前言
当架构一个新的App，最先想到的框架应该要属图片框架和网络框架了，所以我打算花一些时间来研究这两方面框架的源码，图片加载框架我选的是Glide，如果有感兴趣的朋友可以去看看我之前写的Glide的源码分析http://blog.youkuaiyun.com/luofen521/article/details/71213440；网络框架我选的是Okhttp。
正文
这篇文章主要分析OkHttp3.8.1的源码，下面是OkHttp进行一次最基本的Get请求流程：
/**
* Get请求
*/
private void useGet() {
//1.创建OKHttpClient对象
OkHttpClient mOkHttpClient = new OkHttpClient();
//2.创建一个Request对象
Request request = new Request.Builder().url("http://www.baidu.com/").build();
//3.new Call
Call call = mOkHttpClient.newCall(request);
//4.请求加入调度
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
MainActivity.this.runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this, "请求失败", Toast.LENGTH_LONG).show();
}
});
}
@Override
public void onResponse(Call call, Response response) throws IOException {
//注意，这个方法是在子线程中回调的所以不能直接更新UI
final String result = response.body().string();
MainActivity.this.runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this, "请求成功，结果：" + result, Toast.LENGTH_LONG).show();
}
});
}
});
}由上面的代码可知，一次OkHttp的完整请求，主要分为以下4个步骤：
- 创建一个OkHttpClient对象
- 构建一个Request对象，其中包含了url等信息
- 通过OkHttpClient和Request对象，构建出Call对象
- 把Call对象加入到调度中，并注入了一个Callback对象，以便对网络请求结果分发处理
这样就完成了一次完整的网络请求，网络请求完成以后，会回调Callback的响应方法，如果失败，则回调onFailure方法；如果成功，则回调onResponse方法，请求到的结果在其Response对象中，注意的一点就是，这两个方法都是在子线程中回调的。所以，如果要更新UI等操作，需要切换到主线程。
那我们来看看上述的四步，具体执行了哪些操作。
1.创建OkHttpClient
也就是下面这句代码：
OkHttpClient mOkHttpClient = new OkHttpClient();我们到OkHttpClient中看看它的构造方法，如下：
public OkHttpClient() {
this(new Builder());
}可以看到，这里主要是创建了一个Builder对象，然后调用了OkHttpCilent重载的构造方法，我们先看看Builder的构造方法：
public Builder() {
dispatcher = new Dispatcher();
protocols = DEFAULT_PROTOCOLS;
connectionSpecs = DEFAULT_CONNECTION_SPECS;
eventListenerFactory = EventListener.factory(EventListener.NONE);
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;
pingInterval = 0;
}可以看到Builder的构造方法，就是初始化了一堆对象，这些参数在后续的流程中会用到，到时候再回过头来关注细节。很明显，这就是Builder创建对象的模式，也就是用Builder对象来封装OkHttpClient初始化所需要的参数，然后传递Builder对象到OkHttpClient构造方法里，完成OkHttpClient对象属性的初始化。当创建一个对象需要很多参数时，这是一个比较好的方式，在OkHttp中，随处可见这种方式，接下来看看OkHttpClient的有参构造方法：
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;
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;
}可以看到OkHttpClient的构造方法，就是初始化一些参数，这些参数包括证书验证、网络请求超时时间等。
2.创建Request对象
Request request = new Request.Builder().url("http://www.baidu.com/").build();可以看出Request的创建方式也是Builder模式，然后通过链式调用可以给Request指定url、header等。接下里，我们看看具体构建细节。先看看Request中内部类Builder的构造方法：
public Builder() {
this.method = "GET";
this.headers = new Headers.Builder();
}这里首先指定了请求方式为”GET”的方式，然后创建了一个Headers的内部类Builder对象，用于保存header信息。
接下来，看看Request中内部类Builder的url方法，如下：
public Builder url(String url) {
if (url == null) throw new NullPointerException("url == null");
// 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);
if (parsed == null) throw new IllegalArgumentException("unexpected url: " + url);
return url(parsed);
}
public Builder url(HttpUrl url) {
if (url == null) throw new NullPointerException("url == null");
this.url = url;
return this;
}这里，我们只关心主流程，在最后一行，调用了url(HttpUrl url)的重载方法，在这个方法里，为Builder指定了url，并把当前Builder对象返回。
最后看一下build方法
public Request build() {
if (url == null) throw new IllegalStateException("url == null");
return new Request(this);
}这里直接创建了一个Request对象，并把当前Builder的对象传递过去，很明显，就是把之前配置的请求方式、url、header等赋值给Request对象，看看Request的构造方法：
Request(Builder builder) {
this.url = builder.url;
this.method = builder.method;
this.headers = builder.headers.build();
this.body = builder.body;
this.tag = builder.tag != null ? builder.tag : this;
}可以看到，创建Request就是为其指定了请求方式、url、header等信息。
3.构建Call对象
在第1步创建出了OkHttpClient，在第2步，构建了携带请求信息的Request对象。第3步就是通过前两步得到的OkHttpClient对象和Request对象，构建出Call对象。如下：
Call call = mOkHttpClient.newCall(request);我们到OkHttpClient中看看newCall方法，如下：
@Override public Call newCall(Request request) {
return new RealCall(this, request, false /* for web socket */);
}Call是一个接口，这里直接创建了它的实现类RealCall对象，并返回。接下来，看看RealCall的构造方法：
RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
final EventListener.Factory eventListenerFactory = client.eventListenerFactory();
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
// TODO(jwilson): this is unsafe publication and not threadsafe.
