使用Handlerf发送消息或使用Handler轮询时，报错IllegalStateException：This message is already in use.；

 java.lang.IllegalStateException: { when=-107ms what=9 obj=com.saicmaxus.maxuslife.model.CarInfo@be47330 target=com.cc.fragment.OwnerServiceFragment$13 } This message is already in use.

很明显，它的意思是这个消息正在使用中，不可以使用；

                Message msgCarcarstatus = new Message();
                msgCarcarstatus.obj = carInfo;
                msgCarcarstatus.what = CARCARSTATUSUPDATAWHAT;
                mHandler.sendMessageDelayed(msgCarcarstatus,1000*15);
//                mHandler.sendMessageDelayed(msg,1000*15); //IllegalStateException:This message is already in use.

上面代码所示，我们再从新new Message传过去就可以了；

如果感觉这样每次都new效果不过不好，还可以这样使用，Message message = YourHandler.obtainMessage();不会重复创建Message，节省内存开销；

                Message msgCarcarstatus = mHandler.obtainMessage();
                msgCarcarstatus.obj = carInfo;
                msgCarcarstatus.what = CARCARSTATUSUPDATAWHAT;
                mHandler.sendMessageDelayed(msgCarcarstatus,1000*15);
//                mHandler.sendMessageDelayed(msg,1000*15); //IllegalStateException:This message is already in use.

看完整代码：

   private Handler mHandler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
                   if(msg.what == CARCARSTATUSUPDATAWHAT){
               CarInfo carInfo = (CarInfo) msg.obj;
                showCarCarstatus(carInfo.get1(),carInfo.get2(),true); 
                //轮询
                Message msgCarcarstatus = mHandler.obtainMessage();
                msgCarcarstatus.obj = carInfo;
                msgCarcarstatus.what = CARCARSTATUSUPDATAWHAT;
                mHandler.sendMessageDelayed(msgCarcarstatus,1000*15);
//                mHandler.sendMessageDelayed(msg,1000*15); //IllegalStateException:This message is already in use.
            }
        }
    };

//发送消息

private final int CARCARSTATUSUPDATAWHAT = 9;

//定时刷新
Message msg = new Message();
msg.obj = ownerList.get(position);
msg.what = CARCARSTATUSUPDATAWHAT;
mHandler.sendMessageDelayed(msg,1000*15);

移除标记是what的消息：mHandler.removeMessages(CARCARSTATUSUPDATAWHAT);

移除全部的消息：mHandler.removeCallbacksAndMessages(null);

附源码：

/**
 * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
 * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
 *  If you don't want that facility, just call Message.obtain() instead.
 */
public final Message obtainMessage()
{
    return Message.obtain(this);
}

/**
 * Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
 * @param h  Handler to assign to the returned Message object's <em>target</em> member.
 * @return A Message object from the global pool.
 */
public static Message obtain(Handler h) {
    Message m = obtain();
    m.target = h;

    return m;
}

/**
 * Return a new Message instance from the global pool. Allows us to
 * avoid allocating new objects in many cases.
 */
public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

注释说：从全局池返回一个新的Message实例。允许我们避免在很多情况下分配新对象

大概意思就是：如果MessagePool中存在Message，就把这个Message中的内容清空后返回；如果MessagePool中不存在message，就新建一个Message；所以，不管怎样我们拿到的Message都是一个全新的，都需要重新去赋值；