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newCall 实际上是创建了一个 RealCall 有三个参数：OkHttpClient（通用配置，超时时间等） Request(Http请求所用到的条件，url等) 布尔变量forWebSocket（webSocket是一种应用层的交互方式，可双向交互，一般用不到，除非需要频繁刷新数据，股票等。）

	private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {this.client = client;this.originalRequest = originalRequest;this.forWebSocket = forWebSocket;}

Request会被进行多次封装(所以构造函数里对象被命名为originRequest)

在进行newCall().enqueue(),实际就是RealCall的enqueue()

	@Override public void enqueue(Callback responseCallback) {synchronized (this) {if (executed) throw new IllegalStateException("Already Executed");executed = true;}//1transmitter.callStart();//2 关键client.dispatcher().enqueue(new AsyncCall(responseCallback));}

分别看1和2，主要看2

	public void callStart() {//跟踪程序错误this.callStackTrace = Platform.get().getStackTraceForCloseable("response.body().close()");//eventListener是一个监听器，连接的接入和关闭，对程序进行监听eventListener.callStart(call);}

client.dispatcher()返回一个Dispatcher类对象，即线程调度器，然后用这个去进行异步操作，代入参数为一个AsyncCall

	void enqueue(AsyncCall call) {synchronized (this) {//1readyAsyncCalls.add(call);// Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to// the same host.//2if (!call.get().forWebSocket) {AsyncCall existingCall = findExistingCallWithHost(call.host());if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);}}//3promoteAndExecute();}

• 1的readyAsyncCalls 是一个 ArrayDeque<AsyncCall> ，存放 准备要执行但还没有执行，然后会在3的promoteAndExecute()中执行

	private boolean promoteAndExecute() {assert (!Thread.holdsLock(this));List<AsyncCall> executableCalls = new ArrayList<>();boolean isRunning;synchronized (this) {for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {AsyncCall asyncCall = i.next();if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.i.remove();asyncCall.callsPerHost().incrementAndGet();executableCalls.add(asyncCall);runningAsyncCalls.add(asyncCall);}isRunning = runningCallsCount() > 0;}for (int i = 0, size = executableCalls.size(); i < size; i++) {AsyncCall asyncCall = executableCalls.get(i);asyncCall.executeOn(executorService());}return isRunning;}

promoteAndExecute会挑选那些不会导致超负载的call(不超过AsyncCall对应的maxRequest)，放进executableCalls和runningAsyncCalls，然后去执行，就是去遍历executableCalls然后执行。
分别执行就是把调用每一个asyncCall 的 executeOn():

	void executeOn(ExecutorService executorService) {assert (!Thread.holdsLock(client.dispatcher()));boolean success = false;try {executorService.execute(this);success = true;} catch (RejectedExecutionException e) {InterruptedIOException ioException = new InterruptedIOException("executor rejected");ioException.initCause(e);transmitter.noMoreExchanges(ioException);responseCallback.onFailure(RealCall.this, ioException);} finally {if (!success) {client.dispatcher().finished(this); // This call is no longer running!}}}

核心只有一行：

executorService.execute(this);

这里就已经切换线程了，执行的都是传入的 executorService.对象 的execute()方法，都会在后台执行

	@Override protected void execute() {boolean signalledCallback = false;  // 标记回调是否已触发transmitter.timeoutEnter();  // 进入超时处理逻辑try {Response response = getResponseWithInterceptorChain();  // 调用拦截器链获取responsesignalledCallback = true;  // 标记回调已触发responseCallback.onResponse(RealCall.this, response);  // 调用response的回调函数} catch (IOException e) {if (signalledCallback) {// 不要重复触发回调！Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);} else {responseCallback.onFailure(RealCall.this, e);}} catch (Throwable t) {cancel();  // 取消请求if (!signalledCallback) {IOException canceledException = new IOException("canceled due to " + t);canceledException.addSuppressed(t);responseCallback.onFailure(RealCall.this, canceledException);  // 调用失败回调函数}throw t;} finally {client.dispatcher().finished(this);  // 请求结束，将请求移出调度队列}}

其中回调函数就是当初我们在应用层所定义的Callback里边定义的onFailure() 和 onResponse(),然后如果出现异常，会进行调用相应的方法。