this.eventListener = eventListenerFactory.create(this);
}可以看到，RealCall中持有了在前两步初始化的OkHttpClient对象和Request对象。
4.把Call加入调度
前三步都没有看到发起真正的网络请求，没错，网络请求的真正实现就是在第4步中完成的。先看Call怎么加入调度中的：
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
MainActivity.this.runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this, "请求失败", Toast.LENGTH_LONG).show();
}
});
}
@Override
public void onResponse(Call call, Response response) throws IOException {
//注意，这个方法是在子线程中回调的所以不能直接更新UI
final String result = response.body().string();
MainActivity.this.runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this, "请求成功，结果：" + result, Toast.LENGTH_LONG).show();
}
});
}
});enqueue方法会传递一个Callback对象进去，用于请求结束以后，接口回调。由于第3步得到的是一个RealCall对象,我们先来看看RealCall中的enqueue方法：
@Override public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}可以看到，这个方法首先用synchronized关键字，锁住了当前对象，也就是RealCall对象，然后对executed字段进行判断，那executed是什么呢？它是用于表示Call实例有没有执行过，当为true时，表示已经执行过了，这里就会抛出”Alread Executed”的错误信息，可见，第三步得到的Call对象，只能被执行一次。
然后看最后一行，先是通过传递进来的Callback对象封装成了一个AsyncCall对象，那AsyncCall又是个什么类呢？它其实是RealCall中的内部类，我们来看看它的定义：
final class AsyncCall extends NamedRunnable {AsyncCall继承于NamedRunnable，那NamedRunnable是继承Runnable么？我们来看看它的定义：
public abstract class NamedRunnable implements Runnable {果然如此，那我们看看AsyncCall的构造方法：
AsyncCall(Callback responseCallback) {
super("OkHttp %s", redactedUrl());
this.responseCallback = responseCallback;
}可知，这里只是把回调对象赋值给了AsyncCall中的属性，我们继续回到RealCall类中的enqueue方法：
@Override public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}在构建了一个Runnable的实现类AsyncCall以后，直接调用client.dispatcher().enqueue()方法。
client很明显就是一个OkHttpClient对象，看看它的dispatcher方法：
public Dispatcher dispatcher() {
return dispatcher;
}这里其实只是单纯地返回了dispatcher对象，那这个对象是哪里赋予初值的呢？这就要追溯到第1步，构建OkHttpClient对象时，其内部类的Builder的构造方法，部分代码如下：
public Builder() {
dispatcher = new Dispatcher();
protocols = DEFAULT_PROTOCOLS;
//...
