分析的问题

1. Call如何处理同步和异步请求；
2. Dispatcher如何管理请求任务；
3. OkHttp拦截器链；
4. 连接池ConnectionPool。

1. 同步和异步请求

OkHttpClient用内部类Builder的形式进行创建，在请求网络时，通过OkHttpClient类的newCall()方法创建一个Call实例：

  /**
   * 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 */);
  }

newCall()方法实际上调用了RealCall类的静态方法newRealCall()：

  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;
  }

Call是一个接口，RealCall是Call的实现类，在静态方法中创建了RealCall实例并返回。
OkHttp使用execute()和enqueue()方法分别执行同步和异步请求。这两个方法也是Call类的抽象方法，并在RealCall类中实现。

• RealCall类的同步请求方法execute()

  @Override 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);
      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);
    }
  }

client.dispatcher.executed(this)，把任务转交给了Dispatcher类的executed()方法。
getResponseWithInterceptorChain()，执行拦截器链，其中包括发送数据和响应数据过程，都是在拦截器中进行。
client.dispatcher().finished(this)，任务执行结束时，调用了Dispatcher的finished()方法。
此外，还应该知道同步请求会阻塞当前的线程。

• RealCall类的异步请求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));
  }

异步请求，client.dispatcher().enqueue(new AsyncCall(responseCallback));调用了Dispatcher类的enqueue()方法。

所以，在ReaCall类中，execute()和enqueue()方法执行任务时，都是转交给了Dispatcher类处理。异步和同步此处不一样的地方是，enqueue()方法的入参不是RealCall的实例，而是RealCall的一个内部类AsyncCall的实例，AsyncCall继承自NamedRunnable类，以便提交给线程池执行。
同步请求有executed和finished两个过程，异步请求在这里只看到enqueue()开始执行的方法，那么它有没有对应的结束调用呢？贴上内部类AsyncCall看看。

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;
    }

    @Override 
    protected void execute() {
      boolean signalledCallback = false;
      try {
        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);
      }
    }
  }

其父类NamedRunnable是一个抽象类，它继承自Runnable接口，可以指定执行线程名，AsyncCall实现了NamedRunnable的execute()抽象方法，在里面有client.dispatcher().finished(this)，在这里调用Dispatcher类的finished()结束请求任务。

2. Dispatcher任务管理

Dispatcher是在Builder构建时新建的实例，并引用给了OkHttpClient类的成员变量
以下是Dispatcher类的成员变量：

  private int maxRequests = 64;
  private int maxRequestsPerHost = 5;
  private @Nullable Runnable idleCallback;

  /** Executes calls. Created lazily. */
  private @Nullable ExecutorService executorService;

  /** Ready async calls in the order they'll be run. */
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

maxRequests最大请求任务数，64个，maxRequestsPerHost每个主机最大请求任务数，5个。另外还有一个线程池，一个同步请求的队列，一个异步请求的队列，一个备用的异步请求队列。这三个任务队列，都是在Dispatcher的成员中直接创建的，用来存储请求任务。

• 线程池

  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;
  }

这个线程池，是在要使用的时候创建，核心线程数为0，最大非核心线程数为Integer的最大值，可以认为它"无界"，且闲置回收时间为60s，并使用了一个非存储的阻塞队列，用于新任务和空闲线程的配对，类似newCachedThreadPool()。

• 同步请求任务管理
  前面看到执行同步请求，调用Call的execute()同步方法时，调用了Dispatcher的executed()方法和finished()方法，Dispatcher类的这两个方法如下:

  /** Used by {@code Call#execute} to signal it is in-flight. */
  synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }

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

  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();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }  

发现在Dispatcher中，发起请求时，把请求任务加入到了同步队列中，且对任务数没有限制，请求结束时，会把任务从队列中移除。
为什么同步请求需要队列去管理，作用是什么？请求开始时，先将任务添加到队列，再执行，请求响应后，再将任务从队列中移除。
在整个Dispatcher类中，发现runningSyncCalls这个同步队列，还在另几个地方出现，一个取消所有请求的时候，一个获取所有正在执行的请求集合的时候，还有一个是获取所有正在执行的请求数的时候。

  /**
   * Cancel all calls currently enqueued or executing. Includes calls executed both {@linkplain
   * Call#execute() synchronously} and {@linkplain Call#enqueue asynchronously}.
   */
  public synchronized void cancelAll() {
    for (AsyncCall call : readyAsyncCalls) {
      call.get().cancel();
    }

    for (AsyncCall call : runningAsyncCalls) {
      call.get().cancel();
    }

    for (RealCall call : runningSyncCalls) {
      call.cancel();
    }
  }

  /** Returns a snapshot of the calls currently being executed. */
  public synchronized List<Call> runningCalls() {
    List<Call> result = new ArrayList<>();
    result.addAll(runningSyncCalls);
    for (AsyncCall asyncCall : runningAsyncCalls) {
      result.add(asyncCall.get());
    }
    return Collections.unmodifiableList(result);
  }

    public synchronized int runningCallsCount() {
    return runningAsyncCalls.size() + runningSyncCalls.size();
  }

