前言

学会了OkHttp3的用法后，我们当然有必要来了解下OkHttp3的源码，当然现在网上的文章很多，我仍旧希望我这一系列文章篇是最简洁易懂的。

1.从请求处理开始分析

首先OKHttp3如何使用这里就不在赘述了，不明白的同学可以查看Android网络编程（五）OkHttp2.x用法全解析、
Android网络编程（六）OkHttp3用法全解析这两篇文章。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：

  @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：

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

可以看到最终的请求是dispatcher来完成的。

2.Dispatcher任务调度

主要的变量

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。不了解线程池的同学可以查看Android多线程(一)线程池这篇文章。其中用到了SynchronousQueue，不了解它的同学可以查看Java并发编程（六）阻塞队列这篇文章。

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(forWebSocket);
        if (canceled) {
          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!
          logger.log(Level.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)；

 synchronized void finished(AsyncCall call) {
    if (!runningAsyncCalls.remove(call)) throw new AssertionError("AsyncCall wasn't running!");
    promoteCalls();
  }

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。

3.Interceptor拦截器

  private Response getResponseWithInterceptorChain(boolean forWebSocket) throws IOException {
    Interceptor.Chain chain = new ApplicationInterceptorChain(0, originalRequest, forWebSocket);
    return chain.proceed(originalRequest);
  }

getResponseWithInterceptorChain方法，创建了ApplicationInterceptorChain，它是一个拦截器链，这个类也是RealCall的内部类，接下来执行了它的proceed方法：

    @Override public Response proceed(Request request) throws IOException {
      // If there's another interceptor in the chain, call that.
      if (index < client.interceptors().size()) {
        Interceptor.Chain chain = new ApplicationInterceptorChain(index + 1, request, forWebSocket);
        //从拦截器列表取出拦截器
        Interceptor interceptor = client.interceptors().get(index);
        Response interceptedResponse = interceptor.intercept(chain);

        if (interceptedResponse == null) {
          throw new NullPointerException("application interceptor " + interceptor
              + " returned null");
        }

        return interceptedResponse;
      }

      // No more interceptors. Do HTTP.
      return getResponse(request, forWebSocket);
    }

proceed方法每次从拦截器列表中取出拦截器，当存在多个拦截器时都会在第七行阻塞，并等待下一个拦截器的调用返回。下面分别以 拦截器链中有1个、2个拦截器的场景加以模拟：

拦截器主要用来观察，修改以及可能短路的请求输出和响应的回来。通常情况下拦截器用来添加，移除或者转换请求或者响应的头部信息。比如将域名替换为ip地址，将请求头中添加host属性，也可以添加我们应用中的一些公共参数，比如设备id、版本号等等。 不了解拦截器的可以查看Okhttp-wiki 之 Interceptors 拦截器这篇文章。
回到代码上来，我们看最后一行 return getResponse(request, forWebSocket)，如果没有更多的拦截器的话，就会执行网络请求，来看看getResponse方法做了些什么（RealCall.java）：

Response getResponse(Request request, boolean forWebSocket) throws IOException {
 ...省略
    // Create the initial HTTP engine. Retries and redirects need new engine for each attempt.
    engine = new HttpEngine(client, request, false, false, forWebSocket, null, null, null);

    int followUpCount = 0;
    while (true) {
      if (canceled) {
        engine.releaseStreamAllocation();
        throw new IOException("Canceled");
      }

      boolean releaseConnection = true;
      try {
        engine.sendRequest();
        engine.readResponse();
        releaseConnection = false;
      } catch (RequestException e) {
        // The attempt to interpret the request failed. Give up.
        throw e.getCause();
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
  ...省略     
    }
  }

getResponse方法比较长我省略了一些代码，可以看到创建了HttpEngine类并且调用HttpEngine的sendRequest方法和readResponse方法。

