1.前言

• Android开发过程中，使用第三方的框架库已成家常便饭，使用第三方好处避免重复造轮子、降低成本、提升效率、降低风险等等，当遇到框架库不能满足现有业务、框架库设计缺陷或者漏洞、API使用深度不够时，如果停留只会使用层面，就会增加修改过程的难度，所以对于使用的框架库最好还是有个系统的认识；
• 本文带大家深入讲解 OkHttp；
• 文章中实例 linhaojian的Github

2.目录

目录.png

3.定义

• 一款处理网络请求的开源项目，由Square公司贡献。

4.作用

• 通过建造者模式（Builder Pattern）方式，完成复杂的网络请求。

5.特点

• 1.同时支持HTTP1.1与支持HTTP2.0；
• 2.同时支持同步与异步请求；
• 3.同时具备HTTP与WebSocket功能；
• 4.拥有自动维护的socket连接池，减少握手次数；
• 5.拥有队列线程池，轻松写并发；
• 6.拥有Interceptors(拦截器)，轻松处理请求与响应额外需求(例：请求失败重试、响应内容重定向等等)；

6.OkHttp系统图

okhttp系统图.png

7.OkHttpClient(封装请求参数)

• OkHttpClient 通过建造者模式（Builder Pattern）方式，完成请求参数配置。常用如下：
  • connectTimeout ：连接超时
  • readTimeout：读取超时
  • writeTimeout：写入超时
  • pingInterval：websocket情况下连接心跳间隔
  • interceptors：自定义拦截器
  • networkInterceptors：自定义网络连接成功的拦截器
• OkHttpClient 除了完成请求参数的配置之外，还提供获取WebSocket、Call(Call实现类为RealCall，下文会介绍)相关类；

7.1 WebSocket

• WebSocket是一种在单个TCP连接上进行全双工通信的协议，支持服务器想客户端的发送请求，由OkHttpClient创建，源码如下：

  /**
   * Uses {@code request} to connect a new web socket.
   */
  @Override public WebSocket newWebSocket(Request request, WebSocketListener listener) {
    RealWebSocket webSocket = new RealWebSocket(request, listener, new Random(), pingInterval);// 1
    webSocket.connect(this); 
    return webSocket;
  }

注释1：WebSocket是一个接口，它的实现类RealWebSocket，该类完成WebSocket的连接、数据请求与接收功能。

7.2 Call初始化

  //OkHttpClient 初始化Call的函数
  @Override public Call newCall(Request request) {
    return RealCall.newRealCall(this, request, false /* for web socket */);
  }
  // RealCall 初始化函数
  static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    // Safely publish the Call instance to the EventListener.
    RealCall call = new RealCall(client, originalRequest, forWebSocket);
    call.eventListener = client.eventListenerFactory().create(call);
    return call;
  }

8.RealCall同步异步

• RealCall 是真正触发网络请求的类(实现Call接口，一次请求 = 一个RealCall实例)，它提供了同步请求、异步请求；

8.1 同步请求

  @Override public Response execute() throws IOException {
    // 处理不能重复请求，因为一个RealCall对应一个请求。
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      client.dispatcher().executed(this);//1
      Response result = getResponseWithInterceptorChain();//2
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this);
    }
  }
Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      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());//3
    return chain.proceed(originalRequest);//4
  }

注释1：将RealCall实例添加至Dispatcher中(下文会介绍Dispatcher)。
注释2：通过getResponseWithInterceptorChain()获取响应。
注释3：通过封装好的拦截器集合，获取第一个拦截器的任务。
注释4：触发第一个拦截器的任务，该任务就触发一下拦截器的任务，以此类推，原理(Android事件传递机制)如下图：

任务链.png

8.2 异步请求

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

注释1：把AsyncCall请求对象传递进Dispatcher线程池管理；

• AsyncCall 将请求业务放入到Runnable中。

  final class AsyncCall extends NamedRunnable {
      // ......
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
        Response response = getResponseWithInterceptorChain();//2
        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);
      }
    }
  }

