Android笔记|OkHttp3.9请求过程源码解析 Android面试|移动开发后花园|第

首先来看看OkHttp的基础
创建一个普通的同步get请求代码如下：

OkHttpClient client = new OkHttpClient(); //1 String run(String url) throws IOException { Request request = new Request.Builder().url(url).build(); //2 Response response = client.newCall(request).execute(); //3 if (response.isSuccessful()) { return response.body().string(); } else { throw new IOException("Unexpected code " + response); } }

异步get：

OkHttpClient client = new OkHttpClient(); //1 Request request = new Request.Builder() .url("http://xxxxxx") .build(); //2 client.newCall(request).enqueue(new Callback() {//3 @Override public void onFailure(Call call, IOException e) { } @Override public void onResponse(Call call, Response response) throws IOException { if(response.isSuccessful()){//回调的方法执行在子线程。} } });

都是主要三个步骤：1.创建OkHttpClient实例；2.使用构造器创建请求；3.提交请求。
OkHttpClient构造方法
从构造方法开始，看看OkHttpClient的创建过程

public OkHttpClient() { this(new Builder()); } public Builder() { this.dispatcher = builder.dispatcher; this.proxy = builder.proxy; //代理 this.protocols = builder.protocols; //协议 this.connectionSpecs = builder.connectionSpecs; //策略 this.interceptors = Util.immutableList(builder.interceptors); //拦截器 this.networkInterceptors = Util.immutableList(builder.networkInterceptors); //网络拦截器 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; for (ConnectionSpec spec : connectionSpecs) { isTLS = isTLS || spec.isTls(); }if (builder.sslSocketFactory != null || !isTLS) { this.sslSocketFactory = builder.sslSocketFactory; this.certificateChainCleaner = builder.certificateChainCleaner; } else { X509TrustManager trustManager = systemDefaultTrustManager(); this.sslSocketFactory = systemDefaultSslSocketFactory(trustManager); this.certificateChainCleaner = CertificateChainCleaner.get(trustManager); }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; if (interceptors.contains(null)) { throw new IllegalStateException("Null interceptor: " + interceptors); } if (networkInterceptors.contains(null)) { throw new IllegalStateException("Null network interceptor: " + networkInterceptors); } }

可以看到，创建的过程主要是调用了构造器的构造方法，初始化了一些请求相关的参数，这里面几乎都是跟HTTP请求相关的参数，就不深入去追究，主要是要注意这里有三个东西：就是interceptors和networkInterceptors（这两个类就是这篇文章的主题，后面再细说），还有一个Dispatcher，我们需要进去看看，在这里我们可以知道OkHttp请求的任务调度原理：

public final class Dispatcher { private int maxRequests = 64; //最大请求任务个数 private int maxRequestsPerHost = 5; //最大允许同一host的请求任务个数public Dispatcher(ExecutorService executorService) { this.executorService = executorService; }public Dispatcher() { }public synchronized ExecutorService executorService() { if (executorService == null) { executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS, new SynchronousQueue(), Util.threadFactory("OkHttp Dispatcher", false)); } return executorService; } ... }

【Android笔记|OkHttp3.9请求过程源码解析】可以看出来，Dispatcher就像是一个调度中心，这里负责所有的任务执行，值得注意的是我们看到executorService这个对象是个线程池，而且是个不限制大小的线程池，这里用到了SynchronousQueue（没有缓存大小的阻塞队列），这个线程池跟Executors默认的newCachedThreadPool原理相同，当任务加入时如果有空闲的线程就复用，没有的话就创建新的线程，每个线程空闲后60秒被销毁。
创建请求

Request request = new Request.Builder() .url("http://xxxxxx") .build();

这一步很简单，传入请求地址就OK，也没什么多说的
发起请求
到这里算是到了正文，来看看OkHttp的请求到底是怎么来执行的，先看同步的请求：

Response response = client.newCall(request).execute();

client.newCall(request)：

@Override public Call newCall(Request request) { return RealCall.newRealCall(this, request, false /* for web socket */); }

调用RealCall.newRealCall：

static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) { // Safely publish the Call instance to the EventListener. RealCall call = new RealCall(client, originalRequest, forWebSocket); call.eventListener = client.eventListenerFactory().create(call); return call; }

再来到RealCall的构造：

private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) { this.client = client; this.originalRequest = originalRequest; this.forWebSocket = forWebSocket; this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket); }

在这里除了初始化一些对象以外，还有一个很重要的东西，RetryAndFollowUpInterceptor，又看到了一个Interceptor，还记得我们在前面OkHttp的构造中也有一个interceptors和networkInterceptors，看名字也知道他们肯定有什么关系，具体是什么关系我们先买一个伏笔，继续往下看RealCall的同步网络请求execute方法：
RealCall.execute

@Override public Response execute() throws IOException { synchronized (this) { if (executed) throw new IllegalStateException("Already Executed"); executed = true; } captureCallStackTrace(); eventListener.callStart(this); try { client.dispatcher().executed(this); //1 Response result = getResponseWithInterceptorChain(); //2 if (result == null) throw new IOException("Canceled"); return result; } catch (IOException e) { eventListener.callFailed(this, e); throw e; } finally { client.dispatcher().finished(this); } }

到这里，一次同步请求就执行完毕了，不得不感叹OkHttp设计者的强大，API的调用简单明了不说，竟然源码也这么一气呵成。好了，来看看具体是怎么处理的,核心代码在try代码块里面的两行
1. client.dispatcher().executed(this) 这里的dispatcher()也就是之前着重讲到的那个调度器，来看看它的executed方法：
Dispatcher.executed

synchronized void executed(RealCall call) { runningSyncCalls.add(call); }

很简单一句，往运行时队列里面加入这个请求，再顺便看看这个队列吧

/** Running synchronous calls. Includes canceled calls that haven't finished yet. */ private final Deque runningSyncCalls = new ArrayDeque<>();

注释都写的很清楚，就不翻译了
2. Response result = getResponseWithInterceptorChain(); 这里又看到了Interceptor相关的东西，到了这里，箭在弦上，不得不发了，终于可以引出OkHttp最核心的拦截器机制了。
Interceptor
先来看看拦截器的定义：

Java里的拦截器是动态拦截Action调用的对象。它提供了一种机制可以使开发者可以定义在一个action执行的前后执行的代码，也可以在一个action执行前阻止其执行，同时也提供了一种可以提取action中可重用部分的方式。
在AOP（Aspect-Oriented Programming）中拦截器用于在某个方法或字段被访问之前，进行拦截然后在之前或之后加入某些操作。

在OkHttp中，拦截器就是在请求的各个阶段进行一些处理，然后再传递下去继续执行。那OkHttp的拦截器到底是怎么一回事，先回到刚才的那个getResponseWithInterceptorChain方法看看：
RealCall.getResponseWithInterceptorChain

Response getResponseWithInterceptorChain() throws IOException { // Build a full stack of interceptors. List 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, this, eventListener, client.connectTimeoutMillis(), client.readTimeoutMillis(), client.writeTimeoutMillis()); return chain.proceed(originalRequest); }

题目揭晓
在这里看到了，interceptors其实是一个数组，在OkHttp构造里面也初始化了这个数组和networkInterceptors，那这两个数组到底有什么区别呢？感觉篇幅有点多了，下一篇我们来解开它的答案