4.缓存策略

我们先来看看sendRequest方法：

 public void sendRequest() throws RequestException, RouteException, IOException {
    if (cacheStrategy != null) return; // Already sent.
    if (httpStream != null) throw new IllegalStateException();
    //请求头部添加
    Request request = networkRequest(userRequest);
    //获取client中的Cache,同时Cache在初始化的时候会去读取缓存目录中关于曾经请求过的所有信息。
    InternalCache responseCache = Internal.instance.internalCache(client);
    //cacheCandidate为上次与服务器交互缓存的Response
    Response cacheCandidate = responseCache != null
        ? responseCache.get(request)
        : null;

    long now = System.currentTimeMillis();
   
    //创建CacheStrategy.Factory对象，进行缓存配置
    cacheStrategy = new CacheStrategy.Factory(now, request, cacheCandidate).get();
    //网络请求
    networkRequest = cacheStrategy.networkRequest;
    //缓存的响应
    cacheResponse = cacheStrategy.cacheResponse;

    if (responseCache != null) {
     //记录当前请求是网络发起还是缓存发起
      responseCache.trackResponse(cacheStrategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    //不进行网络请求并且缓存不存在或者过期则返回504错误
    if (networkRequest == null && cacheResponse == null) {
      userResponse = new Response.Builder()
          .request(userRequest)
          .priorResponse(stripBody(priorResponse))
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(EMPTY_BODY)
          .build();
      return;
    }
    
    // 不进行网络请求，而且缓存可以使用，直接返回缓存
    if (networkRequest == null) {
      userResponse = cacheResponse.newBuilder()
          .request(userRequest)
          .priorResponse(stripBody(priorResponse))
          .cacheResponse(stripBody(cacheResponse))
          .build();
      userResponse = unzip(userResponse);
      return;
    }

    //需要访问网络时
    boolean success = false;
    try {
      httpStream = connect();
      httpStream.setHttpEngine(this);

      if (writeRequestHeadersEagerly()) {
        long contentLength = OkHeaders.contentLength(request);
        if (bufferRequestBody) {
          if (contentLength > Integer.MAX_VALUE) {
            throw new IllegalStateException("Use setFixedLengthStreamingMode() or "
                + "setChunkedStreamingMode() for requests larger than 2 GiB.");
          }

          if (contentLength != -1) {
            // Buffer a request body of a known length.
            httpStream.writeRequestHeaders(networkRequest);
            requestBodyOut = new RetryableSink((int) contentLength);
          } else {
            // Buffer a request body of an unknown length. Don't write request headers until the
            // entire body is ready; otherwise we can't set the Content-Length header correctly.
            requestBodyOut = new RetryableSink();
          }
        } else {
          httpStream.writeRequestHeaders(networkRequest);
          requestBodyOut = httpStream.createRequestBody(networkRequest, contentLength);
        }
      }
      success = true;
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (!success && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }
  }

上面的代码显然是在发送请求，但是最主要的是做了缓存的策略。cacheCandidate是上次与服务器交互缓存的Response，这里的缓存都是基于Map，key是请求中url的md5，value是在文件中查询到的缓存，页面置换基于LRU算法，我们现在只需要知道它是一个可以读取缓存Header的Response即可。根据cacheStrategy的处理得到了networkRequest和cacheResponse这两个值，根据这两个值的数据是否为null来进行进一步的处理，当networkRequest和cacheResponse都为null的情况也就是不进行网络请求并且缓存不存在或者过期，这时候则返回504错误；当networkRequest 为null时也就是不进行网络请求，而且缓存可以使用时则直接返回缓存；其他的情况则请求网络。
接下来我们查看readResponse方法：

  public void readResponse() throws IOException {
    ...省略
    else{
      //读取网络响应
      networkResponse = readNetworkResponse();
    }
    //将响应头部存入Cookie中
    receiveHeaders(networkResponse.headers());

