Okhttp源码分析--基本使用流程分析
一、 使用
同步请求
OkHttpClient okHttpClient=new OkHttpClient();
Request request=new Request.Builder()
.get()
.url("www.baidu.com")
.build();
Call call =okHttpClient.newCall(request).execute();异步请求
OkHttpClient okHttpClient=new OkHttpClient();
Request request=new Request.Builder()
.get()
.url("www.baidu.com")
.build();
Call call=okHttpClient.newCall(request).enqueue(new Callback() {
@Override public void onFailure(Call call, IOException e) {
Log.i(TAG, "onFailure: ");
}
@Override public void onResponse(Call call, Response response) throws IOException {
Log.i(TAG, "onResponse: ");
}
});可以看出不管是同步还是异步请求，使用okhttp大致分为3个步骤：
1. 创建okhttpclient
2. 创建请求的request
3. 通过client拿到call、发送请求
注：okhttpclient和request的创建均可采用构造者模式，在构造过程中可根据自己的实际需求设置相应的参数，如可在okhttpclient构造时添加自定义拦截器，在request构造过程中设置连接超时时间等。
二、 源码分析
首先看下OkhttpClient这个类，使用步骤的第一步就是构造OkhttpClient对象。
先贴下官方对OkhttpClient的定义
*Factory for {@linkplain Call calls}, which can be used to send HTTP requests and read their
* responses.
* OkHttpClients should be shared
* OkHttp performs best when you create a single {@code OkHttpClient} instance and reuse it for
* all of your HTTP calls. This is because each client holds its own connection pool and thread
* pools. Reusing connections and threads reduces latency and saves memory. Conversely, creating a
* client for each request wastes resources on idle pools.OkhttpClient是用于发送请求和读取响应的，官方建议创建一个单例的OkHttpClient，并且所有的http请求都复用它。因为每个OkHttpClient都有自己的connection pool and thread pool。复用connection pool and thread pool可以节约内存，相反如果为每个请求都创建一个OkHttpClient那么会浪费idle pools中的资源。
创建OkHttpClient有两种方式：
1、通过构造函数
2、通过Builder()创建一个client并自定义设置
同时还提供了一个newBuilder()来自定义client，它跟共享的client拥有相同的connection pool, thread pools, and configuration。我们可以用该方法来获取特定设置的client。
OkHttpClient提供无参构造函数，由代码可知它在内部调用OkHttpClient的有参构造函数
，在有参构造函数里对OkHttpClient主要属性做了初始化赋值。
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;
...//省略N行
}下面贴下OkHttpClient主要的属性
public class OkHttpClient{
final Dispatcher dispatcher;//分发器
final @Nullable 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 （okhttp核心机制）
final List<Interceptor> networkInterceptors;//网络拦截器
final EventListener.Factory eventListenerFactory;
final ProxySelector proxySelector;//代理选择器
final CookieJar cookieJar;//cookie
final @Nullable
Cache cache;//cache 缓存
final @Nullable
InternalCache internalCache;//内部缓存
final SocketFactory socketFactory;//socket 工厂
final @Nullable
SSLSocketFactory sslSocketFactory;//安全套层socket工厂 用于https
final @Nullable
CertificateChainCleaner certificateChainCleaner;//验证确认响应书，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;//本地省份验证
final ConnectionPool connectionPool;//链接池 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;//连接失败是否重试
final int connectTimeout;//连接超时时间
final int readTimeout;//读取超时时间
final int writeTimeout;//写入超时时间
}通过浏览源码我们可以发现OkHttpClient采用了构造者设计模式，这样简化参数设置，降低使用成本。比如我们前面简单使用的例子
OkHttpClient类还有一个需要了解的函数就是newCall，因为OkHttpClient实现Call.Factory接口所以覆写了newCall方法，在方法内部返回的是一个RealCall实例。
/**
* Prepares the {@code request} to be executed at some point in the future.
*/
@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
OkHttpClient构造好了之后接下来就是创建request，request就是我们要发送的请求。它也是通过builder模式构造的。下面贴下Request的主要属性以及其构造函数。
public final class Request {
final HttpUrl url;//请求url地址
final String method;//请求方式
final Headers headers;//请求头
final @Nullable RequestBody body;//请求body
final Map<Class<?>, Object> tags;//请求tags用来标记一类请求如 设置之后可以通过tags取消拥有该tag的请求
Request(Builder builder) {
this.url = builder.url;
this.method = builder.method;
this.headers = builder.headers.build();
this.body = builder.body;
this.tags = Util.immutableMap(builder.tags);
}
...