}就在第一行，在创建OkHttpClient对象时，已经实例化好了Dispatcher对象，其构造方法里没有代码，就不贴出来了。接下里我们继续看得到dispatcher以后，调用其enqueue方法的逻辑：
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}可以看到该方法加了同步锁。并且该方法传递进来了一个Runnable的实现类AsyncCall对象。
这里，首先判断了runningAsyncCalls和runningCallsForHost的个数是否在限制内，如果在，就把传递进来的AsyncCall对象加入到runningAsyncCalls集合中，并调用了executorService().execute(call);去执行call;如果不在限制内，就把AsyncCall对象加入到readyAsyncCalls集合中。
我们来看看上述属性和方法的定义：
private int maxRequests = 64;
private int maxRequestsPerHost = 5;
/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** Returns the number of running calls that share a host with {@code call}. */
private int runningCallsForHost(AsyncCall call) {
int result = 0;
for (AsyncCall c : runningAsyncCalls) {
if (c.host().equals(call.host())) result++;
}
return result;
}可知：
runningAsyncCalls: 正在执行的AsyncCall的个数，包括已经取消了的AsyncCall
readyAsyncCalls：准备执行的AsyncCall的个数
maxRequests：允许正在执行的AsyncCall最大个数，默认64
maxRequestsPerHost：允许每一个Host最多执行的AsyncCall个数
runningCallsForHost(AsyncCall call) ：用于计算正在运行的AsyncCall队列中和当前AsyncCall同Host的个数。
上述代码代码到判断逻辑就是：
发起一个网络请求，会先判断当前正在请求的Runnable个数是否小于64，并且当前网络请求host是否小于5个请求，如果是，则把当前AsyncCall加入到正在请求的AsyncCall队列中去，并从通过线程池去执行该AsyncCall；如果不是，则把当前AsyncCall加入到等待的AsyncCall队列中。
这里，我们先看满足if条件时的逻辑，即调用了executorService().execute(call);方法，我们先来看executorService()方法：
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;
}注意，该方法用synchronized关键字，保证了ExecutorService为单例模式，这个方法其实就是返回了一个线程池对象，该线程池的创建模式为corePoolSiz为0，而maximumPoolSize为Integer的最大值，即2147483647个，当网络请求特别多的时候，会不会开启特别多的线程，导致手机的开销过大，影响性能呢？其实不会，因为maxRequests已经限制了AsyncCall的个数不能超过64。
如果在自己的项目中，有单独的线程池模版，可以自己实现executorService方法，统一管理线程池。
然后就是调用ExecutorService对象的execute方法，即调用了AsyncCall的run方法：
发现AsyncCall没有run方法的实现，我们去从它的父类NamedRunnable中寻找：
@Override public final void run() {
String oldName = Thread.currentThread().getName();
Thread.currentThread().setName(name);
try {
execute();
} finally {
Thread.currentThread().setName(oldName);
}
}在NamedRunnable的run方法中，主要调用类execute方法，但是发现NamedRunnable中的execute方法是个抽象方法，继续从其子类AsyncCall中查看execute方法：
@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);
}
}
}我们主要看主流程：
首先，第4行，调用了getResponseWithInterceptorChain得到了Response对象
然后，第5行，判断一下是否取消了，如果取消了则调用responseCallback.onFailure方法，而responseCallback对象就是第4步call.enqueue(Callback callback)方法的参数callback, 这样就完成当取消了网络请求回调onFailure的流程。
如果没有取消，则在第10行调用responseCallback的onResponse, 这就是我们熟悉的味道了，会把第4行得到的结果当成参数，传递到发起网络请求的地方。
这里已经在子线程中了，也就验证了之前说的回掉onFailure和onResponse方法是在子线程中。
然后，如果请求网络的过程中，进入到了catch块，也是调用onFailure方法，这里就不多说了。
最后，在finally中调用了client.dispatcher().finshed(this)，我们来看看Dispatcher中的finished方法：
void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
int runningCallsCount;
Runnable idleCallback;
synchronized (this) {
if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
if (promoteCalls) promoteCalls();
runningCallsCount = runningCallsCount();
idleCallback = this.idleCallback;
}
if (runningCallsCount == 0 && idleCallback != null) {
idleCallback.run();
}
}这里的finish方法，主要是把当前AsyncCall从正在运行的AsyncCall任务队列里移除掉。
主流程走完，我们回过头来看AsyncCall中execute的第4行，调用了getResponseWithInterceptorChain方法，这就是真正的网络请求的地方了，实现还是有些复杂的：
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);
}首先创建了拦截器Interceptor集合，然后把一堆拦截器加入到集合里，再创建一个RealInterceptorChain对象，它是现实了拦截器Interceptor的内部接口Chain，我们先来看看它的构造方法：
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;
}这里值得注意的就是第1个参数和第5参数，第1个参数是拦截器集合，第5个参数是当前要执行的拦截器在拦截器集合的序号，这里传入的是0，所以接下来会取interceptors的第一个元素，去执行它的方法，后续代码会看到，这里需要知道的就是传入了拦截器集合和需要执行拦截器的序号。
在创建了RealInterceptorChain以后，就调用了它的proceed方法：
@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}这里直接调用了它的重载方法，而后面的三个参数，都是在创建RealInterceptorChain时传入的，这里三个都为null，然后我们继续看看该方法的实现：
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
//省略部分代码...
