使用OKHttp3替换Volley的底层网络请求

一、自定义Volley框架中处理网络请求的HttpStatck，下面我们就使用OKHttp3实现HttpStack的网络请求。
1、在build.gradle文件中添加依赖：

    compile 'com.mcxiaoke.volley:library:1.0.19'
    compile 'com.squareup.okio:okio:1.6.0'
    compile 'com.squareup.okhttp3:okhttp:3.0.0-RC1'

2、实现volley框架里的HttpStack接口，使用OKHttp3替换Volley的底层网络请求。

package com.bolome.network.request;

import com.android.volley.AuthFailureError;
import com.android.volley.Request;
import com.android.volley.toolbox.HttpStack;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.ProtocolVersion;
import org.apache.http.StatusLine;
import org.apache.http.entity.BasicHttpEntity;
import org.apache.http.message.BasicHeader;
import org.apache.http.message.BasicHttpResponse;
import org.apache.http.message.BasicStatusLine;

import java.io.IOException;
import java.util.Map;
import java.util.concurrent.TimeUnit;

import okhttp3.Headers;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Protocol;
import okhttp3.RequestBody;
import okhttp3.Response;
import okhttp3.ResponseBody;

/**
 * Created by android_ls on 16/1/10.
 */
public class OkHttpStack implements HttpStack {
    private final OkHttpClient mClient;

    public OkHttpStack(OkHttpClient client) {
        this.mClient = client;
    }

    private static HttpEntity entityFromOkHttpResponse(Response response) throws IOException {
        BasicHttpEntity entity = new BasicHttpEntity();
        ResponseBody body = response.body();

        entity.setContent(body.byteStream());
        entity.setContentLength(body.contentLength());
        entity.setContentEncoding(response.header("Content-Encoding"));

        if (body.contentType() != null) {
            entity.setContentType(body.contentType().type());
        }
        return entity;
    }

    @SuppressWarnings("deprecation")
    private static void setConnectionParametersForRequest
            (okhttp3.Request.Builder builder, Request request)
            throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            case Request.Method.DEPRECATED_GET_OR_POST:
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    builder.post(RequestBody.create
                            (MediaType.parse(request.getPostBodyContentType()), postBody));
                }
                break;

            case Request.Method.GET:
                builder.get();
                break;

            case Request.Method.DELETE:
                builder.delete();
                break;

            case Request.Method.POST:
                builder.post(createRequestBody(request));
                break;

            case Request.Method.PUT:
                builder.put(createRequestBody(request));
                break;

            case Request.Method.HEAD:
                builder.head();
                break;

            case Request.Method.OPTIONS:
                builder.method("OPTIONS", null);
                break;

            case Request.Method.TRACE:
                builder.method("TRACE", null);
                break;

            case Request.Method.PATCH:
                builder.patch(createRequestBody(request));
                break;

            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }

    private static RequestBody createRequestBody(Request request) throws AuthFailureError {
        final byte[] body = request.getBody();
        if (body == null) return null;

        return RequestBody.create(MediaType.parse(request.getBodyContentType()), body);
    }

    private static ProtocolVersion parseProtocol(final Protocol protocol) {
        switch (protocol) {
            case HTTP_1_0:
                return new ProtocolVersion("HTTP", 1, 0);
            case HTTP_1_1:
                return new ProtocolVersion("HTTP", 1, 1);
            case SPDY_3:
                return new ProtocolVersion("SPDY", 3, 1);
            case HTTP_2:
                return new ProtocolVersion("HTTP", 2, 0);
        }

        throw new IllegalAccessError("Unkwown protocol");
    }

    @Override
    public HttpResponse performRequest(Request request, Map additionalHeaders)
            throws IOException, AuthFailureError {
        int timeoutMs = request.getTimeoutMs();
        OkHttpClient client = mClient.newBuilder()
                .readTimeout(timeoutMs, TimeUnit.MILLISECONDS)
                .connectTimeout(timeoutMs, TimeUnit.MILLISECONDS)
                .writeTimeout(timeoutMs, TimeUnit.MILLISECONDS)
                .build();

        okhttp3.Request.Builder okHttpRequestBuilder = new okhttp3.Request.Builder();
        Map headers = request.getHeaders();
        for (final String name : headers.keySet()) {
            okHttpRequestBuilder.addHeader(name, headers.get(name));
        }

        for (final String name : additionalHeaders.keySet()) {
            okHttpRequestBuilder.addHeader(name, additionalHeaders.get(name));
        }

        setConnectionParametersForRequest(okHttpRequestBuilder, request);

        okhttp3.Request okhttp3Request = okHttpRequestBuilder.url(request.getUrl()).build();
        Response okHttpResponse = client.newCall(okhttp3Request).execute();

        StatusLine responseStatus = new BasicStatusLine
                (
                        parseProtocol(okHttpResponse.protocol()),
                        okHttpResponse.code(),
                        okHttpResponse.message()
                );

        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        response.setEntity(entityFromOkHttpResponse(okHttpResponse));

