OKHttp源码解析
rbr008
8年前
<p>前言：对于 <em>OkHttp</em> 我接触的时间其实不太长，一直都是使用Retrofit + OkHttp 来做网络请求的，但是有同学说面试的时候可能会问框架源码，这样光是会用是不够的，于是便萌生了通一通OkHttp源码的念头。经过大约一周的时间，源码看了个大概（说来惭愧，也就知道里面的原理），这里变向大家介绍一下我的所得，希望对大家能有所帮助。这里推荐两篇博文： <a href="http://www.open-open.com/lib/view/open1462258981866.html" rel="nofollow,noindex">OkHttp 官方教程解析 - 彻底入门 OkHttp 使用</a> 和 <a href="/misc/goto?guid=4959677335833282607" rel="nofollow,noindex">拆轮子系列：拆 OkHttp</a> 前者能够让你入门OkHttp,后者能让你明白OkHttp的原理，我就是看的后者去看的源码，如果看我的不太懂，大家可以去看看上面的。同时，欢迎大家交流，提出意见，谢谢！</p> <h2>总体流程</h2> <p>下面的流程图是由上面的文章抄来的（自己画的图，用的visio）</p> <p><em>整个流程是，通过 OkHttpClient 将构建的 Request 转换为Call，然后在RealCall中进行异步或同步任务，最后通过一些的拦截器 interceptor 发出网络请求和得到返回的 response 。 </em></p> <p>将流程大概是这么个流程，大家可以有个大概的印象，继续向下看：</p> <p><img src="https://simg.open-open.com/show/5e47411a9a996ae276342439cab19c3e.jpg"></p> <p>OkHttp流程图.jpg</p> <p>为了让大家有更深的印象，我准备追踪一个 GET 网络请求的具体流程，来介绍在源码中发生了什么。</p> <h2>GET请求过程</h2> <p>这是利用 OkHttp 写一个Get请求步骤，这里是一个同步的请求,异步的下面也会说：</p> <pre> <code class="language-java">//HTTP GET public String get(String url) throws IOException { //新建OKHttpClient客户端 OkHttpClient client = new OkHttpClient(); //新建一个Request对象 Request request = new Request.Builder() .url(url) .build(); //Response为OKHttp中的响应 Response response = client.newCall(request).execute(); if (response.isSuccessful()) { return response.body().string(); }else{ throw new IOException("Unexpected code " + response); } }</code></pre> <h2>OKHttpClient：流程的总控制者</h2> <p><img src="https://simg.open-open.com/show/69bb681806bbce2bbe6000b45e444ef3.png"></p> <p>OkHttpClient的类设计图</p> <p>使用OkHttp的时候我们都会创建一个OkHttpClient对象:</p> <p>OkHttpClient client = new OkHttpClient();</p> <p>这是做什么的呢？看下builder里面的参数：</p> <pre> <code class="language-java">final Dispatcher dispatcher; //分发器 final Proxy proxy; //代理 final List<Protocol> protocols; //协议 final List<ConnectionSpec> connectionSpecs; //传输层版本和连接协议 final List<Interceptor> interceptors; //拦截器 final List<Interceptor> networkInterceptors; //网络拦截器 final ProxySelector proxySelector; //代理选择 final CookieJar cookieJar; //cookie final Cache cache; //缓存 final InternalCache internalCache; //内部缓存 final SocketFactory socketFactory; //socket 工厂 final SSLSocketFactory sslSocketFactory; //安全套接层socket 工厂，用于HTTPS final CertificateChainCleaner certificateChainCleaner; // 验证确认响应证书 适用 HTTPS 请求连接的主机名。 final HostnameVerifier hostnameVerifier; // 主机名字确认 final CertificatePinner certificatePinner; // 证书链 final Authenticator proxyAuthenticator; //代理身份验证 final Authenticator authenticator; // 本地身份验证 final ConnectionPool connectionPool; //连接池,复用连接 final Dns dns; //域名 final boolean followSslRedirects; //安全套接层重定向 final boolean followRedirects; //本地重定向 final boolean retryOnConnectionFailure; //重试连接失败 final int connectTimeout; //连接超时 final int readTimeout; //read 超时 final int writeTimeout; //write 超时</code></pre> <p>在这些声明的对象中可以看出来，几乎所有用到的类都和 OkHttpClient 有关系。事实上，你能够通过它来设置改变一些参数，因为他是通过 建造者模式 实现的，因此你可以通过 builder() 来设置。如果不进行设置，在 Builder 中就会使用默认的设置：</p> <pre> <code class="language-java">dispatcher = new Dispatcher(); protocols = DEFAULT_PROTOCOLS; connectionSpecs = DEFAULT_CONNECTION_SPECS; proxySelector = ProxySelector.getDefault(); cookieJar = CookieJar.NO_COOKIES; socketFactory = SocketFactory.getDefault(); hostnameVerifier = OkHostnameVerifier.INSTANCE; certificatePinner = CertificatePinner.DEFAULT; proxyAuthenticator = Authenticator.NONE; authenticator = Authenticator.NONE; connectionPool = new ConnectionPool(); dns = Dns.SYSTEM; followSslRedirects = true; followRedirects = true; retryOnConnectionFailure = true; connectTimeout = 10_000; readTimeout = 10_000; writeTimeout = 10_000;</code></pre> <p>看到这，如果你还不明白的话，也没关系，在 OkHttp 中只是设置用的的各个东西。