1.okHttp使用流程分析

      // 1.創(chuàng)建OkHttpClient對(duì)象
        OkHttpClient client = new OkHttpClient.Builder().build();
        // 2.創(chuàng)建Request對(duì)象
        Request request = new Request.Builder().build();
        // 3.創(chuàng)建請(qǐng)求對(duì)象
        Call call = client.newCall(request);
        // 4.同步請(qǐng)求
        try {
            // 同步返回結(jié)果
            Response execute = call.execute();
        } catch (IOException e) {
            e.printStackTrace();
        }
        // 4.異步使用 異步返回結(jié)果
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
            }
            @Override
            public void onResponse(Call call, Response response) throws IOException {
            }
        });

• okhttp請(qǐng)求發(fā)起流程分析
  1.同步請(qǐng)求
  執(zhí)行call.execute()方法，實(shí)際上會(huì)執(zhí)行到RealCall的execute方法智政，方法所示：

 @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      //此處的代碼是將請(qǐng)求加入到okhttp的同步請(qǐng)求隊(duì)列中
      client.dispatcher().executed(this);
      //此處代碼是真正的發(fā)出去同步請(qǐng)求并返回結(jié)果狸剃，這里涉及到okhttp的攔截器，下面會(huì)細(xì)講
      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);
    }
  }

2.異步請(qǐng)求
執(zhí)行 call.enqueue()方法實(shí)際上會(huì)執(zhí)行到RealCall的enqueue方法

 @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    // 獲取okhttp請(qǐng)求調(diào)度器調(diào)用異步請(qǐng)求，AsyncCall方法是一個(gè)實(shí)現(xiàn)Runnable的類
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

AsyncCall繼承NamedRunnable贼陶，NamedRunnable實(shí)現(xiàn)了Runnable，rRunnable的run方法會(huì)執(zhí)行到AsyncCall的execute方法

 final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;

    AsyncCall(Callback responseCallback) {
      super("OkHttp %s", redactedUrl());
      this.responseCallback = responseCallback;
    }

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

    Request request() {
      return originalRequest;
    }

    RealCall get() {
      return RealCall.this;
    }
    // 實(shí)際是Runnable的run方法
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
         //  進(jìn)入攔截器巧娱，發(fā)起請(qǐng)求獲取請(qǐng)求結(jié)果
        Response response = getResponseWithInterceptorChain();
        if (retryAndFollowUpInterceptor.isCanceled()) {
          signalledCallback = true;
          // 請(qǐng)求失敗回調(diào)
          responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
        } else {
          signalledCallback = true;
          // 請(qǐng)求成功回調(diào)
          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);
      }
    }
  }

2.okHttp使用的設(shè)計(jì)模式分析

• Builder設(shè)計(jì)模式
  okHttpClient的Builder類,碉怔，提供默認(rèn)配置參數(shù)

  public Builder() {
      dispatcher = new Dispatcher(); // 默認(rèn)請(qǐng)求調(diào)度器
      protocols = DEFAULT_PROTOCOLS; // 默認(rèn)請(qǐng)求協(xié)議 http/1.1 和http/2
      connectionSpecs = DEFAULT_CONNECTION_SPECS; // 提供默認(rèn)TLS 連接
      eventListenerFactory = EventListener.factory(EventListener.NONE); //指標(biāo)事件監(jiān)聽器，可以監(jiān)控HTTP 呼叫的數(shù)量禁添、大小和持續(xù)時(shí)間
      proxySelector = ProxySelector.getDefault(); // 默認(rèn)代理服務(wù)器
      cookieJar = CookieJar.NO_COOKIES; // 默認(rèn)cookie實(shí)現(xiàn)
      socketFactory = SocketFactory.getDefault(); // 默認(rèn)socket創(chuàng)建工廠
      hostnameVerifier = OkHostnameVerifier.INSTANCE; // 默認(rèn)主機(jī)名驗(yàn)證器  可以進(jìn)行證書校驗(yàn)
      certificatePinner = CertificatePinner.DEFAULT; // 加密證書之類的 俺也不太懂
      proxyAuthenticator = Authenticator.NONE; // 代理請(qǐng)求認(rèn)證 空實(shí)現(xiàn)
      authenticator = Authenticator.NONE; //請(qǐng)求認(rèn)證 空實(shí)現(xiàn)
      connectionPool = new ConnectionPool(); // 默認(rèn)連接池 應(yīng)該socket連接的連接池
      dns = Dns.SYSTEM; // 默認(rèn)dns解析
      followSslRedirects = true;  // ssl重定向
      followRedirects = true; // 默認(rèn)開啟重定向
      retryOnConnectionFailure = true; // 失敗重試
      connectTimeout = 10_000;  // 連接超時(shí)
      readTimeout = 10_000;  // 讀取超時(shí)
      writeTimeout = 10_000; // 寫入超時(shí)
      pingInterval = 0; // 心跳間隔
    }

