OkHttp3源码详解(三) 拦截器

1.构造Demo

首先构造一个简单的异步网络访问Demo:

OkHttpClient client = new OkHttpClient();       
Request request = new Request.Builder()
  .url("http://publicobject.com/helloworld.txt")
  .build();

client.newCall(request).enqueue(new Callback() {
  @Override
  public void onFailure(Call call, IOException e) {
    Log.d("OkHttp", "Call Failed:" + e.getMessage());
  }

  @Override
  public void onResponse(Call call, Response response) throws IOException {
    Log.d("OkHttp", "Call succeeded:" + response.message());
  }
});

2. 发起请求

OkHttpClient.newCall实际是创建一个RealCall实例：

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

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:

@Override 
public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
}

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:

[RealCall.java]    
@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);
      }
    }
  }

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, null, null, null, 0, 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 + 1, 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

https://www.jianshu.com/p/db699081bc38