OKHTTP 源码分析

关于OkHttp网上有很多分析的文章，本篇文章为个人对OkHttp的了解源码分析，自己复习使用，特此总结出来,okHttp为什么这么牛，为什么这么多人使用，以及框架有点，使用的到设计模式等等，从源码角度去分析，也会总结网上比较好的文章分析，欢迎一块探讨交流~

OkHttp 基本使用

本文源码分析基于3.14.2版本

 implementation("com.squareup.okhttp3:okhttp:3.14.2")

        String url = "http://wwww.baidu.com";
        OkHttpClient okHttpClient = new OkHttpClient();
        final Request request = new Request.Builder()
                .url(url)
                .build();
        Call call = okHttpClient.newCall(request);

         //同步请求
//        Response execute = call.execute();
       
        //异步请求
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.d(TAG, "onFailure: ");
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                Log.d(TAG, "onResponse: " + response.body().string());
            }
        });

OkHttp 请求过程

1.1 创建OkHttpClient 对象

public class OkHttpClient implements Cloneable, Call.Factory, WebSocket.Factory {
  static final List<Protocol> DEFAULT_PROTOCOLS = Util.immutableList(
      Protocol.HTTP_2, Protocol.HTTP_1_1);
代码省略 ....
//通过构造者模式初始化一些参数、配置
 public OkHttpClient() {
        this(new Builder());
    }

    OkHttpClient(Builder builder) {
        this.dispatcher = builder.dispatcher;
        this.proxy = builder.proxy;
        this.protocols = builder.protocols;
        this.connectionSpecs = builder.connectionSpecs;
        this.interceptors = Util.immutableList(builder.interceptors);
        this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
        this.eventListenerFactory = builder.eventListenerFactory;
        this.proxySelector = builder.proxySelector;
        this.cookieJar = builder.cookieJar;
        this.cache = builder.cache;
        this.internalCache = builder.internalCache;
        this.socketFactory = builder.socketFactory;

        boolean isTLS = false;
        for (ConnectionSpec spec : connectionSpecs) {
            isTLS = isTLS || spec.isTls();
        }

        if (builder.sslSocketFactory != null || !isTLS) {
            this.sslSocketFactory = builder.sslSocketFactory;
            this.certificateChainCleaner = builder.certificateChainCleaner;
        } else {
            X509TrustManager trustManager = Util.platformTrustManager();
            this.sslSocketFactory = newSslSocketFactory(trustManager);
            this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
        }

        if (sslSocketFactory != null) {
            Platform.get().configureSslSocketFactory(sslSocketFactory);
        }

        this.hostnameVerifier = builder.hostnameVerifier;
        this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
                certificateChainCleaner);
        this.proxyAuthenticator = builder.proxyAuthenticator;
        this.authenticator = builder.authenticator;
        this.connectionPool = builder.connectionPool;
        this.dns = builder.dns;
        this.followSslRedirects = builder.followSslRedirects;
        this.followRedirects = builder.followRedirects;
        this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
        this.callTimeout = builder.callTimeout;
        this.connectTimeout = builder.connectTimeout;
        this.readTimeout = builder.readTimeout;
        this.writeTimeout = builder.writeTimeout;
        this.pingInterval = builder.pingInterval;

