死磕Android_OkHttp3 原理探究

清疚 2021-11-30 00:34 322阅读 0赞

## 1. 前言 ##

Okhttp3 俨然已成为Android的主流网络请求开源框架,它的设计非常巧妙,而且非常灵活,功能强大.它有如下默认特性:

*  支持HTTP/2，允许所有同一个主机地址的请求共享同一个Socket连接
 *  连接池减少请求延时
 *  透明的GZIP压缩减少响应数据的大小
 *  缓存响应内容，避免一些完全重复的请求

现在的Android项目基本上都是以OkHttp来进行高效的网络请求.当然,在使用的同时我们需要去研究它的底层实现,从而让我们写出更好的代码.

## 2. 基本使用 ##

这里简单介绍2种,GET和POST.推荐让 OkHttpClient 保持单例，用同一个 OkHttpClient 实例来执行你的所有请求，因为每一个 OkHttpClient 实例都拥有自己的连接池和线程池，重用这些资源可以减少延时和节省资源，如果为每个请求创建一个 OkHttpClient实例，显然就是一种资源的浪费。

### 1. 使用GET方式请求 ###

public static final String URL = "http://www.baidu.com";
    private OkHttpClient mOkHttpClient = new OkHttpClient();
    private final Request mRequest = new Request.Builder().url(URL).build();
    
    @Override
    public void request() { 
        mOkHttpClient.newCall(mRequest)
                //异步请求
                .enqueue(new Callback() { 
                    @Override
                    public void onFailure(Call call, IOException e) { 
                        e.printStackTrace();
                    }
    
                    @Override
                    public void onResponse(Call call, Response response) throws IOException { 
                        Log.w(TAG, "onResponse: " + response.body().string());
                    }
                });
    }

### 2. 使用POST请求 ###

public static final String URL = "https://api.github.com/markdown/raw";
    private OkHttpClient mOkHttpClient = new OkHttpClient.Builder()
            .build();
    MediaType mMediaType = MediaType.parse("text/x-markdown; charset=utf-8");
    String requestBody = "I am xfhy.";
    private final Request mRequest = new Request.Builder()
            .url(URL)
            .post(RequestBody.create(mMediaType, requestBody))
            .build();
    
    @Override
    public void request() { 
        //每一个Call（其实现是RealCall）只能执行一次，否则会报异常
        mOkHttpClient.newCall(mRequest).enqueue(new Callback() { 
            @Override
            public void onFailure(Call call, IOException e) { 
                e.printStackTrace();
            }
    
            @Override
            public void onResponse(Call call, Response response) throws IOException { 
                Log.w(TAG, "onResponse: " + response.body().string());
            }
        });
    }

## 3. interceptor 拦截器-精髓 ##

使用OkHttp3请求网络还是比较简单,而且异步请求也比较轻松.

### 3.1 构建OkHttpClient ###

正如名字所描述的,OkHttpClient像是一个请求网络的客户端.它内部有很多很多的配置信息(支持协议、任务调度器、连接池、超时时间等),通过构造器模式初始化的这些配置信息.(这里穿插一下,正如你所看到的这种一个类里面很多很多属性需要初始化的,一般就用构造器模式)

public OkHttpClient() { 
        this(new Builder());
    }
    
    public Builder() { 
      //任务调度器
      dispatcher = new Dispatcher();
      //支持的协议
      protocols = DEFAULT_PROTOCOLS;
      connectionSpecs = DEFAULT_CONNECTION_SPECS;
      eventListenerFactory = EventListener.factory(EventListener.NONE);
      proxySelector = ProxySelector.getDefault();
      if (proxySelector == null) { 
        proxySelector = new NullProxySelector();
      }
      cookieJar = CookieJar.NO_COOKIES;
      socketFactory = SocketFactory.getDefault();
      hostnameVerifier = OkHostnameVerifier.INSTANCE;
      certificatePinner = CertificatePinner.DEFAULT;
      proxyAuthenticator = Authenticator.NONE;
      authenticator = Authenticator.NONE;
      //连接池
      connectionPool = new ConnectionPool();
      dns = Dns.SYSTEM;
      followSslRedirects = true;
      followRedirects = true;
      retryOnConnectionFailure = true;
      callTimeout = 0;
      //超时时间
      connectTimeout = 10_000;
      readTimeout = 10_000;
      writeTimeout = 10_000;
      pingInterval = 0;
    }

其中Dispatcher有一个线程池,用于执行异步的请求.并且内部还维护了3个双向任务队列,分别是:准备异步执行的任务队列、正在异步执行的任务队列、正在同步执行的任务队列.

