本文介绍 OkHttp 网络框架，包含简单的使用和源码解析。本文内容基于 OkHttp 3.11.0 版本。
网上关于 OkHttp 源码解析的文章有很多，我在这里参考他们的资料，形成自己的知识体系。
只是停留在应用层面，会使用一些框架是不行的，还需要深入源码、剖析结构。
An HTTP+HTTP/2 client for Android and Java applications. http://square.github.io/okhttp/
支持 HTTP/2 协议，允许连接到同一个主机地址的所有请求共享 Socket 。
在 HTTP/2 协议不可用的情况下，通过连接池减少请求的延迟。
支持 GZip 透明压缩，减少传输的数据包大小。
支持响应缓存，避免同一个重复的网络请求。
OkHttp 的简单使用
一般情况下，对于网络框架有两种常见的使用场景，同步请求和异步请求。
同步请求：
OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
Request request = new Request.Builder().url("https://wshunli.com").build();
Call call = okHttpClient.newCall(request);
Response response = call.execute();
Log.d(TAG, "onCreate: " + response.body().string());异步请求：
OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
Request request = new Request.Builder().url("https://wshunli.com").build();
Call call = okHttpClient.newCall(request);
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
}
@Override
public void onResponse(Call call, Response response) throws IOException {
Log.d(TAG, "onCreate: " + response.body().string());
}
});同步请求和异步请求类似，先实例化 OkHttpClient 和 Request 对象，然后使用 OkHttpClient 对象的 newCall() 方法创建 Call 对象，只不过最后执行 enqueue() 方法，整体和网络请求的思路相似。
OkHttp 的源码分析
OkHttp 网络请求完整的流程图如下：
下面详细介绍。
同步请求
同步请求，先实例化 OkHttpClient 和 Request 对象，然后使用 OkHttpClient 对象的 newCall() 方法创建 Call 对象，最后执行 execute() 方法，整体和网络请求的思路相似。
OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
Request request = new Request.Builder().url("https://wshunli.com").build();
Call call = okHttpClient.newCall(request);
Response response = call.execute();创建 OkHttpClient 对象
我们先看 OkHttp 的构造函数：
public OkHttpClient() {
this(new Builder());
}这里是直接实例化，实质上是使用 建造者模式 构建 OkHttpClient 实例。
下面是 OkHttpClient 内部类 Builder 的构造方法：
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;
}
public OkHttpClient build() {
return new OkHttpClient(this);
}这里 OkHttpClient.Builder 有很多参数，后面再介绍。
创建 Request 对象
和 OkHttpClient 类似，Request 也是是使用 建造者模式 创建实例。
public Builder() {
this.method = "GET";
this.headers = new Headers.Builder();
}
public Request build() {
if (url == null) throw new IllegalStateException("url == null");
return new Request(this);
}其中配置默认请求方法为 GET ，还有一些头部的默认参数。
创建 Call 对象
OkHttpClient 实现了 Call.Factory ，负责根据请求创建新的 Call 对象。
@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}Call 只是个接口，实际是实例化的 RealCall 对象。
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;
}发送同步网络请求
发送请求也是在 RealCall 的 execute() 方法中执行的。
// RealCall#execute()
@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);
}
}在这里主要做了四件事：
1、检查 Call 是否执行过，没有执行将 executed 赋值为 true ，保证每个请求只执行一次；
2、使用 client.dispatcher().executed(this) 来进行实际的请求；
3、调用 getResponseWithInterceptorChain() 方法，获取请求响应的结果；
4、最后 dispatcher 结束自己。