        if (interceptors.contains(null)) {
            throw new IllegalStateException("Null interceptor: " + interceptors);
        }
        if (networkInterceptors.contains(null)) {
            throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
        }
    }
//简单介绍这些参数都是什么含义
  public static final class Builder {
        Dispatcher dispatcher; //调度器
        /**
         * 代理类，默认有三种代理模式DIRECT(直连),HTTP（http代理）,SOCKS（socks代理）
         */
        @Nullable Proxy proxy;
        /**
         * 协议集合，协议类，用来表示使用的协议版本，比如`http/1.0,`http/1.1,`spdy/3.1,`h2等
         */
        List<Protocol> protocols;
        /**
         * 连接规范，用于配置Socket连接层。对于HTTPS，还能配置安全传输层协议（TLS）版本和密码套件
         */
        List<ConnectionSpec> connectionSpecs;
        //拦截器，可以监听、重写和重试请求等
        final List<Interceptor> interceptors = new ArrayList<>();
        final List<Interceptor> networkInterceptors = new ArrayList<>();
        EventListener.Factory eventListenerFactory;
        /**
         * 代理选择类，默认不使用代理，即使用直连方式，当然，我们可以自定义配置，
         * 以指定URI使用某种代理，类似代理软件的PAC功能
         */
        ProxySelector proxySelector;
        //Cookie的保存获取
        CookieJar cookieJar;
        /**
         * 缓存类，内部使用了DiskLruCache来进行管理缓存，匹配缓存的机制不仅仅是根据url，
         * 而且会根据请求方法和请求头来验证是否可以响应缓存。此外，仅支持GET请求的缓存
         */
        @Nullable Cache cache;
        //内置缓存
        @Nullable InternalCache internalCache;
        //Socket的抽象创建工厂，通过createSocket来创建Socket
        SocketFactory socketFactory;
        /**
         * 安全套接层工厂，HTTPS相关，用于创建SSLSocket。一般配置HTTPS证书信任问题都需要从这里着手。
         * 对于不受信任的证书一般会提示
         * javax.net.ssl.SSLHandshakeException异常。
         */
        @Nullable SSLSocketFactory sslSocketFactory;
        /**
         * 证书链清洁器，HTTPS相关，用于从[Java]的TLS API构建的原始数组中统计有效的证书链，
         * 然后清除跟TLS握手不相关的证书，提取可信任的证书以便可以受益于证书锁机制。
         */
        @Nullable CertificateChainCleaner certificateChainCleaner;
        /**
         * 主机名验证器，与HTTPS中的SSL相关，当握手时如果URL的主机名
         * 不是可识别的主机，就会要求进行主机名验证
         */
        HostnameVerifier hostnameVerifier;
        /**
         * 证书锁，HTTPS相关，用于约束哪些证书可以被信任，可以防止一些已知或未知
         * 的中间证书机构带来的攻击行为。如果所有证书都不被信任将抛出SSLPeerUnverifiedException异常。
         */
        CertificatePinner certificatePinner;
        /**
         * 身份认证器，当连接提示未授权时，可以通过重新设置请求头来响应一个
         * 新的Request。状态码401表示远程服务器请求授权，407表示代理服务器请求授权。
         * 该认证器在需要时会被RetryAndFollowUpInterceptor触发。
         */
        Authenticator proxyAuthenticator;
        Authenticator authenticator;
        /**
         * 连接池
         *
         * 我们通常将一个客户端和服务端和连接抽象为一个 connection，
         * 而每一个 connection 都会被存放在 connectionPool 中，由它进行统一的管理，
         * 例如有一个相同的 http 请求产生时，connection 就可以得到复用
         */
        ConnectionPool connectionPool;
        //域名解析系统
        Dns dns;
        //是否遵循SSL重定向
        boolean followSslRedirects;
        //是否重定向
        boolean followRedirects;
        //失败是否重新连接
        boolean retryOnConnectionFailure;
        //回调超时
        int callTimeout;
        //连接超时
        int connectTimeout;
        //读取超时
        int readTimeout;
        //写入超时
        int writeTimeout;
        //与WebSocket有关，为了保持长连接，我们必须间隔一段时间发送一个ping指令进行保活；
        int pingInterval;

        public Builder() {
            dispatcher = new Dispatcher();

            protocols = DEFAULT_PROTOCOLS;

            connectionSpecs = DEFAULT_CONNECTION_SPECS;
            eventListenerFactory = EventListener.factory(EventListener.NONE);
            /**
             * 代理选择类，默认不使用代理，即使用直连方式，当然，我们可以自定义配置，以指定URI使用某种代理，类似代理软件的PAC功能
             */
            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;
            connectionPool = new ConnectionPool();
            dns = Dns.SYSTEM;
            followSslRedirects = true;
            followRedirects = true;
            retryOnConnectionFailure = true;
            callTimeout = 0;
            connectTimeout = 10_000;
            readTimeout = 10_000;
            writeTimeout = 10_000;
            pingInterval = 0;
        }

OkHttp 总的来说通过构造者模式，初始化一些例如调度器、拦截器等参数。
创建OkHttpClient 特别简单，直接new OkHttpClient ()对象出来或者是 OkHttpClient.Builder builder = new OkHttpClient().newBuilder();创建对象都可以。

1.2 创建OkHttpClient 对象

第二部创建Request 对象，下面看一下Request 对象到底做了什么

public final class Request {
  final HttpUrl url;
  final String method;
  final Headers headers;
  final @Nullable RequestBody body;
  final Map<Class<?>, Object> tags;

  private volatile @Nullable CacheControl cacheControl; // Lazily initialized.

  Request(Builder builder) {
    this.url = builder.url;
    this.method = builder.method;
    this.headers = builder.headers.build();
    this.body = builder.body;
    this.tags = Util.immutableMap(builder.tags);
  }

Request 类同样也是通过构造者模式来配置一些请求的参数，包含我们进行网络请求的地址，方法，请求头等。

1.3 创建Call对象 Call call = okHttpClient.newCall(request);我们来跟一下源码

 Call call = okHttpClient.newCall(request);

通过okhttpClient的newCall(request);方法传入我们的请求reauest配置

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

newCall 方法返回了一个RealCall的真正的cal 接着跟踪

 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.transmitter = new Transmitter(client, call);
    return call;
  }

