OkHttp3源码工作流程浅析

细节先放在一边，咱来把工作流程先理一下。

常规调用

• 异步

        Request request = new Request.Builder()
                .url(url)
                .post(requestBody.build())
                .build();
        okHttpClient.newCall(request).enqueue(callback);

• 同步

        Request request = new Request.Builder()
                .url(url)
                .post(requestBody.build())
                .build();
        okHttpClient.newCall(request).execute();

工作流程

先看下OkHttpClient的参数：

//OkHttpClient.java
  public static final class Builder {
  	//重点，分发器，主管分发任务，所以内部肯定有队列之类的存在
    Dispatcher dispatcher;
    @Nullable Proxy proxy;
    List<Protocol> protocols;
    List<ConnectionSpec> connectionSpecs;
    //重点，OkHttp核心工作流程就是顺序执行各种拦截器的
    final List<Interceptor> interceptors = new ArrayList<>();
    final List<Interceptor> networkInterceptors = new ArrayList<>();
    EventListener.Factory eventListenerFactory;
    ProxySelector proxySelector;
    CookieJar cookieJar;
    @Nullable Cache cache;
    @Nullable InternalCache internalCache;
    SocketFactory socketFactory;
    @Nullable SSLSocketFactory sslSocketFactory;
    @Nullable CertificateChainCleaner certificateChainCleaner;
    HostnameVerifier hostnameVerifier;
    CertificatePinner certificatePinner;
    Authenticator proxyAuthenticator;
    Authenticator authenticator;
    ConnectionPool connectionPool;
    Dns dns;
    boolean followSslRedirects;
    boolean followRedirects;
    boolean retryOnConnectionFailure;
    int connectTimeout;
    int readTimeout;
    int writeTimeout;
    int pingInterval;

    public Builder() {
      dispatcher = new Dispatcher();
      protocols = DEFAULT_PROTOCOLS;
      connectionSpecs = DEFAULT_CONNECTION_SPECS;
      eventListenerFactory = EventListener.factory(EventListener.NONE);
      proxySelector = ProxySelector.getDefault();
      cookieJar = CookieJar.NO_COOKIES;
      socketFactory = SocketFactory.getDefault();
      hostnameVerifier = OkHostnameVerifier.INSTANCE;
      certificatePinner = CertificatePinner.DEFAULT;
      proxyAuthenticator = Authenticator.NONE;
      authenticator = Authenticator.NONE;
      connectionPool = new ConnectionPool();
      dns = Dns.SYSTEM;
      followSslRedirects = true;
      followRedirects = true;
      retryOnConnectionFailure = true;
      connectTimeout = 10_000;
      readTimeout = 10_000;
      writeTimeout = 10_000;
      pingInterval = 0;
    }
 }

若只想了解工作流程的话，只需要关注Dispatcher与Interceptor就可以了。
再看newCall干了什么：

//OkHttpClient.java
  @Override public Call newCall(Request request) {
    return RealCall.newRealCall(this, request, false /* for web socket */);
  }

//RealCall.java
  static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
 	//看来这就是请求主体了
    RealCall call = new RealCall(client, originalRequest, forWebSocket);
    call.eventListener = client.eventListenerFactory().create(call);
    return call;
  }

请求被封装成一个RealCall，然后调用了enqueue了扔进了队列。
再看enqueue：

//RealCall.java
  @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    //重点，就是这里了，出现了dispather
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

由Dispatcher接手，并且此处封装了一个带回调的AsyncCall，这个AsyncCall就是任务的实体。
转战Dispatcher：

//Dispatcher.java
  //双向队列，已经准备就绪的异步任务
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
  //双向队列，正在执行的异步任务
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
  //双向队列，正在执行的同步任务
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
  
  //将任务添进队列，线程池开始执行异步任务
  synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      //开始执行
      executorService().execute(call);
    } else {
      //超出最大数量，则放入就绪队列，后面finish时就是从此队列取出下一个任务
      readyAsyncCalls.add(call);
    }
  }

  /**
   * 线程池由此而来，是用SynchronousQueue，一个阻塞队列，
   * 必须等队列中的添加元素被消费后才能继续添加新的元素
   */
  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;
  }

再来看看这个AsynCall怎样封装的：

//RealCall.java
  final class AsyncCall extends NamedRunnable {
  	//应答的回调，enqueue传入的
    private final Callback responseCallback;

    AsyncCall(Callback responseCallback) {
	  //顾名思义，父类就是一个可以命名的线程
      super("OkHttp %s", redactedUrl());
      this.responseCallback = responseCallback;
    }

