深入解析OkHttp3

OkHttp是一个精巧的网络请求库，有如下特性:
1)支持http2，对一台机器的所有请求共享同一个socket
2)内置连接池，支持连接复用，减少延迟
3)支持透明的gzip压缩响应体
4)通过缓存避免重复的请求
5)请求失败时自动重试主机的其他ip，自动重定向
6)好用的API

其本身就是一个很强大的库，再加上Retrofit2、Picasso的这一套组合拳，使其愈发的受到开发者的关注。本篇博客，我将对Okhttp3进行分析（源码基于Okhttp3.4）。

如何引入Okhttp3？

配置Okhttp3非常简单，只需要在Android Studio 的gradle进行如下的配置：

 compile 'com.squareup.okhttp3:okhttp:3.4.1'

添加网络权限：

<uses-permission android:name="android.permission.INTERNET"/>

Okhttp3的基本使用

okHttp的get请求
okHttp的一般使用如下，okHttp默认使用的就是get请求

 String url = "http://write.blog.csdn.net/postlist/0/0/enabled/1";mHttpClient = new OkHttpClient();Request request = new Request.Builder().url(url).build();okhttp3.Response response = null;try {response = mHttpClient.newCall(request).execute();String json = response.body().string();Log.d("okHttp",json);} catch (IOException e) {e.printStackTrace();}}

我们试着将数据在logcat进行打印，发现会报错，原因就是不能在主线程中进行耗时的操作

说明mHttpClient.newCall(request).execute()是同步的，那有没有异步的方法呢，答案是肯定的,就是mHttpClient.newCall(request).enqueue()方法，里面需要new一个callback我们对代码进行修改，如下

public void requestBlog() {String url = "http://write.blog.csdn.net/postlist/0/0/enabled/1";mHttpClient = new OkHttpClient();Request request = new Request.Builder().url(url).build();
/* okhttp3.Response response = null;*//*response = mHttpClient.newCall(request).execute();*/mHttpClient.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {String json = response.body().string();Log.d("okHttp", json);}});}

Okhttp的POST请求

POST提交Json数据

private void postJson() throws IOException {String url = "http://write.blog.csdn.net/postlist/0/0/enabled/1";String json = "haha";OkHttpClient client = new OkHttpClient();RequestBody body = RequestBody.create(JSON, json);Request request = new Request.Builder().url(url).post(body).build();client.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {Log.d(TAG, response.body().string());}});}

POST提交键值对
很多时候我们会需要通过POST方式把键值对数据传送到服务器。 OkHttp提供了很方便的方式来做这件事情。

private void post(String url, String json) throws IOException {OkHttpClient client = new OkHttpClient();RequestBody formBody = new FormBody.Builder().add("name", "liming").add("school", "beida").build();Request request = new Request.Builder().url(url).post(formBody).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {String str = response.body().string();Log.i(TAG, str);}});}

异步上传文件
上传文件本身也是一个POST请求
定义上传文件类型

public static final MediaType MEDIA_TYPE_MARKDOWN= MediaType.parse("text/x-markdown; charset=utf-8");

将文件上传到服务器上：

private void postFile() {OkHttpClient mOkHttpClient = new OkHttpClient();File file = new File("/sdcard/demo.txt");Request request = new Request.Builder().url("https://api.github.com/markdown/raw").post(RequestBody.create(MEDIA_TYPE_MARKDOWN, file)).build();mOkHttpClient.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {Log.i(TAG, response.body().string());}});
}

添加如下权限：

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

提取响应头
典型的HTTP头像是一个 Map<String, String> :每个字段都有一个或没有值。但是一些头允许多个值，像Guava的Multimap。例如：HTTP响应里面提供的Vary响应头，就是多值的。OkHttp的api试图让这些情况都适用。
当写请求头的时候，使用header(name, value)可以设置唯一的name、value。如果已经有值，旧的将被移除，然后添加新的。使用addHeader(name, value)可以添加多值（添加，不移除已有的）。
当读取响应头时，使用header(name)返回最后出现的name、value。通常情况这也是唯一的name、value。如果没有值，那么header(name)将返回null。如果想读取字段对应的所有值，使用headers(name)会返回一个list。
为了获取所有的Header，Headers类支持按index访问。

private final OkHttpClient client = new OkHttpClient();public void run() throws Exception {Request request = new Request.Builder().url("https://api.github.com/repos/square/okhttp/issues").header("User-Agent", "OkHttp Headers.java").addHeader("Accept", "application/json; q=0.5").addHeader("Accept", "application/vnd.github.v3+json").build();Response response = client.newCall(request).execute();if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);System.out.println("Server: " + response.header("Server"));System.out.println("Date: " + response.header("Date"));System.out.println("Vary: " + response.headers("Vary"));
}