}通过Request我们可以得到我们想要的请求，然后下一步就是获取call实例然后发送请求。
在介绍OkHttpClient类的时候我们已经说过call对象是通过OkHttpClient的newCall方法获得的实际返回的是RealCall对象，也就是说真正发送的请求是RealCall，那么我们来看下RealCall这个类
final class RealCall implements Call {
final OkHttpClient client; //realcall持有client
private Transmitter transmitter;//暂时不知道其作用
/** The application's original request unadulterated by redirects or auth headers. */
final Request originalRequest;//原始请求
final boolean forWebSocket;//
// Guarded by this.
private boolean executed;//请求是否执行标志位
private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) { //构造函数
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
}
static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {//okhttpclient即通过该函数返回call
// Safely publish the Call instance to the EventListener.
RealCall call = new RealCall(client, originalRequest, forWebSocket);
call.transmitter = new Transmitter(client, call);
return call;
}RealCall实现的Call接口，其newCall函数内部通过RealCall的构造函数实例化一个call然后返回该call。
最后就是发送请求了，有两种方式：同步和异步。我们先看下同步请求的方式，同步请求是通过execute发送的
@Override public Response execute() throws IOException {
synchronized (this) {//1
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
transmitter.timeoutEnter();
transmitter.callStart();
try {
client.dispatcher().executed(this);//2
return getResponseWithInterceptorChain();//3
} finally {
client.dispatcher().finished(this);//4
}
}
execute首先（注释1处）会synchronized来检查executed值从而确保每个请求只能执行一次。随后调用dispatcher的executed（注释2处）。
来看下Dispatcher这个类
public final class Dispatcher {
private int maxRequests = 64;//最大请求数
private int maxRequestsPerHost = 5;//每个host的最大请求数
private @Nullable Runnable idleCallback;//请求队列空闲回调
/** Executes calls. Created lazily. */
private @Nullable ExecutorService executorService; //执行请求的线程池
/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();//异步准备就绪请求队列
/** Running asynchronous(异步） calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();//异步执行请求队列
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();//同步请求队列可以看出okhttp虽然支持并发请求但是有最大并发请求数的限制。而且okhttp针对不同的请求方式提供了不同的请求队列。dispatcher这个类主要的作用就是根据request的请求方式以及根据当前client的执行情况把新创建的call请求分发至不同的队列中去执行。
了解了dispatcher类作用我们看下它的exectued函数
synchronized void executed(RealCall call) {
runningSyncCalls.add(call);
}很简单它只是把传入的call对象添加到同步请求队列中（runningSyncCalls）。那请求具体是如何发送的呢 我们接着看RealCall的exectued函数。
在注释3处通过调用getResponseWithInterceptorChain()获取respone并返回该respone。
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());//如果在client中设置了自定义interceptor那么会放到interceptors中
interceptors.add(new RetryAndFollowUpInterceptor(client));//添加重试与重定向拦截器
interceptors.add(new BridgeInterceptor(client.cookieJar()));//添加桥接拦截器
interceptors.add(new CacheInterceptor(client.internalCache()));//添加缓存拦截器
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
interceptors.add(new CallServerInterceptor(forWebSocket));//添加CallServer拦截器
Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
originalRequest, this, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());//
创建RealInterceptorChain实例，把interceptors传入
boolean calledNoMoreExchanges = false;
try {
Response response = chain.proceed(originalRequest);//通过proceed链式获取respone
if (transmitter.isCanceled()) {
closeQuietly(response);
throw new IOException("Canceled");
}
return response;//返回respone
} catch (IOException e) {
calledNoMoreExchanges = true;
throw transmitter.noMoreExchanges(e);
} finally {
if (!calledNoMoreExchanges) {
transmitter.noMoreExchanges(null);
}
}
}getResponseWithInterceptorChain首先会把自定义以及okhttp定义的拦截器加到interceptors的list中，然后构造RealInterceptorChain拦截器链，调用chain.proceed链式调用各个拦截器并最终获得respone。
Interceptor可以说是okhttp的核心机制之一，我们一起来看下
public interface Interceptor {
Response intercept(Chain chain) throws IOException;
interface Chain {
Request request();
Response proceed(Request request) throws IOException;
/**
* Returns the connection the request will be executed on. This is only available in the chains
* of network interceptors; for application interceptors this is always null.
