//創(chuàng)建一個Client，相當(dāng)于打開一個瀏覽器
 OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
 //創(chuàng)建一個請求。
        Request request = new Request.Builder()
                .url("http://www.baidu.com")
                .method("GET",null)
                .build();
    //調(diào)用Client 創(chuàng)建一個Call。
        Call call = okHttpClient.newCall(request);
        //Call傳入一個回調(diào)函數(shù)，并加入到請求隊列。
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
            }
            @Override
            public void onResponse(Call call, Response response) throws IOException {
            }
        });
}

 Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://www.baidu.com/")
                .build();
        NetInterface netInterface = retrofit.create(NetInterface.class);
        Call<Person> call = netInterface.getPerson();
        call.enqueue(new Callback<Person>() {
            @Override
            public void onResponse(Call<Person> call, Response<Person> response) {
            }
            @Override
            public void onFailure(Call<Person> call, Throwable t) {
            }
 });

 public void enqueue(Callback responseCallback) {
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

//等待隊列
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
//運行隊列
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
//運行隊列數(shù)量最大值
private int maxRequests = 64;
//訪問不同主機的最大數(shù)量
private int maxRequestsPerHost = 5;

dispatcher.java
 synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      executorService().execute(call);
    } else {
      readyAsyncCalls.add(call);
    }
  }

  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;

 //AsyncCall.java
 @Override protected void execute() {
       try {
       //這個方法去請求網(wǎng)絡(luò)，會返回Respose
        Response response = getResponseWithInterceptorChain();
        //請求成功，回調(diào)接口
         responseCallback.onResponse(RealCall.this, response);
       }catch(Exceptrion e){
            //失敗回調(diào)
         responseCallback.onFailure(RealCall.this, e);
       }finally {
          //從當(dāng)前運行隊列中刪除這個請求
          client.dispatcher().finished(this);
      }
 }

 //這個方法命名：通過攔截器鏈，獲取Response
  Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
     // 這個我們自己定義的攔截器。
    interceptors.addAll(client.interceptors());
    //重試和重定向攔截器
    interceptors.add(retryAndFollowUpInterceptor);
    //請求頭攔截器
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //緩存攔截器
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //連接攔截器
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    //訪問攔截器
    interceptors.add(new CallServerInterceptor(forWebSocket));
    //攔截器責(zé)任鏈
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());
    //執(zhí)行攔截器集合中的攔截器
    return chain.proceed(originalRequest);
  }

 Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0......)
   chain.proceed(originalRequest);
   public Response proceed(Request request,...){
    //構(gòu)建一個index+1的攔截器鏈
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
                connection, index + 1,....);
        //拿到當(dāng)前的攔截器
        Interceptor interceptor = interceptors.get(index);
        //調(diào)用攔截器intercept(next)方法，
        //在這個方法中繼續(xù)調(diào)用realChain.proceed(),從而進行循環(huán)調(diào)用，index索引值再加1.
        Response response = interceptor.intercept(next);
 }