所以通过队列管理同步请求，比直接执行，可以方便地了解所有请求数，以及控制这些请求任务。即使在多线程的应用场景中，因为加了同步锁，依然可以安全地进行管理。

• 异步请求任务管理
  执行异步请求，调用RealCall的enqueue()方法，调用了Dispatcher的enqueue()和finished()方法：

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

  /** Used by {@code AsyncCall#run} to signal completion. */
  void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }

  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();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }

异步任务，会先判断异步任务队列大小是否达到限制值64（maxRequests，64），以及每个地址的请求数是否到达限制值5（maxRequestsPerHost，5），都未达到，把Call实例添加到正在执行的异步任务队列中，并调用线程池来执行任务。如果已达到最大任务数限制，则会加入到备用的异步任务队列中，等待执行。线程池的优点是线程复用，但Dispatcher通过请求数限制，把任务加入到备用队列中，等线程空闲时，再从备用队列中取出任务提交给空闲线程执行，避免了大量线程的创建。此外，同步和异步finished()方法都调用了另一个finished(Deque<T> calls, T call, boolean promoteCalls)范型方法，在它里面进行任务移除，如果是异步任务，则会通过promoteCalls()方法从备用的异步任务队列中，取出任务添加到正在执行的异步任务队列中，同时交给线程池执行。

3. OkHttp拦截器链

无论是同步请求还是异步请求，都是通过拦截器链进行数据请求和响应的处理。同步和异步任务执行中，都调用了RealCall类中getResponseWithInterceptorChain()方法：

  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());

    return chain.proceed(originalRequest);
  }

在拦截器的集合中，首先会加入我们自定义的所有应用拦截器，自定义的所有网络拦截器，和OKHttp默认的一些拦截器，ConnectInterceptor中是打开一个和server的连接，添加网络拦截器是在打开和server的连接之后，所以在网络拦截器可以看到连接信息。最后一个CallServerInterceptor拦截器，它才真正和服务器交互数据。在这个方法中，第一次创建了一个RealInterceptorChain实例，index的入参为0，然后调用RealInterceptorChain类的proceed()方法。

@Override public Response proceed(Request request) throws IOException {
    return proceed(request, streamAllocation, httpCodec, connection);
  }

  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.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    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;
  }

在RealCall类中getResponseWithInterceptorChain()方法中，创建了RealInterceptorChain对象，为何在proceed()方法内部还会出现RealInterceptorChain对象的创建？
其实他们是两个不同的拦截器链对象，第一个拦截器链对象index为0，执行其proceed()方式时，先准备了下一个拦截器链对象，index为1（通过index+1给定）。当interceptors.get(index)拿到第一个拦截器后，执行第一个拦截器的intercept()方法，把刚创建的第二个链对象传了进去，并执行第二个链对象的proceed()方法。这个时候第二链对象的proceed()里面又会准备一个新的链对象(第三个，index为2)，然后一直重复前面的过程，直到最后一个拦截器的intercept()方法被调用。且拦截器链对象的proceed()方法的返回值是Response类型，所以请求过程的处理是从第一个拦截器到最后一个拦截器，响应过程Response的处理，则是从最后一个到第一个的过程。

为什么在介绍OkHttp的拦截器时，都说应用拦截器只执行一次，而网络拦截器会贯穿整个请求过程，可能执行多次呢？要知道这个答案，还得看看默认拦截器的作用分别是什么。

• RetryAndFollowUpInterceptor拦截器
  再贴一次RealCall类的getResponseWithInterceptorChain方法：

//省略
    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));
//省略

interceptors.add(retryAndFollowUpInterceptor);，关键是这一行，它是一个请求重试和重定向的拦截器，其intercept()方法的部分内容如下：

//省略
while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      }
//省略

在这个拦截器里有一个循环，取消请求或者没有重定向的请求了，则退出循环。应用拦截器的添加在RetryAndFollowUpInterceptor之前，所以应用拦截器在整个拦截器链执行过程中只会执行一次，而在RetryAndFollowUpInterceptor之后添加的网络拦截器，以及其他拦截器，则会依情况在循环中执行一次或多次。

• BridgeInterceptor拦截器

public final class BridgeInterceptor implements Interceptor {
  private final CookieJar cookieJar;

  public BridgeInterceptor(CookieJar cookieJar) {
    this.cookieJar = cookieJar;
  }

  @Override 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();
  }

  /** Returns a 'Cookie' HTTP request header with all cookies, like {@code a=b; c=d}. */
  private String cookieHeader(List<Cookie> cookies) {
    StringBuilder cookieHeader = new StringBuilder();
    for (int i = 0, size = cookies.size(); i < size; i++) {
      if (i > 0) {
        cookieHeader.append("; ");
      }
      Cookie cookie = cookies.get(i);
      cookieHeader.append(cookie.name()).append('=').append(cookie.value());
    }
    return cookieHeader.toString();
  }
}