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
    //检查缓存是否可用，如果可用。那么就用当前缓存的Response，关闭网络连接，释放连接。
      if (validate(cacheResponse, networkResponse)) {
        userResponse = cacheResponse.newBuilder()
            .request(userRequest)
            .priorResponse(stripBody(priorResponse))
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();
        releaseStreamAllocation();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        InternalCache responseCache = Internal.instance.internalCache(client);
        responseCache.trackConditionalCacheHit();
        // 更新缓存
        responseCache.update(cacheResponse, stripBody(userResponse));
        userResponse = unzip(userResponse);
        return;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    userResponse = networkResponse.newBuilder()
        .request(userRequest)
        .priorResponse(stripBody(priorResponse))
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (hasBody(userResponse)) {
      maybeCache();
      userResponse = unzip(cacheWritingResponse(storeRequest, userResponse));
    }
  }

这个方法发起刷新请求头部和请求体，解析HTTP响应头部。如果有缓存并且可用则用缓存的数据并更新缓存，否则就用网络请求返回的数据。
我们再来看看validate(cacheResponse, networkResponse)方法是如何判断缓存是否可用的：

  private static boolean validate(Response cached, Response network) {
  //如果服务器返回304则缓存有效
    if (network.code() == HTTP_NOT_MODIFIED) {
      return true;
    }
   //通过缓存和网络请求响应中的Last-Modified来计算是否是最新数据，如果是则缓存有效
    Date lastModified = cached.headers().getDate("Last-Modified");
    if (lastModified != null) {
      Date networkLastModified = network.headers().getDate("Last-Modified");
      if (networkLastModified != null
          && networkLastModified.getTime() < lastModified.getTime()) {
        return true;
      }
    }
    return false;
  }

如缓存果过期或者强制放弃缓存，在此情况下，缓存策略全部交给服务器判断，客户端只用发送条件get请求即可，如果缓存是有效的，则返回304 Not Modifiled，否则直接返回body。条件get请求有两种方式一种是Last-Modified-Date，一种是 ETag。这里采用了Last-Modified-Date，通过缓存和网络请求响应中的Last-Modified来计算是否是最新数据，如果是则缓存有效。

5.失败重连

最后我们再回到RealCall的getResponse方法：

  Response getResponse(Request request, boolean forWebSocket) throws IOException {
  ...省略
      boolean releaseConnection = true;
      try {
        engine.sendRequest();
        engine.readResponse();
        releaseConnection = false;
      } catch (RequestException e) {
        // The attempt to interpret the request failed. Give up.
        throw e.getCause();
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        HttpEngine retryEngine = engine.recover(e.getLastConnectException(), null);
        if (retryEngine != null) {
          releaseConnection = false;
          engine = retryEngine;
          continue;
        }
        // Give up; recovery is not possible.
        throw e.getLastConnectException();
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
        HttpEngine retryEngine = engine.recover(e, null);
        if (retryEngine != null) {
          releaseConnection = false;
          engine = retryEngine;
          continue;
        }

        // Give up; recovery is not possible.
        throw e;
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          StreamAllocation streamAllocation = engine.close();
          streamAllocation.release();
        }
      }
     ...省略
      engine = new HttpEngine(client, request, false, false, forWebSocket, streamAllocation, null,
          response);
    }
  }

查看代码第11行和21行当发生IOException或者RouteException时会执行HttpEngine的recover方法：

  public HttpEngine recover(IOException e, Sink requestBodyOut) {
    if (!streamAllocation.recover(e, requestBodyOut)) {
      return null;
    }

    if (!client.retryOnConnectionFailure()) {
      return null;
    }

    StreamAllocation streamAllocation = close();

    // For failure recovery, use the same route selector with a new connection.
    return new HttpEngine(client, userRequest, bufferRequestBody, callerWritesRequestBody,
        forWebSocket, streamAllocation, (RetryableSink) requestBodyOut, priorResponse);
  }

最后一行可以看到就是重新创建了HttpEngine并返回，用来完成重连。
到这里OkHttp请求网络的流程基本上讲完了，下面是关于OKHttp的请求流程图：

参考资料：
http://www.jianshu.com/p/aad5aacd79bf
http://www.jianshu.com/p/64e256c1dbbf
http://www.cnblogs.com/LuLei1990/p/5534791.html
http://frodoking.github.io/2015/03/12/android-okhttp/