public Response intercept(Chain chain){
      while (true) {
        Response response;
          try {
          //創(chuàng)建StreamAllocation對象，這個對象會在連接攔截器中用到
            StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
                  createAddress(request.url()), call, eventListener, callStackTrace);
              this.streamAllocation = streamAllocation;
             調(diào)用責(zé)任鏈下游攔截器
             response = realChain.proceed(request, streamAllocation, null, null);
            } catch (RouteException e) {
                 // The attempt to connect via a route failed. The request will not have been sent.
                 路由異常，請求還沒發(fā)出去。
                 這樣這個recover(),如果返回的是false，則拋出異常，不再重試
                 如果返回的是true，則執(zhí)行下面的continue，進行下一次while循環(huán)，進行重試，重新發(fā)起網(wǎng)絡(luò)請求。
                 if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
                   throw e.getFirstConnectException();
                 }
                 releaseConnection = false;
                continue;
             } catch (IOException e) {
                 // An attempt to communicate with a server failed. The request may have been sent.
                 請求已經(jīng)發(fā)出去了，但是和服務(wù)器連接失敗了。
                 這個recover()返回值的處理邏輯和上面異常一樣。
                 boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
                 if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
                 releaseConnection = false;
                 continue;
               }
             } finally {//finally是必定會執(zhí)行到的，不管上面的catch中執(zhí)行的是continue還是thow
                // We're throwing an unchecked exception. Release any resources.
                if (releaseConnection) {
                  streamAllocation.streamFailed(null);
                  streamAllocation.release();
                }
              }
             在這個重試攔截器中，okhttp的做法很巧妙。先是在外面有一個while循環(huán)，如果發(fā)生異常，
             會在recover方法中對異常類型進行判斷，如果不符合屬于重試，則返回false，并thow e，結(jié)束while循環(huán)。
             如果符合重試的條件，則返回true，在上面的catch代碼塊中執(zhí)行continue方法，進入下一個while循環(huán)。
            //如果請求正常，并且返回了response，則會進行重定向的邏輯判斷
            followUpRequest在這個方法中會根據(jù)ResponseCode，狀態(tài)碼進行重定向的判斷，
            Request followUp;
                 try {
                   followUp = followUpRequest(response, streamAllocation.route());
                 } catch (IOException e) {
                   streamAllocation.release();
                   throw e;
                 }
                 如果flolowUp 為null，則不需要重定向，直接返回response
                 if (followUp == null) {
                   if (!forWebSocket) {
                     streamAllocation.release();
                   }
                   return response;
                 }
                  如果flolowUp 不為null，則進行重定向了請求
               如果重定向次數(shù)超過MAX_FOLLOW_UPS=20次，則拋出異常，結(jié)束while循環(huán)
              if (++followUpCount > MAX_FOLLOW_UPS) {
                     streamAllocation.release();
                     throw new ProtocolException("Too many follow-up requests: " + followUpCount);
                   }
                   if (followUp.body() instanceof UnrepeatableRequestBody) {
                     streamAllocation.release();
                     throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
                   }
                   if (!sameConnection(response, followUp.url())) {
                     streamAllocation.release();
                     //從重定向請求中拿到url，封裝一個新的streamAllocation對象，
                     streamAllocation = new StreamAllocation(client.connectionPool(),
                         createAddress(followUp.url()), call, eventListener, callStackTrace);
                     this.streamAllocation = streamAllocation;
                   } else if (streamAllocation.codec() != null) {
                     throw new IllegalStateException("Closing the body of " + response
                         + " didn't close its backing stream. Bad interceptor?");
                   }
                   //將重定向請求賦值給request 進入下一個重定向的請求的while循環(huán)，繼續(xù)走上面的while循環(huán)代碼
                   request = followUp;
                   priorResponse = response;
                 }
 }
   //只有這個方法返回值為false都不進行重試。
   private boolean recover(IOException e, StreamAllocation streamAllocation,
       boolean requestSendStarted, Request userRequest) {
     streamAllocation.streamFailed(e);
     // The application layer has forbidden retries.
     應(yīng)用層禁止重試。可以通過OkHttpClient進行配置(默認是允許的)
     if (!client.retryOnConnectionFailure()) return false;
     // We can't send the request body again.
     if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;
     // This exception is fatal. 致命的異常
     判斷是否屬于重試的異常
     if (!isRecoverable(e, requestSendStarted)) return false;
     // No more routes to attempt.
     沒有更多可以連接的路由線路
     if (!streamAllocation.hasMoreRoutes()) return false;
     // For failure recovery, use the same route selector with a new connection.
     return true;
   }
  只有這個方法返回false，都不進行重試。
 private boolean isRecoverable(IOException e, boolean requestSendStarted) {
   // If there was a protocol problem, don't recover.
   出現(xiàn)了協(xié)議異常，不再重試
   if (e instanceof ProtocolException) {
     return false;
   }
   // If there was an interruption don't recover, but if there was a timeout connecting to a route
   // we should try the next route (if there is one).
   requestSendStarted為false時，并且異常類型為Scoket超時異常，將會進行下一次重試
   if (e instanceof InterruptedIOException) {
     return e instanceof SocketTimeoutException && !requestSendStarted;
   }
   // Look for known client-side or negotiation errors that are unlikely to be fixed by trying
   // again with a different route.
   如果是一個握手異常，并且證書出現(xiàn)問題，則不能重試
   if (e instanceof SSLHandshakeException) {
     // If the problem was a CertificateException from the X509TrustManager,
     // do not retry.
     if (e.getCause() instanceof CertificateException) {
       return false;
     }
   }