// 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);
//省略部分代码...
return response;
}主要看第11行，又创建了一个RealInterceptorChain对象，这里第1个参数还是传入了拦截器集合，但是第5个参数是index＋1了，之前是0，所以现在传入的就是1了。然后第12行，这里index 还是为0， 所以就是从拦截器中取出了第一个拦截器，最后再在第13行调用取到的拦截器的intercept方法，进行真正的网络请求，并拿到请求数据Response以后返回。
那么，拦截器集合里的第一个元素是什么呢？我们继续回到之前创建拦截器集合的地方，也就是RealCall的getResponseWithInterceptorChain方法中：
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);
}拦截器的第一个元素应该是第三行代码加入的，我们来看看client.interceptors()方法的实现：
/**
* Returns an immutable list of interceptors that observe the full span of each call: from before
* the connection is established (if any) until after the response source is selected (either the
* origin server, cache, or both).
*/
public List<Interceptor> interceptors() {
return interceptors;
}其实就是直接返回了OkHttpClient中的拦截器集合，那它是在哪里赋予初值的呢？其实就在OkHttpClient的构造方法里：
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);
//省略部分代码...
}在第6行的地方，调用了Util.immutableList方法，传入了builder的interceptors后，得到了OkHttpClient中的拦截器集合，我们来看看immutableList方法：
/** Returns an immutable copy of {@code list}. */
public static <T> List<T> immutableList(List<T> list) {
return Collections.unmodifiableList(new ArrayList<>(list));
}直接调用了Collections.ummodifiableList方法，这个方法的意义其实就是用于复制一个List的数据，但是复制出来的对象是只读的，不可进行add、remove等修改操作，否则就会报：java.lang.UnsupportedOperationException的错误。
所以，OkHttpClient中的拦截器集合，其实就是通过其内部类Builder中的拦截器集合构造出来的，我们看看Builder的interceptors是哪里初始化的：
public static final class Builder {
Dispatcher dispatcher;
@Nullable Proxy proxy;
List<Protocol> protocols;
List<ConnectionSpec> connectionSpecs;
final List<Interceptor> interceptors = new ArrayList<>();
//省略部分代码...第6行，直接创建了拦截器集合，然后我们看看Builder的有参构造方法：
Builder(OkHttpClient okHttpClient) {
this.dispatcher = okHttpClient.dispatcher;
this.proxy = okHttpClient.proxy;
this.protocols = okHttpClient.protocols;
this.connectionSpecs = okHttpClient.connectionSpecs;
this.interceptors.addAll(okHttpClient.interceptors);
//省略部分代码...
}第6行代码，这里把OkHttpClient的拦截器集合加入到Builder的拦截器集合中，但是我们之前在第1步创建OkHttpClient对象时讲到，先是调用了Builder无参的构造方法，初始化了参数以后，才通过Builder对象，去初始化OkHttpClient对象，所以，根本没有调用上述Builder有参构造方法，即OkHttpClient的默认拦截器集合为一个空数组。
我们再回到RealCall的getResponseWithInterceptorChain方法：
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);
}既然client.interceptors默认是空数组，我们来看看第5行的retryAndFollowUpInterceptor会是第一个拦截器么？
我们先说一下结论，其实retryAndFollowUpInterceptor就是第一个拦截器，然后第10行的client.networkInterceptors()的赋值流程和client.interceptors()是一致的，所以最后的值都为size为0的ArrayList。那么传入到RealInterceptorChain的拦截器集合就是加入了以下5个拦截器：
1.RetryAndFollowUpInterceptor
2.BridgeInterceptor
3.CacheInterceptor
4.ConnectInterceptor
5.CallServerInterceptor
然后通过调用这5个拦截器的intercept方法，完成各自的功能。
那我们先来看第一个拦截器RetryAndFollowUpInterceptor初始化的地方：
RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
final EventListener.Factory eventListenerFactory = client.eventListenerFactory();
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
// TODO(jwilson): this is unsafe publication and not threadsafe.