Post方式提交String
使用HTTP POST提交请求到服务。这个例子提交了一个markdown文档到web服务，以HTML方式渲染markdown。因为整个请求体都在内存中，因此避免使用此api提交大文档（大于1MB）。

private void postString() throws IOException {OkHttpClient client = new OkHttpClient();String postBody = ""+ "Releases\n"+ "--------\n"+ "\n"+ " * zhangfei\n"+ " * guanyu\n"+ " * liubei\n";Request request = new Request.Builder().url("https://api.github.com/markdown/raw").post(RequestBody.create(MEDIA_TYPE_MARKDOWN, postBody)).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {System.out.println(response.body().string());}});}

Post方式提交流

以流的方式POST提交请求体。请求体的内容由流写入产生。这个例子是流直接写入Okio的BufferedSink。你的程序可能会使用OutputStream，你可以使用BufferedSink.outputStream()来获取。

public static final MediaType MEDIA_TYPE_MARKDOWN= MediaType.parse("text/x-markdown; charset=utf-8");private void postStream() throws IOException {RequestBody requestBody = new RequestBody() {@Overridepublic MediaType contentType() {return MEDIA_TYPE_MARKDOWN;}@Overridepublic void writeTo(BufferedSink sink) throws IOException {sink.writeUtf8("Numbers\n");sink.writeUtf8("-------\n");for (int i = 2; i <= 997; i++) {sink.writeUtf8(String.format(" * %s = %s\n", i, factor(i)));}}private String factor(int n) {for (int i = 2; i < n; i++) {int x = n / i;if (x * i == n) return factor(x) + " × " + i;}return Integer.toString(n);}};Request request = new Request.Builder().url("https://api.github.com/markdown/raw").post(requestBody).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {System.out.println(response.body().string());}});
}

Post方式提交表单

private void postForm() {OkHttpClient client = new OkHttpClient();RequestBody formBody = new FormBody.Builder().add("search", "Jurassic Park").build();Request request = new Request.Builder().url("https://en.wikipedia.org/w/index.php").post(formBody).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {System.out.println(response.body().string());}});}

Post方式提交分块请求
MultipartBody 可以构建复杂的请求体，与HTML文件上传形式兼容。多块请求体中每块请求都是一个请求体，可以定义自己的请求头。这些请求头可以用来描述这块请求，例如他的Content-Disposition。如果Content-Length和Content-Type可用的话，他们会被自动添加到请求头中。

private static final String IMGUR_CLIENT_ID = "...";
private static final MediaType MEDIA_TYPE_PNG = MediaType.parse("image/png");private void postMultipartBody() {OkHttpClient client = new OkHttpClient();// Use the imgur image upload API as documented at https://api.imgur.com/endpoints/imageMultipartBody body = new MultipartBody.Builder("AaB03x").setType(MultipartBody.FORM).addPart(Headers.of("Content-Disposition", "form-data; name=\"title\""),RequestBody.create(null, "Square Logo")).addPart(Headers.of("Content-Disposition", "form-data; name=\"image\""),RequestBody.create(MEDIA_TYPE_PNG, new File("website/static/logo-square.png"))).build();Request request = new Request.Builder().header("Authorization", "Client-ID " + IMGUR_CLIENT_ID).url("https://api.imgur.com/3/image").post(body).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {System.out.println(response.body().string());}});
}

响应缓存
为了缓存响应，你需要一个你可以读写的缓存目录，和缓存大小的限制。这个缓存目录应该是私有的，不信任的程序应不能读取缓存内容。
一个缓存目录同时拥有多个缓存访问是错误的。大多数程序只需要调用一次new OkHttpClient()，在第一次调用时配置好缓存，然后其他地方只需要调用这个实例就可以了。否则两个缓存示例互相干扰，破坏响应缓存，而且有可能会导致程序崩溃。
响应缓存使用HTTP头作为配置。你可以在请求头中添加Cache-Control: max-stale=3600 ,OkHttp缓存会支持。你的服务通过响应头确定响应缓存多长时间，例如使用Cache-Control: max-age=9600。

int cacheSize = 10 * 1024 * 1024; // 10 MiB
Cache cache = new Cache(cacheDirectory, cacheSize);OkHttpClient.Builder builder = new OkHttpClient.Builder();
builder.cache(cache);
OkHttpClient client = builder.build();Request request = new Request.Builder().url("http://publicobject.com/helloworld.txt").build();Call call = client.newCall(request);
call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {String response1Body = response.body().string();System.out.println("Response 1 response:          " + response);System.out.println("Response 1 cache response:    " + response.cacheResponse());System.out.println("Response 1 network response:  " + response.networkResponse());}});

超时
没有响应时使用超时结束call。没有响应的原因可能是客户点链接问题、服务器可用性问题或者这之间的其他东西。OkHttp支持连接，读取和写入超时。

private void ConfigureTimeouts() {OkHttpClient.Builder builder = new OkHttpClient.Builder();OkHttpClient client = builder.build();client.newBuilder().connectTimeout(10, TimeUnit.SECONDS);client.newBuilder().readTimeout(10,TimeUnit.SECONDS);client.newBuilder().writeTimeout(10,TimeUnit.SECONDS);Request request = new Request.Builder().url("http://httpbin.org/delay/2") // This URL is served with a 2 second delay..build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {System.out.println("Response completed: " + response);}});}

