• https://github.com/square/okhttp
• https://square.github.io/okhttp

源码基于 3.12.0

okhttp3.OkHttpClient#OkHttpClient()

public OkHttpClient() {
    //无参构造就相当于创建了一个空 Builder 对象
    this(new Builder());
}

OkHttpClient(Builder builder) {
    this.dispatcher = builder.dispatcher;
    this.proxy = builder.proxy;
    this.protocols = builder.protocols;
    this.connectionSpecs = builder.connectionSpecs;
    this.interceptors = Util.immutableList(builder.interceptors);
    this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
    this.eventListenerFactory = builder.eventListenerFactory;
    this.proxySelector = builder.proxySelector;
    this.cookieJar = builder.cookieJar;
    this.cache = builder.cache;
    this.internalCache = builder.internalCache;
    this.socketFactory = builder.socketFactory;

    boolean isTLS = false;
    for (ConnectionSpec spec : connectionSpecs) {
      isTLS = isTLS || spec.isTls();
    }

    if (builder.sslSocketFactory != null || !isTLS) {
      this.sslSocketFactory = builder.sslSocketFactory;
      this.certificateChainCleaner = builder.certificateChainCleaner;
    } else {
      X509TrustManager trustManager = Util.platformTrustManager();
      this.sslSocketFactory = newSslSocketFactory(trustManager);
      this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
    }

    if (sslSocketFactory != null) {
      Platform.get().configureSslSocketFactory(sslSocketFactory);
    }

    this.hostnameVerifier = builder.hostnameVerifier;
    this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
        certificateChainCleaner);
    this.proxyAuthenticator = builder.proxyAuthenticator;
    this.authenticator = builder.authenticator;
    this.connectionPool = builder.connectionPool;
    this.dns = builder.dns;
    this.followSslRedirects = builder.followSslRedirects;
    this.followRedirects = builder.followRedirects;
    this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
    this.callTimeout = builder.callTimeout;
    this.connectTimeout = builder.connectTimeout;
    this.readTimeout = builder.readTimeout;
    this.writeTimeout = builder.writeTimeout;
    this.pingInterval = builder.pingInterval;

    if (interceptors.contains(null)) {
      throw new IllegalStateException("Null interceptor: " + interceptors);
    }
    if (networkInterceptors.contains(null)) {
      throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
    }
  }

public Builder newBuilder() {
    return new Builder(this);
}
//静态内部类
public static final class Builder {
   ...
}

okhttp3.OkHttpClient#newCall

//调度器
final Dispatcher dispatcher;
final List<Interceptor> interceptors;
final List<Interceptor> networkInterceptors;
final EventListener.Factory eventListenerFactory;
//
final int callTimeout;
final int connectTimeout;
final int readTimeout;
final int writeTimeout;
/**
 * Prepares the {@code request} to be executed at some point in the future.
 */
@Override public Call newCall(Request request) {
   return RealCall.newRealCall(this, request, false /* for web socket */);
}

okhttp3.RealCall#newRealCall

final OkHttpClient client;
final RetryAndFollowUpInterceptor retryAndFollowUpInterceptor;
final AsyncTimeout timeout;
/**
   * There is a cycle between the {@link Call} and {@link EventListener} that makes this awkward.
   * This will be set after we create the call instance then create the event listener instance.
   */
private @Nullable EventListener eventListener;
/** The application's original request unadulterated by redirects or auth headers. */
final Request originalRequest;
final boolean forWebSocket;

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;
  }

okhttp3.RealCall#execute

  @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    //AsyncTimeout
    timeout.enter();
    eventListener.callStart(this);
    try {
      //
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      e = timeoutExit(e);
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      //
      client.dispatcher().finished(this);
    }
  }

okhttp3.Dispatcher#executed

//
/** Executes calls. Created lazily. */
private @Nullable ExecutorService executorService;
//异步步请求的待执行队列
/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
//异步请求的正在执行队列
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
//同步请求的正在执行队列
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

/** Used by {@code Call#execute} to signal it is in-flight. */
synchronized void executed(RealCall call) {
   //加锁 
   //加入同步请求的正在执行队列
   runningSyncCalls.add(call);
}

okhttp3.Dispatcher#finished

/** Used by {@code Call#execute} to signal completion. */
  void finished(RealCall call) {
    finished(runningSyncCalls, call);
  }

  private <T> void finished(Deque<T> calls, T call) {
    Runnable idleCallback;
    synchronized (this) {
      //移出队列
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      idleCallback = this.idleCallback;
    }
    //后面涉及异步的时候细说
    boolean isRunning = promoteAndExecute();

    if (!isRunning && idleCallback != null) {
      idleCallback.run();
    }
  }

okhttp3.RealCall#getResponseWithInterceptorChain

Response getResponseWithInterceptorChain() throws IOException {
    //拦截器链
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    //okHttpClient 构建时候传入的自定义拦截器
    interceptors.addAll(client.interceptors());
    //失败重试，重定向拦截器
    interceptors.add(retryAndFollowUpInterceptor);
    //请求头
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //缓存拦截器，读取修改缓存
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //网络连接拦截器，正式开启 http 请求，与服务器连接
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      //okHttpClient 构建时候传入的自定义网络拦截器
      interceptors.addAll(client.networkInterceptors());
    }
    //发送网络请求和读取网络响应
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());
    //RealInterceptorChain 通过递归掉头，然每次通过 index+1 分别完成拦截器集合内各个拦截器的处理
    return chain.proceed(originalRequest);
  }

