android:launchMode
An instruction on how the activity should be launched. There are four modes that work in conjunction with activity flags (FLAG_ACTIVITY_* constants) in Intent objects to determine what should happen when the activity is called upon to handle an intent. They are:
"standard"
"singleTop"
"singleTask"
"singleInstance" The default mode is "standard".
Activity有四种启动模式，默认为标准型。还是先给出表格，看的更加清楚。
| Use Cases | Launch Mode | Multiple Instances? | Comments |
| Normal launches for most activities | "standard" | Yes | Default. The system always creates a new instance of the activity in the target task and routes the intent to it. |
| "singleTop" | Conditionally | If an instance of the activity already exists at the top of the target task, the system routes the intent to that instance through a call to its onNewIntent() method, rather than creating a new instance of the activity. | |
| Specialized launches (not recommended for general use) | "singleTask" | No | The system creates the activity at the root of a new task and routes the intent to it. However, if an instance of the activity already exists, the system routes the intent to existing instance through a call to its onNewIntent() method, rather than creating a new one. |
| "singleInstance" | No | Same as "singleTask", except that the system doesn't launch any other activities into the task holding the instance. The activity is always the single and only member of its task. |
"standard"可以有多个相同实例，并且每次启动一个Activity，系统总是在任务栈中创建一个Activity新的实例并且为它设置意图的路径。
"singleTop"的多个实例是有条件的，如果一个Activity的实例已经存在与任务栈顶端，系统通过调用它的onNewIntent()函数来为意图设置到此实例的路径而非创建一个新的相同实例。
以上两个是常用的。
后两个不常用。
"singleTask"只有一个实例。在当前任务栈中，如果实例不存在，则会创建一个，如果这个Activity的实例已经存在，系统通过调用它的onNewIntent()函数来为意图设置到此实例的路径而非创建一个新的实例，即会结束它上面全部的Activity。如果另一个程序启动了这个Activity，并且此Activity已经在当前任务栈中创建，则不会在那个任务栈创建实例，而是使用引用指向当前任务栈中的实例。
例如：
B为singleTask模式
操作：A->B A->B->C A->B->C->B A->B->C->B->C->A A->B->C->B->C->A->B
实际：A->B A->B->C A->B A->B->C->A A->B
"singleInstance"Activity总是独占一个任务栈。如果A中启动B，系统为B另外开辟一个任务栈。