主要參考文章：
1.Okhttp的基本使用
2.Okhttp主流程源碼分析

Okhttp 3.12.0 使用例子

String url = "http://wwww.baidu.com";
OkHttpClient okHttpClient = new OkHttpClient();
final Request request = new Request.Builder()
        .url(url)
        .get()//默認(rèn)就是GET請(qǐng)求襟齿，可以不寫
        .build();
Call call = okHttpClient.newCall(request);
call.enqueue(new Callback() {
    @Override
    public void onFailure(Call call, IOException e) {
        Log.d("ziq", "onFailure: ");
    }

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

1、初始化

建造者模式瓤介，public OkHttpClient() { this(new Builder());} 初始化了一些配置信息：支持協(xié)議灭衷、任務(wù)分發(fā)器（其內(nèi)部包含一個(gè)線程池辆亏，執(zhí)行異步請(qǐng)求）、連接池(其內(nèi)部包含一個(gè)線程池晋涣，維護(hù)connection)、連接/讀/寫超時(shí)時(shí)長(zhǎng)等信息沉桌。

public Builder() {
    dispatcher = new Dispatcher();//任務(wù)調(diào)度器
    protocols = DEFAULT_PROTOCOLS;//支持的協(xié)議 Protocol.HTTP_2, Protocol.HTTP_1_1
    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;//根據(jù)地址與證書 進(jìn)行校驗(yàn)X509Certificate
    certificatePinner = CertificatePinner.DEFAULT;
    proxyAuthenticator = Authenticator.NONE;//身份驗(yàn)證
    authenticator = Authenticator.NONE;//身份驗(yàn)證
    connectionPool = new ConnectionPool();//連接池
    dns = Dns.SYSTEM;
    followSslRedirects = true;
    followRedirects = true;
    retryOnConnectionFailure = true;
    callTimeout = 0;
    connectTimeout = 10_000;
    readTimeout = 10_000;
    writeTimeout = 10_000;
    pingInterval = 0;
}

下面列出 Dispatcher 的主要分析部分

//Dispatcher.java
public final class Dispatcher {
private int maxRequests = 64;//最大請(qǐng)求數(shù)量
private int maxRequestsPerHost = 5;//每臺(tái)主機(jī)最大的請(qǐng)求數(shù)量
private @Nullable Runnable idleCallback;
private @Nullable ExecutorService executorService;//線程池
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();//等待隊(duì)列
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();//異步運(yùn)行中隊(duì)列
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();//同步運(yùn)行中隊(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;
}

谢鹊。。留凭。
void enqueue(AsyncCall call) {
    synchronized (this) {
        readyAsyncCalls.add(call);//添加到異步運(yùn)行隊(duì)列中
    }
    promoteAndExecute();//將 readyAsyncCalls中的call 添加到 runningAsyncCalls并運(yùn)行
}

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();
        //正在運(yùn)行的任務(wù)數(shù)是否大于 64
            if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
        //對(duì)應(yīng)主機(jī) 的 正在運(yùn)行的任務(wù)數(shù)是否大于 5
            if (runningCallsForHost(asyncCall) >= maxRequestsPerHost) continue; // Host max capacity.
            i.remove();
            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;
}

佃扼。。蔼夜。
}

2兼耀、生成call 并 請(qǐng)求

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

//RealCall.java
new RealCall(client, originalRequest, forWebSocket);

@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));
}

那么從dispatcher的代碼中可知道，call直接加入到 readyAsyncCalls隊(duì)列中求冷，然后 在promoteAndExecute方法中判斷是否 可以執(zhí)行 readyAsyncCalls中的任務(wù)瘤运，如果可以則 把任務(wù)加入到runningAsyncCalls ，并 到線程池中執(zhí)行匠题。

3拯坟、執(zhí)行call

asyncCall.executeOn(executorService());
AsyncCall extends NamedRunnable implements Runnable
，所以運(yùn)行run() ->execute() ;

final class AsyncCall extends NamedRunnable {
韭山。郁季。。
@Override protected void execute() {
    boolean signalledCallback = false;
    timeout.enter();
    try {
//1钱磅、進(jìn)過(guò)一系列攔截器后梦裂，返回結(jié)果
        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) {
        e = timeoutExit(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 {
//2、結(jié)束當(dāng)前call,運(yùn)行未執(zhí)行且符合條件的call
        client.dispatcher().finished(this);
    }
}