// Dispatcher#executed()
/** Used by {@code Call#execute} to signal it is in-flight. */
synchronized void executed(RealCall call) {
runningSyncCalls.add(call);
}在同步请求中 dispatcher 只是负责判断请求执行的状态，在异步请求中参与内容过多。
下面我们来看 getResponseWithInterceptorChain() 方法：
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors()); // 加入用户自定义的拦截器
interceptors.add(retryAndFollowUpInterceptor); // 重试和重定向拦截器
interceptors.add(new BridgeInterceptor(client.cookieJar())); // 加入转化请求响应的拦截器
interceptors.add(new CacheInterceptor(client.internalCache())); // 加入缓存拦截器
interceptors.add(new ConnectInterceptor(client)); // 加入连接拦截器
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors()); // 加入用户自定义的网络拦截器
}
interceptors.add(new CallServerInterceptor(forWebSocket)); // 加入请求响应的拦截器
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.readTimeoutMillis());
// 利用 chain 来链式调用拦截器，最后的返回结果就是 Response 对象
return chain.proceed(originalRequest);
}我们都知道，拦截器是 OkHttp 的精髓。
1、client.interceptors() ，首先加入 interceptors 的是用户自定义的拦截器，比如修改请求头的拦截器等；
2、RetryAndFollowUpInterceptor 是用来重试和重定向的拦截器，在下面我们会讲到；
3、BridgeInterceptor 是用来将用户友好的请求转化为向服务器的请求，之后又把服务器的响应转化为对用户友好的响应；
4、CacheInterceptor 是缓存拦截器，若存在缓存并且可用就直接返回该缓存，否则会向服务器请求；
5、ConnectInterceptor 用来建立连接的拦截器；
6、client.networkInterceptors() 加入用户自定义的 networkInterceptors ；
7、CallServerInterceptor是真正向服务器发出请求且得到响应的拦截器；
最后在聚合了这些拦截器后，利用 RealInterceptorChain 来链式调用这些拦截器，利用的就是 责任链模式 。
下面介绍 OkHttp 中的 拦截器
拦截器 Interceptor 是 OkHttp 的核心，实际上它把实际的网络请求、缓存、透明压缩等功能都统一了起来，每一个功能都只是一个 Interceptor，它们再连接成一个 Interceptor.Chain，环环相扣，最终圆满完成一次网络请求。
1、RealInterceptorChain 拦截器链
拦截器链 RealInterceptorChain 是真正把这些拦截器串起来的一个角色，调用 proceed() 方法
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
// If we already have a stream, confirm that the incoming request will use it.
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 we already have a stream, confirm that this is the only call to chain.proceed().
if (this.httpCodec != null && calls > 1) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must call proceed() exactly once");
}
// Call the next interceptor in the chain.
// 得到下一次对应的 RealInterceptorChain
RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
writeTimeout);
// 当前次数的 interceptor
Interceptor interceptor = interceptors.get(index);
// 进行拦截处理，并且在 interceptor 链式调用 next 的 proceed 方法
Response response = interceptor.intercept(next);
// Confirm that the next interceptor made its required call to chain.proceed().
// 确认下一次的 interceptor 调用过 chain.proceed()
if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
throw new IllegalStateException("network interceptor " + interceptor
+ " must call proceed() exactly once");
}
// Confirm that the intercepted response isn't null.