这个拦截器处理了一些默认的通用头，请求头和实体头，而且可以看到，当用户没有对Accept-Encoding请求头设置时，OkHttp默认使用了gzip编码格式对数据进行压缩。同时用户可以通过这个拦截器设置cookies，OkHttp默认是NO_COOKIES。

• CacheInterceptor拦截器

/** Serves requests from the cache and writes responses to the cache. */
public final class CacheInterceptor implements Interceptor {
  //省略
@Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    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.
    }

    // 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();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    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) {
      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());
      }
    }

    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    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;
  }
  //省略
}

这个拦截器中会先依据用户或OkHttp的默认设置生成缓存策略。
网络请求和缓存都不可用时，返回504；
网络请求不可用（缓存策略可以将网路请求置null），返回缓存数据。
网络请求可用，获取响应数据，如果和本地缓存比较，发现未修改，生成一个含有缓存Response和网络Response的新Response返回。如果没有缓存，也重新生成一个含有缓存Response和网络Response的新Response返回。有缓存则将新数据写入缓存。

• 缓存策略CacheStrategy
  在前面那段代码中，通过cacheCandidate).get()返回了CacheStrategy的实例，里面依据各种情况来决定返回的CacheStrategy对象，以及它的两个重要成员networkRequest和cacheResponse，缓存拦截器中大多数的判断都依赖着这两成员对象，决定如何使用网络和缓存数据。

• ConnectInterceptor连接拦截器

/** Opens a connection to the target server and proceeds to the next interceptor. */
public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;

  public ConnectInterceptor(OkHttpClient client) {
    this.client = client;
  }

  @Override 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);
  }
}

如果不是GET请求，会先确认一次连接池中到主机的连接是否可用。这个拦截器的作用主要是从连接池获取一个到server的有效连接。

/** This is the last interceptor in the chain. It makes a network call to the server. */
public final class CallServerInterceptor implements Interceptor {
  private final boolean forWebSocket;

  public CallServerInterceptor(boolean forWebSocket) {
    this.forWebSocket = forWebSocket;
  }

  @Override 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.
      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;
  }

  static final class CountingSink extends ForwardingSink {
    long successfulCount;

    CountingSink(Sink delegate) {
      super(delegate);
    }

    @Override public void write(Buffer source, long byteCount) throws IOException {
      super.write(source, byteCount);
      successfulCount += byteCount;
    }
  }
}

拦截器集合中最后添加的这个拦截器，是真正和server交互数据的地方。

4. 连接池ConnectionPool

在OkHttpClient的Builder类中，connectionPool = new ConnectionPool();创建了一个连接池对象。首先，看一下该类中的成员变量。

/**
   * Background threads are used to cleanup expired connections. There will be at most a single
   * thread running per connection pool. The thread pool executor permits the pool itself to be
   * garbage collected.
   */
  private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
      Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
      new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true));

  /** The maximum number of idle connections for each address. */
  private final int maxIdleConnections;
  private final long keepAliveDurationNs;
  private final Runnable cleanupRunnable = new Runnable() {
    @Override public void run() {
      while (true) {
        long waitNanos = cleanup(System.nanoTime());
        if (waitNanos == -1) return;
        if (waitNanos > 0) {
          long waitMillis = waitNanos / 1000000L;
          waitNanos -= (waitMillis * 1000000L);
          synchronized (ConnectionPool.this) {
            try {
              ConnectionPool.this.wait(waitMillis, (int) waitNanos);
            } catch (InterruptedException ignored) {
            }
          }
        }
      }
    }
  };

  private final Deque<RealConnection> connections = new ArrayDeque<>();
  final RouteDatabase routeDatabase = new RouteDatabase();
  boolean cleanupRunning;

一个线程池，用于清理失效连接，一个阻塞队列，用于管理RealConnection对象的生产和消费，队列中任务对象的数量没有限制。
构造函数

  /**
   * Create a new connection pool with tuning parameters appropriate for a single-user application.
   * The tuning parameters in this pool are subject to change in future OkHttp releases. Currently
   * this pool holds up to 5 idle connections which will be evicted after 5 minutes of inactivity.
   */
  public ConnectionPool() {
    this(5, 5, TimeUnit.MINUTES);
  }

  public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
    this.maxIdleConnections = maxIdleConnections;
    this.keepAliveDurationNs = timeUnit.toNanos(keepAliveDuration);

    // Put a floor on the keep alive duration, otherwise cleanup will spin loop.
    if (keepAliveDuration <= 0) {
      throw new IllegalArgumentException("keepAliveDuration <= 0: " + keepAliveDuration);
    }
  }

从构造函数中，可以了解，其默认的闲置连接是5个，且每个连接的闲置时长为5分钟。在清理任务时，如果发现闲置连接超过5个，则闲置时间最长的连接会被从阻塞队列中移除。如果一个连接的闲置时间超过5分钟，该连接也会被从阻塞队列中移除。到server连接的复用，可以减少三次握手，四次挥手的过程，提高网络请求的效率。