POST的用法：

1.默認值為GET，請求類型更改為POST

StringRequest stringRequest = new StringRequest(Method.POST,url,successListener,ErrorListener);

2.覆寫getParams

@Override  
    protected Map<String, String> getParams() throws AuthFailureError {  
        Map<String, String> map = new HashMap<String, String>();  
        map.put("key", "value");  
        return map;  
    }