注释2：通过getResponseWithInterceptorChain()获取响应;

9.Dispatcher线程池

• Dispatcher 管理网络请求的线程池，其实就是把同步(RealCall)与异步(AsyncCall)的请求放进集合中统一管理，然后通过线程池执行AsyncCall的请求。

9.1 Dispatcher中同步(RealCall)

• RealCall在Dispatcher中，其实主要就是一个存储功能(即用一个集合把RealCall的请求进行存储)。

9.2 Dispatcher中异步(AsyncCall)

• AsyncCall在Dispatcher中，除了使用集合存储AsyncCall的请求，Dispatcher还初始化了一个线程池(ThreadPoolExecutor)处理AsyncCall的网络请求。

public final class Dispatcher {
  //最大请求数量
  private int maxRequests = 64;
  //相同host的最大请求数据
  private int maxRequestsPerHost = 5;
  // ......
  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));//1
    }
    return executorService;
  }
  // ......
  synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {//2
      runningAsyncCalls.add(call);//3
      executorService().execute(call);//4
    } else {
      readyAsyncCalls.add(call);//5
    }
  }
  /** Used by {@code AsyncCall#run} to signal completion. */
  void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }
  /** Used by {@code Call#execute} to signal completion. */
  void finished(RealCall call) {
    finished(runningSyncCalls, call, false);//6
  }
  private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      if (promoteCalls) promoteCalls();//7
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }
    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }
  private void promoteCalls() {
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();
      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        executorService().execute(call);
      }
      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
  }
  // ......
}

注释1：获取自定义线程池；
注释2：判断正在执行的异步请求数量与请求集合中相同host的数量是否满足，如果满足就添加到执行中的集合中，并添加至线程池中执行请求；如果不满足就添加至待执行请求的集合中，等待执行中的请求完成之后，再执行相同host数量判断满足才添加至线程池中执行请求；
注释3：将请求对象AsyncCall添加进请求执行的集合中；
注释4：将请求对象AsyncCall添加进线程池中执行；
注释5：当不满足执行条件时(注释2)，把请求对象添加至待执行的集合中；
注释6：每当一个请求执行完毕时，就会调用finished()去掉对应集合中的存储对象，并在次判断待执行的集合中是否有满足条件的请求，若满足就添加至执行的集合与线程池中执行，若不满足继续等待下一个请求完成再次判断。
注释7：判断待执行的集合中是否满足可执行的对象。

10.Interceptor拦截器及调用链

• Interceptor 拦截器，供使用者可在请求过程或者响应过程中自定义额外的业务处理(例如：最常见的请求失败重试、响应数据的重定向等等)。
• OkHttp3中，除了可自定义额外的拦截器之外，它内部也存储一些固定的拦截器处理其内部业务逻辑，下面就会介绍它们(RetryAndFollowUpInterceptor、BridgeInterceptor、CacheInterceptor、ConnectInterceptor、CallServerInterceptor)；

10.1 RetryAndFollowUpInterceptor

• 定义：重定向拦截器；
• 作用：在无法请求服务器或者请求失败时，服务器会告诉客户端可以处理请求的url，然后重定向拦截器承当重新请求新url的作用(服务器返回3XX错误码为重定向，可以通过响应头的Location获取新请求的url);

10.2 BridgeInterceptor

• 定义：桥拦截器；
• 作用：封装请求头(Content-Type、Connection、Cookie…)与响应头(“Content-Encoding…)的信息。

10.3 CacheInterceptor

• 定义：缓存拦截器；
• 作用：为网络请求提供缓存功能，加快相同请求的访问速度，减少资源损耗。

10.4 ConnectInterceptor

• 定义：连接拦截器；
• 作用：与服务器建立通讯连接。

10.5 CallServerInterceptor

• 定义：请求服务器拦截器；
• 作用：与服务器进行数据通讯(包含请求头、请求内容)。

10.6 调用链

• 上文也提到任务链结构图(责任链模式)：
  任务链.png
• 其实它的原理类似于(Android 事件传递机制)，向下传递请求，向上反馈响应，在调用RealInterceptorChain的proceed()时，创建下一个拦截器的任务，并通过拦截器中intercept()把任务传递至当前拦截器进行关联，然后以此类推，相关代码如下：

