OkHttpClient okHttpClient=new OkHttpClient.Builder().build();
Request request=new Request.Builder()
        .url("http://www.baidu.com")
        .get().build();
Call call = okHttpClient.newCall(request);
try {
    Response execute = call.execute();
    Log.d("lpf","execute"+execute.body());
} catch (IOException e) {
    e.printStackTrace();
}

public Response execute() throws IOException {
    synchronized (this) {
        if (executed) throw new IllegalStateException("Already Executed");
        executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
        client.dispatcher().executed(this);
        // 發(fā)起請求
        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);
    }
}

public void enqueue(Callback responseCallback) {
    synchronized (this) { //不能沖突調(diào)用
        if (executed) throw new IllegalStateException("Already Executed");
        executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback)); //OkHttp會提供默認的分發(fā)器
}

synchronized void enqueue(AsyncCall call) { //分發(fā)器 分發(fā)任務  
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
        runningAsyncCalls.add(call);  //正在執(zhí)行的請求
        executorService().execute(call); //線程池跑任務
    } else {
        readyAsyncCalls.add(call);   //ready 中的任務什么時候執(zhí)行
    }
}

 protected void execute() {
        boolean signalledCallback = false;
        try {
            //執(zhí)行請求 （攔截器）
            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 {
            client.dispatcher().finished(this);  //這里的代碼肯定執(zhí)行
        }
    }
}

private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
        //將任務都運行隊列中移除
        if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
        if (promoteCalls) promoteCalls();
        runningCallsCount = runningCallsCount();
        idleCallback = this.idleCallback;
    }
    if (runningCallsCount == 0 && idleCallback != null) {
        idleCallback.run();
    }
}

 public synchronized ExecutorService executorService() {
        if (executorService == null) {  //這個線程池的特點是高并發(fā) 最大吞吐量
            executorService = 
                new ThreadPoolExecutor(0, //核心線程數(shù)量
                                      Integer.MAX_VALUE,//最大線程數(shù)量
                                       60, 				//空閑線程閑置時間														TimeUnit.SECONDS,   //閑置時間單位
                    				new SynchronousQueue<Runnable>(),//線程等待隊列 
                    				Util.threadFactory("OkHttp Dispatcher",false)//線程工廠
                                      );
        }
        return executorService;
    }

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors()); //自定義攔截器加入到集合
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {//如果不是webSocket
        interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
            originalRequest, this, eventListener, client.connectTimeoutMillis(),
            client.readTimeoutMillis(), client.writeTimeoutMillis());
    return chain.proceed(originalRequest);
}

@Override
public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();
    /**
     *管理類，維護了 與服務器的連接、數(shù)據(jù)流與請求三者的關(guān)系。真正使用的攔截器為 Connect
     */
    StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
            createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;
    int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
        if (canceled) {
            streamAllocation.release();
            throw new IOException("Canceled");
        }
        Response response;
        boolean releaseConnection = true;
        try {
            //請求出現(xiàn)了異常，那么releaseConnection依舊為true。
            response = realChain.proceed(request, streamAllocation, null, null);
            releaseConnection = false;
        } catch (RouteException e) {
            //路由異常，連接未成功，請求還沒發(fā)出去
            //The attempt to connect via a route failed. The request will not have been sent.
            if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
                throw e.getLastConnectException();
            }
            releaseConnection = false;
            continue;
        } catch (IOException e) {
            //請求發(fā)出去了，但是和服務器通信失敗了。(socket流正在讀寫數(shù)據(jù)的時候斷開連接)
            // HTTP2才會拋出ConnectionShutdownException。所以對于HTTP1 requestSendStarted一定是true
            //An attempt to communicate with a server failed. The request may have been sent.
            boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
            if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
            releaseConnection = false;
            continue;
        } finally {
            // We're throwing an unchecked exception. Release any resources.
            //不是前兩種的失敗，那直接關(guān)閉清理所有資源
            if (releaseConnection) {
                streamAllocation.streamFailed(null);
                streamAllocation.release();
            }
        }
        //如果進過重試/重定向才成功的，則在本次響應中記錄上次響應的情況
        //Attach the prior response if it exists. Such responses never have a body.
        if (priorResponse != null) {
            response = response.newBuilder()
                    .priorResponse(
                            priorResponse.newBuilder()
                                    .body(null)
                                    .build()
                    )
                    .build();
        }
        //處理3和4xx的一些狀態(tài)碼，如301 302重定向
        Request followUp = followUpRequest(response, streamAllocation.route());
        if (followUp == null) {
            if (!forWebSocket) {
                streamAllocation.release();
            }
            return response;
        }
        closeQuietly(response.body());
        //限制最大 followup 次數(shù)為20次
        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());
        }
        //todo 判斷是不是可以復用同一份連接
        if (!sameConnection(response, followUp.url())) {
            streamAllocation.release();
            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?");
        }
        request = followUp;
        priorResponse = response;
    }
}

