转载请注明本文出自Cym的博客（http://blog.csdn.net/cym492224103）。谢谢支持。

之前讲了ym—— Android网络框架Volley(体验篇)，大家应该了解了volley的使用，接下来我们要看看怎样把volley使用到实战项目里面，我们先考虑下一些问题：

从上一篇来看 mQueue 仅仅须要一个对象就可以，new RequestQueue对象对资源一种浪费，我们应该在application。以及能够把取消请求的方法也在application进行统一管理，看下面代码：

package com.chronocloud.lib.base;

import android.app.Application;
import android.text.TextUtils;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.VolleyLog;
import com.android.volley.toolbox.Volley;

public class ApplicationController extends Application {

	/**
	 * Log or request TAG
	 */
	public static final String TAG = "VolleyPatterns";

	/**
	 * Global request queue for Volley
	 */
	private RequestQueue mRequestQueue;

	/**
	 * A singleton instance of the application class for easy access in other
	 * places
	 */
	private static ApplicationController sInstance;

	@Override
	public void onCreate() {
		super.onCreate();

		// initialize the singleton
		sInstance = this;
	}

	/**
	 * @return ApplicationController singleton instance
	 */
	public static synchronized ApplicationController getInstance() {
		return sInstance;
	}

	/**
	 * @return The Volley Request queue, the queue will be created if it is null
	 */
	public RequestQueue getRequestQueue() {
		// lazy initialize the request queue, the queue instance will be
		// created when it is accessed for the first time
		if (mRequestQueue == null) {
			// 1
			// 2
			synchronized (ApplicationController.class) {
				if (mRequestQueue == null) {
					mRequestQueue = Volley
							.newRequestQueue(getApplicationContext());
				}
			}
		}
		return mRequestQueue;
	}

	/**
	 * Adds the specified request to the global queue, if tag is specified then
	 * it is used else Default TAG is used.
	 * 
	 * @param req
	 * @param tag
	 */
	public <T> void addToRequestQueue(Request<T> req, String tag) {
		// set the default tag if tag is empty
		req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);

		VolleyLog.d("Adding request to queue: %s", req.getUrl());

		getRequestQueue().add(req);
	}

	/**
	 * Adds the specified request to the global queue using the Default TAG.
	 * 
	 * @param req
	 * @param tag
	 */
	public <T> void addToRequestQueue(Request<T> req) {
		// set the default tag if tag is empty
		req.setTag(TAG);

		getRequestQueue().add(req);
	}

	/**
	 * Cancels all pending requests by the specified TAG, it is important to
	 * specify a TAG so that the pending/ongoing requests can be cancelled.
	 * 
	 * @param tag
	 */
	public void cancelPendingRequests(Object tag) {
		if (mRequestQueue != null) {
			mRequestQueue.cancelAll(tag);
		}
	}
}

接下来我们来看看，怎样自己定义Request

XmlRequest：

public class XMLRequest extends Request<XmlPullParser> {

	private final Listener<XmlPullParser> mListener;

	public XMLRequest(int method, String url, Listener<XmlPullParser> listener,
			ErrorListener errorListener) {
		super(method, url, errorListener);
		mListener = listener;
	}

	public XMLRequest(String url, Listener<XmlPullParser> listener, ErrorListener errorListener) {
		this(Method.GET, url, listener, errorListener);
	}

	@Override
	protected Response<XmlPullParser> parseNetworkResponse(NetworkResponse response) {
		try {
			String xmlString = new String(response.data,
					HttpHeaderParser.parseCharset(response.headers));
			XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
			XmlPullParser xmlPullParser = factory.newPullParser();
			xmlPullParser.setInput(new StringReader(xmlString));
			return Response.success(xmlPullParser, HttpHeaderParser.parseCacheHeaders(response));
		} catch (UnsupportedEncodingException e) {
			return Response.error(new ParseError(e));
		} catch (XmlPullParserException e) {
			return Response.error(new ParseError(e));
		}
	}

	@Override
	protected void deliverResponse(XmlPullParser response) {
		mListener.onResponse(response);
	}

}

public class GsonRequest<T> extends Request<T> {

	private final Listener<T> mListener;

	private Gson mGson;

	private Class<T> mClass;

	public GsonRequest(int method, String url, Class<T> clazz, Listener<T> listener,
			ErrorListener errorListener) {
		super(method, url, errorListener);
		mGson = new Gson();
		mClass = clazz;
		mListener = listener;
	}

	public GsonRequest(String url, Class<T> clazz, Listener<T> listener,
			ErrorListener errorListener) {
		this(Method.GET, url, clazz, listener, errorListener);
	}

