开局一张图，照着时序图，一步步理清
Okhttp主流程.png
1、构建OKHttpClient
OKHttpClient使用了Builder模式构造实例。可以对OKHttpClient进行一系列配置，譬如添加拦截器，设置超时时间等等
OkHttpClient.Builder builder=new OkHttpClient.Builder();
//设置拦截器、超时时间等等
builder.addInterceptor();
builder.connectTimeout();
OkHttpClient client=builder.build();
2、构建Request
Request对象同样也是使用了Buidler模式，可以设置请求地址、请求头、请求方式等等
public Request build() {
if (url == null) throw new IllegalStateException("url == null");
return new Request(this);
}
3、发起请求
当okhttpClient和request都已经准备就绪后，就可以开始发起请求了。
首先调用client的newCall方法,进到这个方法里面
Call call=client.newCall(request);
@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
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;
}
这个过程其实就是构造了一个RealCall的对象，并将之返回。后面我们使用这个对象进行异步调用。
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
}
@Override
public void onResponse(Call call, Response response) throws IOException {
}
});
@Override public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
eventListener.callStart(this);
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
关键代码是调用client的dispatcher()方法返回dispatcher对象，并调用它的enqueue方法。注意 在这个方法参数里 构造一个AsyncCall对象。那么这个对象是什么，我们进到里面去看
final class AsyncCall extends NamedRunnable {
private final Callback responseCallback;
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 {
eventListener.callFailed(RealCall.this, e);
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
}
Asyncall继承了NamedRunnable，但NameRunnable实现了Runnable接口。从这里我们可以猜想到，Runnable对象是用来作什么的，大概率就是用来子线程调用的。所以我们先看下Dispatcher的enqueue方法。
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
如果正在请求的个数没有超过maxRequests（64个）并且所有正在请求的Host个数没有超过maxRequestsPerHost（5个）时，并将Asyncall加入到请求队列中， 再执行executorService().execute(call)方法。否则加入等待队列。
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;
}
到这里就很明显了，这是一个线程池，通过线程池进行请求。那么最终应该会执行Runnable的run方法。
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);
}
}
NameRunnable重写了run方法，并最终调用了AsyncCall的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 {
eventListener.callFailed(RealCall.this, e);
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
Response response = getResponseWithInterceptorChain() 看到这行代码，返回了请求结果Response，再通过Callback回调出去。
至于getResponseWithInterceptorChain里面到底做了 下篇再分析