简单封装okHttp框架

新建一个工具类OkHttpUtils
OkHttpClient必须是单例的，所以这里我们需要使用到单例设计模式,私有化构造函数，提供一个方法给外界获取OkHttpUtils实例对象

public class OkHttpUtils {private  static  OkHttpUtils mInstance;private OkHttpClient mHttpClient;private OkHttpUtils() {};public static  OkHttpUtils getInstance(){return  mInstance;}}

一般网络请求分为get和post请求两种，但无论哪种请求都是需要用到request的，所以我们首先封装一个request,创建一个doRequest方法，在其内先编写mHttpClient.newCall(request).enqueue(new Callback())相关逻辑

public  void doRequest(final Request request){mHttpClient.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {}});
}

我们需要自定义一个callback，BaseCallback,并将其传入request方法中

public class BaseCallback  {}

在OkHttpUtils中编写get和post方法

public void get(String url){}public void post(String url,Map<String,Object> param){}

post方法中构建request对象，这里我们需要创建一个buildRequest方法，用于生成request对象

private  Request buildRequest(String url,HttpMethodType methodType,Map<String,Object> params){return null;
}

这里需要定一个枚举对象HttpMethodType，用于区分是get还是post

enum  HttpMethodType{GET,POST,}

buildRequest方法根据HttpMethodType不同有相应的逻辑处理

private  Request buildRequest(String url,HttpMethodType methodType,Map<String,Object> params){Request.Builder builder = new Request.Builder().url(url);if (methodType == HttpMethodType.POST){builder.post(body);}else if(methodType == HttpMethodType.GET){builder.get();}return builder.build();}

builder.post()方法中需要一个body,所以我们需要创建一个方法builderFormData()方法用于返回RequestBody,这里内部逻辑后面再进行完善

private RequestBody builderFormData(Map<String,Object> params){return null;
}

于是buildRequest方法变成了这样

private  Request buildRequest(String url,HttpMethodType methodType,Map<String,Object> params){Request.Builder builder = new Request.Builder().url(url);if (methodType == HttpMethodType.POST){RequestBody body = builderFormData(params);builder.post(body);}else if(methodType == HttpMethodType.GET){builder.get();}return builder.build();}

get方法进行修改：

public void get(String url,BaseCallback callback){Request request = buildRequest(url,HttpMethodType.GET,null);doRequest(request,callback);}

post方法进行修改：

public void post(String url,Map<String,Object> params,BaseCallback callback){Request request = buildRequest(url,HttpMethodType.POST,params);doRequest(request,callback);
}

完善builderFormData()方法

private RequestBody builderFormData(Map<String,String> params){FormBody.Builder builder =  new FormBody.Builder();if(params!=null){for(Map.Entry<String,String> entry:params.entrySet()){builder.add(entry.getKey(),entry.getValue());}}return builder.build();
}

BaseCallback中定义一个抽象方法onBeforeRequest，这样做的理由是我们在加载网络数据成功前，一般都有进度条等显示，这个方法就是用来做这些处理的

public abstract class BaseCallback  {public  abstract void onBeforeRequest(Request request);}

OkHttpUtils的doRequest方法增加如下语句：

baseCallback.onBeforeRequest(request);

BaseCallback中多定义2个抽象方法

public abstract  void onFailure(Request request, Exception e) ;/***请求成功时调用此方法* @param response*/
public abstract  void onResponse(Response response);

由于Response的状态有多种，比如成功和失败，所以需要onResponse分解为3个抽象方法

/**** 状态码大于200，小于300 时调用此方法* @param response* @param t* @throws*/
public abstract void onSuccess(Response response,T t) ;/*** 状态码400，404，403，500等时调用此方法* @param response* @param code* @param e*/
public abstract void onError(Response response, int code,Exception e) ;/*** Token 验证失败。状态码401,402,403 等时调用此方法* @param response* @param code*/
public abstract void onTokenError(Response response, int code);

response.body.string()方法返回的都是String类型，而我们需要显示的数据其实是对象，所以我们就想抽取出方法，直接返回对象，由于我们不知道对象的类型是什么，所以我们在BaseCallback中使用范型

public abstract class BaseCallback<T>

BaseCallback中需要将泛型转换为Type，所以要声明Type类型

public   Type mType;

BaseCallback中需要如下一段代码，将泛型T转换为Type类型

static Type getSuperclassTypeParameter(Class<?> subclass)
{Type superclass = subclass.getGenericSuperclass();if (superclass instanceof Class){throw new RuntimeException("Missing type parameter.");}ParameterizedType parameterized = (ParameterizedType) superclass;return $Gson$Types.canonicalize(parameterized.getActualTypeArguments()[0]);
}

