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OkHttp 版源码分析

OkHttp 版源码分析

作者: deriklpw | 来源:发表于2020-03-16 23:23 被阅读0次

    分析的问题

    1. Call如何处理同步和异步请求;
    2. Dispatcher如何管理请求任务;
    3. OkHttp拦截器链;
    4. 连接池ConnectionPool

    1. 同步和异步请求

    OkHttpClient用内部类Builder的形式进行创建,在请求网络时,通过OkHttpClient类的newCall()方法创建一个Call实例:

      /**
       * 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 */);
      }
    

    newCall()方法实际上调用了RealCall类的静态方法newRealCall()

      static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
        // Safely publish the Call instance to the EventListener.
        RealCall call = new RealCall(client, originalRequest, forWebSocket);
        call.eventListener = client.eventListenerFactory().create(call);
        return call;
      }
    

    Call是一个接口,RealCallCall的实现类,在静态方法中创建了RealCall实例并返回。
    OkHttp使用execute()enqueue()方法分别执行同步和异步请求。这两个方法也是Call类的抽象方法,并在RealCall类中实现。

    • RealCall类的同步请求方法execute()
      @Override public Response execute() throws IOException {
        synchronized (this) {
          if (executed) throw new IllegalStateException("Already Executed");
          executed = true;
        }
        captureCallStackTrace();
        eventListener.callStart(this);
        try {
          client.dispatcher().executed(this);
          Response result = getResponseWithInterceptorChain();
          if (result == null) throw new IOException("Canceled");
          return result;
        } catch (IOException e) {
          eventListener.callFailed(this, e);
          throw e;
        } finally {
          client.dispatcher().finished(this);
        }
      }
    

    client.dispatcher.executed(this),把任务转交给了Dispatcher类的executed()方法。
    getResponseWithInterceptorChain(),执行拦截器链,其中包括发送数据和响应数据过程,都是在拦截器中进行。
    client.dispatcher().finished(this),任务执行结束时,调用了Dispatcherfinished()方法。
    此外,还应该知道同步请求会阻塞当前的线程。

    • RealCall类的异步请求enqueue()
      @Override public void enqueue(Callback responseCallback) {
        synchronized (this) {
          if (executed) throw new IllegalStateException("Already Executed");
          executed = true;
        }
        captureCallStackTrace();
        eventListener.callStart(this);
        client.dispatcher().enqueue(new AsyncCall(responseCallback));
      }
    

    异步请求,client.dispatcher().enqueue(new AsyncCall(responseCallback));调用了Dispatcher类的enqueue()方法。

    所以,在ReaCall类中,execute()enqueue()方法执行任务时,都是转交给了Dispatcher类处理。异步和同步此处不一样的地方是,enqueue()方法的入参不是RealCall的实例,而是RealCall的一个内部类AsyncCall的实例,AsyncCall继承自NamedRunnable类,以便提交给线程池执行。
    同步请求有executedfinished两个过程,异步请求在这里只看到enqueue()开始执行的方法,那么它有没有对应的结束调用呢?贴上内部类AsyncCall看看。

    final class AsyncCall extends NamedRunnable {
        private final Callback responseCallback;
    
        AsyncCall(Callback responseCallback) {
          super("OkHttp %s", redactedUrl());
          this.responseCallback = responseCallback;
        }
    
        String host() {
          return originalRequest.url().host();
        }
    
        Request request() {
          return originalRequest;
        }
    
        RealCall get() {
          return RealCall.this;
        }
    
        @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 {
              eventListener.callFailed(RealCall.this, e);
              responseCallback.onFailure(RealCall.this, e);
            }
          } finally {
            client.dispatcher().finished(this);
          }
        }
      }
    

    其父类NamedRunnable是一个抽象类,它继承自Runnable接口,可以指定执行线程名,AsyncCall实现了NamedRunnableexecute()抽象方法,在里面有client.dispatcher().finished(this),在这里调用Dispatcher类的finished()结束请求任务。

    2. Dispatcher任务管理

    Dispatcher是在Builder构建时新建的实例,并引用给了OkHttpClient类的成员变量
    以下是Dispatcher类的成员变量:

      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<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<>();
    

    maxRequests最大请求任务数,64个,maxRequestsPerHost每个主机最大请求任务数,5个。另外还有一个线程池,一个同步请求的队列,一个异步请求的队列,一个备用的异步请求队列。这三个任务队列,都是在Dispatcher的成员中直接创建的,用来存储请求任务。

