Okhttp网络访问过程解析

无聊啊无聊！！！

本文仅仅记录了网络访问的过程

1. 发送请求

1.1  Call 接口 

Call接口对http请求的抽象封装。而它的实现类是RealCall

public interface Call extends Cloneable {
 
  Request request();
  //同步执行
  Response execute() throws IOException;
  //异步执行
  void enqueue(Callback responseCallback);
  //是否执行了
  boolean isExecuted();
  //是否取消了
  boolean isCanceled();
  Call clone();
  interface Factory {
    Call newCall(Request request);
  }
}

1.2 例子

 val okHttpClient = OkHttpClient.Builder().build()
        val request = Request.Builder()
                .url("")
                .build()
        okHttpClient.newCall(request).enqueue(object :Callback{
            override fun onFailure(call: Call, e: IOException) {}

            override fun onResponse(call: Call, response: Response) {}
        })

1.2.1  OkhttpClient对象的初始化

 public Builder() {
      dispatcher = new Dispatcher();
      protocols = DEFAULT_PROTOCOLS;
      connectionSpecs = DEFAULT_CONNECTION_SPECS;
      eventListenerFactory = EventListener.factory(EventListener.NONE);
      proxySelector = ProxySelector.getDefault();
      cookieJar = CookieJar.NO_COOKIES;
      socketFactory = SocketFactory.getDefault();
      hostnameVerifier = OkHostnameVerifier.INSTANCE;
      certificatePinner = CertificatePinner.DEFAULT;
      proxyAuthenticator = Authenticator.NONE;
      authenticator = Authenticator.NONE;
      connectionPool = new ConnectionPool();
      dns = Dns.SYSTEM;
      followSslRedirects = true;
      followRedirects = true;
      retryOnConnectionFailure = true;
      connectTimeout = 10_000;
      readTimeout = 10_000;
      writeTimeout = 10_000;
      pingInterval = 0;
    }

public OkHttpClient build() {
      return new OkHttpClient(this);
    }

OkhttpClient通过建造者模式初始化了自己需要的一些属性。其中最重要的是dispatcher 调度器。

 

1.2.2  Request 初始化

public Builder() {
      this.method = "GET";
      this.headers = new Headers.Builder();
    }

public Request build() {
      if (url == null) throw new IllegalStateException("url == null");
      return new Request(this);
    }

也是通过建造者模式初始化属性。也可以通过方法进行配置。

 

1.2.3 okHttpClient.newCall(request).enqueue（callback）

创建了一个RealCall对象 ，然后调用RealCall的enqueue方法。RealCall类实现了Call接口，而Call接口是抽象了http的请求的行为。

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

@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);
    }
  }


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

可以看到RealCall中不管是execute（）同步执行  还是 enqueue异步执行都回需要用到dispatcher 这个对象。

不同的是execute（）直接调用了 Response result = getResponseWithInterceptorChain() 请求数据，并返回请求后的结果。

而enqueue（）方法则是 将回调接口封装到一个AsyncCall类中，然后再执行AcyncCall对象。

 

2.请求调度

请求调度就需要用到Dispatcher类了。它在Okhttpclient初始化的时候已经初始化了。

public final class Dispatcher {
  private int maxRequests = 64;
  private int maxRequestsPerHost = 5;
  private @Nullable Runnable idleCallback;

  /** Executes calls. Created lazily. */
  private @Nullable ExecutorService executorService;

  /** Ready async calls in the order they'll be run. */
  private final Deque readyAsyncCalls = new ArrayDeque<>();

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque runningAsyncCalls = new ArrayDeque<>();

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque runningSyncCalls = new ArrayDeque<>();

在调度器中，允许最大请求数为64，允许的最大主机数为5。同时它维护了三个队列。分别是准备执行的异步请求队列，正在执行的异步请求队列 和 正在执行的同步请求队列。

synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }

当执行同步请求时，dispather调度器直接将RealCall添加到正在执行的同步请求队列中。

synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      executorService().execute(call);
    } else {
      readyAsyncCalls.add(call);
    }
  }

当执行异步请求时，会先判断 正在运行的异步请求队列个数小于允许的最大请求数64 同时 主机数不能大于5个。满足这两个条件则 将当前请求加入到 正在运行的异步队列中，并且开始执行请求。否则将请求添加到 准备执行的异步请求队列中。executorService().execute(call);线程池执行AsyncCall对象

 

AsyncCall类

final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;

    AsyncCall(Callback responseCallback) {
      super("OkHttp %s", redactedUrl());
      this.responseCallback = responseCallback;
    }

    @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);
      }
    }
  }

AsyncCall 继承了NamedRunnable类，而NamedRunnable 又实现了Runnable接口。所以可将AsyncCall类看作是一个runnable。

在AsyncCall的execute（）方法中 调用了 Response response = getResponseWithInterceptorChain(); 获取请求结果。

其实不管是同步还是异步都是通过getResponseWithInterceptorChain()方法获取请求结果的。

 

3. 拦截器 

在okHttp中每个拦截器都会负责不同的功能。

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List 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);
  }

在这个方法中 依次添加拦截器。首先是添加开发者自定义的拦截器，然后再添加RetryAndFollowUpInterceptor，BridgeInterceptor，CacheInterceptor，ConnectInterceptor，CallServerInterceptor组成拦截器链。然后创建RealInterceptorChain 类，执行process方法。

public final class RealInterceptorChain implements Interceptor.Chain {

    public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection connection) throws IOException {
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);
    // Confirm that the next interceptor made its required call to chain.proceed().
    if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
      throw new IllegalStateException("network interceptor " + interceptor
          + " must call proceed() exactly once");
    }
    if (response == null) {
      throw new NullPointerException("interceptor " + interceptor + " returned null");
    }
    return response;
  }   
}