*/
@Nullable Connection connection();
Call call();
int connectTimeoutMillis();
Chain withConnectTimeout(int timeout, TimeUnit unit);
int readTimeoutMillis();
Chain withReadTimeout(int timeout, TimeUnit unit);
int writeTimeoutMillis();
Chain withWriteTimeout(int timeout, TimeUnit unit);
}
}它是okhttp定义的一个接口类，并且okhttp提供了5个实现类，他们就是getResponseWithInterceptorChain()中添加到interceptors中的5个Interceptor，他们作用各不相同，这个以后会单独分析。除此之外我们还可以自定义自己的拦截器。
了解了拦截器的概念之后我们看下RealInterceptorChain及其proceed函数
public final class RealInterceptorChain implements Interceptor.Chain {
private final List<Interceptor> interceptors;//拦截器list
private final Transmitter transmitter;
private final @Nullable Exchange exchange;
private final int index;
private final Request request;//请求
private final Call call;
private final int connectTimeout;
private final int readTimeout;
private final int writeTimeout;
private int calls;
public RealInterceptorChain(List<Interceptor> interceptors, Transmitter transmitter,
@Nullable Exchange exchange, int index, Request request, Call call,
int connectTimeout, int readTimeout, int writeTimeout) {//构造函数
this.interceptors = interceptors;
this.transmitter = transmitter;
this.exchange = exchange;
this.index = index;
this.request = request;
this.call = call;
this.connectTimeout = connectTimeout;
this.readTimeout = readTimeout;
this.writeTimeout = writeTimeout;
}
@Override public Response proceed(Request request) throws IOException {//proceed方法实际调用同名的proceed方法
return proceed(request, transmitter, exchange);
}
public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange)
throws IOException {//1
if (index >= interceptors.size()) throw new AssertionError();
calls++;
// If we already have a stream, confirm that the incoming request will use it.
if (this.exchange != null && !this.exchange.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.exchange != null && calls > 1) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must call proceed() exactly once");
}
// 2
//Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(interceptors, transmitter, exchange,
index + 1, request, call, connectTimeout, readTimeout, writeTimeout);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);
// Confirm that the next interceptor made its required call to chain.proceed().
if (exchange != 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;
}RealInterceptorChain作为拦截器链它持有整个应用的拦截器以及网络拦截器。该类的proceed方法实际是调用了该类重载的proceed方法（注释1处）。在注释2处，调用拦截器链中的下一个拦截器。在这里new了一个RealInterceptorChain，注意这里传入的index加了1（getResponseWithInterceptorChain传入的index为0），这代表拦截器链中的下一个拦截器的index，之后根据index获取当前的拦截器并调用其intercept方法。intercept是接口Interceptor的一个方法，由具体的实现类实现，此处我们以RetryAndFollowUpInterceptor为例看下intercept方法中做了什么事情。
public final class RetryAndFollowUpInterceptor implements Interceptor {
private final OkHttpClient client;//持有的client
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();//获取传入的chain的request 此处的request是next的request
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Transmitter transmitter = realChain.transmitter();
int followUpCount = 0;
Response priorResponse = null;
while (true) {
transmitter.prepareToConnect(request);
if (transmitter.isCanceled()) {
throw new IOException("Canceled");
}
Response response;
boolean success = false;
try {
response = realChain.proceed(request, transmitter, null);//调用next的proceed方法
success = true;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
if (!recover(e.getLastConnectException(), transmitter, false, request)) {
throw e.getFirstConnectException();
}
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, transmitter, requestSendStarted, request)) throw e;
continue;
} finally {
// The network call threw an exception. Release any resources.