    • 线程池
      public synchronized ExecutorService executorService() {
        if (executorService == null) {
          executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
              new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
        }
        return executorService;
      }
    

    这个线程池,是在要使用的时候创建,核心线程数为0,最大非核心线程数为Integer的最大值,可以认为它"无界",且闲置回收时间为60s,并使用了一个非存储的阻塞队列,用于新任务和空闲线程的配对,类似newCachedThreadPool()

    • 同步请求任务管理
      前面看到执行同步请求,调用Callexecute()同步方法时,调用了Dispatcherexecuted()方法和finished()方法,Dispatcher类的这两个方法如下:
      /** Used by {@code Call#execute} to signal it is in-flight. */
      synchronized void executed(RealCall call) {
        runningSyncCalls.add(call);
      }
    
      /** Used by {@code Call#execute} to signal completion. */
      void finished(RealCall call) {
        finished(runningSyncCalls, call, false);
      }
    
      private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
        int runningCallsCount;
        Runnable idleCallback;
        synchronized (this) {
          if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
          if (promoteCalls) promoteCalls();
          runningCallsCount = runningCallsCount();
          idleCallback = this.idleCallback;
        }
    
        if (runningCallsCount == 0 && idleCallback != null) {
          idleCallback.run();
        }
      }  
    

    发现在Dispatcher中,发起请求时,把请求任务加入到了同步队列中,且对任务数没有限制,请求结束时,会把任务从队列中移除。
    为什么同步请求需要队列去管理,作用是什么?请求开始时,先将任务添加到队列,再执行,请求响应后,再将任务从队列中移除。
    在整个Dispatcher类中,发现runningSyncCalls这个同步队列,还在另几个地方出现,一个取消所有请求的时候,一个获取所有正在执行的请求集合的时候,还有一个是获取所有正在执行的请求数的时候。

      /**
       * Cancel all calls currently enqueued or executing. Includes calls executed both {@linkplain
       * Call#execute() synchronously} and {@linkplain Call#enqueue asynchronously}.
       */
      public synchronized void cancelAll() {
        for (AsyncCall call : readyAsyncCalls) {
          call.get().cancel();
        }
    
        for (AsyncCall call : runningAsyncCalls) {
          call.get().cancel();
        }
    
        for (RealCall call : runningSyncCalls) {
          call.cancel();
        }
      }
    
      /** Returns a snapshot of the calls currently being executed. */
      public synchronized List<Call> runningCalls() {
        List<Call> result = new ArrayList<>();
        result.addAll(runningSyncCalls);
        for (AsyncCall asyncCall : runningAsyncCalls) {
          result.add(asyncCall.get());
        }
        return Collections.unmodifiableList(result);
      }
    
        public synchronized int runningCallsCount() {
        return runningAsyncCalls.size() + runningSyncCalls.size();
      }
    

    所以通过队列管理同步请求,比直接执行,可以方便地了解所有请求数,以及控制这些请求任务。即使在多线程的应用场景中,因为加了同步锁,依然可以安全地进行管理。

    • 异步请求任务管理
      执行异步请求,调用RealCallenqueue()方法,调用了Dispatcherenqueue()finished()方法:
      synchronized void enqueue(AsyncCall call) {
        if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
          runningAsyncCalls.add(call);
          executorService().execute(call);
        } else {
          readyAsyncCalls.add(call);
        }
      }
    
      /** Used by {@code AsyncCall#run} to signal completion. */
      void finished(AsyncCall call) {
        finished(runningAsyncCalls, call, true);
      }
    
      private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
        int runningCallsCount;
        Runnable idleCallback;
        synchronized (this) {
          if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
          if (promoteCalls) promoteCalls();
          runningCallsCount = runningCallsCount();
          idleCallback = this.idleCallback;
        }
    
        if (runningCallsCount == 0 && idleCallback != null) {
          idleCallback.run();
        }
      }
    

    异步任务,会先判断异步任务队列大小是否达到限制值64(maxRequests64),以及每个地址的请求数是否到达限制值5(maxRequestsPerHost5),都未达到,把Call实例添加到正在执行的异步任务队列中,并调用线程池来执行任务。如果已达到最大任务数限制,则会加入到备用的异步任务队列中,等待执行。线程池的优点是线程复用,但Dispatcher通过请求数限制,把任务加入到备用队列中,等线程空闲时,再从备用队列中取出任务提交给空闲线程执行,避免了大量线程的创建。此外,同步和异步finished()方法都调用了另一个finished(Deque<T> calls, T call, boolean promoteCalls)范型方法,在它里面进行任务移除,如果是异步任务,则会通过promoteCalls()方法从备用的异步任务队列中,取出任务添加到正在执行的异步任务队列中,同时交给线程池执行。