public final class RealInterceptorChain implements Interceptor.Chain {
 public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection connection) throws IOException {
    // ......
    // 创建下一个拦截器的任务
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    // 获取当前任务中的拦截器
    Interceptor interceptor = interceptors.get(index);
    // 将下一个任务传递至当前拦截器中进行关联，并在拦截器中传入的任务触发执行下一个拦截器
    Response response = interceptor.intercept(next);
    // ......
    return response;
  }
}

11.缓存机制CacheInterceptor

• 看看缓存机制CacheInterceptor的实现原理：

public final class CacheInterceptor implements Interceptor {
  // ....
  @Override public Response intercept(Chain chain) throws IOException {
    // 根据请求内容通过Cache类判断是否已经存在响应的缓存信息
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;
    long now = System.currentTimeMillis();
    // 通过请求对象与缓存对象获取缓存策略，根据请求头的内容(Date、Expires、Last-Modified....)制定缓存策略
    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;
    // 记录请求过程的相关数据（请求次数、缓存次数....）
    if (cache != null) {
      cache.trackResponse(strategy);
    }
    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }
    // 当请求头中包含only-if-cached时，networkRequest 与 cacheResponse 都为空，表示不进行网络请求
    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();
    }
    // 不进行网络请求，并请求头包含缓存标识时，构建缓存内容.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }
    Response networkResponse = null;
    try {
      // 执行下一个拦截器任务
      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());
      }
    }
    // 如果本地有缓存，并且响应结果为没有修改时，直接从本地缓存获取相关信息数据
    if (cacheResponse != null) {
      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();
        // 更新响应对象至缓存中
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }
   // 没有响应缓存时，封装请求返回的响应并添加至本地缓存中
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();
    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // 添加本地缓存中
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }
      // 去掉（POST\PUT\DELETE\MOVE\PATCH）请求方法的本地缓存
      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }
    return response;
  }
}

12.连接与请求（StreamAllocation，RealConnection，HttpCodec）

• OkHttp3把网络连接、请求数据通讯过程封装StreamAllocation，RealConnection，HttpCodec中；
• StreamAllocation：负责初始化RealConnection、HttpCodec，并将前2者与RealCall进行关联；
  1.StreamAllocation初始化(在RetryAndFollowUpInterceptor进行实例化)

public final class RetryAndFollowUpInterceptor implements Interceptor {
  @Override public Response intercept(Chain chain) throws IOException {
    //......
    // 实例化
    StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;
    //......
    }
}

2.RealConnection与HttpCodec初始化(RealConnection在ConnectInterceptor中通过StreamAllocation的newStream()初始化，而HttpCodec在RealConnection中被初始化)

public final class ConnectInterceptor implements Interceptor {
  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    // 获取HttpCodec 对象
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();
    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }
}

public final class StreamAllocation {
    //......
  public HttpCodec newStream(
      OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
    int connectTimeout = chain.connectTimeoutMillis();
    int readTimeout = chain.readTimeoutMillis();
    int writeTimeout = chain.writeTimeoutMillis();
    int pingIntervalMillis = client.pingIntervalMillis();
    boolean connectionRetryEnabled = client.retryOnConnectionFailure();
    try {
      // 初始化RealConnection，最后会调用RealConnection的connect函数建立网络连接
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
     // 通过RealConnection 实例化HttpCpdec
      HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);
      synchronized (connectionPool) {
        codec = resultCodec;
        return resultCodec;
      }
    } catch (IOException e) {
      throw new RouteException(e);
    }
  }
}

public final class RealConnection extends Http2Connection.Listener implements Connection {
  public HttpCodec newCodec(OkHttpClient client, Interceptor.Chain chain,
      StreamAllocation streamAllocation) throws SocketException {
    //判断HTTP是否为Http2，如果是实例化Http2Codec，否则实例化Http1Codec
    if (http2Connection != null) {
      return new Http2Codec(client, chain, streamAllocation, http2Connection);
    } else {
      socket.setSoTimeout(chain.readTimeoutMillis());
      source.timeout().timeout(chain.readTimeoutMillis(), MILLISECONDS);
      sink.timeout().timeout(chain.writeTimeoutMillis(), MILLISECONDS);
      return new Http1Codec(client, streamAllocation, source, sink);
    }
  }
}