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());
        }
        long contentLength = body.contentLength();
        if (contentLength != -1) {
            requestBuilder.header("Content-Length", Long.toString(contentLength));
            requestBuilder.removeHeader("Transfer-Encoding");
        } else {
            requestBuilder.header("Transfer-Encoding", "chunked");
            requestBuilder.removeHeader("Content-Length");
        }
    }
    if (userRequest.header("Host") == null) {
        requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }
    if (userRequest.header("Connection") == null) {
        requestBuilder.header("Connection", "Keep-Alive");
    }
    // If we add an "Accept-Encoding: gzip" header field we're responsible for also
  // decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
        transparentGzip = true;
        requestBuilder.header("Accept-Encoding", "gzip");
    }
    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
        requestBuilder.header("Cookie", cookieHeader(cookies));
    }
    if (userRequest.header("User-Agent") == null) {
        requestBuilder.header("User-Agent", Version.userAgent());
    }
    Response networkResponse = chain.proceed(requestBuilder.build());
    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
    Response.Builder responseBuilder = networkResponse.newBuilder()
            .request(userRequest);
    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) throws IOException {
    //通過url的md5數(shù)據(jù) 從文件緩存查找 （GET請求才有緩存）
    Response cacheCandidate = cache != null
            ? cache.get(chain.request())
            : null;
    long now = System.currentTimeMillis();
    //緩存策略:根據(jù)各種條件(請求頭)組成 請求與緩存
    CacheStrategy strategy =
            new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    //
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;
    if (cache != null) {
        cache.trackResponse(strategy);
    }
    if (cacheCandidate != null && cacheResponse == null) {
        closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }
    //沒有網(wǎng)絡請求也沒有緩存
    //If we're forbidden from using the network and the cache is insufficient, fail.
    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();
    }
    //沒有請求，肯定就要使用緩存
    //If we don't need the network, we're done.
    if (networkRequest == null) {
        return cacheResponse.newBuilder()
                .cacheResponse(stripBody(cacheResponse))
                .build();
    }
    //去發(fā)起請求
    Response networkResponse = null;
    try {
        networkResponse = chain.proceed(networkRequest);
    } finally {
        // If we're crashing on I/O or otherwise, don't leak the cache body.
        if (networkResponse == null && cacheCandidate != null) {
            closeQuietly(cacheCandidate.body());
        }
    }
    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
        //服務器返回304無修改，那就使用緩存的響應修改了時間等數(shù)據(jù)后作為本次請求的響應
        if (networkResponse.code() == HTTP_NOT_MODIFIED) {
            Response response = cacheResponse.newBuilder()
                    .headers(combine(cacheResponse.headers(), networkResponse.headers()))
                    .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
                    .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
                    .cacheResponse(stripBody(cacheResponse))
                    .networkResponse(stripBody(networkResponse))
                    .build();
            networkResponse.body().close();
            // Update the cache after combining headers but before stripping the
            // Content-Encoding header (as performed by initContentStream()).
            cache.trackConditionalCacheHit();
            cache.update(cacheResponse, response);
            return response;
        } else {
            closeQuietly(cacheResponse.body());
        }
    }
    //走到這里說明緩存不可用 那就使用網(wǎng)絡的響應
    Response response = networkResponse.newBuilder()
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
    //進行緩存
    if (cache != null) {
        if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response,
                networkRequest)) {
            // Offer this request to the cache.
            CacheRequest cacheRequest = cache.put(response);
            return cacheWritingResponse(cacheRequest, response);
        }
        if (HttpMethod.invalidatesCache(networkRequest.method())) {
            try {
                cache.remove(networkRequest);
            } catch (IOException ignored) {
                // The cache cannot be written.
            }
        }
    }
    return response;
}