    3. OkHttp拦截器链

    无论是同步请求还是异步请求,都是通过拦截器链进行数据请求和响应的处理。同步和异步任务执行中,都调用了RealCall类中getResponseWithInterceptorChain()方法:

      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, this, eventListener, client.connectTimeoutMillis(),
            client.readTimeoutMillis(), client.writeTimeoutMillis());
    
        return chain.proceed(originalRequest);
      }
    

    在拦截器的集合中,首先会加入我们自定义的所有应用拦截器,自定义的所有网络拦截器,和OKHttp默认的一些拦截器,ConnectInterceptor中是打开一个和server的连接,添加网络拦截器是在打开和server的连接之后,所以在网络拦截器可以看到连接信息。最后一个CallServerInterceptor拦截器,它才真正和服务器交互数据。在这个方法中,第一次创建了一个RealInterceptorChain实例,index的入参为0,然后调用RealInterceptorChain类的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 {
        if (index >= interceptors.size()) throw new AssertionError();
    
        calls++;
    
        // If we already have a stream, confirm that the incoming request will use it.
        if (this.httpCodec != null && !this.connection.supportsUrl(request.url())) {
          throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
              + " must retain the same host and port");
        }
    
        // If we already have a stream, confirm that this is the only call to chain.proceed().
        if (this.httpCodec != null && calls > 1) {
          throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
              + " must call proceed() exactly once");
        }
    
        // Call the next interceptor in the chain.
        RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
            connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
            writeTimeout);
        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");
        }
    
        // Confirm that the intercepted response isn't null.
        if (response == null) {
          throw new NullPointerException("interceptor " + interceptor + " returned null");
        }
    
        if (response.body() == null) {
          throw new IllegalStateException(
              "interceptor " + interceptor + " returned a response with no body");
        }
    
        return response;
      }
    

    RealCall类中getResponseWithInterceptorChain()方法中,创建了RealInterceptorChain对象,为何在proceed()方法内部还会出现RealInterceptorChain对象的创建?
    其实他们是两个不同的拦截器链对象,第一个拦截器链对象index为0,执行其proceed()方式时,先准备了下一个拦截器链对象,index为1(通过index+1给定)。当interceptors.get(index)拿到第一个拦截器后,执行第一个拦截器的intercept()方法,把刚创建的第二个链对象传了进去,并执行第二个链对象的proceed()方法。这个时候第二链对象的proceed()里面又会准备一个新的链对象(第三个,index为2),然后一直重复前面的过程,直到最后一个拦截器的intercept()方法被调用。且拦截器链对象的proceed()方法的返回值是Response类型,所以请求过程的处理是从第一个拦截器到最后一个拦截器,响应过程Response的处理,则是从最后一个到第一个的过程。

    为什么在介绍OkHttp的拦截器时,都说应用拦截器只执行一次,而网络拦截器会贯穿整个请求过程,可能执行多次呢?要知道这个答案,还得看看默认拦截器的作用分别是什么。

    • RetryAndFollowUpInterceptor拦截器
      再贴一次RealCall类的getResponseWithInterceptorChain方法:
    //省略
        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));
    //省略
    

    interceptors.add(retryAndFollowUpInterceptor);,关键是这一行,它是一个请求重试和重定向的拦截器,其intercept()方法的部分内容如下:

    //省略
    while (true) {
          if (canceled) {
            streamAllocation.release();
            throw new IOException("Canceled");
          }
    
          Response response;
          boolean releaseConnection = true;
          try {
            response = realChain.proceed(request, streamAllocation, null, null);
            releaseConnection = false;
          }
    //省略
    

    在这个拦截器里有一个循环,取消请求或者没有重定向的请求了,则退出循环。应用拦截器的添加在RetryAndFollowUpInterceptor之前,所以应用拦截器在整个拦截器链执行过程中只会执行一次,而在RetryAndFollowUpInterceptor之后添加的网络拦截器,以及其他拦截器,则会依情况在循环中执行一次或多次。

