OkHttp3是Square出品的高质量Http网络请求库，目前在GitHub上的star数超过17000。很多Android项目的网络组件都是基于OkHttp封装的，还有著名的Retrofit也是基于OkHttp封装的。

OkHttp的基本使用

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OkHttpClient client = new OkHttpClient();
Request request = new Request.Builder()
    .url(ENDPOINT)
    .build();
//同步请求    
Response response = client.newCall(request).execute();
//异步请求
client.newCall(request).enqueue(new Callback() {
  @Override
  public void onFailure(Call call, IOException e) {
    
  }

  @Override
  public void onResponse(Call call, Response response) throws IOException {

  }
});

最基本的用法就是先创建一个OkHttpClient，然后build出一个Requset对象，最后发送请求，可以是同步请求，也可以是异步请求。使用起来很简单，但背后是怎么实现的，下面从源码层面来分析下。

OkHttp 调用流程

OkHttp内部调用流程图

第一步: new OkHttpClient(Builder)

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//OkHttpClient.java
public OkHttpClient() {
    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.readTimeout = builder.readTimeout;
    this.writeTimeout = builder.writeTimeout;
    this.pingInterval = builder.pingInterval;
}

这里创建了一个默认的OkHttpCient.Builder，用于配置各种参数。

第二步:okhttpclient.newCall(request)

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//OkHttpClient.java
@Override public Call newCall(Request request) {
return new RealCall(this, request, false /* for web socket */);
}
//RealCall.java
RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    this.client = client;
    this.originalRequest = originalRequest;
    this.forWebSocket = forWebSocket;
    this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
}

这里用request对象创建了一个RealCall对象，把一些参数传到RealCall。

第三步:execute() or enqueue()

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//RealCall.java
@Override public Response execute() throws IOException {
synchronized (this) {
  if (executed) throw new IllegalStateException("Already Executed");
  executed = true;
}
captureCallStackTrace();
try {
  client.dispatcher().executed(this);
  //核心的函数
  Response result = getResponseWithInterceptorChain();
  if (result == null) throw new IOException("Canceled");
  return result;
} finally {
  client.dispatcher().finished(this);
}
}

同步请求，很直接就调用到了最核心的函数getResponseWithInterceptorChain()。再看下异步请求。

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

而异步请求，将用户接口的responseCallback对象封装成一个AsyncCall对象提交给Dispather来处理，这里的AsyncCall是RealCall的一个内部类。再看下这个Dispather怎么处理这个AsyncCall的。

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

Dispather管理了一些请求队列，如果正在执行的异步请求没有达到上限，就直接将这个请求提交给线程池，否则加入到等待队列中。而且这里直接把AsyncCall的对象给了线程池，其实这个AsyncCall就是一个Runnable的实现类。

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//RealCall.java
final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;

    AsyncCall(Callback responseCallback) {
      super("OkHttp %s", redactedUrl());
      this.responseCallback = responseCallback;
    }
    ......
    @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 {
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        client.dispatcher().finished(this);
      }
    }
  }

AsyncCall父类的run()方法会调用抽象方法execute()，也就是将在Dispather里的线程池执行AsyncCall对象的时候，就会执行到execute()，在这个方法里同样调用了核心的网络请求方法getResponseWithInterceptorChain()。
而且在execute()里会回调用户接口responseCallback的回调方法。注意:这里的回调是在非主线程直接回调的，也就是在Android里使用的话要注意这里面不能直接更新UI操作。
至此，同步请求和异步请求最终都是调用的getResponseWithInterceptorChain();来发送网络请求，只是异步请求涉及到一些线程池操作，包括请求的队列管理、调度。

第四步:getResponseWithInterceptorChain()

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//RealCall.java
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);
return chain.proceed(originalRequest);
}

在这个方法里就是添加了一些拦截器，然后启动一个拦截器调用链，拦截器递归调用之后最后返回请求的响应Response。这里的拦截器分层的思想就是借鉴的网络里的分层模型的思想。请求从最上面一层到最下一层，响应从最下一层到最上一层，每一层只负责自己的任务，对请求或响应做自己负责的那块的修改。

Q1:这里为什么每次都重新创建RealInterceptorChain对象，为什么不直接复用上一层的RealInterceptorChain对象？(文末给出答案)

OkHttp拦截器分层结构

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//RealInterceptorChain.java
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
  Connection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
......
RealInterceptorChain next = new RealInterceptorChain(
    interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);
...
return response;
}

RealInterceptorChain的proceed()，每次重新创建一个RealInterceptorChain对象，然后调用下一层的拦截器的interceptor.intercept()方法。
每一个拦截器的intercept()方法都是这样的模型

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@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    // 1、该拦截器在Request阶段负责的事情
    
    // 2、调用RealInterceptorChain.proceed()，其实是递归调用下一层拦截器的intercept方法
    response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);

    //3、该拦截器在Response阶段负责的事情，然后返回到上一层拦截器的 response阶段
    return  response;     
    }
  }

这差不多就是OkHttp的分层拦截器模型，借鉴了网络里的OSI七层模型的思想。最底层是CallServerInterceptor，类比网络里的物理层。OkHttp还支持用户自定义拦截器，插入到最顶层和CallServerInterceptor上一层的位置。比如官方写了一个Logging Interceptor，用于打印网络请求日志的拦截器。