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;
}在代码中是一次次链式调用拦截器。
2、RetryAndFollowUpInterceptor 重试和重定向的拦截器
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Call call = realChain.call();
EventListener eventListener = realChain.eventListener();
StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(request.url()), call, eventListener, callStackTrace);
this.streamAllocation = streamAllocation;
int followUpCount = 0;
Response priorResponse = null;
while (true) {
// 如果取消，就释放资源
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response;
boolean releaseConnection = true;
try {
// 调用下一个拦截器
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
// 路由连接失败，请求将不会被发送
if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
throw e.getFirstConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
// 服务器连接失败，请求可能已被发送
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
// 抛出未检查的异常，释放资源
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
// 如果不需要重定向，那么 followUp 为空，会根据响应码判断
Request followUp;
try {
followUp = followUpRequest(response, streamAllocation.route());
} catch (IOException e) {
streamAllocation.release();
throw e;
}
// 释放资源，返回 response
if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}
// 关闭 response 的 body
closeQuietly(response.body());
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
// response 和 followUp 比较是否为同一个连接
// 若为重定向就销毁旧连接，创建新连接
if (!sameConnection(response, followUp.url())) {
streamAllocation.release();
streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(followUp.url()), call, eventListener, callStackTrace);
this.streamAllocation = streamAllocation;
} else if (streamAllocation.codec() != null) {
throw new IllegalStateException("Closing the body of " + response
+ " didn't close its backing stream. Bad interceptor?");
}
// 将重定向操作得到的新请求设置给 request
request = followUp;
priorResponse = response;
}
}总体来说，RetryAndFollowUpInterceptor 是用来失败重试以及重定向的拦截器。
3、BridgeInterceptor 桥街和适配拦截器
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
// 将用户友好的 request 构造为发送给服务器的 request
RequestBody body = userRequest.body();
// 若有请求体，则构造
if (body != null) {
MediaType contentType = body.contentType();
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());
}
long contentLength = body.contentLength();
if (contentLength != -1) {
requestBuilder.header("Content-Length", Long.toString(contentLength));
requestBuilder.removeHeader("Transfer-Encoding");
} else {
requestBuilder.header("Transfer-Encoding", "chunked");
requestBuilder.removeHeader("Content-Length");
}
}
if (userRequest.header("Host") == null) {
requestBuilder.header("Host", hostHeader(userRequest.url(), false));
}
if (userRequest.header("Connection") == null) {
requestBuilder.header("Connection", "Keep-Alive");
}
// If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
// the transfer stream.
// 使用 gzip 压缩
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}
// 设置 cookie
List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}
// 设置 UA
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
// 构造完后，将 request 交给下一个拦截器去处理。最后又得到服务端响应 networkResponse
Response networkResponse = chain.proceed(requestBuilder.build());
// 保存 networkResponse 的 cookie
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
// 将 networkResponse 构造为对用户友好的 response
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
// 如果 networkResponse 使用 gzip 并且有响应体的话，给用户友好的 response 设置响应体
if (transparentGzip
&& "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
&& HttpHeaders.hasBody(networkResponse)) {
GzipSource responseBody = new GzipSource(networkResponse.body().source());
Headers strippedHeaders = networkResponse.headers().newBuilder()
.removeAll("Content-Encoding")
.removeAll("Content-Length")
.build();
responseBuilder.headers(strippedHeaders);
String contentType = networkResponse.header("Content-Type");
responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
}
return responseBuilder.build();
}在 BridgeInterceptor 这一步，先把用户友好的请求进行重新构造，变成了向服务器发送的请求。
之后调用 chain.proceed(requestBuilder.build()) 进行下一个拦截器的处理。
等到后面的拦截器都处理完毕，得到响应。再把 networkResponse 转化成对用户友好的 response 。
4、CacheInterceptor 缓存拦截器
分析 CacheInterceptor 拦截器 intercept() 方法的源代码
@Override public Response intercept(Chain chain) throws IOException {
// 得到 request 对应缓存中的 response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
// 获取当前时间，会和之前缓存的时间进行比较
long now = System.currentTimeMillis();
// 得到缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
// 追踪缓存，其实就是计数
if (cache != null) {
cache.trackResponse(strategy);
}
// 缓存不适用，关闭
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
// If we're forbidden from using the network and the cache is insufficient, fail.
// 禁止网络并且没有缓存的话，返回失败
if (networkRequest == null && cacheResponse == null) {
return new Response.Builder()
.request(chain.request())
.protocol(Protocol.HTTP_1_1)
.code(504)
.message("Unsatisfiable Request (only-if-cached)")
.body(Util.EMPTY_RESPONSE)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
// If we don't need the network, we're done.