Request類的Builder類撮胧，提供默認(rèn)請(qǐng)求方式

public Builder() {
      this.method = "GET"; //默認(rèn)get請(qǐng)求
      this.headers = new Headers.Builder();
    }

• 責(zé)任鏈設(shè)計(jì)模式
  前邊已經(jīng)介紹過(guò)，這里是okhttp發(fā)起請(qǐng)求的地方老翘，從這里開始通過(guò)責(zé)任鏈模式走okHttp的所有攔截器

  Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors()); // 加載用戶自定義攔截器
    interceptors.add(retryAndFollowUpInterceptor); // 重試攔截器
    interceptors.add(new BridgeInterceptor(client.cookieJar())); // 橋接攔截器
    interceptors.add(new CacheInterceptor(client.internalCache())); // 緩存攔截器
    interceptors.add(new ConnectInterceptor(client)); // 連接攔截器
    if (!forWebSocket) {  //如果不是webSocket協(xié)議 添加
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket)); // 請(qǐng)求攔截器 最后的一個(gè)攔截器
    // 構(gòu)建責(zé)任鏈對(duì)象
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());
    // 進(jìn)入責(zé)任鏈
    return chain.proceed(originalRequest);
  }
}

通過(guò)上述代碼芹啥，最終會(huì)進(jìn)入到RealInterceptorChain的proceed方法

 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.
    // 構(gòu)建攔截器責(zé)任鏈的下一個(gè)節(jié)點(diǎn)
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    // 獲取當(dāng)前攔截器
    Interceptor interceptor = interceptors.get(index);
    // 執(zhí)行當(dāng)前攔截器 注意這里的參數(shù)是next，也就是說(shuō)是下一個(gè)責(zé)任鏈上的節(jié)點(diǎn)
    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;
  }

此處Response response = interceptor.intercept(next);會(huì)執(zhí)行攔截器的intercept方法铺峭，以第一個(gè)攔截器RetryAndFollowUpInterceptor為例墓怀，查看此攔截器的intercept方法

 @Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
        //... 此處省略部分代碼
      try {
        // 執(zhí)行責(zé)任鏈節(jié)點(diǎn)的處理方法  記得上邊的攔截器中傳入的下一個(gè)節(jié)點(diǎn)的對(duì)象，所以這里就調(diào)到下一個(gè)節(jié)點(diǎn)的proceed方法
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
          throw e.getFirstConnectException();
        }
         //... 此處省略部分代碼
      priorResponse = response;
    }
  }

這樣責(zé)任鏈的每一個(gè)節(jié)點(diǎn)都會(huì)先執(zhí)行攔截的intercept方法卫键，然后執(zhí)行下一個(gè)節(jié)點(diǎn)的proceed的方法傀履，直到執(zhí)行到最后一個(gè)攔截器
CallServerInterceptor的intercept方法，會(huì)發(fā)起真正的請(qǐng)求永罚，拿到響應(yīng)體啤呼，依次返回結(jié)果。（具體請(qǐng)求過(guò)程待補(bǔ)充）