在BaseCallback的构造函数中进行mType进行赋值

public BaseCallback()
{mType = getSuperclassTypeParameter(getClass());
}

OkHttpUtils中doRequest方法的onFailure与onResponse方法会相应的去调用baseCallback的方法

mHttpClient.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {baseCallback.onFailure(request,e);}@Overridepublic void onResponse(Call call, Response response) throws IOException {if(response.isSuccessful()) {baseCallback.onSuccess(response,null);}else {baseCallback.onError(response,response.code(),null);}/*mGson.fromJson(response.body().string(),baseCallback.mType);*/}});

onResponse方法中成功的情况又有区分，根据mType的类型不同有相应的处理逻辑，同时还要考虑Gson解析错误的情况

@Override
public void onResponse(Call call, Response response) throws IOException {if(response.isSuccessful()) {String resultStr = response.body().string();if (baseCallback.mType == String.class){baseCallback.onSuccess(response,resultStr);}else {try {Object obj = mGson.fromJson(resultStr, baseCallback.mType);baseCallback.onSuccess(response,obj);}catch (com.google.gson.JsonParseException e){ // Json解析的错误baseCallback.onError(response,response.code(),e);}}}else {baseCallback.onError(response,response.code(),null);}}

构造函数中进行一些全局变量的初始化的操作，还有一些超时的设计

private OkHttpUtils() {mHttpClient = new OkHttpClient();OkHttpClient.Builder builder = mHttpClient.newBuilder();builder.connectTimeout(10, TimeUnit.SECONDS);builder.readTimeout(10,TimeUnit.SECONDS);builder.writeTimeout(30,TimeUnit.SECONDS);mGson = new Gson();};

静态代码块初始化OkHttpUtils对象

static {mInstance = new OkHttpUtils();
}

在okHttpUtils内，需要创建handler进行UI界面的更新操作，创建callbackSuccess方法

private void callbackSuccess(final  BaseCallback callback , final Response response, final Object obj ){mHandler.post(new Runnable() {@Overridepublic void run() {callback.onSuccess(response, obj);}});
}

doRequest方法的onResponse方法也进行相应的改写

if (baseCallback.mType == String.class){/*baseCallback.onSuccess(response,resultStr);*/callbackSuccess(baseCallback,response,resultStr);
}

创建callbackError方法

private void callbackError(final BaseCallback callback, final Response response, final Exception e) {mHandler.post(new Runnable() {@Overridepublic void run() {callback.onError(response, response.code(), e);}});
}

将doRequest方法的onResponse方法中的baseCallback.onError(response,response.code(),e);替换为callbackError(baseCallback,response,e);方法

@Override
public void onResponse(Call call, Response response) throws IOException {if(response.isSuccessful()) {String resultStr = response.body().string();if (baseCallback.mType == String.class){/*baseCallback.onSuccess(response,resultStr);*/callbackSuccess(baseCallback,response,resultStr);}else {try {Object obj = mGson.fromJson(resultStr, baseCallback.mType);/*baseCallback.onSuccess(response,obj);*/callbackSuccess(baseCallback,response,obj);}catch (com.google.gson.JsonParseException e){ // Json解析的错误/*baseCallback.onError(response,response.code(),e);*/callbackError(baseCallback,response,e);}}}else {callbackError(baseCallback,response,null);/*baseCallback.onError(response,response.code(),null);*/}}

至此，我们的封装基本完成。

OkHttp3源码分析

请求处理分析
当我们要请求网络的时候我们需要用OkHttpClient.newCall(request)进行execute或者enqueue操作，当我们调用newCall时：

/*** Prepares the {@code request} to be executed at some point in the future.*/
@Override public Call newCall(Request request) {return new RealCall(this, request);
}

实际返回的是一个RealCall类，我们调用enqueue异步请求网络实际上是调用了RealCall的enqueue方法：

@Override public void enqueue(Callback responseCallback) {synchronized (this) {if (executed) throw new IllegalStateException("Already Executed");executed = true;}client.dispatcher().enqueue(new AsyncCall(responseCallback));
}

最终的请求是dispatcher来完成的。

Dispatcher任务调度

Dispatcher的本质是异步请求的管理器，控制最大请求并发数和单个主机的最大并发数，并持有一个线程池负责执行异步请求。对同步的请求只是用作统计。他是如何做到控制并发呢，其实原理就在上面的2个execute代码里面,真正网络请求执行前后会调用executed和finished方法，而对于AsyncCall的finished方法后，会根据当前并发数目选择是否执行队列中等待的AsyncCall。并且如果修改Dispatcher的maxRequests或者maxRequestsPerHost也会触发这个过程。
Dispatcher主要用于控制并发的请求，它主要维护了以下变量：

/** 最大并发请求数*/
private int maxRequests = 64;
/** 每个主机最大请求数*/
private int maxRequestsPerHost = 5;
/** 消费者线程池 */
private ExecutorService executorService;
/** 将要运行的异步请求队列 */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/**正在运行的异步请求队列 */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** 正在运行的同步请求队列 */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

构造函数

public Dispatcher(ExecutorService executorService) {this.executorService = executorService;
}public Dispatcher() {
}public synchronized ExecutorService executorService() {if (executorService == null) {executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));}return executorService;
}