真正的流程要从里面的 newCall() 方法中说起：</p> <pre> <code class="language-java">/** * Prepares the {@code request} to be executed at some point in the future. * 准备将要被执行的request */ @Override public Call newCall(Request request) { return new RealCall(this, request); }</code></pre> <p>当通过 建造者模式 创建了 Request 之后（这个没什么好说），紧接着就通过下面的代码来获得 Response</p> <p>大家还记得上面做 GET 请求时的这句代码吧：</p> <p>Response response = client.newCall(request).execute(); 这就代码就开启了整个GET请求的流程：</p> <h2>RealCall：真正的请求执行者。</h2> <p>先看一下他的构造方法：</p> <pre> <code class="language-java">protected RealCall(OkHttpClient client, Request originalRequest) { this.client = client; this.originalRequest = originalRequest; this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client); }</code></pre> <p>可以看到他传过来一个 OkHttpClient 对象和一个 originalRequest （我们创建的 Request ）。</p> <p>接下来看它的 execute() 方法：</p> <pre> <code class="language-java">@Override public Response execute() throws IOException { synchronized (this) { if (executed) throw new IllegalStateException("Already Executed"); //(1) executed = true; } try { client.dispatcher.executed(this);//(2) Response result = getResponseWithInterceptorChain();//(3) if (result == null) throw new IOException("Canceled"); return result; }finally { client.dispatcher.finished(this);//(4) } }</code></pre> <ol> <li>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 all#clone 方法进行克隆。</li> <li>利用 client.dispatcher().executed(this) 来进行实际执行， dispatcher 是刚才看到的 OkHttpClient.Builder 的成员之一，它的文档说自己是异步 HTTP 请求的执行策略，现在看来，同步请求它也有掺和。</li> <li>调用 getResponseWithInterceptorChain() 函数获取 HTTP 返回结果，从函数名可以看出，这一步还会进行一系列“拦截”操作。</li> <li> <p>最后还要通知 dispatcher 自己已经执行完毕。</p> <p>dispatcher 这里我们不过度关注，在同步执行的流程中，涉及到 dispatcher 的内容只不过是告知它我们的执行状态，比如开始执行了（调用 executed ），比如执行完毕了（调用 finished ），在异步执行流程中它会有更多的参与。</p> <p>真正发出网络请求，解析返回结果的，还是 getResponseWithInterceptorChain ：</p> <pre> <code class="language-java">//拦截器的责任链。 private Response getResponseWithInterceptorChain() throws IOException { // Build a full stack of interceptors. List<Interceptor> interceptors = new ArrayList<>(); interceptors.addAll(client.interceptors()); //(1) interceptors.add(retryAndFollowUpInterceptor); //(2) interceptors.add(new BridgeInterceptor(client.cookieJar())); //(3) interceptors.add(new CacheInterceptor(client.internalCache())); //(4) interceptors.add(new ConnectInterceptor(client)); //(5) if (!retryAndFollowUpInterceptor.isForWebSocket()) { interceptors.addAll(client.networkInterceptors()); //(6) } interceptors.add(new CallServerInterceptor( retryAndFollowUpInterceptor.isForWebSocket())); //(7) Interceptor.Chain chain = new RealInterceptorChain( interceptors, null, null, null, 0, originalRequest); return chain.proceed(originalRequest); // <<=========开始链式调用 }</code></pre> </li> </ol> <ol> <li>在配置 OkHttpClient 时设置的 interceptors ；</li> <li>负责失败重试以及重定向的 RetryAndFollowUpInterceptor ；</li> <li>负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 BridgeInterceptor ；</li> <li>负责读取缓存直接返回、更新缓存的 CacheInterceptor ；</li> <li>负责和服务器建立连接的 ConnectInterceptor ；</li> <li>配置 OkHttpClient 时设置的 networkInterceptors ；</li> <li>负责向服务器发送请求数据、从服务器读取响应数据的 CallServerInterceptor 。</li> <li>在 return chain.proceed(originalRequest); 中开启链式调用：</li> </ol> <h2>RealInterceptorChain</h2> <pre> <code class="language-java">public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, Connection 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. //如果我们已经有一个stream。确定即将到来的request会使用它 if (this.httpCodec != null && !sameConnection(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(). //如果我们已经有一个stream， 确定chain.proceed()唯一的call 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. //调用链的下一个拦截器 RealInterceptorChain next = new RealInterceptorChain( interceptors, streamAllocation, httpCodec, connection, index + 1, request); 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"); } return response; } `</code></pre> <p>代码很多，但是主要是进行一些判断，主要的代码在这：</p> <pre> <code class="language-java">// Call the next interceptor in the chain. //调用链的下一个拦截器 RealInterceptorChain next = new