责任链模式的应用：Retrofit2 + OkHttp3 配置及Interceptor原理

Okhttp的Interceptor拦截器源码解析

OkHttp的调用流程：
在调用client.newCall(request)将会调用到RealCall.newRealCall(this, request, false);方法，之后将会调用到RealCall.getResponseWithInterceptorChain()函数，而在其中将进行拦截器链的构建。

RealCall.java
核心代码：
Response result = getResponseWithInterceptorChain();
通过getResponseWithInterceptorChain完成对拦截器的构建

  @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } finally {
      client.dispatcher().finished(this);
    }
  }

getResponseWithInterceptorChain()
包含自定义拦截器
重定向与失败重试retryAndFollowUpInterceptor
用户的请求头处理，响应处理Cookie持久性策略 BridgeInterceptor

缓存请求、响应缓存的写入 (客户端设置的缓存策略)CacheInterceptor
发送请求，读取服务器的响应CallServerInterceptor
最后通过RealInterceptorChain调用proceed接口启动拦截器。

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);
  }

几个管件类和接口

Interceptor.java

public interface Interceptor {
  Response intercept(Chain chain) throws IOException;

  interface Chain {
    Request request();

    Response proceed(Request request) throws IOException;

    Connection connection();
  }
}

RealInterceptorChain.java

public final class RealInterceptorChain implements Interceptor.Chain {
  private final List<Interceptor> interceptors;
  private final StreamAllocation streamAllocation;
  private final HttpCodec httpCodec;
  private final Connection connection;
  private final int index;
  private final Request request;
  private int calls;

  public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
      HttpCodec httpCodec, Connection 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 Response proceed(Request request) throws IOException {
    return proceed(request, streamAllocation, httpCodec, connection);
  }

  public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      Connection 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;
  }

  
}

RealInterceptorChain实现了Interceptor接口,调用源头来自于RealCall的getResponseWithInterceptorChain方法

调用流程：
1.加入自定义拦截器。然后将okhttp内置的一些拦截器按照功能顺序add进list里。
2.定义整个拦截器工作的源头。new一个RealInterceptorChain，idnex为0，把初始的request传了进去,然后调用proceed获取请求的response
3.RealInterceptorChain实现了Interceptor.Chain接口，在proceed方法中获取上一个拦截器包装的Request，然后将下一个拦截器和整个Request的信息包装成RealInterceptorChain，并且会调用当前拦截器的intercept方法把下一个拦截器(RealInterceptorChain)传进去，并且获取到Response
4.CallServerInterceptor的Interceptor完成最终的网络请求并返回response