FLAG_ACTIVITY_CLEAR_TASK :如果在调用Context.startActivity时传递这个标记，将会导致任何用来放置该activity的已经存在的task里面的已经存在

的activity先清空，然后该activity再在该task中启动，也就是说，这个新启动的activity变为了这个空tas的根activity.所有老的activity都结束掉。

该标志必须和FLAG_ACTIVITY_NEW_TASK一起使用。

示例：

我们有三个activity:MainActivity、ActivityA、ActivityB. 依次从MainActivity中启动ActivityA，从ActivityA中启动ActivityB.

首先我们所有的都使用缺省的，也即没有设置taskAffinity、launchMode，启动activity时Intent也不带标记，我们看一下启动B之后的堆栈：

ACTIVITY MANAGER ACTIVITIES (dumpsys activity activities)    Main stack:    TaskRecord{4199dd68 #56 A com.leaves.ipanel U 0}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.leaves.ipanel/.MainActivity }    Hist #3: ActivityRecord{4140fdf8 u0 com.leaves.ipanel/.ActivityB}    Intent { act=com.leaves.ipanel.ActivityB cmp=com.leaves.ipanel/.ActivityB }    ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    Hist #2: ActivityRecord{413ff7e8 u0 com.leaves.ipanel/.ActivityA}    Intent { act=com.leaves.ipanel.ActivityA cmp=com.leaves.ipanel/.ActivityA }    ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    Hist #1: ActivityRecord{413f8b28 u0 com.leaves.ipanel/.MainActivity}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.leaves.ipanel/.MainActivity }    ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    TaskRecord{41350f60 #2 A com.android.launcher U 0}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10600000 cmp=com.android.launcher/com.android.launcher2.Launcher }    Hist #0: ActivityRecord{41616790 u0 com.android.launcher/com.android.launcher2.Launcher}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10000000 cmp=com.android.launcher/com.android.launcher2.Launcher }    ProcessRecord{41615818 628:com.android.launcher/1000}   

可以看到三者在同一个task中，且都存在。

接下来我们在启动ActivityB时添加FLAG_ACTIVITY_CLEAR_TASK和FLAG_ACTIVITY_NEW_TASK

public void onClick(View v) {    // TODO Auto-generated method stub    Log.i(TAG, "--onClick--task id = " + getCurrentTaskId());       Intent intent = new Intent("com.leaves.ipanel.ActivityB");      intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);    startActivity(intent);     
}  

再看下启动ActivityB之后的堆栈：

    ACTIVITY MANAGER ACTIVITIES (dumpsys activity activities)    Main stack:    TaskRecord{4245bd10 #57 A com.leaves.ipanel U 0}    Intent { act=com.leaves.ipanel.ActivityB flg=0x10008000 cmp=com.leaves.ipanel/.ActivityB }    Hist #1: ActivityRecord{413f8b28 u0 com.leaves.ipanel/.ActivityB}    Intent { act=com.leaves.ipanel.ActivityB flg=0x10008000 cmp=com.leaves.ipanel/.ActivityB }    ProcessRecord{41413010 13441:com.leaves.ipanel/u0a10061}    TaskRecord{41350f60 #2 A com.android.launcher U 0}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10600000 cmp=com.android.launcher/com.android.launcher2.Launcher }    Hist #0: ActivityRecord{41616790 u0 com.android.launcher/com.android.launcher2.Launcher}    Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10000000 cmp=com.android.launcher/com.android.launcher2.Launcher }    ProcessRecord{41615818 628:com.android.launcher/1000}    

可以看到,MainActivity和ActivityA都被结束掉了，原因就是因为该task用来装载ActivityB，而启动时又设置了FLAG_ACTIVITY_CLEAR_TASK和FLAG_ACTIVITY_NEW_TASK