StringRequest的用法：

1.發出http請求，需要傳入三個參數(url,success response,error response)。

 StringRequest stringRequest = new StringRequest(url,  
                        new Response.Listener<String>() {  
                            @Override  
                            public void onResponse(String response) {  
                                Log.d("TAG", response);  
                            }  
                        }, new Response.ErrorListener() {  
                            @Override  
                            public void onErrorResponse(VolleyError error) {  
                                Log.e("TAG", error.getMessage(), error);  
                            }  
                        });

2.加入RequestQueue

mQueue.add(stringRequest);