/** Executes calls. Created lazily. */
    //这个线程池是需要的时候才会被初始化
    private @Nullable ExecutorService executorService;
    
    /** Ready async calls in the order they'll be run. */
    private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
    
    /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
    private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
    
    /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
    private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
    
    public synchronized ExecutorService executorService() {
        if (executorService == null) {
           //注意,该线程池没有核心线程,线程数量可以是Integer.MAX_VALUE个(相当于没有限制),超过60秒没干事就要被回收
          executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
              new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
        }
        return executorService;
    }

### 3.2 构建Request ###

Request感觉就是一个请求的封装.它里面封装了url、method、header、body,该有的都有了.而且它也是用构造器模式来构建的,它默认的请求方式是GET

public final class Request { 
      final HttpUrl url;
      final String method;
      final Headers headers;
      final @Nullable RequestBody body;
      final Map<Class<?>, Object> tags;
    
    
     public Builder() { 
      this.method = "GET";
      this.headers = new Headers.Builder();
     }
        
      public static class Builder { 
        @Nullable HttpUrl url;
        String method;
        Headers.Builder headers;
        @Nullable RequestBody body;
    
        /** A mutable map of tags, or an immutable empty map if we don't have any. */
        Map<Class<?>, Object> tags = Collections.emptyMap();
    
        public Builder() { 
          this.method = "GET";
          this.headers = new Headers.Builder();
        }
    }

### 3.3 开始请求 ###

我们进入mOkHttpClient的newCall方法,它构造的是一个Call对象,实际上是一个RealCall

/** * Prepares the {@code request} to be executed at some point in the future. */
    @Override public Call newCall(Request request) { 
        return RealCall.newRealCall(this, request, false /* for web socket */);
    }

**RealCall\#enqueue(Callback)**

所以示例中的enqueue实际上是RealCall中的方法

@Override public void enqueue(Callback responseCallback) { 
        ......
        //将AsyncCall传入任务调度器,
        client.dispatcher().enqueue(new AsyncCall(responseCallback));
    }

将AsyncCall(这个我们稍后再说)传入任务调度器,任务任务调度器会将其存入待执行的请求队列(上面提到的readyAsyncCalls)中,然后条件允许的话再加入到运行中的请求队列(runningAsyncCalls)中,然后将这个请求放到任务调度器中的线程池中进行消费.下面是详细代码

----Dispatcher#enqueue(AsyncCall)
    void enqueue(AsyncCall call) {
        synchronized (this) {
          readyAsyncCalls.add(call);
    
          // Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
          // the same host.
          if (!call.get().forWebSocket) {
            AsyncCall existingCall = findExistingCallWithHost(call.host());
            if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);
          }
        }
        promoteAndExecute();
      }
    
    private boolean promoteAndExecute() {
        List<AsyncCall> executableCalls = new ArrayList<>();
        boolean isRunning;
        synchronized (this) {
          //从待执行队列中取出来
          for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
            AsyncCall asyncCall = i.next();
            //如果正在执行的任务>=64  那么就算了,先缓一缓
            if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
            if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.
        
            i.remove();
            asyncCall.callsPerHost().incrementAndGet();
            executableCalls.add(asyncCall);
            //加入到运行队列中
            runningAsyncCalls.add(asyncCall);
          }
          isRunning = runningCallsCount() > 0;
        }
        
        for (int i = 0, size = executableCalls.size(); i < size; i++) {
          AsyncCall asyncCall = executableCalls.get(i);
          //一个个地开始执行    executorService方法是获取线程池
          asyncCall.executeOn(executorService());
        }
        
        return isRunning;
    }
    
    //获取线程池代码
    public synchronized ExecutorService executorService() {
        if (executorService == null) {
          executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
              new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
        }
        return executorService;
      }

