基本概念

首先从使用出发，其次再结合源码来分析OkHttp3的内部实现的，建议大家下载 OkHttp 源码跟着本文，过一遍源码。首先来看一下OkHttp3的请求代码。

OkHttpClient client = new OkHttpClient();String run(String url) throws IOException {Request request = new Request.Builder().url(url).build();Response response = client.newCall(request).execute();return response.body().string();
}

OkHttp3的执行流程

创建OkHttpClient对象。OkHttpClient为网络请求执行的一个中心，它会管理连接池，缓存，SocketFactory，代理，各种超时时间，DNS，请求执行结果的分发等许多内容。
创建Request对象。Request用于描述一个HTTP请求，比如请求的方法是GET还是POST，请求的URL，请求的header，请求的body，请求的缓存策略等。
创建Call对象。Call是一次HTTP请求的Task，它会执行网络请求以获得响应。OkHttp中的网络请求执行Call既可以同步进行，也可以异步进行。调用call.execute()将直接执行网络请求，阻塞直到获得响应。而调用call.enqueue()传入回调，则会将Call放入一个异步执行队列，由ExecutorService在后台执行。
执行网络请求并获取响应。

上面的代码中涉及到几个常用的类：Request、Response和Call。下面就这几个类做详细的介绍。

Request

每一个HTTP请求包含一个URL、一个方法（GET或POST或其他）、一些HTTP头，请求还可能包含一个特定内容类型的数据类的主体部分。

Response

响应是对请求的回复，包含状态码、HTTP头和主体部分。

Call

OkHttp使用Call抽象出一个满足请求的模型，尽管中间可能会有多个请求或响应。执行Call有两种方式，同步或异步。

那么首先来看一下OkHttpClient的源码实现。

public class OkHttpClient implements Cloneable, Call.Factory, WebSocket.Factory {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.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;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.connectTimeout = builder.connectTimeout;this.readTimeout = builder.readTimeout;this.writeTimeout = builder.writeTimeout;this.pingInterval = builder.pingInterval;}
}

然后使用okHttpClient发起请求。例如：

okHttpClient.newCall(request).enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Override
public void onResponse(Call call, Response response) throws IOException {}
});

那接下来我们在看下Request。例如：

Request request = new Request.Builder().url("url").build();

该段代码主要实现初始化构建者模式和请求对象，并且用URL替换Web套接字URL。其源码如下：

public final class Request {public Builder() {this.method = "GET";this.headers = new Headers.Builder();}public Builder url(String url) {......// Silently replace web socket URLs with HTTP URLs.if (url.regionMatches(true, 0, "ws:", 0, 3)) {url = "http:" + url.substring(3);} else if (url.regionMatches(true, 0, "wss:", 0, 4)) {url = "https:" + url.substring(4);}HttpUrl parsed = HttpUrl.parse(url);......return url(parsed);}public Request build() {......return new Request(this);}
}

我们来看一下okHttpClient的异步请求方式。

okHttpClient.newCall(request).enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {}@Override
public void onResponse(Call call, Response response) throws IOException {}
});

而newCall又调用了RealCall函数，来看源码：

public class OkHttpClient implements Cloneable, Call.Factory, WebSocket.Factory {@Override public Call newCall(Request request) {return new RealCall(this, request, false/* for web socket */);}
}

RealCall实现了Call.Factory接口创建了一个RealCall的实例，而RealCall是Call接口的实现。继续看代码：

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

由上面的代码可以得出：

检查这个 call 是否已经被执行了，每个 call 只能被执行一次，如果想要一个完全一样的 call，可以利用 call#clone方法进行克隆。
利用 client.dispatcher().enqueue(this) 来进行实际执行，dispatcher 是刚才看到的OkHttpClient.Builder 的成员之一。
AsyncCall是RealCall的一个内部类并且继承NamedRunnable。

final class AsyncCall extends NamedRunnable {private final Callback responseCallback;AsyncCall(Callback responseCallback) {super("OkHttp %s", new Object[]{RealCall.this.redactedUrl()});this.responseCallback = responseCallback;}...
}