Dispatcher有两个构造函数，可以使用自己设定线程池，如果没有设定线程池则会在请求网络前自己创建线程池，这个线程池类似于CachedThreadPool比较适合执行大量的耗时比较少的任务。

异步请求

synchronized void enqueue(AsyncCall call) {if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {runningAsyncCalls.add(call);executorService().execute(call);} else {readyAsyncCalls.add(call);}
}

当正在运行的异步请求队列中的数量小于64并且正在运行的请求主机数小于5时则把请求加载到runningAsyncCalls中并在线程池中执行，否则就再入到readyAsyncCalls中进行缓存等待。

AsyncCall
线程池中传进来的参数就是AsyncCall它是RealCall的内部类，内部也实现了execute方法：

 @Override protected void execute() {boolean signalledCallback = false;try {Response response = getResponseWithInterceptorChain();if (retryAndFollowUpInterceptor.isCanceled()) {signalledCallback = true;responseCallback.onFailure(RealCall.this, new IOException("Canceled"));} else {signalledCallback = true;responseCallback.onResponse(RealCall.this, response);}} catch (IOException e) {if (signalledCallback) {// Do not signal the callback twice!Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);} else {responseCallback.onFailure(RealCall.this, e);}} finally {client.dispatcher().finished(this);}}
}

首先我们来看看最后一行，无论这个请求的结果如何都会执行client.dispatcher().finished(this)；

/** Used by {@code AsyncCall#run} to signal completion. */
void finished(AsyncCall call) {finished(runningAsyncCalls, call, true);
}/** Used by {@code Call#execute} to signal completion. */
void finished(RealCall call) {finished(runningSyncCalls, call, false);
}private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {int runningCallsCount;Runnable idleCallback;synchronized (this) {if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");if (promoteCalls) promoteCalls();runningCallsCount = runningCallsCount();idleCallback = this.idleCallback;}if (runningCallsCount == 0 && idleCallback != null) {idleCallback.run();}
}
finished方法将此次请求从runningAsyncCalls移除后还执行了promoteCalls方法：
private void promoteCalls() {if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {AsyncCall call = i.next();if (runningCallsForHost(call) < maxRequestsPerHost) {i.remove();runningAsyncCalls.add(call);executorService().execute(call);}if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.}
}

可以看到最关键的点就是会从readyAsyncCalls取出下一个请求，并加入runningAsyncCalls中并交由线程池处理。好了让我们再回到上面的AsyncCall的execute方法，我们会发getResponseWithInterceptorChain方法返回了Response，很明显这是在请求网络。

Interceptor拦截器
在回到RealCall中，我们看到无论是execute还是enqueue，真正的Response是通过这个函数getResponseWithInterceptorChain获取的，其他的代码都是用作控制与回调。而这里就是真正请求的入口，也是到了OkHttp的一个很精彩的设计:Interceptor与Chain
看一下RealCall中的getResponseWithInterceptorChain方法

private Response getResponseWithInterceptorChain() throws IOException {// Build a full stack of interceptors.List<Interceptor> interceptors = new ArrayList<>();interceptors.addAll(client.interceptors());interceptors.add(retryAndFollowUpInterceptor);interceptors.add(new BridgeInterceptor(client.cookieJar()));interceptors.add(new CacheInterceptor(client.internalCache()));interceptors.add(new ConnectInterceptor(client));if (!retryAndFollowUpInterceptor.isForWebSocket()) {interceptors.addAll(client.networkInterceptors());}interceptors.add(new CallServerInterceptor(retryAndFollowUpInterceptor.isForWebSocket()));Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest);return chain.proceed(originalRequest);
}

这也是与旧版本不一致的地方，在3.4.x以前，没有这些内部的这些拦截器，只有用户的拦截器与网络拦截器。而Request和Response是通过HttpEngine来完成的。在RealCall实现了用户拦截器与RetryAndFollowUp的过程，而在HttpEngine内部处理了请求转换、Cookie、Cache、网络拦截器、连接网络的过程。值得一提的是，在旧版是获取到Response后调用网络拦截器的拦截。
而在这里，RealInterceptorChain会递归的创建并以此调用拦截器，去掉诸多异常，简化版代码如下:

public Response proceed(Request request, StreamAllocation streamAllocation, HttpStream httpStream,Connection connection) throws IOException {if (index >= interceptors.size()) throw new AssertionError();calls++;// If we already have a stream, confirm that the incoming request will use it.if (this.httpStream != null && !sameConnection(request.url())) {throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)+ " must retain the same host and port");}// If we already have a stream, confirm that this is the only call to chain.proceed().if (this.httpStream != null && calls > 1) {throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)+ " must call proceed() exactly once");}// Call the next interceptor in the chain.RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpStream, connection, index + 1, request);Interceptor interceptor = interceptors.get(index);Response response = interceptor.intercept(next);// Confirm that the next interceptor made its required call to chain.proceed().if (httpStream != null && index + 1 < interceptors.size() && next.calls != 1) {throw new IllegalStateException("network interceptor " + interceptor+ " must call proceed() exactly once");}// Confirm that the intercepted response isn't null.if (response == null) {throw new NullPointerException("interceptor " + interceptor + " returned null");}return response;
}