	@Override
	protected Response<T> parseNetworkResponse(NetworkResponse response) {
		try {
			String jsonString = new String(response.data,
					HttpHeaderParser.parseCharset(response.headers));
			return Response.success(mGson.fromJson(jsonString, mClass),
					HttpHeaderParser.parseCacheHeaders(response));
		} catch (UnsupportedEncodingException e) {
			return Response.error(new ParseError(e));
		}
	}

	@Override
	protected void deliverResponse(T response) {
		mListener.onResponse(response);
	}

}

	// 共用失败回调
	private class StrErrListener implements ErrorListener {

		@Override
		public void one rrorResponse(VolleyError arg0) {
			Toast.makeText(mContext,
					VolleyErrorHelper.getMessage(arg0, mContext),
					Toast.LENGTH_LONG).show();
		}

	}

package com.example.volley;

import java.util.HashMap;
import java.util.Map;

import android.content.Context;

import com.android.volley.AuthFailureError;
import com.android.volley.NetworkError;
import com.android.volley.NetworkResponse;
import com.android.volley.NoConnectionError;
import com.android.volley.ServerError;
import com.android.volley.TimeoutError;
import com.android.volley.VolleyError;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
//正如前面代码看到的。在创建一个请求时，须要加入一个错误监听onErrorResponse。假设请求发生异常，会返回一个VolleyError实例。
//下面是Volley的异常列表：
//AuthFailureError：假设在做一个HTTP的身份验证，可能会发生这个错误。
//NetworkError：Socket关闭，服务器宕机，DNS错误都会产生这个错误。

//NoConnectionError：和NetworkError相似，这个是client没有网络连接。
//ParseError：在使用JsonObjectRequest或JsonArrayRequest时，假设接收到的JSON是畸形，会产生异常。
//SERVERERROR：服务器的响应的一个错误，最有可能的4xx或5xx HTTP状态代码。
//TimeoutError：Socket超时，服务器太忙或网络延迟会产生这个异常。
默认情况下，Volley的超时时间为2.5秒。
假设得到这个错误能够使用RetryPolicy。

public class VolleyErrorHelper {

	/**
	 * Returns appropriate message which is to be displayed to the user against
	 * the specified error object.
	 * 
	 * @param error
	 * @param context
	 * @return
	 */
	public static String getMessage(Object error, Context context) {
		if (error instanceof TimeoutError) {
			return context.getResources().getString(
					R.string.generic_server_down);
		} else if (isServerProblem(error)) {
			return handleServerError(error, context);
		} else if (isNetworkProblem(error)) {
			return context.getResources().getString(R.string.no_internet);
		}
		return context.getResources().getString(R.string.generic_error);
	}

	/**
	 * Determines whether the error is related to network
	 * 
	 * @param error
	 * @return
	 */
	private static boolean isNetworkProblem(Object error) {
		return (error instanceof NetworkError)
				|| (error instanceof NoConnectionError);
	}

	/**
	 * Determines whether the error is related to server
	 * 
	 * @param error
	 * @return
	 */
	private static boolean isServerProblem(Object error) {
		return (error instanceof ServerError)
				|| (error instanceof AuthFailureError);
	}

	/**
	 * Handles the server error, tries to determine whether to show a stock
	 * message or to show a message retrieved from the server.
	 * 
	 * @param err
	 * @param context
	 * @return
	 */
	private static String handleServerError(Object err, Context context) {
		VolleyError error = (VolleyError) err;

		NetworkResponse response = error.networkResponse;

		if (response != null) {
			switch (response.statusCode) {
			case 404:
			case 422:
			case 401:
				try {
					// server might return error like this { "error":
					// "Some error occured" }
					// Use "Gson" to parse the result
					HashMap<String, String> result = new Gson().fromJson(
							new String(response.data),
							new TypeToken<Map<String, String>>() {
							}.getType());

					if (result != null && result.containsKey("error")) {
						return result.get("error");
					}

				} catch (Exception e) {
					e.printStackTrace();
				}
				// invalid request
				return error.getMessage();

			default:
				return context.getResources().getString(
						R.string.generic_server_down);
			}
		}
		return context.getResources().getString(R.string.generic_error);
	}
}

	<string name="no_internet">无网络连接~！
</string>
	<string name="generic_server_down">连接服务器失败~！
</string>
	<string name="generic_error">网络异常,请稍后再试~！</string>