this.eventListener = eventListenerFactory.create(this);
}在RealCall的构造方法里的第7行，初始化了retryAndFollowUpInterceptor，它是一个RetryAndFollowUpInterceptor对象，而RealCall的构造方法是在第3步，OkHttpClient调用newCall方法时调用的，所以也验证了retryAndFollowUpInterceptor就是RealCall中拦截器的第一个元素。
然后我们回到主流程，得到了第一个拦截器以后，会调用其intercept方法，并把一个新的RealInterceptorChain对象传递进去，我们先看看RetryAndFollowUpInterceptor中的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Call call = realChain.call();
EventListener eventListener = realChain.eventListener();
streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
call, eventListener, callStackTrace);
int followUpCount = 0;
Response priorResponse = null;
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response = null;
boolean releaseConnection = true;
try {
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
if (!recover(e.getLastConnectException(), false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, requestSendStarted, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
Request followUp = followUpRequest(response);
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) {
streamAllocation.release();
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()), call, eventListener, callStackTrace);
} 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;
}
}首先，在第7行的地方，创建了一个StreamAllocation对象。然后在第12行，写了一个while(true)的循环，其实当一次性请求成功时，while里面的代码块只会执行一次，得到Response结果以后，直接return了，所以我们先不用纠结这个while(true)。继续往下看，第21行代码处，调用了realChain.proceed方法，而这个realChain对象，就是在取出拦截器集合里第一个元素RetryAndFollowUpInterceptor对象以后，调用intercept时传递进来的参数RealInterceptorChain对象，还记得当时创建时，构造参数传递的第5个参数为index+1么？也就是这时的realChain的index＝1。
这里有些难理解，总体来说就是：
1.首先，创建了一系列的拦截器
2.然后创建了一个RealInterceptorChain对象，调用了它的proceed方法，并告知取拦截器的序号
3.在proceed方法里，从拦截器集合里，取出对应序号的拦截器，然后调用拦截器的intercept方法
4.在intercept方法里，继续调用RealInterceptorChain的proceed方法
5.一直循环3－4，直到取到最后一个拦截器CallServerInterceptor，并返回网络请求的结果Response。
如图：
那我们继续看RealInterceptorChain的proceed方法，就是上述流程的第4步：
这时候取出的就是第2个拦截器BridgeInterceptor对象，然后我们看看它的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
RequestBody body = userRequest.body();
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();
}可以看到，这个方法主要是处理了网络请求头的问题，包括文本类型、Host、User-Agent、Keep-Alive等配置。这里主要看47行，这里继续调用了RealInterceptorChain的peroceed方法，不过这里的方法参数Request已经是封装过Request了。
根据之前讲到的，我们很容易想到，这时候应该是取第3个拦截器CacheInterceptor，并执行它的intercept方法，我们来看看这个方法：
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
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.
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.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
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.
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);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}看名称就知道了，这显然是一个处理缓存相关的拦截器。我们直接看核心代码，就是第42行，这里继续调用了chain.proceed方法,通过调用下一个拦截器的intercept方法得到请求网络结果networkResponse，然后在第72行通过返回的结果，构造Response出对象，并返回。
而下一个拦截器就是ConnectInterceptor,我们来看看它的intercept方法：
@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);
}容易看出ConnectInterceptor就是一个建立网络连接的拦截器，第9行代码处，就是得到了一个网络链接RealConnection，就像HttpUrlConnection中的new URL().openConnection()一样，然后在第11行，调用了realChain.proceed方法，并把刚才得到RealConnection对象传递进去，这里很容易推测，最后一个拦截器的作用应该就是做真正的网络请求。
下一个拦截器，也是最后一个拦截器，就是CallServerInterceptor，我们来看看它的intercept方法：
@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;
}我们先看第9行，直接通过调用了httpCodec.writeRequestHeaders方法把请求头写入，而httpCodec其实是在ConnectInterceptor的intercept方法中创建的，它其实是个Http1Codec对象。如果是post请求并且有请求body,则会执行第23行代码去调用httpCodec.createRequestBody去构建请求body，并写入。这里为Get请求，所以不会执行这段代码。继续往下看，第35行调用了httpCodec的finishRequest方法，其实就是flush了一把，用于请求数据写入完成。然后第38行，通过httpCodec的readResponseHeaders方法得到ResponseBuilder对象，然后再在第41行，去得到Response对象，最后，在55行去调用httpCodec.openResponseBody方法，构造出最后的Response并返回。
这样就完成了所有拦截器的流程，然后就逐级返回把这个Response给到Callback的回调方法onResponse。
整个流程，可能就是拦截器的部分复杂一些，不过也是OkHttp的精髓所在，把不同的功能拆分成不同的拦截器来实现。我们总结下这5个拦截器的功能：
1.RetryAndFollowUpInterceptor：处理失败、转发等网络请求
2.BridgeInterceptor：处理请求头
3.CacheInterceptor：处理缓存相关的逻辑
4.ConnectInterceptor：构造网络连接
5.CallServerInterceptor：真正网络请求，并得到Response最后返回
我们会发现，这5个拦截器的排序也是恰到好处。
最后，一图抵千言：
总结：
这篇博客主要是对OkHttp一次简单的Get请求的源码分析，还有很多细节性的地方没有涉及到，后面会写一篇OkHttp实现细节的的博客。
欢迎交流、指正。
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