Chain与Interceptor会互相递归调用，直到链的尽头。
我们看到，通过职责链模式，清楚地切开了不同的逻辑，每个拦截器完成自己的职责，从而完成用户的网络请求。
大概流程是:
1)先经过用户拦截器
2)RetryAndFollowUpInterceptor负责自动重试和进行必要的重定向
3)BridgeIntercetor负责将用户Request转换成一个实际的网络请求的Request，再调用下层的拦截器获取Response，最后再将网络Response转换成用户的Reponse
4)CacheInterceptor负责控制缓存
5)ConnectInterceptor负责进行连接主机
6)网络拦截器进行拦截
7)CallServerInterceptor是真正和服务器通信，完成http请求

连接与通信
在RetryAndFollowUpInterceptor中,会创建StreamAllocation，然后交给下游的ConnectInterceptor

@Override public Response intercept(Chain chain) throws IOException {RealInterceptorChain realChain = (RealInterceptorChain) chain;Request request = realChain.request();StreamAllocation streamAllocation = realChain.streamAllocation();// We need the network to satisfy this request. Possibly for validating a conditional GET.boolean doExtensiveHealthChecks = !request.method().equals("GET");HttpStream httpStream = streamAllocation.newStream(client, doExtensiveHealthChecks);RealConnection connection = streamAllocation.connection();return realChain.proceed(request, streamAllocation, httpStream, connection);
}

这里会创建一个HttpStream，并且取到一个RealConnection，继续交给下游的CallServerInterceptor。
我们跟踪进去看看，StreamAllocation里面做了什么

public HttpStream newStream(OkHttpClient client, boolean doExtensiveHealthChecks) {int connectTimeout = client.connectTimeoutMillis();int readTimeout = client.readTimeoutMillis();int writeTimeout = client.writeTimeoutMillis();boolean connectionRetryEnabled = client.retryOnConnectionFailure();try {RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,writeTimeout, connectionRetryEnabled, doExtensiveHealthChecks);HttpStream resultStream;if (resultConnection.framedConnection != null) {resultStream = new Http2xStream(client, this, resultConnection.framedConnection);} else {resultConnection.socket().setSoTimeout(readTimeout);resultConnection.source.timeout().timeout(readTimeout, MILLISECONDS);resultConnection.sink.timeout().timeout(writeTimeout, MILLISECONDS);resultStream = new Http1xStream(client, this, resultConnection.source, resultConnection.sink);}synchronized (connectionPool) {stream = resultStream;return resultStream;}} catch (IOException e) {throw new RouteException(e);}
}

这里的代码逻辑是这样的，找一个健康的连接，设置超时时间，然后根据协议创建一个HttpStream并返回。
继续跟进去看findHealthyConnection:

private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,int writeTimeout, boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)throws IOException {while (true) {RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,connectionRetryEnabled);// If this is a brand new connection, we can skip the extensive health checks.synchronized (connectionPool) {if (candidate.successCount == 0) {return candidate;}}// Do a (potentially slow) check to confirm that the pooled connection is still good. If it// isn't, take it out of the pool and start again.if (!candidate.isHealthy(doExtensiveHealthChecks)) {noNewStreams();continue;}return candidate;}
}

上面的逻辑也很简单，在findConnection中找一个连接，然后做健康检查，如果不健康就回收，并再次循环，那么真正寻找连接的代码就在findConnection里面了:

/*** Returns a connection to host a new stream. This prefers the existing connection if it exists,* then the pool, finally building a new connection.*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,boolean connectionRetryEnabled) throws IOException {Route selectedRoute;synchronized (connectionPool) {if (released) throw new IllegalStateException("released");if (stream != null) throw new IllegalStateException("stream != null");if (canceled) throw new IOException("Canceled");RealConnection allocatedConnection = this.connection;if (allocatedConnection != null && !allocatedConnection.noNewStreams) {return allocatedConnection;}// Attempt to get a connection from the pool.RealConnection pooledConnection = Internal.instance.get(connectionPool, address, this);if (pooledConnection != null) {this.connection = pooledConnection;return pooledConnection;}selectedRoute = route;}if (selectedRoute == null) {selectedRoute = routeSelector.next();synchronized (connectionPool) {route = selectedRoute;refusedStreamCount = 0;}}RealConnection newConnection = new RealConnection(selectedRoute);acquire(newConnection);synchronized (connectionPool) {Internal.instance.put(connectionPool, newConnection);this.connection = newConnection;if (canceled) throw new IOException("Canceled");}newConnection.connect(connectTimeout, readTimeout, writeTimeout, address.connectionSpecs(),connectionRetryEnabled);routeDatabase().connected(newConnection.route());return newConnection;
}

这里大概分成分成3大步:
1)如果当前有连接并且符合要求的话，就直接返回
2)如果线程池能取到一个符合要求的连接的话，就直接返回
3)如果Route为空，从RouteSelector取一个Route，然后新建一个RealConnection，并放入ConnectionPool，随后调用connect，再返回