 @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    // ...省略部分代碼
    Request request = realChain.request();
      if (responseBuilder == null) {
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
    // ...省略部分代碼
        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } else if (!connection.isMultiplexed()) {
        streamAllocation.noNewStreams();
      }
    }
    // ...省略部分代碼
    httpCodec.finishRequest();
    // 構(gòu)建返回體對(duì)象  真正返回呢袱，這里采用okIo進(jìn)行網(wǎng)絡(luò)請(qǐng)求官扣，后期有時(shí)間補(bǔ)充對(duì)應(yīng)的請(qǐng)求過(guò)程
    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
      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) {
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }
    return response;
  }

3.okHttp的線程調(diào)度器（Dispatcher）分析

從上邊的請(qǐng)求過(guò)程可以看到，不管同步請(qǐng)求或者異步請(qǐng)求羞福，都會(huì)調(diào)用okHttpClient的dispatcher()來(lái)操作惕蹄，

  //同步請(qǐng)求
 client.dispatcher().executed(this);

  // 異步請(qǐng)求
 client.dispatcher().enqueue(new AsyncCall(responseCallback));

Dispatcher調(diào)度器的所有成員變量

  private int maxRequests = 64;  //同時(shí)最大請(qǐng)求數(shù)量
  private int maxRequestsPerHost = 5; // 同時(shí)最大同一域名的請(qǐng)求數(shù)量
  private @Nullable Runnable idleCallback;  // 調(diào)度器空閑狀態(tài)的回調(diào)

  /** Executes calls. Created lazily. */
  private @Nullable ExecutorService executorService; // 請(qǐng)求線程池

  /** Ready async calls in the order they'll be run. */
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();  // 異步請(qǐng)求等待隊(duì)列

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();  // 異步請(qǐng)求運(yùn)行隊(duì)列

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();  // 同步請(qǐng)求運(yùn)行隊(duì)列

同步請(qǐng)求隊(duì)列分析,從RealCall的同步請(qǐng)求方法開始

 @Override public Response execute() throws IOException {
    synchronized (this) {
    // ...省略部分代碼
    try {
      client.dispatcher().executed(this);  // 加載同步請(qǐng)求隊(duì)列
      Response result = getResponseWithInterceptorChain(); //獲取請(qǐng)求結(jié)果
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this); //完成同步請(qǐng)求
    }
  }

異步請(qǐng)求隊(duì)列分析，從RealCall的異步請(qǐng)求enqueue()方法開始分析

@Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
    // ...省略部分代碼
    client.dispatcher().enqueue(new AsyncCall(responseCallback)); // 加入異步請(qǐng)求隊(duì)列  
  }

具體實(shí)現(xiàn)治专，看Dispatcher的enqueue方法

synchronized void enqueue(AsyncCall call) {
      // 如果正在運(yùn)行異步請(qǐng)求數(shù)量小于最大請(qǐng)求數(shù)量限制 
      //并且 同一域名的請(qǐng)求數(shù)量小于maxRequestsPerHost卖陵，將請(qǐng)求加入運(yùn)行中隊(duì)列，
     // 并直接加入的線程池執(zhí)行张峰，否則添加到等待隊(duì)列
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call); //加入異步請(qǐng)求運(yùn)行中隊(duì)列
      executorService().execute(call); // 執(zhí)行異步請(qǐng)求
    } else {
      readyAsyncCalls.add(call); // 加入異步請(qǐng)求等待隊(duì)列
    }
  }

