一俘闯、okhttp簡(jiǎn)介

OKHttp是一個(gè)處理網(wǎng)絡(luò)請(qǐng)求的開(kāi)源項(xiàng)目潭苞，Android 當(dāng)前最火熱網(wǎng)絡(luò)框架，由移動(dòng)支付Square公司貢獻(xiàn)真朗，用于替代HttpUrlConnection和Apache HttpClient(android API23 6.0里已移除HttpClient）〈苏睿現(xiàn)在在Android開(kāi)發(fā)中最火的應(yīng)用最多的聯(lián)網(wǎng)框架毫無(wú)疑問(wèn)是okhttp，目前已經(jīng)大量替代了Volley蜜猾、HttpUrlConnection秀菱。

二、okhttp用法
1）導(dǎo)包：

    compile 'com.squareup.okhttp3:okhttp:3.9.1'

2）通過(guò)Builder類實(shí)例化客戶端類OkhttpClient蹭睡。

 private val client: OkHttpClient by lazy {
        OkHttpClient.Builder()
            .connectTimeout(30, TimeUnit.SECONDS)
            .readTimeout(60, TimeUnit.SECONDS)
            .writeTimeout(60, TimeUnit.SECONDS)
            .build()
    }

實(shí)例化OkhttpClient時(shí)可以通過(guò)Builder類添加一系列客戶端參數(shù)衍菱。
3）通過(guò)Builder類實(shí)例化請(qǐng)求類Request，通過(guò)OkHttpClient的newCall方法執(zhí)行請(qǐng)求肩豁。

 fun get(url: String, callBack: ResponseCallBack) {
        var request = Request.Builder()
            .url(Api.baseUrl + url)
            .build()
        client.newCall(request).enqueue(
            object : Callback {
                override fun onFailure(call: Call, e: IOException) {
                    callBack?.onFail(e.toString())
                }

                override fun onResponse(call: Call, response: Response) {
                    callBack?.onSuccess(response.body()!!.string())
                }

            }
        )
    }

    fun post(url: String, requestParams: RequestParams, callBack: ResponseCallBack) {
        var requestBody = FormBody.Builder()
        requestParams.forEach {
            requestBody.add(it.key, it.value.toString())
        }
        var request = Request.Builder()
            .url(Api.baseUrl + url)
            .post(requestBody.build())
            .build()
        client.newCall(request).enqueue(
            object : Callback {
                override fun onFailure(call: Call, e: IOException) {
                    callBack?.onFail(e.toString())
                }

                override fun onResponse(call: Call, response: Response) {
                    callBack?.onSuccess(response.body()!!.string())
                }
            }
        )
    }

實(shí)例化Request時(shí)可以通過(guò)Builder類添加一系列請(qǐng)求參數(shù)（例如Header信息）脊串，這里也不進(jìn)行詳細(xì)描述，在分析源碼時(shí)進(jìn)行具體分析清钥。
4）同步琼锋、異步請(qǐng)求。execute()執(zhí)行同步請(qǐng)求祟昭，enqueue()執(zhí)行異步請(qǐng)求缕坎，同步請(qǐng)求方法返回Response，異步請(qǐng)求通過(guò)CallBack返回Response篡悟。

三谜叹、源碼分析
1）OkHttpClient客戶端類，主要看一下各個(gè)成員變量搬葬。

final Dispatcher dispatcher;//請(qǐng)求分發(fā)器荷腊，分發(fā)執(zhí)行Request
  final @Nullable Proxy proxy;//代理
  final List<Protocol> protocols;//協(xié)議，管理協(xié)議版本
  final List<ConnectionSpec> connectionSpecs;//傳輸層版本和連接協(xié)議
  final List<Interceptor> interceptors;//攔截器
  final List<Interceptor> networkInterceptors;//網(wǎng)絡(luò)攔截器
  final EventListener.Factory eventListenerFactory;//請(qǐng)求監(jiān)聽(tīng)器
  final ProxySelector proxySelector;//代理選擇
  final CookieJar cookieJar;//cookie
  final @Nullable Cache cache;//緩存
  final @Nullable InternalCache internalCache;//內(nèi)部緩存
  final SocketFactory socketFactory;//socket工廠
  final @Nullable SSLSocketFactory sslSocketFactory;//HTTPS ssl安全套接層工廠
  final @Nullable CertificateChainCleaner certificateChainCleaner;//驗(yàn)證證書(shū)
  final HostnameVerifier hostnameVerifier;//確認(rèn)主機(jī)名
  final CertificatePinner certificatePinner;//證書(shū)鏈
  final Authenticator proxyAuthenticator;//代理身份驗(yàn)證
  final Authenticator authenticator;//本地身份驗(yàn)證
  final ConnectionPool connectionPool;//連接池
  final Dns dns;//DNS域名解析
  final boolean followSslRedirects;//安全套接層重定向
  final boolean followRedirects;//本地重定向
  final boolean retryOnConnectionFailure;//連接失敗重試
  final int connectTimeout;//連接超時(shí)
  final int readTimeout;//讀取超時(shí)
  final int writeTimeout;//寫(xiě)入超時(shí)
  final int pingInterval;//ping命令時(shí)間間隔

