 activity 在开启新 activity 的 intent 设置此 flag， 当前 activity 之前的 activity 将被当视为 top，当前的 activity 将立即结束。
例如：栈中情况 A,B,C，C 启动 D 时使用此标志，在启动时 C 不会被当成栈顶 Activity，而是 B 作为栈顶启动 D，然后 C 会 finish()。经常与 FLAG_ACTIVITY_FORWARD_RESULT 一起配合使用。
8、 FLAG_ACTIVITY_NO_HISTORY
/**
* If set, the new activity is not kept in the history stack. As soon as
* the user navigates away from it, the activity is finished. This may also
* be set with the {@link android.R.styleable#AndroidManifestActivity_noHistory
* noHistory} attribute.
*
* <p>If set, {@link android.app.Activity#onActivityResult onActivityResult()}
* is never invoked when the current activity starts a new activity which
* sets a result and finishes.
*/
public static final int FLAG_ACTIVITY_NO_HISTORY = 0x40000000;
如果设置了此 flag，开启的 activity 将不会存在历史堆栈中。一旦用户离开它，activity 就结束了。也可以用{@link android.R来设置。styleable # AndroidManifestActivity_noHistory noHistory}属性。如果设置了此 flag，activity 将不会回调 onActivityResult()。
9、 FLAG_ACTIVITY_TASK_ON_HOME
/**
* If set in an Intent passed to {@link Context#startActivity Context.startActivity()},
* this flag will cause a newly launching task to be placed on top of the current
* home activity task (if there is one). That is, pressing back from the task
* will always return the user to home even if that was not the last activity they
* saw. This can only be used in conjunction with {@link #FLAG_ACTIVITY_NEW_TASK}.
*/
public static final int FLAG_ACTIVITY_TASK_ON_HOME = 0X00004000;
如果 intent 中设置此 flag 将使新启动的 task 置于当前 home activity 任务之上(如果有的话)。也就是说，从任务中返回总是会将用户返回到home，即使这不是他们看到的最后一个 activity。此 flag 只能与 FLAG_ACTIVITY_NEW_TASK 配合使用。
10、FLAG_EXCLUDE_STOPPED_PACKAGES
* If set, this intent will not match any components in packages that
* are currently stopped. If this is not set, then the default behavior
* is to include such applications in the result.
如果设置，intent 将与当前停止的包中的任何组件不匹配。如果未设置此flag，则默认行为是在结果中包含此类应用程序。