異步請(qǐng)求就會(huì)在線程中執(zhí)行Runnable的run方法泪蔫，也就會(huì)執(zhí)行到AsyncCall 的execute方法，這樣就可以正在發(fā)起請(qǐng)求獲取響應(yīng)結(jié)果喘批。

  final class AsyncCall extends NamedRunnable {
  // ...省略部分代碼
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
        Response response = getResponseWithInterceptorChain(); //獲取請(qǐng)求結(jié)果
        if (retryAndFollowUpInterceptor.isCanceled()) {
          signalledCallback = true;
          responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
        } else {
          signalledCallback = true;
          responseCallback.onResponse(RealCall.this, response);
        }
      } catch (IOException e) {
        // ...省略部分代碼
      } finally {
        client.dispatcher().finished(this); //請(qǐng)求結(jié)束
      }
    }
  }

不管是同步請(qǐng)求還是異步請(qǐng)求撩荣，最后都會(huì)執(zhí)行到 client.dispatcher().finished(this); 這句代碼铣揉，我們看一下具體的實(shí)現(xiàn)

private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
      //執(zhí)行結(jié)束，將請(qǐng)求從隊(duì)列中移除
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      // promoteCalls 同步請(qǐng)求為false,異步請(qǐng)求為true餐曹，調(diào)用promoteCalls來(lái)判斷是否可以將等待隊(duì)列的請(qǐng)求放到運(yùn)行隊(duì)列
      if (promoteCalls) promoteCalls();
      runningCallsCount = runningCallsCount(); //獲取請(qǐng)求的數(shù)量
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {  //如果請(qǐng)求隊(duì)列中沒(méi)有數(shù)據(jù)逛拱，也就是說(shuō)Dispatcher處于空閑狀態(tài)，調(diào)用idleCallback的run方法
      idleCallback.run();
    }
  }

promoteCalls()方法的實(shí)現(xiàn)

private void promoteCalls() {
    //運(yùn)行隊(duì)列的數(shù)量是否大于最大請(qǐng)求數(shù)量
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    // 等到隊(duì)列是否為空
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
    // 迭代等待隊(duì)列
    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();
      // 判定同一域名的請(qǐng)求是否消息最大數(shù)量
      if (runningCallsForHost(call) < maxRequestsPerHost) {
      // 移除等待隊(duì)列數(shù)據(jù)
        i.remove();
      //加入運(yùn)行中隊(duì)列
        runningAsyncCalls.add(call);
        // 加入線程池
        executorService().execute(call);
      }
      // 這句代碼個(gè)人感覺(jué)冗余了
      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
  }

• Dispatcher線程池的配置

 public synchronized ExecutorService executorService() {
    if (executorService == null) {
      // 第一個(gè)參數(shù) 核心線程數(shù)
      // 第二個(gè)參數(shù)  最大線程數(shù)
      // 第三個(gè)參數(shù)  線程碧ê铮活等到時(shí)間
     // 第四個(gè)參數(shù) 單位秒
     // 第五個(gè)參數(shù) 線程阻塞隊(duì)列
     // 第六個(gè)參數(shù) 線程構(gòu)造工廠
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

1.線程池的核心線程數(shù)為什么設(shè)置成0朽合，最小線程數(shù)設(shè)置成最大值
由于okHttp已經(jīng)自己利用readyAsyncCalls和runningAsyncCalls管理請(qǐng)求隊(duì)列和等待隊(duì)列，沒(méi)必要再通過(guò)線程池來(lái)控制了
2.采用SynchronousQueue來(lái)實(shí)現(xiàn)等待隊(duì)列
SynchronousQueue是無(wú)界的饱狂，是一種無(wú)緩沖的等待隊(duì)列曹步，但是由于該Queue本身的特性，在某次添加元素后必須等待其他線程取走后才能繼續(xù)添加休讳；可以認(rèn)為SynchronousQueue是一個(gè)緩存值為1的阻塞隊(duì)列箭窜，但是 isEmpty()方法永遠(yuǎn)返回是true，remainingCapacity() 方法永遠(yuǎn)返回是0衍腥，remove()和removeAll() 方法永遠(yuǎn)返回是false，iterator()方法永遠(yuǎn)返回空纳猫，peek()方法永遠(yuǎn)返回null婆咸。