    • BridgeInterceptor拦截器
    public final class BridgeInterceptor implements Interceptor {
      private final CookieJar cookieJar;
    
      public BridgeInterceptor(CookieJar cookieJar) {
        this.cookieJar = cookieJar;
      }
    
      @Override public Response intercept(Chain chain) throws IOException {
        Request userRequest = chain.request();
        Request.Builder requestBuilder = userRequest.newBuilder();
    
        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<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());
        }
    
        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);
          String contentType = networkResponse.header("Content-Type");
          responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
        }
    
        return responseBuilder.build();
      }
    
      /** Returns a 'Cookie' HTTP request header with all cookies, like {@code a=b; c=d}. */
      private String cookieHeader(List<Cookie> cookies) {
        StringBuilder cookieHeader = new StringBuilder();
        for (int i = 0, size = cookies.size(); i < size; i++) {
          if (i > 0) {
            cookieHeader.append("; ");
          }
          Cookie cookie = cookies.get(i);
          cookieHeader.append(cookie.name()).append('=').append(cookie.value());
        }
        return cookieHeader.toString();
      }
    }
    

    这个拦截器处理了一些默认的通用头,请求头和实体头,而且可以看到,当用户没有对Accept-Encoding请求头设置时,OkHttp默认使用了gzip编码格式对数据进行压缩。同时用户可以通过这个拦截器设置cookiesOkHttp默认是NO_COOKIES

    • CacheInterceptor拦截器
    /** Serves requests from the cache and writes responses to the cache. */
    public final class CacheInterceptor implements Interceptor {
      //省略
    @Override public Response intercept(Chain chain) throws IOException {
        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.
        }
    
        // If we're forbidden from using the network and the cache is insufficient, fail.
        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.
        if (networkRequest == null) {
          return cacheResponse.newBuilder()
              .cacheResponse(stripBody(cacheResponse))
              .build();
        }
    
        Response networkResponse = null;
        try {
          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());
          }
        }
    
        // If we have a cache response too, then we're doing a conditional get.
        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());
          }
        }
    
        Response response = networkResponse.newBuilder()
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
    
        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.
            }
          }
        }
    
        return response;
      }
      //省略
    }
    

    这个拦截器中会先依据用户或OkHttp的默认设置生成缓存策略。
    网络请求和缓存都不可用时,返回504;
    网络请求不可用(缓存策略可以将网路请求置null),返回缓存数据。
    网络请求可用,获取响应数据,如果和本地缓存比较,发现未修改,生成一个含有缓存Response和网络Response的新Response返回。如果没有缓存,也重新生成一个含有缓存Response和网络Response的新Response返回。有缓存则将新数据写入缓存。

    • 缓存策略CacheStrategy
      在前面那段代码中,通过cacheCandidate).get()返回了CacheStrategy的实例,里面依据各种情况来决定返回的CacheStrategy对象,以及它的两个重要成员networkRequestcacheResponse,缓存拦截器中大多数的判断都依赖着这两成员对象,决定如何使用网络和缓存数据。

    • ConnectInterceptor连接拦截器

    /** Opens a connection to the target server and proceeds to the next interceptor. */
    public final class ConnectInterceptor implements Interceptor {
      public final OkHttpClient client;
    
      public ConnectInterceptor(OkHttpClient client) {
        this.client = client;
      }
    
      @Override public Response intercept(Chain chain) throws IOException {
        RealInterceptorChain realChain = (RealInterceptorChain) chain;
        Request request = realChain.request();
        StreamAllocation streamAllocation = realChain.streamAllocation();
    
        // We need the network to satisfy this request. Possibly for validating a conditional GET.
        boolean doExtensiveHealthChecks = !request.method().equals("GET");
        HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
        RealConnection connection = streamAllocation.connection();
    
        return realChain.proceed(request, streamAllocation, httpCodec, connection);
      }
    }
    

    如果不是GET请求,会先确认一次连接池中到主机的连接是否可用。这个拦截器的作用主要是从连接池获取一个到server的有效连接。

    /** This is the last interceptor in the chain. It makes a network call to the server. */
    public final class CallServerInterceptor implements Interceptor {
      private final boolean forWebSocket;
    
      public CallServerInterceptor(boolean forWebSocket) {
        this.forWebSocket = forWebSocket;
      }
    