    String host() {
      return originalRequest.url().host();
    }

    Request request() {
      return originalRequest;
    }

    RealCall get() {
      return RealCall.this;
    }

	//重点，真正的执行流程从此开始
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
      	//重点，应答在此已经得到
        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 {
          eventListener.callFailed(RealCall.this, e);
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        //重点，结束当前请求后的后续操作
        client.dispatcher().finished(this);
      }
    }
  }

AsyncCall就是一个Runnable，它自己的execute就是任务执行的具体操作了，而response应该就在getResponseWithInterceptorChain中拿到了：

//RealCall.java
  Response getResponseWithInterceptorChain() throws IOException {
    List<Interceptor> interceptors = new ArrayList<>();
    //所有自定义的拦截器
    interceptors.addAll(client.interceptors());
    //重定向拦，主要负责失败重连工作
    interceptors.add(retryAndFollowUpInterceptor);
    //桥接，主要负责设置内容长度、编码方式、设置gzip压缩、添加请求头、cookie等相关功能
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //缓存，处理相关缓存策略
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //连接，创建真实的connection，打开socket链接
    interceptors.add(new ConnectInterceptor(client));
    //forWebSocket默认为false，传入一般也为false
    if (!forWebSocket) {
      //所有自定义的网络拦截器
      interceptors.addAll(client.networkInterceptors());
    }
    //向服务器发送请求，返回Response
    interceptors.add(new CallServerInterceptor(forWebSocket));
	//组成了一个链
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,originalRequest, this, eventListener, client.connectTimeoutMillis(), client.readTimeoutMillis(), client.writeTimeoutMillis());
	//重点，最终在这里得到了response
    return chain.proceed(originalRequest);
  }

各种Interceptor就是网络请求所作的具体通讯操作，到这里，已经接近通讯层了，继续看RealInterceptorChain：

//RealInterceptorChain.java
  public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection 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.httpCodec != null && !this.connection.supportsUrl(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.httpCodec != 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, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

    // Confirm that the next interceptor made its required call to chain.proceed().
    if (httpCodec != 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");
    }

    if (response.body() == null) {
      throw new IllegalStateException(
          "interceptor " + interceptor + " returned a response with no body");
    }

    return response;
  }

没错了，就在这里返回了最终的response，至于Interceptor究竟做了什么最终获取到了Response，这里就不讨论了。
应答找到了，看看应答后会发生什么事：

//Dispatcher.java
  /** Used by {@code AsyncCall#run} to signal completion. */
  void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }

  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!");
      //这里传入了true
      if (promoteCalls) promoteCalls();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }
	
	//当前没有任务运行且“空闲任务”不为空时，开始执行这个“空闲任务”
    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }

  //当一个网络请求任务完成后，默认是要到此方法来的
  private void promoteCalls() {
  	//正在执行的任务超限，不再往下走了
    if (runningAsyncCalls.size() >= maxRequests) return; 
    //就绪的任务队列空了，没有任务可执行了，也不再往下走了，看来这个方法就是为了接着执行下一个方法而写的了
    if (readyAsyncCalls.isEmpty()) return; 

	//把就绪队列的任务取出来，可以是多个，取决于runningAsyncCalls的上限，缺多少补多少个，然后这N个任务中某个或多个任务执行完后，又来到此处取出M个就绪的任务，一直到将readyAsyncCalls队列中所有的任务取完
    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();

      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        //走正常的异步执行流程
        executorService().execute(call);
      }
	  //当前正在执行的任务数量到上限了，只有等待下一次添加任务了
      if (runningAsyncCalls.size() >= maxRequests) return; 
    }
  }

可以看到，当一个AsynCall执行完毕后，会根据相关限制从readyAsyncCalls取出更多的AsynCall进行执行，直到所有AsynCall执行完毕。

回调就不必多说了，在AsynCall.responseCallback中传回了response。
这就是异步的工作流程。

同步有一点区别，在RealCall中调用的方法不一样：

//RealCall.java
 @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      //只做了添加正在执行的队列中
      client.dispatcher().executed(this);
      //直接执行了，进入了同样的方法
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this);
    }
  }

看得出来，同步是直接进行了获取response的操作，异步相比之下就多了线程池的调度操作，也是应有之意。

简而言之，OkHttp3的网络请求就是通过一个InterceptorChain获取response的过程，想深入了解okhttp3的原理，就要研究各个Interceptor的作用了。

以上。