通过此方法我们我们可以看到返回了RealCall 对象，Call 是一个接口，RealCall 是Call具体实现类，我们这个时候拿到了RealCall 对象

final class RealCall implements Call {
  final OkHttpClient client;

  /**
   * There is a cycle between the {@link Call} and {@link Transmitter} that makes this awkward.
   * This is set after immediately after creating the call instance.
   */
  private Transmitter transmitter;

1.4 进行请求操作

1.4.1 我们通过拿到的call 对象直接调用call.execute();方法可以进行同步请求，但是一般情况下这个方法我们使用的很少，因为Android要求主线程不能执行耗时的操作否则容易出现ANR。

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

同步操作最终会调用getResponseWithInterceptorChain()这个方法，来返回我们的请求结果Response,我们接口返回的数据就封装在这个类里面，关于getResponseWithInterceptorChain();这个方法具体执行了什么操作，我们暂时不进行分析，稍后和异步同时进行分析。这个时候我们整个流程的同步操作流程就完成了。
1.4.2call.enqueue(new Callback() {}异步操作，我们来看看内部具体的实现:

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

这个方法又通过okHttpClient中的dispatcher 调度器调了异步方法

 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();
  }

此处又调用了promoteAndExecute()方法，接着跟

 /** 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<>();

 private boolean promoteAndExecute() {
    assert (!Thread.holdsLock(this));

    List<AsyncCall> executableCalls = new ArrayList<>();
    boolean isRunning;
    synchronized (this) {
      for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
        AsyncCall asyncCall = i.next();

        if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
        if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.

        i.remove();
        asyncCall.callsPerHost().incrementAndGet();
        executableCalls.add(asyncCall);
        runningAsyncCalls.add(asyncCall);
      }
      isRunning = runningCallsCount() > 0;
    }

    for (int i = 0, size = executableCalls.size(); i < size; i++) {
      AsyncCall asyncCall = executableCalls.get(i);
      asyncCall.executeOn(executorService());
    }

    return isRunning;
  }

此处出现了三个集合，来保存我们的请求 然后通过asyncCall.executeOn(executorService());方法来执行我们的请求

 void executeOn(ExecutorService executorService) {
      assert (!Thread.holdsLock(client.dispatcher()));
      boolean success = false;
      try {
        executorService.execute(this);
        success = true;
      } catch (RejectedExecutionException e) {
        InterruptedIOException ioException = new InterruptedIOException("executor rejected");
        ioException.initCause(e);
        transmitter.noMoreExchanges(ioException);
        responseCallback.onFailure(RealCall.this, ioException);
      } finally {
        if (!success) {
          client.dispatcher().finished(this); // This call is no longer running!
        }
      }
    }

在此处我们发现了一个线程池，

public final class Dispatcher {
private int maxRequests = 64;  //线程池最多同时执行64个请求
private int maxRequestsPerHost = 5;  //相同host 最5个执行

 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;
  }

final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;
    private volatile AtomicInteger callsPerHost = new AtomicInteger(0);

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

    AtomicInteger callsPerHost() {
      return callsPerHost;
    }

    void reuseCallsPerHostFrom(AsyncCall other) {
      this.callsPerHost = other.callsPerHost;
    }

    String host() {
      return originalRequest.url().host();
    }

    Request request() {
      return originalRequest;
    }

    RealCall get() {
      return RealCall.this;
    }

    /**
     * Attempt to enqueue this async call on {@code executorService}. This will attempt to clean up
     * if the executor has been shut down by reporting the call as failed.
     */
    void executeOn(ExecutorService executorService) {
      assert (!Thread.holdsLock(client.dispatcher()));
      boolean success = false;
      try {
        executorService.execute(this);
        success = true;
      } catch (RejectedExecutionException e) {
        InterruptedIOException ioException = new InterruptedIOException("executor rejected");
        ioException.initCause(e);
        transmitter.noMoreExchanges(ioException);
        responseCallback.onFailure(RealCall.this, ioException);
      } finally {
        if (!success) {
          client.dispatcher().finished(this); // This call is no longer running!
        }
      }
    }

    @Override protected void execute() {
      boolean signalledCallback = false;
      transmitter.timeoutEnter();
      try {
        Response response = getResponseWithInterceptorChain();
        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);
      }
    }
  }

通过模板模式来执行线程操作，最后我们会发现调用了这个方法getResponseWithInterceptorChain()来返回我们的结果，也就是说和同步操作一样，最终都是调用同一个方法来返回。区别在于异步OkHttp 默认为我们创建了线程池以及队列让我们进行异步请求。
到这基本上我们的同步异步流程就走完了。下篇文章我们分析getResponseWithInterceptorChain()这个方法到底执行了什么以及我们的请求到底是怎么请求的，怎么进行连接怎么返回的结果。以及OkHttp最核心的东西拦截器的作用。