      @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();
    
        realChain.eventListener().requestHeadersStart(realChain.call());
        httpCodec.writeRequestHeaders(request);
        realChain.eventListener().requestHeadersEnd(realChain.call(), 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();
            realChain.eventListener().responseHeadersStart(realChain.call());
            responseBuilder = httpCodec.readResponseHeaders(true);
          }
    
          if (responseBuilder == null) {
            // Write the request body if the "Expect: 100-continue" expectation was met.
            realChain.eventListener().requestBodyStart(realChain.call());
            long contentLength = request.body().contentLength();
            CountingSink requestBodyOut =
                new CountingSink(httpCodec.createRequestBody(request, contentLength));
            BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
    
            request.body().writeTo(bufferedRequestBody);
            bufferedRequestBody.close();
            realChain.eventListener()
                .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
          } 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();
    
        if (responseBuilder == null) {
          realChain.eventListener().responseHeadersStart(realChain.call());
          responseBuilder = httpCodec.readResponseHeaders(false);
        }
    
        Response response = responseBuilder
            .request(request)
            .handshake(streamAllocation.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
          responseBuilder = httpCodec.readResponseHeaders(false);
    
          response = responseBuilder
                  .request(request)
                  .handshake(streamAllocation.connection().handshake())
                  .sentRequestAtMillis(sentRequestMillis)
                  .receivedResponseAtMillis(System.currentTimeMillis())
                  .build();
    
          code = response.code();
        }
    
        realChain.eventListener()
                .responseHeadersEnd(realChain.call(), 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(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());
        }
    
        return response;
      }
    
      static final class CountingSink extends ForwardingSink {
        long successfulCount;
    
        CountingSink(Sink delegate) {
          super(delegate);
        }
    
        @Override public void write(Buffer source, long byteCount) throws IOException {
          super.write(source, byteCount);
          successfulCount += byteCount;
        }
      }
    }
    

    拦截器集合中最后添加的这个拦截器,是真正和server交互数据的地方。

    4. 连接池ConnectionPool

    OkHttpClientBuilder类中,connectionPool = new ConnectionPool();创建了一个连接池对象。首先,看一下该类中的成员变量。

    /**
       * Background threads are used to cleanup expired connections. There will be at most a single
       * thread running per connection pool. The thread pool executor permits the pool itself to be
       * garbage collected.
       */
      private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
          Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true));
    
      /** The maximum number of idle connections for each address. */
      private final int maxIdleConnections;
      private final long keepAliveDurationNs;
      private final Runnable cleanupRunnable = new Runnable() {
        @Override public void run() {
          while (true) {
            long waitNanos = cleanup(System.nanoTime());
            if (waitNanos == -1) return;
            if (waitNanos > 0) {
              long waitMillis = waitNanos / 1000000L;
              waitNanos -= (waitMillis * 1000000L);
              synchronized (ConnectionPool.this) {
                try {
                  ConnectionPool.this.wait(waitMillis, (int) waitNanos);
                } catch (InterruptedException ignored) {
                }
              }
            }
          }
        }
      };
    
      private final Deque<RealConnection> connections = new ArrayDeque<>();
      final RouteDatabase routeDatabase = new RouteDatabase();
      boolean cleanupRunning;
    

    一个线程池,用于清理失效连接,一个阻塞队列,用于管理RealConnection对象的生产和消费,队列中任务对象的数量没有限制。
    构造函数

      /**
       * Create a new connection pool with tuning parameters appropriate for a single-user application.
       * The tuning parameters in this pool are subject to change in future OkHttp releases. Currently
       * this pool holds up to 5 idle connections which will be evicted after 5 minutes of inactivity.
       */
      public ConnectionPool() {
        this(5, 5, TimeUnit.MINUTES);
      }
    
      public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
        this.maxIdleConnections = maxIdleConnections;
        this.keepAliveDurationNs = timeUnit.toNanos(keepAliveDuration);
    
        // Put a floor on the keep alive duration, otherwise cleanup will spin loop.
        if (keepAliveDuration <= 0) {
          throw new IllegalArgumentException("keepAliveDuration <= 0: " + keepAliveDuration);
        }
      }
    

    从构造函数中,可以了解,其默认的闲置连接是5个,且每个连接的闲置时长为5分钟。在清理任务时,如果发现闲置连接超过5个,则闲置时间最长的连接会被从阻塞队列中移除。如果一个连接的闲置时间超过5分钟,该连接也会被从阻塞队列中移除。到server连接的复用,可以减少三次握手,四次挥手的过程,提高网络请求的效率。

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