1.构造Demo

 

首先构造一个简单的异步网络访问Demo:

OkHttpClient client = new OkHttpClient();       Request request = new Request.Builder()  .url("http://publicobject.com/helloworld.txt")  .build();client.newCall(request).enqueue(new Callback() {  @Override  public void onFailure(Call call, IOException e) {    Log.d("OkHttp", "Call Failed:" + e.getMessage());  }  @Override  public void onResponse(Call call, Response response) throws IOException {    Log.d("OkHttp", "Call succeeded:" + response.message());  }});

2. 发起请求

OkHttpClient.newCall实际是创建一个RealCall实例：

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

RealCall.enqueue实际就是讲一个RealCall放入到任务队列中，等待合适的机会执行:

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

从代码中可以看到最终RealCall被转化成一个AsyncCall并被放入到任务队列中，任务队列中的分发逻辑这里先不说，相关实现会放在疑问进行介绍。这里只需要知道AsyncCall的excute方法最终将会被执行:

[RealCall.java]    @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);      }    }  }

execute方法的逻辑并不复杂，简单的说就是：

• 调用getResponseWithInterceptorChain获取服务器返回
• 通知任务分发器(client.dispatcher)该任务已结束

getResponseWithInterceptorChain构建了一个拦截器链，通过依次执行该拦截器链中的每一个拦截器最终得到服务器返回。

3. 构建拦截器链

首先来看下getResponseWithInterceptorChain的实现：

源码路径：okhttp3/RealCall.java

1 // 开始执行整个请求 2 Response getResponseWithInterceptorChain() throws IOException { 3   // Build a full stack of interceptors. 4   // 拦截器栈 5   List
     
       interceptors = new ArrayList<>(); 6   // 前文说过的 普通拦截器 7   interceptors.addAll(client.interceptors()); 8   // 重试拦截器，网络错误、请求失败等 9   interceptors.add(retryAndFollowUpInterceptor);10   // 桥接拦截器，主要是重构请求头即header11   interceptors.add(new BridgeInterceptor(client.cookieJar()));12   // 缓存拦截器13   interceptors.add(newCacheInterceptor(client.internalCache()));14   // 连接拦截器，连接服务器，https包装15   interceptors.add(new ConnectInterceptor(client));16   // 网络拦截器，websockt不支持，同样是自定义17   if (!forWebSocket) {18     interceptors.addAll(client.networkInterceptors());19   }20   // 服务拦截器，主要是发送（write、input）、读取（read、output）数据21   interceptors.add(new CallServerInterceptor(forWebSocket));22 23   // 开启调用链24   Interceptor.Chain chain = new RealInterceptorChain(25       interceptors, null, null, null, 0, originalRequest);26   return chain.proceed(originalRequest);27 }
     

其逻辑大致分为两部分：

• 创建一系列拦截器，并将其放入一个拦截器数组中。这部分拦截器即包括用户自定义的拦截器也包括框架内部拦截器
• 创建一个拦截器链RealInterceptorChain,并执行拦截器链的proceed方法

接下来看下RealInterceptorChain的实现逻辑：

1 public final class RealInterceptorChain implements Interceptor.Chain { 2   private final List
       
         interceptors; 3   private final StreamAllocation streamAllocation; 4   private final HttpCodec httpCodec; 5   private final RealConnection connection; 6   private final int index; 7   private final Request request; 8   private int calls; 9 10   public RealInterceptorChain(List
        
          interceptors, StreamAllocation streamAllocation,11                               HttpCodec httpCodec, RealConnection connection, int index, Request request) {12     this.interceptors = interceptors;13     this.connection = connection;14     this.streamAllocation = streamAllocation;15     this.httpCodec = httpCodec;16     this.index = index;17     this.request = request;18   }19 20   @Override public Connection connection() {21     return connection;22   }23 24   public StreamAllocation streamAllocation() {25     return streamAllocation;26   }27 28   public HttpCodec httpStream() {29     return httpCodec;30   }31 32   @Override public Request request() {33     return request;34   }35 36   @Override public Response proceed(Request request) throws IOException {37     return proceed(request, streamAllocation, httpCodec, connection);38   }39 40   public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,41       RealConnection connection) throws IOException {42 43     ......44     // Call the next interceptor in the chain.45     RealInterceptorChain next = new RealInterceptorChain(46         interceptors, streamAllocation, httpCodec, connection, index + 1, request);47     Interceptor interceptor = interceptors.get(index);48     Response response = interceptor.intercept(next);49     50     ...... 51 52     return response;53   }54 }
        
       

在proceed方法中的核心代码可以看到，proceed实际上也做了两件事：

• 创建下一个拦截链。传入index + 1使得下一个拦截器链只能从下一个拦截器开始访问
• 执行索引为index的intercept方法，并将下一个拦截器链传入该方法