也就是说不管当前走的是步骤1还是2，一开始一定是从3开始的，也就是在RealConnection的connect中真正完成了socket连接。
connect里面代码比较长，真正要做的就是一件事，如果是https请求并且是http代理，则建立隧道连接，隧道连接请参考RFC2817，否则建立普通连接。
这两者都调用了2个函数:connectSocket(connectTimeout, readTimeout); establishProtocol(readTimeout, writeTimeout, connectionSpecSelector);
但是隧道连接则多了一个代理认证的过程，可能会反复的connectSocket和构造请求。
看一下connectSocket:

private void connectSocket(int connectTimeout, int readTimeout) throws IOException {Proxy proxy = route.proxy();Address address = route.address();rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP? address.socketFactory().createSocket(): new Socket(proxy);rawSocket.setSoTimeout(readTimeout);try {Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);} catch (ConnectException e) {throw new ConnectException("Failed to connect to " + route.socketAddress());}source = Okio.buffer(Okio.source(rawSocket));sink = Okio.buffer(Okio.sink(rawSocket));
}

就是根据Route来创建socket，在connect，随后将rawSocket的InputStream与OutputStream包装成Source与Sink。这里提一下，OkHttp是依赖Okio的，Okio封装了Java的IO API，如这里的Source与Sink，非常简洁实用。

而establishProtocol里，如果是https则走TLS协议，生成一个SSLSocket，并进行握手和验证，同时如果是HTTP2或者SPDY3的话，则生成一个FrameConnection。这里不再多提，HTTP2和HTTP1.X大相径庭，我们这里主要是分析HTTP1.X的连接，后面有机会我们会单独开篇讲HTTP2。同时TLS相关的话题这里也一并略过，想了解的朋友可以看一看相应的Java API和HTTPS连接的资料。

再回到StreamAllcation.newStream的代码resultStream = new Http1xStream( client, this, resultConnection.source, resultConnection.sink);实质上HttpStream其实就是Request和Response读写Socket的抽象，我们看到Http1xStream取到了Socket输入输出流，随后在CallServerInterceptor可以拿来做读写。

我们看CallServerInterceptor做了什么:

@Override public Response intercept(Chain chain) throws IOException {HttpStream httpStream = ((RealInterceptorChain) chain).httpStream();StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();Request request = chain.request();long sentRequestMillis = System.currentTimeMillis();httpStream.writeRequestHeaders(request);if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {Sink requestBodyOut = httpStream.createRequestBody(request, request.body().contentLength());BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);request.body().writeTo(bufferedRequestBody);bufferedRequestBody.close();}httpStream.finishRequest();Response response = httpStream.readResponseHeaders().request(request).handshake(streamAllocation.connection().handshake()).sentRequestAtMillis(sentRequestMillis).receivedResponseAtMillis(System.currentTimeMillis()).build();if (!forWebSocket || response.code() != 101) {response = response.newBuilder().body(httpStream.openResponseBody(response)).build();}if ("close".equalsIgnoreCase(response.request().header("Connection"))|| "close".equalsIgnoreCase(response.header("Connection"))) {streamAllocation.noNewStreams();}int code = response.code();if ((code == 204 || code == 205) && response.body().contentLength() > 0) {throw new ProtocolException("HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());}return response;
}

CallServerInterceptor顾名思义，就是真正和Server进行通信的地方。这里也是按照HTTP协议，依次写入请求头，还有根据情况决定是否写入请求体。随后读响应头闭构造一个Response。
里面具体是如何实现呢，我们看Http1xStream：
首先是写头:

@Override public void writeRequestHeaders(Request request) throws IOException {String requestLine = RequestLine.get(request, streamAllocation.connection().route().proxy().type());writeRequest(request.headers(), requestLine);
}

构造好请求行，进入writeRequest:

/** Returns bytes of a request header for sending on an HTTP transport. */
public void writeRequest(Headers headers, String requestLine) throws IOException {if (state != STATE_IDLE) throw new IllegalStateException("state: " + state);sink.writeUtf8(requestLine).writeUtf8("\r\n");for (int i = 0, size = headers.size(); i < size; i++) {sink.writeUtf8(headers.name(i)).writeUtf8(": ").writeUtf8(headers.value(i)).writeUtf8("\r\n");}sink.writeUtf8("\r\n");state = STATE_OPEN_REQUEST_BODY;
}

这里就一目了然了，就是一行行的写请求行和请求头到sink中
再看readResponse:

/** Parses bytes of a response header from an HTTP transport. */
public Response.Builder readResponse() throws IOException {if (state != STATE_OPEN_REQUEST_BODY && state != STATE_READ_RESPONSE_HEADERS) {throw new IllegalStateException("state: " + state);}try {while (true) {StatusLine statusLine = StatusLine.parse(source.readUtf8LineStrict());Response.Builder responseBuilder = new Response.Builder().protocol(statusLine.protocol).code(statusLine.code).message(statusLine.message).headers(readHeaders());if (statusLine.code != HTTP_CONTINUE) {state = STATE_OPEN_RESPONSE_BODY;return responseBuilder;}}} catch (EOFException e) {// Provide more context if the server ends the stream before sending a response.IOException exception = new IOException("unexpected end of stream on " + streamAllocation);exception.initCause(e);throw exception;}
}