okhttp3.RealCall#enqueue

@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));
  }

okhttp3.Dispatcher#enqueue

 void enqueue(AsyncCall call) {
    synchronized (this) {
      readyAsyncCalls.add(call);
    }
    promoteAndExecute();
  }

okhttp3.Dispatcher#promoteAndExecute

  /**
   * Promotes eligible calls from {@link #readyAsyncCalls} to {@link #runningAsyncCalls} and runs
   * them on the executor service. Must not be called with synchronization because executing calls
   * can call into user code.
   *
   * @return true if the dispatcher is currently running calls.
   */
  private boolean promoteAndExecute() {
    assert (!Thread.holdsLock(this));

    List<AsyncCall> executableCalls = new ArrayList<>();
    boolean isRunning;
    synchronized (this) {
      for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
        AsyncCall asyncCall = i.next();

        if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
        if (runningCallsForHost(asyncCall) >= maxRequestsPerHost) continue; // Host max capacity.

        i.remove();
        executableCalls.add(asyncCall);
        runningAsyncCalls.add(asyncCall);
      }
      isRunning = runningCallsCount() > 0;
    }

    for (int i = 0, size = executableCalls.size(); i < size; i++) {
      AsyncCall asyncCall = executableCalls.get(i);
      //
      asyncCall.executeOn(executorService());
    }

    return isRunning;
  }

okhttp3.Dispatcher#executorService

• 和 okhttp3.ConnectionPool 里的线程池一致
• 逻辑相当于带参的 java.util.concurrent.Executors#newCachedThreadPool

 public synchronized ExecutorService executorService() {
    if (executorService == null) {
        //延迟初始化
        //corePoolSize maximumPoolSize keepAliveTime unit workQueue
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

okhttp3.RealCall.AsyncCall#executeOn

 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;
    }

    /**
     * Attempt to enqueue this async call on {@code executorService}. This will attempt to clean up
     * if the executor has been shut down by reporting the call as failed.
     */
    void executeOn(ExecutorService executorService) {
      assert (!Thread.holdsLock(client.dispatcher()));
      boolean success = false;
      try {
        //执行 NamedRunnable -> run -> okhttp3.RealCall.AsyncCall#execute
        executorService.execute(this);
        success = true;
      } catch (RejectedExecutionException e) {
        InterruptedIOException ioException = new InterruptedIOException("executor rejected");
        ioException.initCause(e);
        eventListener.callFailed(RealCall.this, ioException);
        responseCallback.onFailure(RealCall.this, ioException);
      } finally {
        if (!success) {
          client.dispatcher().finished(this); // This call is no longer running!
        }
      }
    }
    //okhttp3.RealCall.AsyncCall#execute
    //此方法内部可以看出 回掉是在子线程里
     @Override protected void execute() {
      boolean signalledCallback = false;
      timeout.enter();
      try {
        Response response = getResponseWithInterceptorChain();
        if (retryAndFollowUpInterceptor.isCanceled()) {
          signalledCallback = true;
          //回调失败方法：取消
          responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
        } else {
          signalledCallback = true;
          //回调成功方法
          responseCallback.onResponse(RealCall.this, response);
        }
      } catch (IOException e) {
        e = timeoutExit(e);
        if (signalledCallback) {
          // Do not signal the callback twice!
          Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
        } else {
          eventListener.callFailed(RealCall.this, e);
            //回调失败方法
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        client.dispatcher().finished(this);
      }
    }
  }

okhttp3.internal.NamedRunnable

/**
 * Runnable implementation which always sets its thread name.
 */
public abstract class NamedRunnable implements Runnable {
  protected final String name;

  public NamedRunnable(String format, Object... args) {
    this.name = Util.format(format, args);
  }

  @Override public final void run() {
    String oldName = Thread.currentThread().getName();
    Thread.currentThread().setName(name);
    try {
      execute();
    } finally {
      Thread.currentThread().setName(oldName);
    }
  }
  //具体实现 okhttp3.RealCall.AsyncCall#execute
  protected abstract void execute();
}

RetryAndFollowUpInterceptor

okhttp3.internal.http.RetryAndFollowUpInterceptor#intercept

 @Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();
    //okhttp3.internal.connection.StreamAllocation
    StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;

    int followUpCount = 0;
    Response priorResponse = null;
    //
    while (true) {
      //循环 取消
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
        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();
        }
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        // 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.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // 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();
      }

      Request followUp;
      try {
        followUp = followUpRequest(response, streamAllocation.route());
      } catch (IOException e) {
        streamAllocation.release();
        throw e;
      }

      if (followUp == null) {
        streamAllocation.release();
        return response;
      }

      closeQuietly(response.body());

      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();
        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;
    }
  }

OkHttp 流程图