RealInterceptorChain( interceptors, streamAllocation, httpCodec, connection, index + 1, request); //(1) Interceptor interceptor = interceptors.get(index); //(2) Response response = interceptor.intercept(next); //(3)</code></pre> <ol> <li>实例化下一个拦截器对应的 RealIterceptorChain 对象，这个对象会在传递给当前的拦截器</li> <li>得到当前的拦截器： interceptors 是存放拦截器的 ArryList</li> <li>调用当前拦截器的 intercept() 方法，并将下一个拦截器的 RealIterceptorChain 对象传递下去<br> 除了在client中自己设置的 interceptor ,第一个调用的就是 retryAndFollowUpInterceptor <h2>RetryAndFollowUpInterceptor:负责失败重试以及重定向</h2> 直接上代码</li> </ol> <pre> <code class="language-java">@Override public Response intercept(Chain chain) throws IOException { Request request = chain.request(); streamAllocation = new StreamAllocation( client.connectionPool(), createAddress(request.url())); int followUpCount = 0; Response priorResponse = null; while (true) { if (canceled) { streamAllocation.release(); throw new IOException("Canceled"); } Response response = null; boolean releaseConnection = true; try { response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null); //(1) releaseConnection = false; } catch (RouteException e) { // The attempt to connect via a route failed. The request will not have been sent. //通过路线连接失败，请求将不会再发送 if (!recover(e.getLastConnectException(), true, request)) throw e.getLastConnectException(); releaseConnection = false; continue; } catch (IOException e) { // An attempt to communicate with a server failed. The request may have been sent. // 与服务器尝试通信失败，请求不会再发送。 if (!recover(e, false, request)) throw e; releaseConnection = false; continue; } finally { // We're throwing an unchecked exception. Release any resources. //抛出未检查的异常，释放资源 if (releaseConnection) { streamAllocation.streamFailed(null); streamAllocation.release(); } } // Attach the prior response if it exists. Such responses never have a body. // 附加上先前存在的response。这样的response从来没有body // TODO: 2016/8/23 这里没赋值，岂不是一直为空？ if (priorResponse != null) { // (2) response = response.newBuilder() .priorResponse(priorResponse.newBuilder() .body(null) .build()) .build(); } Request followUp = followUpRequest(response); //判断状态码 (3) if (followUp == null){ if (!forWebSocket) { 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) { 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())); } 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; } }</code></pre> <ol> <li>这里是最关键的代码，可以看出在 response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null); 中直接调用了下一个拦截器，然后捕获可能的异常来进行操作</li> <li>这里没看太懂，有点坑，以后补</li> <li>这里对于返回的response的状态码进行判断，然后进行处理</li> </ol> <h2>BridgeInterceptor：</h2> <p>负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 。</p> <pre> <code class="language-java">@Override public Response intercept(Chain chain) throws IOException { Request userRequest = chain.request(); Request.Builder requestBuilder = userRequest.newBuilder(); //检查request。将用户的request转换为发送到server的请求 RequestBody body = userRequest.body(); //(1) 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. //GZIP压缩 boolean transparentGzip = false; if (userRequest.header("Accept-Encoding") == 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()); //(2) HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers()); //(3) 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(); }</code></pre> <ol> <li>在（1）和（2）之间， BridgeInterceptor 对于 request 的格式进行检查，让构建了一个新的 request</li> <li>调用下一个 interceptor 来得到response</li> <li>（3）下面就是对得到的response进行一些判断操作，最后将结果返回。</li> </ol> <pre> <code class="language-java">@Override public Response intercept(Chain chain) throws IOException { Response cacheCandidate = cache != null //=============(1) ? cache.get(chain.request()) //通过request得到缓存 : null; long now = System.currentTimeMillis(); CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); //根据request来得到缓存策略===========(2) Request networkRequest = strategy.networkRequest; Response cacheResponse = strategy.cacheResponse; if (cache != null) { cache.trackResponse(strategy); } if (cacheCandidate != null && cacheResponse == null) { //存在缓存的response，但是不允许缓存 closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it. 