}

4续搀、Interceptor攔截器

到了這里塞琼，到達(dá)了okhttp最核心的部分， Interceptor禁舷，采用了責(zé)任鏈的設(shè)計(jì)模式

4.1client.dispatcher().finished(this)彪杉；

//Dispatcher.java
private <T> void finished(Deque<T> calls, T call) {
    Runnable idleCallback;
    synchronized (this) {
    if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
        idleCallback = this.idleCallback;
    }
    boolean isRunning = promoteAndExecute();
    if (!isRunning && idleCallback != null) {
        idleCallback.run();
    }
}

可以看到毅往，移除call后，又回到 一開始 的promoteAndExecute() 方法派近，去把readyAsyncCalls中的call 拿出來(lái)運(yùn)行攀唯。

4.2 責(zé)任鏈

Response response = getResponseWithInterceptorChain();進(jìn)過(guò)一系列攔截器后，返回結(jié)果

//RealCall.java
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);
}

添加的各Interceptor渴丸，分別負(fù)責(zé)功能：
client.interceptors() 用戶自定義的Interceptor侯嘀，能攔截到所有的請(qǐng)求
RetryAndFollowUpInterceptor 負(fù)責(zé)失敗重連和重定向相關(guān)
BridgeInterceptor 負(fù)責(zé)配置請(qǐng)求的頭信息，比如Keep-Alive谱轨、gzip戒幔、Cookie等可以優(yōu)化請(qǐng)求
CacheInterceptor 負(fù)責(zé)緩存管理，使用DiskLruCache做本地緩存土童，CacheStrategy決定緩存策略
ConnectInterceptor 開始與目標(biāo)服務(wù)器建立連接诗茎，獲得RealConnection
client.networkInterceptors() 用戶自定義的Interceptor，僅在生產(chǎn)網(wǎng)絡(luò)請(qǐng)求時(shí)生效
CallServerInterceptor 實(shí)際網(wǎng)絡(luò)請(qǐng)求的地方献汗。

遞歸實(shí)現(xiàn)：
1敢订、RealInterceptorChain.proceed(...) 為開頭敦腔，會(huì)生成 下一步的RealInterceptorChain next1疟赊，與當(dāng)前的RealInterceptorChain參數(shù)一樣，區(qū)別是index 加1了,所以會(huì)調(diào)用 下一級(jí) 攔截器的interceptor1.intercept(next)
2湘捎、interceptor1.intercept(next)調(diào)用 下一個(gè)鏈 next1.proceed(...）方法
3尿招、next1.proceed(...）調(diào)用下一個(gè)interceptor2.intercept(next) 形成 遞歸
4矾柜、最后的攔截器是 CallServerInterceptor，返回response泊业，然后一級(jí)一級(jí)返回response把沼。

//RealInterceptorChain.java
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,RealConnection connection)throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();
    calls++;
    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 (this.httpCodec != null && calls > 1) {
        throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
        + " must call proceed() exactly once");
    }
//1、生成 下一步的RealInterceptorChain吁伺，與當(dāng)前的RealInterceptorChain參數(shù)一樣饮睬，區(qū)別是index 加1 了
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
 //2、Interceptor interceptor內(nèi)會(huì)調(diào)用 next.proceed(...) 又回到這里的第一步篮奄，再生成一個(gè)chain 形成遞歸
    Response response = interceptor.intercept(next);
//結(jié)束遞歸

    if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
        throw new IllegalStateException("network interceptor " + interceptor
        + " must call proceed() exactly once");
    }
    if (response == null) {
        throw new NullPointerException("interceptor " + interceptor + " returned null");
    }
    if (response.body() == null) {
        throw new IllegalStateException(
        "interceptor " + interceptor + " returned a response with no body");
    }
    return response;
}

各個(gè)攔截器部分細(xì)節(jié)描述：