public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

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();
    long sentRequestMillis = System.currentTimeMillis();
    realChain.eventListener().requestHeadersStart(realChain.call());
    httpCodec.writeRequestHeaders(request);
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);
    Response.Builder responseBuilder = null;
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
        // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
        // Continue" response before transmitting the request body. If we don't get that, return
        // what we did get (such as a 4xx response) without ever transmitting the request body.
         //這個請求頭代表了在發(fā)送請求體之前需要和服務器確定是否愿意接受客戶端發(fā)送的請求體
        //但是如果服務器不同意接受請求體，那么我們就需要標記該連接不能再被復用，調(diào)用 noNewStreams() 關(guān)閉相關(guān)的Socket。
        if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
            httpCodec.flushRequest();
            realChain.eventListener().responseHeadersStart(realChain.call());
            responseBuilder = httpCodec.readResponseHeaders(true);
        }
        if (responseBuilder == null) {
            // Write the request body if the "Expect: 100-continue" expectation was met.
            realChain.eventListener().requestBodyStart(realChain.call());
            long contentLength = request.body().contentLength();
            CountingSink requestBodyOut =
                    new CountingSink(httpCodec.createRequestBody(request, contentLength));
            BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
            request.body().writeTo(bufferedRequestBody);
            bufferedRequestBody.close();
            realChain.eventListener()
                    .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
        } else if (!connection.isMultiplexed()) {
            // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1
          // connection
            // from being reused. Otherwise we're still obligated to transmit the request
          // body to
            // leave the connection in a consistent state.
            streamAllocation.noNewStreams();
        }
    }
    httpCodec.finishRequest();
    if (responseBuilder == null) {
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(false);
    }
    Response response = responseBuilder
            .request(request)
            .handshake(streamAllocation.connection().handshake())
            .sentRequestAtMillis(sentRequestMillis)
            .receivedResponseAtMillis(System.currentTimeMillis())
            .build();
    int code = response.code();
    if (code == 100) {
        // server sent a 100-continue even though we did not request one.
        // try again to read the actual response
        responseBuilder = httpCodec.readResponseHeaders(false);
        response = responseBuilder
                .request(request)
                .handshake(streamAllocation.connection().handshake())
                .sentRequestAtMillis(sentRequestMillis)
                .receivedResponseAtMillis(System.currentTimeMillis())
                .build();
        code = response.code();
    }
    realChain.eventListener()
            .responseHeadersEnd(realChain.call(), response);
    if (forWebSocket && code == 101) {
        // Connection is upgrading, but we need to ensure interceptors see a non-null
      // response body.
        response = response.newBuilder()
                .body(Util.EMPTY_RESPONSE)
                .build();
    } else {
        response = response.newBuilder()
                .body(httpCodec.openResponseBody(response))
                .build();
    }
    if ("close".equalsIgnoreCase(response.request().header("Connection"))
            || "close".equalsIgnoreCase(response.header("Connection"))) {
        streamAllocation.noNewStreams();
    }
    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
        throw new ProtocolException(
                "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }
    return response;
}