// 不用网络请求，返回缓存
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
// 交给下一个拦截器，返回 networkResponse
networkResponse = chain.proceed(networkRequest);
} finally {
// If we're crashing on I/O or otherwise, don't leak the cache body.
if (networkResponse == null && cacheCandidate != null) {
closeQuietly(cacheCandidate.body());
}
}
// 如果我们同时有缓存和 networkResponse ，根据情况使用
if (cacheResponse != null) {
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
Response response = cacheResponse.newBuilder()
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.sentRequestAtMillis(networkResponse.sentRequestAtMillis())
.receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
networkResponse.body().close();
// 更新原来的缓存至最新
// Update the cache after combining headers but before stripping the
// Content-Encoding header (as performed by initContentStream()).
cache.trackConditionalCacheHit();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
// 保存之前未缓存的缓存
if (cache != null) {
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
CacheRequest cacheRequest = cache.put(response);
return cacheWritingResponse(cacheRequest, response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}CacheInterceptor 做的事情就是根据请求拿到缓存，若没有缓存或者缓存失效，就进入网络请求阶段，否则会返回缓存。
5、ConnectInterceptor 拦截器
@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Request request = realChain.request();
StreamAllocation streamAllocation = realChain.streamAllocation();
// We need the network to satisfy this request. Possibly for validating a conditional GET.
boolean doExtensiveHealthChecks = !request.method().equals("GET");
HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}实际上建立连接就是创建了一个 HttpCodec 对象，它是对 HTTP 协议操作的抽象，有两个实现：Http1Codec 和 Http2Codec，顾名思义，它们分别对应 HTTP/1.1 和 HTTP/2 版本的实现。
6、CallServerInterceptor 拦截器，发送和接收数据
@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
HttpCodec httpCodec = realChain.httpStream();
StreamAllocation streamAllocation = realChain.streamAllocation();
RealConnection connection = (RealConnection) realChain.connection();
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
// 整理请求头并写入
httpCodec.writeRequestHeaders(request);
Response.Builder responseBuilder = null;
// 检查是否为有 body 的请求方法
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.
// 如果有 Expect: 100-continue 在请求头中，那么要等服务器的响应
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
responseBuilder = httpCodec.readResponseHeaders(true);
}
if (responseBuilder == null) {
// Write the request body if the "Expect: 100-continue" expectation was met.
// 写入请求体
Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
} 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();
// 得到响应头
if (responseBuilder == null) {
responseBuilder = httpCodec.readResponseHeaders(false);
}
// 构造 response
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
int code = response.code();
// 如果为 web socket 且状态码是 101 ，那么 body 为空
if (forWebSocket && code == 101) {
// Connection is upgrading, but we need to ensure interceptors see a non-null response body.
response = response.newBuilder()
.body(Util.EMPTY_RESPONSE)
.build();
} else {
// 读取 body
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
// 如果请求头中有 close 那么断开连接
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}
// 抛出协议异常
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}在 CallServerInterceptor 中可见，关于请求和响应部分都是通过 HttpCodec 来实现的。而在 HttpCodec 内部又是通过 sink 和 source 来实现的。所以说到底还是 IO 流在起作用。
异步请求
和同步请求类似，先实例化 OkHttpClient 和 Request 对象，然后使用 OkHttpClient 对象的 newCall() 方法创建 Call 对象，只不过最后执行 enqueue() 方法，整体和网络请求的思路相似。
OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
Request request = new Request.Builder().url("https://wshunli.com").build();
Call call = okHttpClient.newCall(request);