if (!success) {
transmitter.exchangeDoneDueToException();
}
}
// 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();
}
Exchange exchange = Internal.instance.exchange(response);
Route route = exchange != null ? exchange.connection().route() : null;
Request followUp = followUpRequest(response, route);//处理请求重定向
if (followUp == null) {
if (exchange != null && exchange.isDuplex()) {
transmitter.timeoutEarlyExit();
}
return response;//直到没有重定向之后返回respone
}
RequestBody followUpBody = followUp.body();
if (followUpBody != null && followUpBody.isOneShot()) {
return response;
}
closeQuietly(response.body());
if (transmitter.hasExchange()) {
exchange.detachWithViolence();
}
if (++followUpCount > MAX_FOLLOW_UPS) {
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
request = followUp;
priorResponse = response;
}
}
}我们看到在RetryAndFollowUpInterceptor的intercept方法中会调用传入的next（即拦截器链中当前拦截器的下一个拦截器）的proceed方法，这样就可以链式的依次调用chain中所有拦截器，每个拦截器都执行自己的任务最终返回respone。该respone通过RealCall的getResponseWithInterceptorChain返回到execute方法并最终变成我们获得的respone。至此同步请求获得了respone，最后的操作就是在RealCall的execute方法中调用finished方法
@Override public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
transmitter.timeoutEnter();
transmitter.callStart();
try {
client.dispatcher().executed(this);
return getResponseWithInterceptorChain();
} finally {
client.dispatcher().finished(this);//拿到respone后调用finish方法
}
}该方法是dispatcher提供的
void finished(RealCall call) {
finished(runningSyncCalls, call);
}
private <T> void finished(Deque<T> calls, T call) {
Runnable idleCallback;
synchronized (this) {
if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");//从同步执行队列移除该call 移除失败会抛出异常
idleCallback = this.idleCallback;
}
boolean isRunning = promoteAndExecute();//1
if (!isRunning && idleCallback != null) {
idleCallback.run();//如果isRuning为false并且idleCallback不为空就执行idleCallback。
}
}我们看下promoteAndExecute()这个方法
private boolean promoteAndExecute() {
assert (!Thread.holdsLock(this));
List<AsyncCall> executableCalls = new ArrayList<>();
boolean isRunning;
synchronized (this) {
for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {//1
AsyncCall asyncCall = i.next();
if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.
i.remove();
asyncCall.callsPerHost().incrementAndGet();
executableCalls.add(asyncCall);
runningAsyncCalls.add(asyncCall);
}
isRunning = runningCallsCount() > 0;
}
for (int i = 0, size = executableCalls.size(); i < size; i++) {//2
AsyncCall asyncCall = executableCalls.get(i);
asyncCall.executeOn(executorService());
}
return isRunning;
}
该方法主要是两个for循环，首先第一个for循环（注释1处）遍历准备就绪队列如果不为空且满足一定条件则添加到executableCalls中，但是在同步请求时准备就绪队列（readyAsyncCalls）为空，executableCalls也为空，所以两个for循环都不会进入（实际上该方法是为异步请求准备的）函数最终返回false。
此时回到dispatcher的finish（）方法，它会判断promoteAndExecute()返回值和idleCallback是否为空，如果isRuning为false并且idleCallback不为空就执行idleCallback，否则就什么都不做。
至此同步请求流程分析完毕。
异步请求
异步请求跟同步请求不同的地方就是它是调用RealCall的enqueue方法
@Override public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
transmitter.callStart();
client.dispatcher().enqueue(new AsyncCall(responseCallback));//执行dispatcher的enqueue
}在方法内部调用dispatcher的enqueue并传入AsyncCall参数，我们先来看下AsyncCall再分析异步请求流程
final class AsyncCall extends NamedRunnable {
private final Callback responseCallback;
private volatile AtomicInteger callsPerHost = new AtomicInteger(0);
AsyncCall(Callback responseCallback) {
super("OkHttp %s", redactedUrl());
this.responseCallback = responseCallback;
}
}AsyncCall是RealCall的一个内部类它继承自NamedRunnable
public abstract class NamedRunnable implements Runnable {
protected final String name;
public NamedRunnable(String format, Object... args) {
this.name = Util.format(format, args);
}
@Override public final void run() {
String oldName = Thread.currentThread().getName();
Thread.currentThread().setName(name);
try {
execute();
} finally {
Thread.currentThread().setName(oldName);
}
}
protected abstract void execute();
}
NamedRunnable其实就是一个实现Runnable的线程类。在它的run方法中调用了其提供的抽象函数execute()，execute()的实现是在AsyncCall
@Override protected void execute() {
boolean signalledCallback = false;
transmitter.timeoutEnter();
try {
Response response = getResponseWithInterceptorChain();
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);
}
}
可以看到这跟同步请求的过程是一样的先通过getResponseWithInterceptorChain()链式调用拦截链去获得resopne，之后是通过callback回调结果，最后调用finished。
分析AsyncCall之后我们可以大致猜测出异步请求的实现是通过线程去执行Call，请求的执行过程跟同步请求是一样的只不过最后是通过callbcak返回。
好了，我们来看下具体的异步请求流程，之前说到dispatcher的enqueue方法，那我们来看下这个方法都做了什么
void enqueue(AsyncCall call) {
synchronized (this) {
readyAsyncCalls.add(call);//添加到异步准备就绪队列
// Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
// the same host.