• RealConnection：真正的负责完成网络连接；

public final class RealConnection extends Http2Connection.Listener implements Connection {
public void connect(int connectTimeout, int readTimeout, int writeTimeout,
      int pingIntervalMillis, boolean connectionRetryEnabled, Call call,
      EventListener eventListener) {
    if (protocol != null) throw new IllegalStateException("already connected");
    RouteException routeException = null;
    List<ConnectionSpec> connectionSpecs = route.address().connectionSpecs();
    ConnectionSpecSelector connectionSpecSelector = new ConnectionSpecSelector(connectionSpecs);
    if (route.address().sslSocketFactory() == null) {
      if (!connectionSpecs.contains(ConnectionSpec.CLEARTEXT)) {
        throw new RouteException(new UnknownServiceException(
            "CLEARTEXT communication not enabled for client"));
      }
      String host = route.address().url().host();
      if (!Platform.get().isCleartextTrafficPermitted(host)) {
        throw new RouteException(new UnknownServiceException(
            "CLEARTEXT communication to " + host + " not permitted by network security policy"));
      }
    } else {
      if (route.address().protocols().contains(Protocol.H2_PRIOR_KNOWLEDGE)) {
        throw new RouteException(new UnknownServiceException(
            "H2_PRIOR_KNOWLEDGE cannot be used with HTTPS"));
      }
    }
    while (true) {
      try {
        if (route.requiresTunnel()) {
          connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener);
          if (rawSocket == null) {
            // We were unable to connect the tunnel but properly closed down our resources.
            break;
          }
        } else {
          // 最后会调用socket.connect()进行网络连接
          connectSocket(connectTimeout, readTimeout, call, eventListener);
        }
        establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener);
        eventListener.connectEnd(call, route.socketAddress(), route.proxy(), protocol);
        break;
      } catch (IOException e) {
        closeQuietly(socket);
        closeQuietly(rawSocket);
        socket = null;
        rawSocket = null;
        source = null;
        sink = null;
        handshake = null;
        protocol = null;
        http2Connection = null;
        eventListener.connectFailed(call, route.socketAddress(), route.proxy(), null, e);
        if (routeException == null) {
          routeException = new RouteException(e);
        } else {
          routeException.addConnectException(e);
        }
        if (!connectionRetryEnabled || !connectionSpecSelector.connectionFailed(e)) {
          throw routeException;
        }
      }
    }
    if (route.requiresTunnel() && rawSocket == null) {
      ProtocolException exception = new ProtocolException("Too many tunnel connections attempted: "
          + MAX_TUNNEL_ATTEMPTS);
      throw new RouteException(exception);
    }
    if (http2Connection != null) {
      synchronized (connectionPool) {
        allocationLimit = http2Connection.maxConcurrentStreams();
      }
    }
  }
}

• HttpCodec：负责完成发送请求头与数据内容(使用okio完成数据的写入与读出)；
  • Http1Codec：处理HTTP1.1协议的数据传递。
  • Http2Codec：处理HTTP2.0协议的数据传递。

13.实例

• 编写一个简单GET请求：

public void get(){
        OkHttpClient okHttpClient = new OkHttpClient()
                .newBuilder()
//                .addInterceptor()
                .readTimeout(30, TimeUnit.SECONDS)
                .build();
        Request request = new Request.Builder()
//                .header()
                .url("http://www.baidu.com")
                .build();
        okHttpClient.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {             
            }
            @Override
            public void onResponse(Call call, Response response) throws IOException {
            }
        });
}

14.总结

• 到此，Android OkHttp3就讲解完毕。
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