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
}
@Override
public void onResponse(Call call, Response response) throws IOException {
}
});异步请求在 Callback 回调中获取响应，有 onResponse() 、 onFailure() 两个方法。
发送异步网络请求
前面三个步骤完全一致，我们从发送异步网络请求开始，异步请求是调用 RealCall 实例的 enqueue() 方法。。
// RealCall#enqueue()
@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));
}这里使用 Dispatcher 分发器我来处理请求。
// Dispatcher#enqueue()
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}实质上异步网络请求是在 Dispatcher 中做到任务调度。
下面介绍 OkHttp 中的 任务调度
我们来看 Dispatcher 类的源代码。
public final class Dispatcher {
private int maxRequests = 64;
private int maxRequestsPerHost = 5;
private @Nullable Runnable idleCallback;
/** Executes calls. Created lazily. */
// 线程池的实现
private @Nullable ExecutorService executorService;
/** Ready async calls in the order they'll be run. */
// 就绪等待网络请求的异步队列
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
// 正在执行网络请求的异步队列
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
// 正在执行网络请求的同步队列
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
public Dispatcher(ExecutorService executorService) {
this.executorService = executorService;
}
public Dispatcher() {
}
// 创建线程池
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;
}
/* 省略部分无关代码*/
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
/* 省略部分无关代码*/
}异步请求是放在线程池中执行的，如果最大异步请求数小于 64 并且 单个 HOST 的异步请求数小于 5 ，将请求添加到 runningAsyncCalls 中，否则添加到 readyAsyncCalls 中。
我们来看添加进线程池的 AsyncCall 类，实际上 AsyncCall 是继承自 NamedRunnable 的 RealCall 内部类。NamedRunnable 是实现了 Runnable 接口的抽象类。
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 {
// 和同步请求相同，调用拦截器，得到响应
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 {
// 在 runningAsyncCalls 中移除
client.dispatcher().finished(this);
}
}
}在 AsyncCall 的 execute() 方法中，也是调用了 getResponseWithInterceptorChain() 方法来得到 Response 对象。从这里开始，就和同步请求的流程是一样的，就没必要讲了。
不同的是在得到 Response 后，进行结果的回调。
在 AsyncCall 的最后调用了 Dispatcher 的 finished() 方法。
// Dispatcher#finished()
/** Used by {@code AsyncCall#run} to signal completion. */
void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
/** Used by {@code Call#execute} to signal completion. */
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!");
// 将 readyAsyncCalls 中的 call 移动到 runningAsyncCalls 中，并加入到线程池中
if (promoteCalls) promoteCalls();
runningCallsCount = runningCallsCount();
idleCallback = this.idleCallback;
}
if (runningCallsCount == 0 && idleCallback != null) {
idleCallback.run();
}
}这里所做的工作就是把执行过的 Call 移除，然后将 readyAsyncCalls 中的 Call 移动到 runningAsyncCalls 中并加入线程池中。
基本上 OkHttp 的请求响应的流程就介绍完了，主要是关于 OkHttp 的 拦截器链 和 任务调度 原理。
还有很多细节没有涉及，需要花费很大的精力，才能理解分析透彻，后面有机会再介绍。
参考资料：
1、拆轮子系列：拆 OkHttp - Piasy的博客 | Piasy Blog
https://blog.piasy.com/2016/07/11/Understand-OkHttp/
2、OkHttp源码解析 | 俞其荣的博客 | Qirong Yu's Blog
http://yuqirong.me/2017/07/25/OkHttp源码解析/
3、OkHttp源码分析 - 掘金
https://juejin.im/post/5af4482951882567286064e6
4、okhttp源码分析（一）——基本流程（超详细） - 简书
https://www.jianshu.com/p/37e26f4ea57b
5、OKHttp源码解析 | Frodo's Blog
http://frodoking.github.io/2015/03/12/android-okhttp/
6、OkHttp 源码解析（一）：基本流程 - Coding - SegmentFault 思否
https://segmentfault.com/a/1190000012656606
7、【Android】OkHttp源码分析 - CSDN博客
https://blog.csdn.net/u010983881/article/details/79175824
8、深入浅出 OkHttp 源码 - DiyCode
https://www.diycode.cc/topics/640
9、Okhttp框架源码分析 - 简书
https://www.jianshu.com/p/18a4861600d1
10、OkHttp 3.7源码分析（一）——整体架构 - CSDN博客
https://blog.csdn.net/asiaLIYAZHOU/article/details/72598320
11、okhttp网络框架源码解析 - CSDN博客
https://blog.csdn.net/fanguangjun123/article/details/78621585
12、OKHttp网络框架源码解析（一）okHttp框架同步异步请求流程和源码分析 - CSDN博客
https://blog.csdn.net/qq_24675479/article/details/79483193
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