前言
说句废话，作为一个工作几年的程序员，在日常工作中，难免会用一些三方封装库，来方便我们的开发，但是不能仅仅会用就满足，我们还了解它的工作原理。
正文
转入正题，看到本文的朋友应该知道了OK给出的API的基本调用（本文不对基本使用做介绍，有需要自行看https://github.com/square/okhttp/wiki/Recipes），
那下面试着揭开OK的面纱，查看内部是如何实现的。
准备工作： 首先去下载https://github.com/square/okhttp源码，然后maven编译完成后导入IDEA中。
在分析工作流程前，需要先了解几个类：OkHttpClient，Call,Request,Response。
OkHttpClient： Call的工厂，可以发送HTTP请求和读取响应，推荐多个请求共享同一个client，每一个 client都持有自己的connection pool（连接池）和（线程池），reuse 连接和线程，减少了延迟和节省了内存的开支；相反，对每一个Request创建一个client浪费空闲池中的资源。
推荐创建singleton HTTP client的方式
使用：
public final OkHttpClient client = new OkHttpClient();
或者：
public final OkHttpClient client = new OkHttpClient.Builder()
.addInterceptor(new HttpLoggingInterceptor())
.cache(new Cache(cacheDir, cacheSize))
.build();
线程和连接在空闲时会自动释放。
关闭 dispatcher's executor service ，This will also cause future calls to the client to be rejected
client.dispatcher().executorService().shutdown()： 主动释放后，以后的Call 调用也会被拒绝。
Clear the connection pool with client.connectionPool().evictAll() Note that the
清除连接池的操作：client.connectionPool().evictAll()，connection pool's daemon thread may not exit immediately.
连接池的守护线程可能不会立即退出。
关闭客户端缓存的方法
如果client 有cache，调用client.cache().close()，注：创建Calls 使用cache,但是设置了关闭cache会导致报错？
If your client has a cache, call {@link Cache#close close()}. Note that it is an error to
create calls against a cache that is closed, and doing so will cause the call to crash.
Call ：A call is a request that has been prepared for execution. 准备被执行的请求，可以取消，一个call实例表示一个请求/响应对(流)，只能执行一次。
Request：An HTTP request.
Response：An HTTP response 这个类的实例不是不可变的，ResponseBody(响应体)是一个一次性的值，只被使用一次，然后被关闭。所有其他属性都是不可变的。接下来，就拿下面这个Get请求为例，看下Ok的内部工作流程吧。
package okhttp3.guide;
import java.io.IOException;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
public class GetExample {
OkHttpClient client = new OkHttpClient();
String run(String url) throws IOException {
Request request = new Request.Builder()
.url(url)
.build();
try (Response response = client.newCall(request).execute()) {
return response.body().string();
}
}
public static void main(String[] args) throws IOException {
GetExample example = new GetExample();
String response = example.run("https://raw.github.com/square/okhttp/master/README.md");
System.out.println(response);
}
}| 流程描述：初始化OkHttpClient 对象，接下来执行run()方法，里面对Request对象进行初始化，然后client.newCall(request).execute()发起请求，返回结果Response。 |
第一步OkHttpClient初始化
首先对OkHttpClient初始化操作，发起Request前进行默认初始化设置，相当于准备工作。
下面是OkHttpClient 类中的部分属性。
final Dispatcher dispatcher; final @Nullable Proxy proxy; final List<Protocol> protocols; final List<ConnectionSpec> connectionSpecs; final List<Interceptor> interceptors; final List<Interceptor> networkInterceptors; final EventListener.Factory eventListenerFactory; final ProxySelector proxySelector; final CookieJar cookieJar; final @Nullable Cache cache; final @Nullable InternalCache internalCache; final SocketFactory socketFactory; final @Nullable SSLSocketFactory sslSocketFactory; final @Nullable CertificateChainCleaner certificateChainCleaner; final HostnameVerifier hostnameVerifier; final CertificatePinner certificatePinner; final Authenticator proxyAuthenticator; final Authenticator authenticator; final ConnectionPool connectionPool; final Dns dns; final boolean followSslRedirects; final boolean followRedirects; final boolean retryOnConnectionFailure; final int connectTimeout; final int readTimeout; final int writeTimeout; final int pingInterval;
第二步 构建Request
接下来构建Request，指定是设置Request的 url ，请求类型 such as get/post …
以及request headers or request body , tag 。
下面是Request类中定义的部分属性。
/**
* An HTTP request. Instances of this class are immutable if their {@link #body} is null or itself
* immutable.