上面我们提到了很多次AsyncCall,它其实是一个RealCall的非静态内部类,所以能直接访问到RealCall的属性啥的,方便.同时,AsyncCall继承自NamedRunnable,NamedRunnable实现了NamedRunnable.

public abstract class NamedRunnable implements Runnable { 
      protected final String name;
    
      public NamedRunnable(String format, Object... args) { 
        this.name = Util.format(format, args);
      }
    
      @Override public final void run() { 
        String oldName = Thread.currentThread().getName();
        Thread.currentThread().setName(name);
        try { 
          execute();
        } finally { 
          Thread.currentThread().setName(oldName);
        }
      }
    
      protected abstract void execute();
    }

NamedRunnable中使用了模板方法模式,子类必须实现execute方法,并且将逻辑放在execute中.并且NamedRunnable中还设置了自己线程的名字,实属方便管理.

上面的任务调度器中执行的AsyncCall,相当于就是执行的AsyncCall的execute的逻辑

@Override protected void execute() { 
      boolean signalledCallback = false;
      transmitter.timeoutEnter();
      try { 
        
        //-----------------------重点代码 华丽的分割线围起来---------------------------------
        //1. 通过拦截器链条,获取最终的网络请求结果
        Response response = getResponseWithInterceptorChain();
        //2. 标记已执行 不能再执行第二次了
        signalledCallback = true;
        //3. 将结果回调给调用处
        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);
      }
    }

开始了,开始了,重点来了,通过getResponseWithInterceptorChain方法这条拦截器链路可以获取到网络请求的结果.然后我们通过CallBack接口回调回调用处.

![在这里插入图片描述][20190722224150465.png]

在开始之前,大家先看两张图,这张图是整个拦截器的流程,也是OkHttp的精华,设计之巧妙.

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3hmaHlf_size_16_color_FFFFFF_t_70]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3hmaHlf_size_16_color_FFFFFF_t_70 1]

从上面的代码也可以看到,getResponseWithInterceptorChain方法是获取到了网络请求的最终数据的.紧接着根据我画了两张图,这两张图主要是描绘了从getResponseWithInterceptorChain进去之后发生的事,它内部会串行的执行一些特定的拦截器(interceptors),每个拦截器负责一个特殊的职责.最后那个拦截器负责请求服务器,然后服务器返回了数据再根据这个拦截器的顺序逆序返回回去,最终就得到了网络数据.

下面先简单介绍一下这些拦截器,方便后面的源码梳理

*  **RetryAndFollowUpInterceptor** 负责请求的重定向操作，用于处理网络请求中，请求失败后的重试机制。
 *  **BridgeInterceptor** 主要是添加一些header
 *  **CacheInterceptor** 负责缓存
 *  **ConnectInterceptor** 打开与目标服务器的连接
 *  **CallServerInterceptor** 最后一个拦截器,负责请求网络

### 3.4 进入拦截器调用链 ###

有了上面的简单介绍,我们直接进入getResponseWithInterceptorChain方法一探究竟.

Response getResponseWithInterceptorChain() throws IOException { 
        // Build a full stack of interceptors.
        //用来盛放所有的拦截器的
        List<Interceptor> interceptors = new ArrayList<>();
        
        //1. 添加用户定义的拦截器
        interceptors.addAll(client.interceptors());
        //2. 添加一些OkHttp自带的拦截器
        interceptors.add(new RetryAndFollowUpInterceptor(client));
        interceptors.add(new BridgeInterceptor(client.cookieJar()));
        interceptors.add(new CacheInterceptor(client.internalCache()));
        interceptors.add(new ConnectInterceptor(client));
        
        if (!forWebSocket) { 
          //这里还有一个网络拦截器,也是可以用户自定义的
          interceptors.addAll(client.networkInterceptors());
        }
        
        //最终访问服务器的拦截器
        interceptors.add(new CallServerInterceptor(forWebSocket));
        
