为啥要有再次分析OkHttp呢，因为作为android 网络底层框架，它的使用范围和稳定性都是非常高的。那么我们需要对他进行充分的熟悉。 我们还是从他自身的同步和异步请求方法来分析吧，因为这个是落脚方法。位于NewCall里面。
@Override public Response execute() throws IOException { //首先锁住自身，这个synchronized是一个非重入锁。如果同一线程或者别的线程想再次调用对象的execute 方法，executed 是个布尔变量，线程的可见性可以保障。如果发现正在执行就会抛出异常。 synchronized (this) { if (executed) throw new IllegalStateException("Already Executed"); executed = true; } captureCallStackTrace(); timeout.enter(); eventListener.callStart(this); try { //还是利用Dispatch 来执行这个RealCall.那么下面我们着重分析这个。 client.dispatcher().executed(this); Response result = getResponseWithInterceptorChain(); if (result == null) throw new IOException("Canceled"); return result; } catch (IOException e) { e = timeoutExit(e); eventListener.callFailed(this, e); throw e; } finally { client.dispatcher().finished(this); } }Dispatcher的executed 实际也只是把他加入ArrayDeque 这个双向队列，跟ArrayList一样，不过也是线程不安全的，所以加上了锁。
synchronized void executed(RealCall call) { runningSyncCalls.add(call); }那么执行方法应该就是getResponseWithInterceptorChain（）了，我们来分析下。
Response getResponseWithInterceptorChain() throws IOException { // Build a full stack of interceptors. List<Interceptor> interceptors = new ArrayList<>(); //自己传入进去的拦截器。 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()); } //这个拦截器是最终执行请求的。一会看下怎么通过责任链的模式，请求到这里。 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); }落脚方法跑到了RealInterceptorChain的proceed,可以猜测这个是把所有拦截器统一起来的方法，看看他的名字Real。开始分析proceed。
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, RealConnection connection) throws IOException { //index代表正在执行的拦截器的索引。 if (index >= interceptors.size()) throw new AssertionError(); calls++; // If we already have a stream, confirm that the incoming request will use it. //httpCode不为空代表正在请求。 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); //这个地方实际上就是代表把当前的后一个传入进去，然后对当前的拦截器调用intercept方法。因为每个拦截器都会有一个方法就是 (RealInterceptorChain)chain.proceed 这样保证拦截器都能够执行到intercept方法同时也能下一个执行到，这个算法还是值得一看的。 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; }那么我们还是一个一个从OkHttp的拦截来说吧，因为他为实现http请求做的一些列工作。retryAndFollowUpInterceptor属于重复请求的，暂时不管。 BridgeInterceptor
@Override public Response intercept(Chain chain) throws IOException { Request userRequest = chain.request(); Request.Builder requestBuilder = userRequest.newBuilder(); RequestBody body = userRequest.body(); //如果是post请求，RequestBody 有数据，如果是get就没有。有个类似的概念叫做ReponseBody 我们返回的数据可以在这个里面去获取。 if (body != null) { MediaType contentType = body.contentType(); //添加 Content-Type 到header。 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"); } //添加Cookie. 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()); } //传递出去给到下面拦截器。 Response networkResponse = chain.proceed(requestBuilder.build()); //实际这一步是保存Cookie信息需要更进去看。 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(); }可以看到主要是做了一些Cook和header的设置工作。 接下来看下面一个。CacheInterceptor 主要是跟缓存相关。
@Override public Response intercept(Chain chain) throws IOException { //首先分析此request的缓存是否存在，就是同样的请求的Response是否存在。 Response cacheCandidate = cache != null ? cache.get(chain.request()) : null; long now = System.currentTimeMillis(); //获取到CacheStrategy。 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. //networkRequest 代表网络请求，如果这个不存在那么直接返回了，不会继续往下走了。 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. //如果网络请求不存在，但是缓存存在直接利用缓存的Reponse. if (networkRequest == null) { return cacheResponse.newBuilder() .cacheResponse(stripBody(cacheResponse)) .build(); } //不利用缓存开始请求了。 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. //更新缓存。 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); } //验证下缓存是否合法。否则移除。 if (HttpMethod.invalidatesCache(networkRequest.method())) { try { cache.remove(networkRequest); } catch (IOException ignored) { // The cache cannot be written. } } } return response; }接下来ConnectInterceptor。这个也是一个对连接产生直接影响的类还是非常短的。
@Override public Response intercept(Chain chain) throws IOException { RealInterceptorChain realChain = (RealInterceptorChain) chain; Request request = realChain.request(); //这个StreamAllocation 是在RetryAndFollowUpInterceptor创建的 // StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(), // createAddress(request.url()), call, eventListener, callStackTrace); StreamAllocation streamAllocation = realChain.streamAllocation(); // We need the network to satisfy this request. Possibly for validating a conditional GET. boolean doExtensiveHealthChecks = !request.method().equals("GET"); //获取到HttpCodec对象 HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks); //会复用RealConnection. RealConnection connection = streamAllocation.connection(); return realChain.proceed(request, streamAllocation, httpCodec, connection); }接下来来到最后一个CallServerInterceptor。内容比较多，打算再开一篇文章来分析。