        Headers responseHeaders = okHttpResponse.headers();
        for (int i = 0, len = responseHeaders.size(); i < len; i++) {
            final String name = responseHeaders.name(i), value = responseHeaders.value(i);
            if (name != null) {
                response.addHeader(new BasicHeader(name, value));
            }
        }

        return response;
    }

}

3、创建Volley队列RequestQueue

RequestQueue requestQueue = Volley.newRequestQueue(this, new OkHttpStack(new OkHttpClient()));

二、Volley是Google官方发布的异步网络请求框架，试用场景数据量小，通信频繁的网络操作。Volley 异步网络请求分析：

上图是官方给出的Volley架构图，蓝色为主线程，绿色为缓存线程，橙色是网络线程。总的来说，就是一个请求队列和三种线程，UI线程（1个），Cache线程（1个）和Network线程（默认是4个）。

1、UI线程负责添加请求任务，执行任务结果；

2、Cache线程负责检查缓存，命中后直接将任务结果分发到主线程；

3、Network线程由多个任务线程（NetworkDispatcher）组成的，相当于一个大小为size的线程池，这些线程会同时启动，并持续的从任务队列中获取待执行的任务，任务执行完后会将结果分发到UI线程。

4、Request：请求的抽象类。StringRequest、JsonRequest、ImageRequest 都是它的子类，表示某种类型的请求。可扩展性强。

5、RequestQueue.java：请求队列，里面包含一个CacheDispatcher(用于处理走缓存请求的调度线程)、NetworkDispatcher数组(用于处理走网络请求的调度线程)，一个ResponseDelivery(返回结果分发接口)，通过 start() 方法启动时会启动CacheDispatcher和NetworkDispatchers。然后看创建请求队列方法内的代码：

public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

        String userAgent = "volley/0";
        try {
            String packageName = context.getPackageName();
            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
            userAgent = packageName + "/" + info.versionCode;
        } catch (NameNotFoundException e) {
        }

        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);

        RequestQueue queue;
        if (maxDiskCacheBytes <= -1)
        {
            // No maximum size specified
            queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        }
        else
        {
            // Disk cache size specified
            queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
        }

        queue.start();

        return queue;
    }

看到这里，需要了解三个类的作用：
HttpStack.java：处理HTTP请求，返回请求结果。目前Volley中有基于 HttpURLConnection 的 HurlStack 和 基于 Apache HttpClient 的HttpClientStack。

Network.java：调用HttpStack处理请求，并将结果转换为可被ResponseDelivery处理的NetworkResponse。

Cache.java：缓存请求结果，Volley默认使用的是基于sdcard缓存的DiskBasedCache。NetworkDispatcher得到请求结果后判断是否需要存储在 Cache，CacheDispatcher会从 Cache 中取缓存结果。

创建Network需要HttpStatck，如果newRequestQueue传入的stack为null，API Level >= 9，采用基于 HttpURLConnection 的 HurlStack；小于 9，采用基于 HttpClient 的 HttpClientStack。

if (stack == null) {
    if (Build.VERSION.SDK_INT >= 9) {
        stack = new HurlStack();
    } else {
        // Prior to Gingerbread, HttpUrlConnection was unreliable.
        // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
        stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
    }
}

接下来启动所需的所有线程：

public void start() {
    stop();  // Make sure any currently running dispatchers are stopped.
    // Create the cache dispatcher and start it.
    mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
    mCacheDispatcher.start();

    // Create network dispatchers (and corresponding threads) up to the pool size.
    for (int i = 0; i < mDispatchers.length; i++) {
        NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
                mCache, mDelivery);
        mDispatchers[i] = networkDispatcher;
        networkDispatcher.start();
    }
}

CacheDispatcher.java：继承自Thread，用于调度处理「缓存请求」。启动后会不断从缓存请求队列中取请求处理，队列为空则等待，请求处理结束则将结果传递给ResponseDelivery去执行后续处理。当结果未缓存过、缓存失效或缓存需要刷新的情况下，该请求都需要重新进入NetworkDispatcher去调度处理。

NetworkDispatcher.java：继承自Thread，用于调度处理「网络请求」。启动后会不断从网络请求队列中取请求处理，队列为空则等待，请求处理结束则将结果传递给ResponseDelivery去执行后续处理，并判断结果是否要进行缓存。

ResponseDelivery.java：分发结果的interface，postResponse以及postError。

接下来再回头看一下Volley的架构图。
第一步：把请求加入缓存队列
第二步：「缓存调度线程」CacheDispatcher从缓存队列中取出一个请求，如果缓存命中，就读取缓存响应并解析，然后将结果返回到主线程
第三步：缓存未命中，请求被加入网络请求队列，「网络调度线程」NetworkDispatcher轮询取出请求，HTTP请求传输，解析响应，写入缓存，然后将结果返回到主线程

总结：
在主线程中调用RequestQueue的add()方法来添加一个网络请求任务。
1、以请求方法(GET、POST、PUT…) + URL作为缓存KEY，根据cacheKey从缓存队列里获取该网络请求任务。
a、如果为空的话则把这个任务加入到网络请求队列中；
b、如果不为空的话再判断该缓存是否已过期，如果已经过期了则同样把这个任务 加入到网络请求队列中；
c、否则就认为不需要重发网络请求，直接使用缓存中的数据即可。
2、加入到网络请求队列中的任务
a、发送HTTP请求
b、解析响应结果，写入缓存，并回调主线程。