if (!call.get().forWebSocket) {
AsyncCall existingCall = findExistingCallWithHost(call.host());
if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);
}
}
promoteAndExecute();//执行请求
}很简单在方法内部先是把call添加到异步准备就绪队列然后调用了 promoteAndExecute，promoteAndExecute我们之前在同步请求分析过它内部主要是两个for循环，在同步请求时这两个for循环都是不满足条件的，那我们看下异步请求时
private boolean promoteAndExecute() {
assert (!Thread.holdsLock(this));
List<AsyncCall> executableCalls = new ArrayList<>();
boolean isRunning;
synchronized (this) {
for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {//把异步准备就绪对列中的call取出
AsyncCall asyncCall = i.next();
//判断最大请求数以及每个host请求数是否符合要求
if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.
//移除准备就绪队列中的call
i.remove();
asyncCall.callsPerHost().incrementAndGet();//记录该call的host的请求数
executableCalls.add(asyncCall);//添加到executableCalls
runningAsyncCalls.add(asyncCall);//添加到异步执行队列
}
isRunning = runningCallsCount() > 0;
}
for (int i = 0, size = executableCalls.size(); i < size; i++) {//executableCalls不为空，取出执行
AsyncCall asyncCall = executableCalls.get(i);
asyncCall.executeOn(executorService());//call实际执行
}
return isRunning;
}因为在之前已经把call添加到了异步准备就绪队列（readyAsyncCalls
），所以第一个for是可以进入的，在第一个for循环内部首先会先判断当前准备就绪队列中的call是否达到了最大请求数即最大并发请求数，然后判断单个host是否达到最大请求数。之后就是把当前的call添加到executableCalls和runningAsyncCalls两个队列中。之后进入第二个for循环，在这个for循环中依次其中取出call对象并调用其executeO函数。
注意在execute函数中传入的executorService其实是一个线程池
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;
}可以看出executorService是一个仅有非核心线程，且非核心线程数无限大的线程池。
好了简单了解了executorService我们返回来接着看下executeOn，
在executeOn中调用了executorService.execute(this)，executeOn是AsyncCall内部的函数而AsyncCall是一个线程类所以该操作会执行线程的run方法，这里具体来说就是NamedRunnable的run方法，我们知道在这个run方法中调用了execute()方法，execute()我们上面分析过了跟同步请求过程一样链式调用拦截器最终获取respone。
void executeOn(ExecutorService executorService) {
assert (!Thread.holdsLock(client.dispatcher()));
boolean success = false;
try {
executorService.execute(this);//实际执行call
success = true;
} catch (RejectedExecutionException e) {
InterruptedIOException ioException = new InterruptedIOException("executor rejected");
ioException.initCause(e);
transmitter.noMoreExchanges(ioException);
responseCallback.onFailure(RealCall.this, ioException);
} finally {
if (!success) {
client.dispatcher().finished(this); // This call is no longer running!
}
}
}至此异步请求流程也分析完了。
最后我们来总结下Okhttp的请求流程：
首先不管同步还是异步都会先初始化一个OkhttpClient之后是Request、Call。
不同之处在于同步请求把请求添加到runningSyncCalls
然后直接调用execute，链式调用拦截器获取respone并返回。异步请求则是把请求先添加到readyAsyncCalls，之后执行的时候再把其添加到runningAsyncCalls并且把请求放到子线程中取执行，即链式调用拦截器获取respone是在子线程中完成的。
还有就是不管是同步还是异步请求完成后都会把Call移除。
以上只是Okhttp最简单的流程分析，其实Okhttp还有很多值得我们学习的地方，之后我会继续更新相关内容来更深入了解Okhttp。