        //3. 将拦截器,当前拦截器索引等传入Interceptor.Chain
        Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
            originalRequest, this, client.connectTimeoutMillis(),
            client.readTimeoutMillis(), client.writeTimeoutMillis());
        
        boolean calledNoMoreExchanges = false;
        try { 
          //4. 请求访问下一个拦截器
          Response response = chain.proceed(originalRequest);
          if (transmitter.isCanceled()) { 
            closeQuietly(response);
            throw new IOException("Canceled");
          }
          return response;
        } catch (IOException e) { 
          calledNoMoreExchanges = true;
          throw transmitter.noMoreExchanges(e);
        } finally { 
          if (!calledNoMoreExchanges) { 
            transmitter.noMoreExchanges(null);
          }
        }
    }

可以看到,OkHttp这个拦截器链的大体流程,最开始是用户自定义的拦截器,然后才是OkHttp自己默认的拦截器(需要注意的是,最后一个拦截器是CallServerInterceptor).然后将拦截器集合和当前拦截器的索引等数据传入RealInterceptorChain,调用RealInterceptorChain对象的proceed,并最终得到执行结果.看来逻辑在RealInterceptorChain的proceed方法内部

public final class RealInterceptorChain implements Interceptor.Chain { 
        private final List<Interceptor> interceptors;
        private final Transmitter transmitter;
        private final @Nullable Exchange exchange;
        private final int index;
        private final Request request;
        private final Call call;
        private final int connectTimeout;
        private final int readTimeout;
        private final int writeTimeout;
        private int calls;
        
        public RealInterceptorChain(List<Interceptor> interceptors, Transmitter transmitter,
        @Nullable Exchange exchange, int index, Request request, Call call,
        int connectTimeout, int readTimeout, int writeTimeout) { 
            this.interceptors = interceptors;
            this.transmitter = transmitter;
            this.exchange = exchange;
            this.index = index;
            this.request = request;
            this.call = call;
            this.connectTimeout = connectTimeout;
            this.readTimeout = readTimeout;
            this.writeTimeout = writeTimeout;
        }
    
        @Override 
        public Response proceed(Request request) throws IOException { 
            return proceed(request, transmitter, exchange);
        }
        
        public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange)
          throws IOException { 
            calls++;
            
            // Call the next interceptor in the chain.
            //调用下一个interceptor.注意到,这里的index索引+1了的,所以是下一个interceptor
            RealInterceptorChain next = new RealInterceptorChain(interceptors, transmitter, exchange,
                index + 1, request, call, connectTimeout, readTimeout, writeTimeout);
            //当前interceptor
            Interceptor interceptor = interceptors.get(index);
            //调用interceptor的intercept方法
            Response response = interceptor.intercept(next);
            
            return response;
        }
    }

在proceed方法里面主要是将下一个拦截器的RealInterceptorChain构建出来,然后传入当前拦截器的intercept方法里面,方便在intercept方法里面执行下一个RealInterceptorChain的proceed方法.intercept方法返回的是获取数据之后的Response.

下面进入intercept方法内部,Interceptor其实是一个接口,然后所有的拦截器都实现了这个接口Interceptor.如果没有用户自定义的拦截器,那么第一个拦截器就是RetryAndFollowUpInterceptor

RetryAndFollowUpInterceptor\#intercept

@Override public Response intercept(Chain chain) throws IOException { 
        Request request = chain.request();
        RealInterceptorChain realChain = (RealInterceptorChain) chain;
        Transmitter transmitter = realChain.transmitter();
    
        int followUpCount = 0;
        Response priorResponse = null;
        
        //死循环 直到达到重定向的最大次数
        while (true) { 
          //准备一个流来承载request,如果存在则复用
          transmitter.prepareToConnect(request);
    
          if (transmitter.isCanceled()) { 
            throw new IOException("Canceled");
          }
    
          Response response;
          boolean success = false;
          try { 
            //调用下一个拦截器 
            response = realChain.proceed(request, transmitter, null);
            success = true;
          } catch (RouteException e) { 
          
            //下面是一些失败,然后又重新请求的代码
          
            // The attempt to connect via a route failed. The request will not have been sent.
            if (!recover(e.getLastConnectException(), transmitter, false, request)) { 
              throw e.getFirstConnectException();
            }
            continue;
          } catch (IOException e) { 
            // An attempt to communicate with a server failed. The request may have been sent.
            boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
            if (!recover(e, transmitter, requestSendStarted, request)) throw e;
            continue;
          } finally { 
            // The network call threw an exception. Release any resources.
            if (!success) { 
              transmitter.exchangeDoneDueToException();
            }
          }
    