缓存不适合，关闭 } // If we're forbidden from using the network and the cache is insufficient, fail. //如果我们禁止使用网络，且缓存为null，失败 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(EMPTY_BODY) .sentRequestAtMillis(-1L) .receivedResponseAtMillis(System.currentTimeMillis()) .build(); } // If we don't need the network, we're done. if (networkRequest == null) { //没有网络请求，跳过网络，返回缓存 return cacheResponse.newBuilder() .cacheResponse(stripBody(cacheResponse)) .build(); } Response networkResponse = null; try { networkResponse = chain.proceed(networkRequest);//网络请求拦截器 //======(3) } finally { // If we're crashing on I/O or otherwise, don't leak the cache body. //如果我们因为I/O或其他原因崩溃，不要泄漏缓存体 if (networkResponse == null && cacheCandidate != null) { closeQuietly(cacheCandidate.body()); } } // If we have a cache response too, then we're doing a conditional get.========(4) //如果我们有一个缓存的response，然后我们正在做一个条件GET if (cacheResponse != null) { if (validate(cacheResponse, networkResponse)) { //比较确定缓存response可用 Response response = cacheResponse.newBuilder() .headers(combine(cacheResponse.headers(), networkResponse.headers())) .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()). //更新缓存，在剥离content-Encoding之前 cache.trackConditionalCacheHit(); cache.update(cacheResponse, response); return response; } else { closeQuietly(cacheResponse.body()); } } Response response = networkResponse.newBuilder() .cacheResponse(stripBody(cacheResponse)) .networkResponse(stripBody(networkResponse)) .build(); if (HttpHeaders.hasBody(response)) { // =========(5) CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache); response = cacheWritingResponse(cacheRequest, response); } return response; }</code></pre> <ol> <li>首先，根据 request 来判断 cache 中是否有缓存的 response ，如果有，得到这个 response ，然后进行判断当前 response 是否有效，没有将 cacheCandate 赋值为空。</li> <li>根据request判断缓存的策略，是否要使用了网络，缓存 或两者都使用</li> <li>调用下一个拦截器，决定从网络上来得到 response</li> <li>如果本地已经存在 cacheResponse ，那么让它和网络得到的 networkResponse 做比较，决定是否来更新缓存的 cacheResponse</li> <li> <p>缓存未经缓存过的 response</p> <h2>ConnectInterceptor:建立连接</h2> <pre> <code class="language-java">@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); }</code></pre> </li> </ol> <p>实际上建立连接就是创建了一个 HttpCodec 对象，它将在后面的步骤中被使用，那它又是何方神圣呢？它是对 HTTP 协议操作的抽象，有两个实现： Http1Codec 和 Http2Codec ，顾名思义，它们分别对应 HTTP/1.1 和 HTTP/2 版本的实现。</p> <p>在Http1Codec中，它利用 <a href="/misc/goto?guid=4959677335926595225" rel="nofollow,noindex">Okio</a> 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。</p> <p>而创建 HttpCodec 对象的过程涉及到 StreamAllocation、RealConnection ，代码较长，这里就不展开，这个过程概括来说，就是找到一个可用的 RealConnection ，再利用 RealConnection 的输入输出（ BufferedSource 和 BufferedSink ）创建 HttpCodec 对象，供后续步骤使用。</p> <h2>NetworkInterceptors</h2> <p>配置OkHttpClient时设置的 NetworkInterceptors。</p> <h2>CallServerInterceptor：发送和接收数据</h2> <pre> <code class="language-java">@Override public Response intercept(Chain chain) throws IOException { HttpCodec httpCodec = ((RealInterceptorChain) chain).httpStream(); StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation(); Request request = chain.request(); long sentRequestMillis = System.currentTimeMillis(); httpCodec.writeRequestHeaders(request); if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) { //===(1) Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength()); BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut); request.body().writeTo(bufferedRequestBody); bufferedRequestBody.close(); } httpCodec.finishRequest(); Response response = httpCodec.readResponseHeaders() //====(2) .request(request) .handshake(streamAllocation.connection().handshake()) .sentRequestAtMillis(sentRequestMillis) .receivedResponseAtMillis(System.currentTimeMillis()) .build(); if (!forWebSocket || response.code() != 101) { response = response.newBuilder() .body(httpCodec.openResponseBody(response)) .build(); } if ("close".equalsIgnoreCase(response.request().header("Connection")) || "close".equalsIgnoreCase(response.header("Connection"))) { streamAllocation.noNewStreams(); } int code = response.code(); if ((code == 204 || code == 205) && response.body().contentLength() > 0) { throw new ProtocolException( "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength()); } return response; }</code></pre> <ol> <li>检查请求方法，用 Httpcodec 处理 request</li> <li>进行网络请求得到 response</li> <li>返回r esponse</li> </ol> <h3>总结</h3> <p>前面说了拦截器用了 责任链设计模式 ,它将请求一层一层向下传，知道有一层能够得到Resposne就停止向下传递，然后将 response 向上面的拦截器传递，然后各个拦截器会对 respone 进行一些处理，最后会传到 RealCall 类中通过 execute 来得到 esponse 。</p> <h2>异步请求的流程：</h2> <p>异步get请求示例如下：</p> <pre> <code class="language-java">private final OkHttpClient client = new OkHttpClient(); public void run() throws Exception { Request request = new Request.Builder() .url("http://publicobject.com/helloworld.txt") .build(); client.newCall(request).enqueue(new Callback() { @Override public void onFailure(Call call, IOException e) { e.printStackTrace(); } @Override public void onResponse(Call call, Response response) throws IOException { if (!response.isSuccessful()) throw new IOException("Unexpected code " + response); Headers responseHeaders = response.headers(); for (int i = 0, size = responseHeaders.size(); i < size; i++) { System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i)); } System.out.println(response.body().string()); } }); }</code></pre> <p>由代码中 client.newCall(request).enqueue(Callback) ，开始我们知道 client.newCall(request) 方法返回的是 RealCall 对象，接下来继续向下看 enqueue() 方法:</p> <pre> <code class="language-java">//异步任务使用 @Override public void enqueue(Callback responseCallback) { synchronized (this) { if (executed) throw new IllegalStateException("Already Executed"); executed = true; } client.dispatcher().enqueue(new AsyncCall(responseCallback)); }</code></pre> <p>调用了上面我们没有详细说的 Dispatcher 类中的 enqueue(Call ) 方法.接着继续看：</p> <pre> <code class="language-java">synchronized void enqueue(AsyncCall call) { if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) { runningAsyncCalls.add(call); executorService().execute(call); } else { readyAsyncCalls.add(call); } }</code></pre> <p>如果中的 runningAsynCalls 不满，且 call 占用的 host 小于最大数量，则将 call 加入到 runningAsyncCalls 中执行，同时利用线程池执行 call ；否者将 call 加入到 readyAsyncCalls 中。 runningAsyncCalls 和 readyAsyncCalls 是什么呢？看下面：</p> <pre> <code class="language-java">/** Ready async calls in the order they'll be run. */ private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>(); //正在准备中的异步请求队列 /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */ private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>(); //运行中的异步请求 /** Running synchronous calls. Includes canceled calls that haven't finished yet. */ private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); //同步请求</code></pre> <p>call 加入到线程池中执行了。现在再看 AsynCall的 代码，它是 RealCall 中的内部类:</p> <pre> <code class="language-java">//异步请求 final class AsyncCall extends NamedRunnable { private final Callback responseCallback; private 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 { responseCallback.onFailure(RealCall.this, e); } } finally { client.dispatcher().finished(this); } } }</code></pre> <p>AysncCall 中的 execute() 中的方法，同样是通过 Response response = getResponseWithInterceptorChain(); 来获得response，这样异步任务也同样通过了interceptor，剩下的流程就和上面一样了。</p> <h3>结语：</h3> <p>看到这，不知道你是否明白了OkHttp的请求过程，如果有什么问题或意见，欢迎私信。</p> <p>参考</p> <ol> <li><a href="http://www.open-open.com/lib/view/open1462258981866.html" rel="nofollow,noindex">OkHttp 官方教程解析 - 彻底入门 OkHttp 使用</a></li> <li><a href="/misc/goto?guid=4959677335833282607" rel="nofollow,noindex">拆轮子系列：拆 OkHttp</a></li> </ol> <p> </p> <p>来自：http://www.jianshu.com/p/27c1554b7fee</p> <p> </p>