HTTP是我们交换数据和媒体流的现代应用网络，有效利用HTTP可以使我们节省带宽和更快地加载数据，Square公司开源的OkHttp网络请求是有效率的HTTP客户端。之前的知识面仅限于框架API的调用，接触到实际的工作之后深知自己知识的不足，故而深挖框架源码尽力吸取前辈的设计经验。关于此框架的源码解析网上的教程多不胜数，此文名为源码解析，实则是炒冷饭之作，如有错误和不足之处还望各位看官指出。

拦截器

拦截器是OkHttp框架设计的精髓所在，拦截器所定义的是Request的所通过的责任链而不管Request的具体执行过程，并且可以让开发人员自定义自己的拦截器功能并且插入到责任链中

1. 用户自定义的拦截器位于 OkHttpClient.addInterceptor() 添加到interceptors责任链中

2. RealCall.execute()执行的时候调用RealCall.getResponseWithInterceptorChain()将 来自 OkHttpClient的interceptors以及默认的拦截器一并加入到RealInterceptorChain责任链中并调用, 代码并没有对originalRequest进行封装, InterceptorChain和originalRequest一并流转到 RealInterceptorChain类中处理

   CustomInterceptor
   RetryAndFollowUpInterceptor
   BridgeInterceptor
   CacheInterceptor
   ConnectInterceptor
   NetworkInterceptors
   CallServiceInterceptor

3. RealInterceptorChain.proceed()

4. EventListener.callStart()也是在RealCall.execute()嵌入到Request调用过程, EventListener.callEnd()位于StreamAllocation中调用

5. Request.Builder

   • url (String/URL/HttpUrl)
   • header
   • CacheControl
   • Tag (Use this API to attach timing, debugging, or other application data to a request so that you may read it in interceptors, event listeners, or callbacks.)

BridgeInterceptor

Bridges from application code to network code. First it builds a network request from a user request. Then it proceeds to call the network. Finally it builds a user response from the network response.

此拦截器是应用码到网络码的桥接。它会将用户请求封装成一个网络请求并且执行请求，同时它还完成从网络响应到用户响应的转化. 最后Chain.proceed() 方法启动拦截器责任链， RealInterceptorChain中通过递归调用将网络请求以及响应的任务分别分配到各个拦截器中， 然后通过ResponseBuilder.build()方法将网络响应封装, 然后递归调用责任链模式使得调用以及Response处理的过程可以一并写入BridgeInterceptor中

public final class RealInterceptorChain implements Interceptor.Chain {
   public Response proceed(Request request, StreamAllocation streamAllocation, 
   HttpCodec httpCodec, RealConnection connection) throws IOException {
       if (index >= interceptors.size()) throw new AssertionError();

       calls++;

       ...
       // Call the next interceptor in the chain.
       RealInterceptorChain next = new RealInterceptorChain(interceptors, 
          streamAllocation, httpCodec,connection, index + 1, request, call, 
          eventListener, connectTimeout, readTimeout,writeTimeout);
       Interceptor interceptor = interceptors.get(index);
       Response response = interceptor.intercept(next);
       ...
       return response;
     }
}

CallServiceInterceptor

Interceptor的逻辑均在intercept()方法中实现, 在通过Chain实体类获取到请求主题之后，通过BufferedSink接口将请求转发到Okio接口，在拦截过程中通过EventListener接口将拦截器处理状态（主要是RequestBodyStart和RequestBodyEnd两个状态）发送出去

public final class CallServiceInterceptor implements Interceptor {
    @Override public Response intercept(Chain chain) throws IOException {
          Response.Builder responseBuilder = null;
          if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
          // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
          // Continue" response before transmitting the request body. If we don't get that, return
          // what we did get (such as a 4xx response) without ever transmitting the request body.
          if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
            httpCodec.flushRequest();
            realChain.eventListener().responseHeadersStart(realChain.call());
            responseBuilder = httpCodec.readResponseHeaders(true);
          }

          if (responseBuilder == n