可以看到所有成員變量都是final的急凰，這樣所有的變量賦值就全部交給Builder類女仰，Builder類是OkHttpClient的靜態(tài)內(nèi)部類。可以通過(guò)builder()來(lái)完成設(shè)置疾忍，在不進(jìn)行設(shè)置情況下會(huì)使用Builder的默認(rèn)設(shè)置乔外。

  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;
    }

2）Request請(qǐng)求信息類。成員變量如下：

  final HttpUrl url;//請(qǐng)求url信息
  final String method;//請(qǐng)求方式(post get)
  final Headers headers;//請(qǐng)求header
  final @Nullable RequestBody body;//請(qǐng)求Body
  final Object tag;//請(qǐng)求tag

3）執(zhí)行newCall()方法創(chuàng)建Call對(duì)象锭碳，Call接口唯一實(shí)現(xiàn)類是RealCall袁稽，是執(zhí)行請(qǐng)求的真正對(duì)象。

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

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;
  }

RealCall請(qǐng)求類的源碼如下：

final class RealCall implements Call {
  final OkHttpClient client;
  final RetryAndFollowUpInterceptor retryAndFollowUpInterceptor;//請(qǐng)求重定向攔截器
  private EventListener eventListener;
  final Request originalRequest;//request信息
  final boolean forWebSocket;
  // Guarded by this.
  private boolean executed;
  private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    this.client = client;
    this.originalRequest = originalRequest;
    this.forWebSocket = forWebSocket;
    this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
  }

  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;
  }
  //同步請(qǐng)求
  @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);
    }
  }
.....
//異步請(qǐng)求
  @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));
  }

  @Override public void cancel() {
    retryAndFollowUpInterceptor.cancel();
  }

  @Override public synchronized boolean isExecuted() {
    return executed;
  }

  @Override public boolean isCanceled() {
    return retryAndFollowUpInterceptor.isCanceled();
  }

  @SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
  @Override public RealCall clone() {
    return RealCall.newRealCall(client, originalRequest, forWebSocket);
  }

  StreamAllocation streamAllocation() {
    return retryAndFollowUpInterceptor.streamAllocation();
  }

  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);
      }
    }
  }

//鏈?zhǔn)秸?qǐng)求方法
  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);
  }

先來(lái)分析同步請(qǐng)求擒抛，從源碼中可以看到同步請(qǐng)求中會(huì)先通過(guò)

client.dispatcher().executed(this);

完成對(duì)請(qǐng)求的監(jiān)聽(tīng)推汽，本質(zhì)上是將當(dāng)前請(qǐng)求放入一個(gè)隊(duì)列中，當(dāng)請(qǐng)求結(jié)束之后再移除隊(duì)列歧沪。

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

對(duì)于異步請(qǐng)求來(lái)說(shuō)同樣會(huì)進(jìn)行請(qǐng)求的監(jiān)聽(tīng)歹撒，唯一的區(qū)別在于異步請(qǐng)求是用兩個(gè)隊(duì)列完成，一個(gè)是等待請(qǐng)求的隊(duì)列诊胞，一個(gè)是正在請(qǐng)求的隊(duì)列暖夭。

/** 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<>();