          // 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();
          }
    
          Exchange exchange = Internal.instance.exchange(response);
          Route route = exchange != null ? exchange.connection().route() : null;
          Request followUp = followUpRequest(response, route);
    
          if (followUp == null) { 
            if (exchange != null && exchange.isDuplex()) { 
              transmitter.timeoutEarlyExit();
            }
            return response;
          }
    
          RequestBody followUpBody = followUp.body();
          if (followUpBody != null && followUpBody.isOneShot()) { 
            return response;
          }
    
          closeQuietly(response.body());
          if (transmitter.hasExchange()) { 
            exchange.detachWithViolence();
          }
    
          if (++followUpCount > MAX_FOLLOW_UPS) { 
            throw new ProtocolException("Too many follow-up requests: " + followUpCount);
          }
    
          request = followUp;
          priorResponse = response;
        }
      }

RetryAndFollowUpInterceptor主要是负责错误处理,以及重定向.当然重定向是有最大次数的,OkHttp规定是20次.

RetryAndFollowUpInterceptor执行proceed方法是来到了BridgeInterceptor,它是一个连接桥.添加了很多header

@Override public Response intercept(Chain chain) throws IOException { 
        Request userRequest = chain.request();
        Request.Builder requestBuilder = userRequest.newBuilder();
        
        //进行header的包装
        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");
        }
    
        //添加Accept-Encoding：gzip
        // 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");
        }
    
        //创建OkhttpClient配置的cookieJar
        List<Cookie> 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());
        }
        
        //执行下一个Interceptor
        Response networkResponse = chain.proceed(requestBuilder.build());
    
        HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
    
        Response.Builder responseBuilder = networkResponse.newBuilder()
            .request(userRequest);
    
        //先判断服务器是否支持gzip压缩,支持则交给Okio处理
        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);
          String contentType = networkResponse.header("Content-Type");
          responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
        }
        
        //最后将结果返回
        return responseBuilder.build();
      }

BridgeInterceptor就跟它的名字那样，它是一个连接桥.它负责把用户构造的请求转换成发送给服务器的请求,就是添加了不少的header,其中还有gzip等.

BridgeInterceptor的下一个拦截器是CacheInterceptor

@Override public Response intercept(Chain chain) throws IOException { 
        如果配置了缓存：优先从缓存中读取Response
        Response cacheCandidate = cache != null
            ? cache.get(chain.request())
            : null;
    
        long now = System.currentTimeMillis();
        
        //缓存策略，该策略通过某种规则来判断缓存是否有效
        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.
        }
    
        // If we're forbidden from using the network and the cache is insufficient, fail.
        //如果根据缓存策略strategy禁止使用网络，并且缓存无效，直接返回空的Response
        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();
        }
    
        // If we don't need the network, we're done.
        //如果根据缓存策略strategy禁止使用网络，且有缓存则直接使用缓存
        if (networkRequest == null) { 
          return cacheResponse.newBuilder()
              .cacheResponse(stripBody(cacheResponse))
              .build();
        }
    
        //需要网络
        Response networkResponse = null;
        try { 
          //执行下一个拦截器,发起网路请求
          networkResponse = chain.proceed(networkRequest);
        } finally { 
          // If we're crashing on I/O or otherwise, don't leak the cache body.
          if (networkResponse == null && cacheCandidate != null) { 
            closeQuietly(cacheCandidate.body());
          }
        }
    
        //本地有缓存，
        // If we have a cache response too, then we're doing a conditional get.
        if (cacheResponse != null) { 
            //并且服务器返回304状态码（说明缓存还没过期或服务器资源没修改）
          if (networkResponse.code() == HTTP_NOT_MODIFIED) { 
            //使用缓存数据
            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());
          }
        }
    
        //如果网络资源已经修改：使用网络响应返回的最新数据
        Response response = networkResponse.newBuilder()
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
    
        //将最新的数据缓存起来
        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.
            }
          }
        }
        