而NamedRunnable又实现了Runnable接口，来看代码：

public abstract class NamedRunnable implements Runnable {......@Override public final void run() {......try {execute();}......}protected abstract void execute();
}

可以看到NamedRunnable实现了Runnbale接口并且是个抽象类，其抽象方法是execute()，该方法是在run方法中被调用的，这也就意味着NamedRunnable是一个任务，并且其子类应该实现execute方法。下面再看AsyncCall的实现：

final class AsyncCall extends NamedRunnable {private final Callback responseCallback;AsyncCall(Callback responseCallback) {super("OkHttp %s", redactedUrl());this.responseCallback = responseCallback;}......
final class RealCall implements Call {@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) {......responseCallback.onFailure(RealCall.this, e);} finally {client.dispatcher().finished(this);}
}

AsyncCall实现了execute方法，首先是调用getResponseWithInterceptorChain()方法获取响应，然后获取成功后，就调用回调的onReponse方法，如果失败，就调用回调的onFailure方法，并调用Dispatcher的finished方法。

Dispatcher线程池介绍

那还看一下Dispatcher类的相关代码：

public final class Dispatcher {/** 最大并发请求数为64 */private int maxRequests = 64;/** 每个主机最大请求数为5 */private int maxRequestsPerHost = 5;/** 线程池 */private ExecutorService executorService;/** 准备执行的请求 */private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();/** 正在执行的异步请求，包含已经取消但未执行完的请求 */private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();/** 正在执行的同步请求，包含已经取消单未执行完的请求 */private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();}

在OkHttp，使用如下构造了单例线程池，相关源码如下：

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;}

executorService函数会构造一个线程池ExecutorService：

executorService = new ThreadPoolExecutor(
//corePoolSize 最小并发线程数,如果是0的话，空闲一段时间后所有线程将全部被销毁0,
//maximumPoolSize: 最大线程数，当任务进来时可以扩充的线程最大值，当大于了这个值就会根据丢弃处理机制来处理Integer.MAX_VALUE,
//keepAliveTime: 当线程数大于corePoolSize时，多余的空闲线程的最大存活时间60,
//单位秒TimeUnit.SECONDS,
//工作队列,先进先出new SynchronousQueue<Runnable>(),
//单个线程的工厂         Util.threadFactory("OkHttp Dispatcher", false));

可以看出，在Okhttp中，构建了一个核心为[0, Integer.MAX_VALUE]的线程池，它不保留任何最小线程数，随时创建更多的线程数，当线程空闲时只能活60秒，它使用了一个不存储元素的阻塞工作队列，一个叫做”OkHttp Dispatcher”的线程工厂。也就是说，在实际运行中，当收到10个并发请求时，线程池会创建十个线程，当工作完成后，线程池会在60s后相继关闭所有线程。

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

从上述源码分析，如果当前还能执行一个并发请求，则加入 runningAsyncCalls ，立即执行，否则加入 readyAsyncCalls 队列。由此，可以得出Dispatcher的以下作用。

调度线程池Disptcher实现了高并发，低阻塞的实现；
采用Deque作为缓存，先进先出的顺序执行；
任务在try/finally中调用了finished函数，控制任务队列的执行顺序，而不是采用锁，减少了编码复杂性提高性能。

try {Response response = getResponseWithInterceptorChain();if (retryAndFollowUpInterceptor.isCanceled()) {signalledCallback = true;responseCallback.onFailure(RealCall.this, new IOException("Canceled"));} else {signalledCallback = true;responseCallback.onResponse(RealCall.this, response);}} finally {client.dispatcher().finished(this);}

其流程可以用下图表示：

getResponseWithInterceptorChain方法

相关的方法源码如下：

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);}

从上述源码得知，不管okhttp有多少拦截器最后都会走，如下方法：

Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest);
return chain.proceed(originalRequest);