最終進(jìn)行網(wǎng)絡(luò)請(qǐng)求是通過(guò)鏈?zhǔn)椒椒╣etResponseWithInterceptorChain來(lái)完成的。這個(gè)方法也是網(wǎng)絡(luò)請(qǐng)求的精髓所在撵孤。這個(gè)方法里通過(guò)獲取設(shè)置的一系列攔截器來(lái)完成網(wǎng)絡(luò)請(qǐng)求工作迈着，每個(gè)攔截器有不同的分工。最后通過(guò)構(gòu)建的攔截器鏈調(diào)用proceed()方法完成網(wǎng)絡(luò)請(qǐng)求邪码。由于不同的攔截器分別負(fù)責(zé)網(wǎng)絡(luò)請(qǐng)求中的不同工作裕菠。分析起來(lái)內(nèi)容比較多，這里先不進(jìn)行具體各個(gè)攔截器的源碼分析闭专。

4）Dispatcher請(qǐng)求分發(fā)器奴潘，在okhttp中網(wǎng)絡(luò)請(qǐng)求分發(fā)是通過(guò)Dispatcher分發(fā)器來(lái)維護(hù)的，Dispatcher控制著當(dāng)前網(wǎng)絡(luò)請(qǐng)求是否執(zhí)行還是進(jìn)入等待狀態(tài)影钉，這里涉及一些網(wǎng)絡(luò)請(qǐng)求的域名等判斷信息画髓。Dispatcher類源碼如下：

public final class Dispatcher {
  private int maxRequests = 64;//最大請(qǐng)求數(shù)
  private int maxRequestsPerHost = 5;//最大主機(jī)名
  private @Nullable Runnable idleCallback;
  private @Nullable ExecutorService executorService;//請(qǐng)求線程池，核心線程0平委，非核心線程最大值Integer.MAX_VALUE
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();//異步等待隊(duì)列
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();//異步請(qǐng)求隊(duì)列
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();//同步請(qǐng)求隊(duì)列
.......
  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;
  }

  public synchronized void setMaxRequests(int maxRequests) {
    if (maxRequests < 1) {
      throw new IllegalArgumentException("max < 1: " + maxRequests);
    }
    this.maxRequests = maxRequests;
    promoteCalls();
  }

  public synchronized int getMaxRequests() {
    return maxRequests;
  }
  public synchronized void setMaxRequestsPerHost(int maxRequestsPerHost) {
    if (maxRequestsPerHost < 1) {
      throw new IllegalArgumentException("max < 1: " + maxRequestsPerHost);
    }
    this.maxRequestsPerHost = maxRequestsPerHost;
    promoteCalls();
  }

  public synchronized int getMaxRequestsPerHost() {
    return maxRequestsPerHost;
  }

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

  private void promoteCalls() {
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();
      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        executorService().execute(call);
      }
      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
  }

  private int runningCallsForHost(AsyncCall call) {
    int result = 0;
    for (AsyncCall c : runningAsyncCalls) {
      if (c.host().equals(call.host())) result++;
    }
    return result;
  }

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

  void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }

  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();
    }
  }
  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 queuedCallsCount() {
    return readyAsyncCalls.size();
  }
  public synchronized int runningCallsCount() {
    return runningAsyncCalls.size() + runningSyncCalls.size();
  }
}

在同步請(qǐng)求時(shí)直接加入同步請(qǐng)求隊(duì)列中奈虾，持有同步請(qǐng)求的請(qǐng)求引用。對(duì)于異步請(qǐng)求來(lái)講廉赔，會(huì)判斷當(dāng)前正在執(zhí)行的請(qǐng)求是否達(dá)到最大請(qǐng)求限制愚墓，并且正在執(zhí)行請(qǐng)求的主機(jī)名是否達(dá)到個(gè)數(shù)限制，如果都沒(méi)有達(dá)到限制昂勉，則立即通過(guò)線程池進(jìn)行網(wǎng)絡(luò)請(qǐng)求，否則放入異步等待隊(duì)列扫腺。那么什么時(shí)候進(jìn)行執(zhí)行等待隊(duì)列中的請(qǐng)求呢岗照？
在一個(gè)網(wǎng)絡(luò)請(qǐng)求結(jié)束之后會(huì)調(diào)用Dispatcher的finished方法，在finished方法中會(huì)調(diào)用promoteCalls()方法來(lái)判斷時(shí)候可以執(zhí)行等待隊(duì)列中的網(wǎng)絡(luò)請(qǐng)求。

文中demo代碼：https://github.com/24KWYL/okhttp.git