        //返回最新的数据
        return response;
      }

CacheInterceptor是进行一些缓存上面的处理,接下来是ConnectInterceptor

@Override 
    public Response intercept(Chain chain) throws IOException { 
        RealInterceptorChain realChain = (RealInterceptorChain) chain;
        Request request = realChain.request();
        Transmitter transmitter = realChain.transmitter();
        
        // We need the network to satisfy this request. Possibly for validating a conditional GET.
        //判断请求是不是GET方法, 不是的情况下,需要进行有效监测
        boolean doExtensiveHealthChecks = !request.method().equals("GET");
        Exchange exchange = transmitter.newExchange(chain, doExtensiveHealthChecks);
        
        //执行下一个拦截器
        return realChain.proceed(request, transmitter, exchange);
    }

ConnectInterceptor的下一个拦截器就是最好一个拦截器CallServerInterceptor了.

@Override public Response intercept(Chain chain) throws IOException { 
        RealInterceptorChain realChain = (RealInterceptorChain) chain;
        Exchange exchange = realChain.exchange();
        Request request = realChain.request();
    
        long sentRequestMillis = System.currentTimeMillis();
        //整理请求头并写入
        exchange.writeRequestHeaders(request);
    
        boolean responseHeadersStarted = false;
        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"))) { 
            exchange.flushRequest();
            responseHeadersStarted = true;
            exchange.responseHeadersStart();
            responseBuilder = exchange.readResponseHeaders(true);
          }
    
          if (responseBuilder == null) { 
            if (request.body().isDuplex()) { 
              // Prepare a duplex body so that the application can send a request body later.
              exchange.flushRequest();
              BufferedSink bufferedRequestBody = Okio.buffer(
                  exchange.createRequestBody(request, true));
              request.body().writeTo(bufferedRequestBody);
            } else { 
              // Write the request body if the "Expect: 100-continue" expectation was met.
              BufferedSink bufferedRequestBody = Okio.buffer(
                  exchange.createRequestBody(request, false));
              request.body().writeTo(bufferedRequestBody);
              bufferedRequestBody.close();
            }
          } else { 
            exchange.noRequestBody();
            if (!exchange.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.
              exchange.noNewExchangesOnConnection();
            }
          }
        } else { 
          exchange.noRequestBody();
        }
    
        if (request.body() == null || !request.body().isDuplex()) { 
          //发送最终的请求
          exchange.finishRequest();
        }
    
        if (!responseHeadersStarted) { 
          exchange.responseHeadersStart();
        }
    
        if (responseBuilder == null) { 
          //响应头
          responseBuilder = exchange.readResponseHeaders(false);
        }
    
        Response response = responseBuilder
            .request(request)
            .handshake(exchange.connection().handshake())
            .sentRequestAtMillis(sentRequestMillis)
            .receivedResponseAtMillis(System.currentTimeMillis())
            .build();
    
        int code = response.code();
        if (code == 100) { 
          // server sent a 100-continue even though we did not request one.
          // try again to read the actual response
          response = exchange.readResponseHeaders(false)
              .request(request)
              .handshake(exchange.connection().handshake())
              .sentRequestAtMillis(sentRequestMillis)
              .receivedResponseAtMillis(System.currentTimeMillis())
              .build();
    
          code = response.code();
        }
    
        exchange.responseHeadersEnd(response);
    
        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(exchange.openResponseBody(response))
              .build();
        }
    
        //断开连接
        if ("close".equalsIgnoreCase(response.request().header("Connection"))
            || "close".equalsIgnoreCase(response.header("Connection"))) { 
          exchange.noNewExchangesOnConnection();
        }
    
        //抛出协议异常
        if ((code == 204 || code == 205) && response.body().contentLength() > 0) { 
          throw new ProtocolException(
              "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
        }
    
        return response;
      }

这是链中最后一个拦截器，它向 服务器 发起了一次网络访问.负责向服务器发送请求数据、从服务器读取响应数据.拿到数据之后再沿着链返回.

## 4. 总结 ##

OkHttp的拦截器链设计得非常巧妙,是典型的责任链模式.并最终由最后一个链处理了网络请求,并拿到结果.本文主要是对OkHttp主流程进行了梳理,通过本文能对OkHttp有一个整体的了解.

[20190722224150465.png]: /images/20211129/c04eeaa3ec4f4e2fbbb7866f65e6f007.png
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