从方法名字基本可以猜到是干嘛的，调用 chain.proceed(originalRequest); 将request传递进来，从拦截器链里拿到返回结果。那么看一下RealInterceptorChain类。

public final class RealInterceptorChain implements Interceptor.Chain {public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,HttpCodec httpCodec, RealConnection connection, int index, Request request) {this.interceptors = interceptors;this.connection = connection;this.streamAllocation = streamAllocation;this.httpCodec = httpCodec;this.index = index;this.request = request;}......@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++;......// Call the next interceptor in the chain.RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec, connection, index + 1, request);Interceptor interceptor = interceptors.get(index);Response response = interceptor.intercept(next);......return response;}protected abstract void execute();
}

该类实现了Chain接口，在getResponseWithInterceptorChain调用时好几个参数都传的null。主要看proceed方法，proceed方法中判断index（此时为0）是否大于或者等于client.interceptors(List )的大小。由于httpStream为null，所以首先创建next拦截器链，主需要把索引置为index+1即可；然后获取第一个拦截器，调用其intercept方法。Interceptor 代码如下：

public interface Interceptor {Response intercept(Chain chain) throws IOException;interface Chain {Request request();Response proceed(Request request) throws IOException;Connection connection();}
}

BridgeInterceptor从用户的请求构建网络请求，然后提交给网络，最后从网络响应中提取出用户响应。从最上面的图可以看出，BridgeInterceptor实现了适配的功能。下面是其intercept方法：

public final class BridgeInterceptor implements Interceptor {......@Override
public Response intercept(Chain chain) throws IOException {Request userRequest = chain.request();Request.Builder requestBuilder = userRequest.newBuilder();RequestBody body = userRequest.body();//如果存在请求主体部分，那么需要添加Content-Type、Content-Length首部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);responseBuilder.body(new RealResponseBody(strippedHeaders, 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();}
}

从上面的代码可以看出，首先获取原请求，然后在请求中添加头，比如Host、Connection、Accept-Encoding参数等，然后根据看是否需要填充Cookie，在对原始请求做出处理后，使用chain的procced方法得到响应，接下来对响应做处理得到用户响应，最后返回响应。再看下一个拦截器ConnectInterceptor的处理：

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();// 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);}
}

实际上建立连接就是创建了一个 HttpCodec 对象，它利用 Okio 对 Socket 的读写操作进行封装，Okio 以后有机会再进行分析，现在让我们对它们保持一个简单地认识：它对 java.io 和 java.nio 进行了封装，让我们更便捷高效的进行 IO 操作。

CallServerInterceptor

CallServerInterceptor是拦截器链中最后一个拦截器，负责将网络请求提交给服务器。

@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();httpCodec.writeRequestHeaders(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();responseBuilder = httpCodec.readResponseHeaders(true);}if (responseBuilder == null) {// Write the request body if the "Expect: 100-continue" expectation was met.Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);request.body().writeTo(bufferedRequestBody);bufferedRequestBody.close();} 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) {responseBuilder = httpCodec.readResponseHeaders(false);}Response response = responseBuilder.request(request).handshake(streamAllocation.connection().handshake()).sentRequestAtMillis(sentRequestMillis).receivedResponseAtMillis(System.currentTimeMillis()).build();int code = response.code();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;}

从上面的代码中可以看出，首先获取HttpStream对象，然后调用writeRequestHeaders方法写入请求的头部，然后判断是否需要写入请求的body部分，最后调用finishRequest()方法将所有数据刷新给底层的Socket，接下来尝试调用readResponseHeaders()方法读取响应的头部，然后再调用openResponseBody()方法得到响应的body部分，最后返回响应。

总结

最后我们用一张图来总结ohhttp的整个请求流程。

OkHttp的底层是通过Java的Socket发送HTTP请求与接受响应的(，但是OkHttp实现了连接池的概念，即对于同一主机的多个请求，其实可以公用一个Socket连接，而不是每次发送完HTTP请求就关闭底层的Socket，这样就实现了连接池的概念，而且OkHttp对Socket的读写操作使用的OkIo库进行了一层封装。

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