BridgeInterceptor

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Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
// Request阶段
RequestBody body = userRequest.body();
if (body != null) {
  MediaType contentType = body.contentType();
    ......
  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("Connection") == null) {
  requestBuilder.header("Connection", "Keep-Alive");
 }
}
    .....
Response networkResponse = chain.proceed(requestBuilder.build());
// Response阶段
    .....
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);
  responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}

BridgeInterceptor拦截器在Request阶段，将用户的配置信息，重新创建Request.Builder对象，重新build出Request对象，并添加一些请求头，比如:host，content-length，keep-alive等。
BridgeInterceptor在Response阶段做gzip解压操作。

CacheInterceptor

CacheInterceptor拦截器在Request阶段判断该请求是否有缓存，是否需要重新请求，如果不需要重新请求，直接从缓存里取出内容，封装一个Response返回，不需要再调用下一层。
CacheInterceptor拦截器在Response阶段，就是把下面一层的Response做缓存。

ConnectInterceptor

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//ConnectInterceptor.java
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, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);

ConnectInterceptor拦截器只在Request阶段建立连接，Response阶段直接把下一层的Response返回给上一层。再看下建立连接的过程。

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public HttpCodec newStream(OkHttpClient client, boolean doExtensiveHealthChecks) {
....
try {
  RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
      writeTimeout, connectionRetryEnabled, doExtensiveHealthChecks);
  HttpCodec resultCodec = resultConnection.newCodec(client, this);
......
} catch (IOException e) {
  throw new RouteException(e);
}
}

findHealthyConnection()函数寻找一条健康的网络连接，其内部主要调用了findConnection()。

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private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
  boolean connectionRetryEnabled) throws IOException {
Route selectedRoute;
synchronized (connectionPool) {
 .....
  // Attempt to get a connection from the pool.
  Internal.instance.get(connectionPool, address, this);
  if (connection != null) {
    return connection;
  }

  selectedRoute = route;
}

// If we need a route, make one. This is a blocking operation.
if (selectedRoute == null) {
  selectedRoute = routeSelector.next();
}

// Create a connection and assign it to this allocation immediately. This makes it possible for
// an asynchronous cancel() to interrupt the handshake we're about to do.
RealConnection result;
synchronized (connectionPool) {
  route = selectedRoute;
  refusedStreamCount = 0;
  result = new RealConnection(connectionPool, selectedRoute);
  acquire(result);
  if (canceled) throw new IOException("Canceled");
}

// Do TCP + TLS handshakes. This is a blocking operation.
result.connect(connectTimeout, readTimeout, writeTimeout, connectionRetryEnabled);
routeDatabase().connected(result.route());

Socket socket = null;
synchronized (connectionPool) {
  // Pool the connection.
  Internal.instance.put(connectionPool, result);
 .....
}
closeQuietly(socket);
return result;
}

这里面大概就是从连接池里去找已有的网络连接，如果有，则复用，减少三次握手；没有的话，则创建一个RealConnection对象，三次握手，建立连接，然后将连接放到连接池。具体的内部connect过程，就不深入了。

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public ConnectionPool() {
this(5, 5, TimeUnit.MINUTES);
}
public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
}

ConnectionPool最多支持保持5个地址的连接keep-alive，每个keep-alive 5分钟，并有异步线程循环清理无效的连接。

CallServerInterceptor

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@Override public Response intercept(Chain chain) throws IOException {
...
httpCodec.writeRequestHeaders(request);
Response.Builder responseBuilder = null;
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
  ......
  if (responseBuilder == null) {
    Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
    BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
    request.body().writeTo(bufferedRequestBody);
    bufferedRequestBody.close();
  }
}
httpCodec.finishRequest();
if (responseBuilder == null) {
  responseBuilder = httpCodec.readResponseHeaders(false);
}
Response response = responseBuilder
    .request(request)
    .handshake(streamAllocation.connection().handshake())
    .sentRequestAtMillis(sentRequestMillis)
    .receivedResponseAtMillis(System.currentTimeMillis())
    .build();

int code = response.code();
.....
return response;
}

CallServerInterceptor 精简出来的代码就是writeRequestHeaders()，flushRequest()，finishRequest()，发送请求，然后readResponseHeaders，openResponseBody读取response。
CallServerInterceptor底层的IO流读写依赖于Square自家的Okio项目，HttpCodec是封装的IO编码和解码的实现。

至此，OkHttp中几个核心的拦截器就到此为止了，OkHttp最精髓的部分也就体现在这个拦截器上。最后补充几个关于OkHttp的面试问题。

  • OkHttp是如何做链路复用？
  • OkHttp的Intereptor能不能取消一个request？
    这两个问题在分析源码之后应该很容易回答了。

回答上面留的一个问题:
每次重新创建一个RealInterceptorChain对象，因为这里是递归调用，在调用下一层拦截器的interupter()方法的时候，本层的 response阶段还没有执行完成，如果复用RealInterceptorChain对象，必然导致下一层修改RealInterceptorChain，所以需要重新创建RealInterceptorChain对象。

许可协议

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