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(); } 

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OkHttp3的执行流程

1. 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。
2. 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。
3. 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。
4. 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; } } 

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然后使用okHttpClient发起请求。例如：

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

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那接下来我们在看下Request。例如：

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

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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); } } 

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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 { } }); 

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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 */); } } 

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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)); } } 

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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; } ... } 

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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(); } 

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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); } } 

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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<>(); } 

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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; } 

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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)); 

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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); } } 

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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); } 

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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); } 

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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); 

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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(); } 

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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(); } } 

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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(); } } 

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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); } } 

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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; } 

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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。