读OkHttp3源码（一）：OkHttpClient

1，简介：OkHttpClient是Call的的工厂，可用于发送HTTP请求并读取其响应。 大多数应用程序可以对所有HTTP请求使用一个OkHttpClient，这得益于共享的响应缓存、线程池、连接重用等等。

• 要使用默认设置创建OkHttpClient，请使用默认构造函数。
• 或者使用OkHttpClient.Builder创建自定义配置的实例。
• 要在发出请求之前调整现有客户端，请使用newBuilder()。

2，构造方法：

public OkHttpClient() {
    this(new Builder());
}

默认构造方法调用另外一个私有构造方法，需要传入一个Builder对象：

private 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.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;
        } else {
            try {
                SSLContext sslContext = SSLContext.getInstance("TLS");
                sslContext.init(null, null, null);
                this.sslSocketFactory = sslContext.getSocketFactory();
            } catch (GeneralSecurityException e) {
                //系统没有TLS。就放弃。
                throw new AssertionError();
            }
        }
        if (sslSocketFactory != null && builder.trustRootIndex == null) {
            X509TrustManager trustManager = Platform.get().trustManager(sslSocketFactory);
            if (trustManager == null) {
                throw new IllegalStateException("Unable to extract the trust manager on " + Platform.get()
                        + ", sslSocketFactory is " + sslSocketFactory.getClass());
            }
            this.trustRootIndex = Platform.get().trustRootIndex(trustManager);
            this.certificatePinner = builder.certificatePinner.newBuilder()
                    .trustRootIndex(trustRootIndex)
                    .build();
        } else {
            this.trustRootIndex = builder.trustRootIndex;
            this.certificatePinner = builder.certificatePinner;
        }
        this.hostnameVerifier = builder.hostnameVerifier;
        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.connectTimeout = builder.connectTimeout;
        this.readTimeout = builder.readTimeout;
        this.writeTimeout = builder.writeTimeout;
    }

通过builder对象默认的属性对OkHttpClient里的dispatcher（调度器）、proxy（代理）、protocols（支持协议）、connectionSpecs（连接策略）、interceptors（拦截器）、networkInterceptors（网络拦截器）、proxySelector（代理选择器）、cookieJar（cookie存储）、cache（缓存）、internalCache（okHttp自己使用的缓存，不对外）、socketFactory（套接字工厂）、sslSocketFactory（ssl套接字工厂）、trustRootIndex（可信赖根）、hostnameVerifier（主机名称校验器）、certificatePinner（固定证书）、proxyAuthenticator（代理服务器授权验证）

 

/**
 * 请求（calls）的的工厂类，可用于发送HTTP请求并读取其响应。
 * 大多数应用程序可以对所有HTTP请求使用一个OkHttpClient，这得益于共享的响应缓存、线程池、连接重用等等。
 * 要使用默认设置创建OkHttpClient，请使用默认构造函数。或者使用OkHttpClient.Builder创建自定义配置的实例。
 * 要在发出请求之前调整现有客户端，请使用newBuilder()。
 * 这个例子显示了一个超时30秒的调用：
 * <pre>
 *     OkHttpClient client = ...
 *     OkHttpClient clientWith30sTimeout = client.newBuilder()
 *          .readTimeout(30, TimeUnit.SECONDS)
 *          .build();
 *    Response response = clientWith30sTimeout.newCall(request).execute();
 * </pre>
 */
public class OkHttpClient implements Cloneable, Call.Factory {
    private static final List<Protocol> DEFAULT_PROTOCOLS = Util.immutableList(
            Protocol.HTTP_2, Protocol.SPDY_3, Protocol.HTTP_1_1);

    private static final List<ConnectionSpec> DEFAULT_CONNECTION_SPECS = Util.immutableList(
            ConnectionSpec.MODERN_TLS, ConnectionSpec.COMPATIBLE_TLS, ConnectionSpec.CLEARTEXT);

    static {
        Internal.instance = new Internal() {
            @Override
            public void addLenient(Headers.Builder builder, String line) {
                builder.addLenient(line);
            }

            @Override
            public void addLenient(Headers.Builder builder, String name, String value) {
                builder.addLenient(name, value);
            }

            @Override
            public void setCache(Builder builder, InternalCache internalCache) {
                builder.setInternalCache(internalCache);
            }

            @Override
            public InternalCache internalCache(OkHttpClient client) {
                return client.internalCache();
            }

            @Override
            public boolean connectionBecameIdle(
                    ConnectionPool pool, RealConnection connection) {
                return pool.connectionBecameIdle(connection);
            }

            @Override