也是一样的，从source中读请求行和请求头
最后看openResponseBody:

@Override public ResponseBody openResponseBody(Response response) throws IOException {Source source = getTransferStream(response);return new RealResponseBody(response.headers(), Okio.buffer(source));
}

这里说一下就是根据请求的响应把包裹InputStream的source再次封装，里面做一些控制逻辑，然后再封装成ResponseBody。
例如FiexdLengthSource，就是期望获取到byte的长度是固定的值:

/** An HTTP body with a fixed length specified in advance. */
private class FixedLengthSource extends AbstractSource {private long bytesRemaining;public FixedLengthSource(long length) throws IOException {bytesRemaining = length;if (bytesRemaining == 0) {endOfInput(true);}}@Override public long read(Buffer sink, long byteCount) throws IOException {if (byteCount < 0) throw new IllegalArgumentException("byteCount < 0: " + byteCount);if (closed) throw new IllegalStateException("closed");if (bytesRemaining == 0) return -1;long read = source.read(sink, Math.min(bytesRemaining, byteCount));if (read == -1) {endOfInput(false); // The server didn't supply the promised content length.throw new ProtocolException("unexpected end of stream");}bytesRemaining -= read;if (bytesRemaining == 0) {endOfInput(true);}return read;}@Override public void close() throws IOException {if (closed) return;if (bytesRemaining != 0 && !Util.discard(this, DISCARD_STREAM_TIMEOUT_MILLIS, MILLISECONDS)) {endOfInput(false);}closed = true;}
}

当读完期望的长度时就把这个RealConnection回收，如果少于期望的长度则抛异常。

ConnectionPool
到了OkHttp3时代，ConnectionPool就是每个Client独享的了，我们刚才提到了ConnectionPool，那么他到底是如何运作呢。
ConnectionPool持有一个静态的线程池。
StreamAllocation不管通过什么方式，在获取到RealConnection后，RealConnection会添加一个对StreamAllocation的引用。
在每个RealConnection加入ConnectionPool后，如果当前没有在清理，就会把cleanUpRunnable加入线程池。
cleanUpRunnable里面是一个while(true),一个循环包括:
调用一次cleanUp方法进行清理并返回一个long, 如果是-1则退出，否则调用wait方法等待这个long值的时间
cleanUp代码如下:

ong cleanup(long now) {int inUseConnectionCount = 0;int idleConnectionCount = 0;RealConnection longestIdleConnection = null;long longestIdleDurationNs = Long.MIN_VALUE;// Find either a connection to evict, or the time that the next eviction is due.synchronized (this) {for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {RealConnection connection = i.next();// If the connection is in use, keep searching.if (pruneAndGetAllocationCount(connection, now) > 0) {inUseConnectionCount++;continue;}idleConnectionCount++;// If the connection is ready to be evicted, we're done.long idleDurationNs = now - connection.idleAtNanos;if (idleDurationNs > longestIdleDurationNs) {longestIdleDurationNs = idleDurationNs;longestIdleConnection = connection;}}if (longestIdleDurationNs >= this.keepAliveDurationNs|| idleConnectionCount > this.maxIdleConnections) {// We've found a connection to evict. Remove it from the list, then close it below (outside// of the synchronized block).connections.remove(longestIdleConnection);} else if (idleConnectionCount > 0) {// A connection will be ready to evict soon.return keepAliveDurationNs - longestIdleDurationNs;} else if (inUseConnectionCount > 0) {// All connections are in use. It'll be at least the keep alive duration 'til we run again.return keepAliveDurationNs;} else {// No connections, idle or in use.cleanupRunning = false;return -1;}}closeQuietly(longestIdleConnection.socket());// Cleanup again immediately.return 0;
}

遍历每一个RealConnection，通过引用数目确定哪些是空闲的，哪些是在使用中，同时找到空闲时间最长的RealConnection。
如果空闲数目超过最大空闲数或者空闲时间超过最大空闲时间，则清理掉这个RealConnection，并返回0，表示需要立刻再次清理
否则如果空闲的数目大于0个，则等待最大空闲时间-已有的最长空闲时间
否则如果使用中的数目大于0，则等待最大空闲时间
否则返回 -1，并标识退出清除状态
同时如果某个RealConnection空闲后，会进入ConnectionPool.connectionBecameIdle方法,如果不可被复用，则被移除，否则立刻唤醒上面cleanUp的wait，再次清理，因为可能超过了最大空闲数目
这样通过一个静态的线程池，ConnectionPool做到了每个实例定期清理，保证不会超过最大空闲时间和最大空闲数目的策略。

OkHttp3分析就到此结束了。

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