在process方法中 会取出下一个interceptor 执行 process方法。然后再一层一层的将结果往上传递。

所有的拦截器的执行过程都是：

@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    //1 Request阶段，该拦截器在Request阶段负责做的事情

    //2 调用RealInterceptorChain.proceed()，其实是在递归调用下一个拦截器的intercept()方法
    response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);

    //3 Response阶段，完成了该拦截器在Response阶段负责做的事情，然后返回到上一层的拦截器。
    return response;     
    }
  }

 

3.2 RetryAndFollowUpInterceptor 失败重试以及重定向

public final class RetryAndFollowUpInterceptor implements Interceptor {

  //最大重定向次数
  private static final int MAX_FOLLOW_UPS = 20;

  
 @Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();

    streamAllocation = new StreamAllocation(
        client.connectionPool(), createAddress(request.url()), callStackTrace);
    //重定向次数为0
    int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
      //当前的请求是否取消
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response = null;
      boolean releaseConnection = true;
      try {
        //获取下一个拦截器拿到请求结果
        response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
        //出现异常，检测链接是否还可以继续使用
        if (!recover(e.getLastConnectException(), false, request)) {
          throw e.getLastConnectException();
        }
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        //与服务器建立链接失败
        boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
        if (!recover(e, requestSendStarted, request)) throw e;
        releaseConnection = false;
        continue;
      } finally {
        //检测到其他未知异常，则释放连接和资源
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }
      //根据不同的响应码处理请求。
      Request followUp = followUpRequest(response);
      //当followUp为空时不做重定向，释放链接
      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        //返回请求结果
        return response;
      }

      closeQuietly(response.body());
      //重定向次数不能大于20次
      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }

      if (!sameConnection(response, followUp.url())) {
        streamAllocation.release();
        streamAllocation = new StreamAllocation(
            client.connectionPool(), createAddress(followUp.url()), callStackTrace);
      } else if (streamAllocation.codec() != null) {
        throw new IllegalStateException("Closing the body of " + response
            + " didn't close its backing stream. Bad interceptor?");
      }

      request = followUp;
      priorResponse = response;
    }
  }


}

 RetryAndFollowUpInterceptor的作用是处理一些连接异常和重定向问题。

 

3.3  BridgeInterceptor

public final class BridgeInterceptor implements Interceptor {

  @Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();
    // 获取用户的请求，添加或删除操作，将用户的请求转化为服务器的请求。
    RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }

      long contentLength = body.contentLength();
      if (contentLength != -1) {
        requestBuilder.header("Content-Length", Long.toString(contentLength));
        requestBuilder.removeHeader("Transfer-Encoding");
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked");
        requestBuilder.removeHeader("Content-Length");
      }
    }

    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }

    if (userRequest.header("Connection") == null) {
      requestBuilder.header("Connection", "Keep-Alive");
    }

    // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }

    List cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }
    
    //获取请求结果后，转化为对用户友好的响应 
    Response networkResponse = chain.proceed(requestBuilder.build());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);

    if (transparentGzip
        && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
        && HttpHeaders.hasBody(networkResponse)) {
      GzipSource responseBody = new GzipSource(networkResponse.body().source());
      Headers strippedHeaders = networkResponse.headers().newBuilder()
          .removeAll("Content-Encoding")
          .removeAll("Content-Length")
          .build();
      responseBuilder.headers(strippedHeaders);
      responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
    }
    //返回请求结果
    return responseBuilder.build();
  }
}

BridgeInterceptor的作用是 它将用户的请求转化为发送给服务器的请求。并且对服务器传过来的结果，转化为对用户友好的响应。

 

3.4 CacheInterceptor 缓存拦截器

public final class CacheInterceptor implements Interceptor {

  @Override public Response intercept(Chain chain) throws IOException {
    //获取缓存信息
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    //当没有网络时，同时也没有缓存时，返回504 
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    //当没有网络，但有缓存时，返回缓存
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      //执行下一个拦截器，并获取请求结果
      networkResponse = chain.proceed(networkRequest);
    } finally {
      //发生IO异常时，释放掉缓存
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    //当缓存不为空，并且请求的结果没有改变时，直接返回缓存
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }
    
    //取出结果
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();
    //添加到缓存 
    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }
    //返回请求结果
    return response;
  }



}

 

3.4 ConnectInterceptor 连接拦截器

public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;


  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();

    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }
}

ConnectInterceptor拦截器的作用是与服务器做连接。具体如果连接，可以在StreamAllocation类中实现。

 

3.5 CallServerInterceptor

public final class CallServerInterceptor implements Interceptor {

  
  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();
    //写入请求头
    httpCodec.writeRequestHeaders(request);

    Response.Builder responseBuilder = null;
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
      // Continue" response before transmitting the request body. If we don't get that, return what
      // we did get (such as a 4xx response) without ever transmitting the request body.
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        httpCodec.flushRequest();
        responseBuilder = httpCodec.readResponseHeaders(true);
      }
      //写入请求体
      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
      } else if (!connection.isMultiplexed()) {
        // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection from
        // being reused. Otherwise we're still obligated to transmit the request body to leave the
        // connection in a consistent state.
        streamAllocation.noNewStreams();
      }
    }

    httpCodec.finishRequest();
    
    //读取响应头
    if (responseBuilder == null) {
      responseBuilder = httpCodec.readResponseHeaders(false);
    }

    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();
    //读取响应体
    int code = response.code();
    if (forWebSocket && code == 101) {
      // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      streamAllocation.noNewStreams();
    }

    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }
    //返回结果
    return response;
  }

}

CallServerInterceptor负责从服务器中读取响应的数据。