首先构造一个简单的异步网络访问Demo:
1. OkHttpClient client = new OkHttpClient();
2. Request request = new Request.Builder()
3. .url("http://publicobject.com/helloworld.txt")
4. .build();
6. client.newCall(request).enqueue(new Callback() {
7. @Override
8. public void onFailure(Call call, IOException e) {
9. Log.d("OkHttp", "Call Failed:" + e.getMessage());
10. }
12. @Override
13. public void onResponse(Call call, Response response) throws IOException {
14. Log.d("OkHttp", "Call succeeded:" + response.message());
15. }
16. });
2. 发起请求
OkHttpClient.newCall实际是创建一个RealCall实例：
1. @Override
2. public Call newCall(Request request) {
3. return new RealCall(this, request, false /* for web socket */);
4. }
RealCall.enqueue实际就是讲一个RealCall放入到任务队列中，等待合适的机会执行:
1. @Override
2. public void enqueue(Callback responseCallback) {
3. synchronized (this) {
4. if (executed) throw new IllegalStateException("Already Executed");
5. executed = true;
6. }
7. captureCallStackTrace();
8. client.dispatcher().enqueue(new AsyncCall(responseCallback));
9. }
从代码中可以看到最终RealCall被转化成一个AsyncCall并被放入到任务队列中，任务队列中的分发逻辑这里先不说，相关实现会放在OkHttp源码分析——任务队列疑问进行介绍。这里只需要知道AsyncCall的excute方法最终将会被执行:
1. [RealCall.java]
2. @Override protected void execute() {
3. boolean signalledCallback = false;
4. try {
5. Response response = getResponseWithInterceptorChain();
6. if (retryAndFollowUpInterceptor.isCanceled()) {
7. signalledCallback = true;
8. responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
9. } else {
10. signalledCallback = true;
11. responseCallback.onResponse(RealCall.this, response);
12. }
13. } catch (IOException e) {
14. if (signalledCallback) {
15. // Do not signal the callback twice!
16. Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
17. } else {
18. responseCallback.onFailure(RealCall.this, e);
19. }
20. } finally {
21. client.dispatcher().finished(this);
22. }
23. }
24. }
execute方法的逻辑并不复杂，简单的说就是：
调用getResponseWithInterceptorChain获取服务器返回
通知任务分发器(client.dispatcher)该任务已结束
getResponseWithInterceptorChain构建了一个拦截器链，通过依次执行该拦截器链中的每一个拦截器最终得到服务器返回。
3. 构建拦截器链
首先来看下getResponseWithInterceptorChain的实现：
源码路径：okhttp3/RealCall.java
// 开始执行整个请求
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
// 拦截器栈
List<Interceptor> interceptors = new ArrayList<>();
// 前文说过的 普通拦截器
interceptors.addAll(client.interceptors());
// 重试拦截器，网络错误、请求失败等
interceptors.add(retryAndFollowUpInterceptor);
// 桥接拦截器，主要是重构请求头即header
interceptors.add(new BridgeInterceptor(client.cookieJar()));
// 缓存拦截器
interceptors.add(newCacheInterceptor(client.internalCache()));
// 连接拦截器，连接服务器，https包装
interceptors.add(new ConnectInterceptor(client));
// 网络拦截器，websockt不支持，同样是自定义
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
// 服务拦截器，主要是发送（write、input）、读取（read、output）数据
interceptors.add(new CallServerInterceptor(forWebSocket));
// 开启调用链
Interceptor.Chain chain = new RealInterceptorChain(
interceptors, , originalRequest);
return chain.proceed(originalRequest);
}
其逻辑大致分为两部分：
创建一系列拦截器，并将其放入一个拦截器数组中。这部分拦截器即包括用户自定义的拦截器也包括框架内部拦截器
创建一个拦截器链RealInterceptorChain,并执行拦截器链的proceed方法
接下来看下RealInterceptorChain的实现逻辑：
public final class RealInterceptorChain implements Interceptor.Chain {
private final List<Interceptor> interceptors;
private final StreamAllocation streamAllocation;
private final HttpCodec httpCodec;
private final RealConnection connection;
private final int index;
private final Request request;
private int calls;
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;
}
@Override public Connection connection() {
return connection;
}
public StreamAllocation streamAllocation() {
return streamAllocation;
}
public HttpCodec httpStream() {
return httpCodec;
}
@Override public Request request() {
return request;
}
@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
......
// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + , request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);
......
return response;
}
}
在proceed方法中的核心代码可以看到，proceed实际上也做了两件事：
创建下一个拦截链。传入index + 1使得下一个拦截器链只能从下一个拦截器开始访问
执行索引为index的intercept方法，并将下一个拦截器链传入该方法。
原文链接：https://www.bbsmax.com/A/MAzAEmQMz9/