*/
public final class Request {
final HttpUrl url;
final String method;
final Headers headers;
final @Nullable RequestBody body;
final Object tag;
private volatile CacheControl cacheControl; // Lazily initialized.第三步发起Request返回Response
下面是发起请求的代码，我们拆开来看
try (Response response = client.newCall(request).execute()) {
return response.body().string();
}client.newCall(request) 执行完后返回了Call 对象。
/**
* 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 */);
}我们可以看出RealCall.newRealCall(),返回了Call对象，看下RealCall
inal class RealCall implements Call {
/**省略部分代码*/
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;
}
@Override public Response execute() throws IOException {
/**省略部分代码*/
try {
client.dispatcher().executed(this);
//=============关键代码==============
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);
}
/**省略部分代码*/
}可以看出RealCall 实现了 Call接口，静态方法newRealCall初始化一个RealCall对象，并且有一个execute()方法，这个方法返回值就是Response 对象，也就是说，我们调用client.newCall(request).execute()发起Request，然后返回 Response。
Response response = getResponseWithInterceptorChain();
RealCall的execute()方法中有这样一行代码，很显然，是getResponseWithInterceptorChain()拿到了对应的Response 信息，一起看下RealCall中的这个方法
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);
}getResponseWithInterceptorChain方法首先把所有的interceptors封装到一个集合中，然后把这个集合作为参数，初始化RealInterceptorChain【RealInterceptorChain是拦截器链，承载所有的拦截器链，包含：所有的自定义拦截器，OK内核，所有的网络拦截器，及网络调用。】
最后执行proceed(),现在我们一步步靠近Ok请求的核心了，看下proceed方法
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
/**省略部分代码*/
// 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);
//=============关键代码==============
Response response = interceptor.intercept(next);
/**省略部分代码*/
return response;
}可以看到 chain内部的proceed方法最终还是执行的interceptor.intercept()方法拿到了Response,那几回过头看下前面getResponseWithInterceptorChain()方法中都有那些interceptor
//只看interceptor
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
//client我们自己设置的interceptor
interceptors.addAll(client.interceptors());
//retryAndFollowUpInterceptor：请求失败时，重试和重定向拦截器。
interceptors.add(retryAndFollowUpInterceptor);
//从应用程序代码到网络代码的桥梁
interceptors.add(new BridgeInterceptor(client.cookieJar()));
//缓存拦截器 从cache中读取response和写response到 cache
interceptors.add(new CacheInterceptor(client.internalCache()));
//打开到目标服务器的连接，并继续执行下一个拦截器
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
//拦截器链中最后一个interceptor，It makes a network call to the server
interceptors.add(new CallServerInterceptor(forWebSocket));
/**省略部分代码*/
}注释很明显，最后一个CallServerInterceptor拦截器的任务是向Remote Server发起Request。又前面我们知道 调用的都是Interceptor的 intercept()方法，那最终，我们跟踪一个发起的Request到了 CallServerInterceptor的intercept方法中。
@Override public Response intercept(Chain chain) throws IOException {
/**
* * A concrete interceptor chain that carries the entire interceptor chain: all application
* interceptors, the OkHttp core, all network interceptors, and finally the network caller.
* 拦截器 链 ，ok的核心，拦截器是网咯的调用者
*/
RealInterceptorChain realChain = (RealInterceptorChain) chain;
/**
* Encodes HTTP requests and decodes HTTP responses. 编码http请求，解码http响应
*/
HttpCodec httpCodec = realChain.httpStream();
/**
* 协调 Connections 、Stream、 Calls 之间关系的类。
* Connections： physical socket connections to remote servers ：物理Socket 连接远程服务器
* Streams：HTTP request/response pairs HTTP 请求/响应对。
* Calls：a logical sequence of streams, typically an initial request and its follow up requests.
* 一系列的流，通常是初始请求及后续请求，推荐每个call上面所有的流，使用相同的链接。
* We prefer to keep all streams of a single call on the same connection for better behavior and locality.
*/
StreamAllocation streamAllocation = realChain.streamAllocation();
/**
* HTTP套接字和流 发送和接收数据
*/
RealConnection connection = (RealConnection) realChain.connection();
/**
* HTTP请求
*/
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
/**
* 开始处理请求头
*/
realChain.eventListener().requestHeadersStart(realChain.call());
httpCodec.writeRequestHeaders(request);
realChain.eventListener().requestHeadersEnd(realChain.call(), request);
Response.Builder responseBuilder = null;
/**
* 判断请求方法 和请求body 不为空
*/
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 (responseBuilder == null) {
/**
* Write the request body if the "Expect: 100-continue" expectation was met.
* 如果“期望:100继续”的期望得到满足，请编写请求体。
*/
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);
}
/**
* 初始化response对象
*/
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
/**
* 响应头
*/
realChain.eventListener()
.responseHeadersEnd(realChain.call(), response);
int code = response.code();
if (forWebSocket && code == 101) {
/**
*Connection is upgrading, but we need to ensure interceptors see a non-null response body.
连接正在升级，但是我们需要确保拦截器看到一个非空响应体
*/
response = response.newBuilder()
.body(Util.EMPTY_RESPONSE)
.build();
} else {
/**
* 最终执行到这里将response对象，构建完成，然后返回
* 构建response 的ResponseBody
*/
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
//===========删除部分代码========================
return response;
}这个方法比较重要，所以要看仔细一点，由此，我们可以得到我们想要的结果了。
结论
一个Request的发起到接收Response 经历的过程：
首先构建OkHttpClient 对象，接下来使用构建好的Request作为参数传递给OkHttpClient 的newCall() 生成RealCall 对象，然后执行call的execute方法，最终这个方法内部通过CallServerInterceptor返回了Response，Response 封装了服务器返回的数据。
大体流程就是这样，更多细节与流程图稍后补充。
作者：u011733020