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OKHttp涉及到拦截器大概的执行步骤为：

1.通过newCall生成RealCall对象

具体代码如下：

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

2.调用Call的execute方法

当然这也可以是执行异步任务的enqueue方法，我们这里主要分析execute方法，在这个方法中调用getResponseWithInterceptorChain()从而调用拦截器。
具体代码如下：

/*** 同步执行网络请求并返回响应** @return 返回响应对象* @throws IOException 如果在执行请求过程中发生I/O错误*/@Override public Response execute() throws IOException {// 同步锁，确保同一时间只有一个线程执行该方法synchronized (this) {// 检查是否已经执行过，如果已经执行过则抛出异常if (executed) throw new IllegalStateException("Already Executed");// 标记为已执行executed = true;}// 捕获调用栈信息，用于调试和错误追踪captureCallStackTrace();try {// 将当前请求添加到调度器中，以便管理和执行client.dispatcher().executed(this);// 通过拦截器链获取响应Response result = getResponseWithInterceptorChain();// 如果响应为空，则抛出异常if (result == null) throw new IOException("Canceled");// 返回响应return result;} finally {// 请求完成后，从调度器中移除当前请求client.dispatcher().finished(this);}}

3.调用拦截器，处理相关拦截操作（责任链模式）

  /*** 通过拦截器链获取响应** @return 返回响应对象* @throws IOException 如果在获取响应过程中发生I/O错误*/Response getResponseWithInterceptorChain() throws IOException {// 构建一个完整的拦截器栈List<Interceptor> interceptors = new ArrayList<>();// 添加应用程序提供的拦截器interceptors.addAll(client.interceptors());// 添加重试和重定向拦截器interceptors.add(retryAndFollowUpInterceptor);// 添加桥接拦截器，用于处理请求和响应的头部信息interceptors.add(new BridgeInterceptor(client.cookieJar()));// 添加缓存拦截器，用于处理缓存interceptors.add(new CacheInterceptor(client.internalCache()));// 添加连接拦截器，用于建立网络连接interceptors.add(new ConnectInterceptor(client));// 如果不是WebSocket请求，添加网络拦截器if (!forWebSocket) {interceptors.addAll(client.networkInterceptors());}// 添加调用服务器拦截器，用于发送请求和接收响应interceptors.add(new CallServerInterceptor(forWebSocket));// 创建一个拦截器链Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest);// 执行拦截器链并返回响应return chain.proceed(originalRequest);}

4.拦截器执行，链式调用

核心类为：RealInterceptorChain

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;}// 这个是拦截器中执行的proceed方法@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.// 传入下一个拦截器的索引和拦截器List，生成RealInterceptorChain对象RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec, connection, index + 1, request);// 根据当前索引获取拦截器Interceptor interceptor = interceptors.get(index);// 我们的拦截器实现了intercept(chain)方法，通过intercept实现了调用拦截器链的操作Response response = interceptor.intercept(next);... return response;}
}

具体我们实现拦截器的简单例子如下：

public class CustomHeaderInterceptor implements Interceptor {@Overridepublic Response intercept(Chain next) throws IOException {Request originalRequest = next.request();// 添加自定义头部信息Request newRequest = originalRequest.newBuilder().header("Custom-Header", "Custom-Value").build();// 继续处理请求，我们可以看到，Response response = interceptor.intercept(next)传下来的next在这里执行了proceed方法。从而实现了链式调用return next.proceed(newRequest);}
}