public Response intercept(Chain chain) throws IOException {
     Request userRequest = chain.request();
     Request.Builder requestBuilder = userRequest.newBuilder();
     RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }
      ..................
    }
    在請求頭中添加gizp，是否壓縮
  boolean transparentGzip = false;
     if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
       transparentGzip = true;
       requestBuilder.header("Accept-Encoding", "gzip");
     }
    //cookies
     List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
     if (!cookies.isEmpty()) {
       requestBuilder.header("Cookie", cookieHeader(cookies));
     }
     調(diào)用責(zé)任鏈中下一個攔截器的方法，網(wǎng)絡(luò)請求得到的數(shù)據(jù)封裝到networkResponse中
     Response networkResponse = chain.proceed(requestBuilder.build());
    對cookie進行處理
    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
    如果設(shè)置了gzip，則會對networkResponse進行解壓縮。
     if (transparentGzip
            && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
            && HttpHeaders.hasBody(networkResponse)) {
          GzipSource responseBody = new GzipSource(networkResponse.body().source());
          Headers strippedHeaders = networkResponse.headers().newBuilder()
              .removeAll("Content-Encoding")
              .removeAll("Content-Length")
              .build();
          responseBuilder.headers(strippedHeaders);
          String contentType = networkResponse.header("Content-Type");
          responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
        }
    return responseBuilder.build();
}

public Response intercept(Chain chain){
   //  this.cache = DiskLruCache.create(fileSystem, directory, 201105, 2, maxSize);
    這個緩存在底層使用的是DiskLruCache
    //以request為key從緩存中拿到response。
     Response cacheCandidate = cache != null
            ? cache.get(chain.request()): null;
     long now = System.currentTimeMillis();
     //緩存策略
     CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
     Request networkRequest = strategy.networkRequest;
     Response cacheResponse = strategy.cacheResponse;
   // If we're forbidden from using the network and the cache is insufficient, fail.
   //如果請求和響應(yīng)都為null，直接返回504
   if (networkRequest == null && cacheResponse == null) {
     return new Response.Builder()
         .request(chain.request())
         .protocol(Protocol.HTTP_1_1)
         .code(504)
         .message("Unsatisfiable Request (only-if-cached)")
         .body(Util.EMPTY_RESPONSE)
         .sentRequestAtMillis(-1L)
         .receivedResponseAtMillis(System.currentTimeMillis())
         .build();
   }
   // If we don't need the network, we're done.
   //如果請求為null，緩存不為null，則直接使用緩存。
       if (networkRequest == null) {
         return cacheResponse.newBuilder()
             .cacheResponse(stripBody(cacheResponse))
             .build();
       }
     Response networkResponse = null;
        try {
          //調(diào)用責(zé)任鏈下一個攔截器
          networkResponse = chain.proceed(networkRequest);
        } finally {
        }
      Response response = networkResponse.newBuilder()
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
     // Offer this request to the cache.
     //將響應(yīng)存入緩存。
      CacheRequest cacheRequest = cache.put(response);
}

public Response intercept(Chain chain) throws IOException {
     RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    //在重定向攔截器中創(chuàng)建，
    StreamAllocation streamAllocation = realChain.streamAllocation();
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    //從連接池中，找到一個可以復(fù)用的連接，
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
   // RealConnection 中封裝了一個Socket和一個Socket連接池
    RealConnection connection = streamAllocation.connection();
    //調(diào)用下一個攔截器
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
//遍歷連接池
RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {
    assert (Thread.holdsLock(this));
    for (RealConnection connection : connections) {
      if (connection.isEligible(address, route)) {
        streamAllocation.acquire(connection, true);
        return connection;
      }
    }
    return null;
  }
  public boolean isEligible(Address address, @Nullable Route route) {
    // If this connection is not accepting new streams, we're done.
    if (allocations.size() >= allocationLimit || noNewStreams) return false;
    // If the non-host fields of the address don't overlap, we're done.
    if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;
    // If the host exactly matches, we're done: this connection can carry the address.
    從連接池中找到一個連接參數(shù)一致且并未占用的連接
    if (address.url().host().equals(this.route().address().url().host())) {
      return true; // This connection is a perfect match.
  }

/** This is the last interceptor in the chain. It makes a network call to the server. */
這是責(zé)任鏈中最后一個攔截器，這個會去請求服務(wù)器。
 public Response intercept(Chain chain) throws IOException {
      RealInterceptorChain realChain = (RealInterceptorChain) chain;
      HttpCodec httpCodec = realChain.httpStream();
      StreamAllocation streamAllocation = realChain.streamAllocation();
      RealConnection connection = (RealConnection) realChain.connection();
      Request request = realChain.request();
      //將請求頭寫入緩存
      httpCodec.writeRequestHeaders(request);
      return response;