开源框架源码解析系列（1）—

开源框架源码解析系列（1）——进入OkHttp的世界

以前曾经写过一篇关于Okhttp的使用的文章深入解析OkHttp3，通过这篇文章可以了解OkHttp的各种基本用法，光会使用并不算好汉，我们还要深入理解源码，学习优秀的设计思想，本篇我就带大家一起分析源码，基于Okhttp 3.10.0版本。

1.请求部分源码解析
1.1 回顾请求的基本用法
1.1.1 发送同步请求

Request request = new Request.Builder().url(url).build();
try {
//同步请求
Call call = mOkHttpClient.newCall(request);
Response response = call.execute();
String json = response.body().string();
Log.d(TAG, json);

} catch (IOException e) {
e.printStackTrace();
}
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1.1.2 发送异步请求

//异步请求
Call call = mOkHttpClient.newCall(request);
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
Log.d(TAG, "onFailure:" + e.getMessage());
}

@Override
public void onResponse(Call call, Response response) throws IOException {
String json = response.body().string();
Log.d(TAG, json);
}
});
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1.2 同步请求源码分析
首先，我们要理解，无论是同步请求还是异步请求，我们都需要先编写以下代码：

OkHttpClient mOkHttpClient = new OkHttpClient();
Request request = new Request.Builder().url(url).build();
Call call = mOkHttpClient.newCall(request);
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Call是请求的关键对象，是通过调用Call的execute方法之后，就会进入请求的逻辑

Response response = call.execute();
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1.2.1 OkHttpClient# newCall

@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
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我们可以看到OkHttpClient# newCall会调用RealCall.newRealCall方法

1.2.2 RealCall # newRealCall

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;
}
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查看一下构造函数

private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
}
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RealCall的创建过程中会持有OkHttpClient，请求的 Request还有创建了一个拦截器RetryAndFollowUpInterceptor（这个后面会详细说明），同时创建eventListener 。

1.2.3 Call# execute()
创建好Call之后，调用execute()方法就开始了请求的流程，Call是一个借口，所以我们要查看它的实现类RealCall。

1.2.4 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);
}
}
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这里重点在于如下两句

client.dispatcher().executed(this);
Response result = getResponseWithInterceptorChain();
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调用OkHttpClient持有的Dispatcher对象执行call，Dispatcher是非常重要的一环，后面详细介绍。

1.2.5 Dispatcher# execute()

synchronized void executed(RealCall call) {
runningSyncCalls.add(call);
}
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private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
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将同步请求的RealCall 添加到同步的队列中。

1.2.6 RealCall# getResponseWithInterceptorChain()
通过RealCall# getResponseWithInterceptorChain()方法就可以获取请求返回的Response，返回到调用者

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);
}
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这里主要是一系列拦截器的添加操作，然后调用Interceptor.Chain的proceed方法去执行请求

chain.proceed(originalRequest)
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拦截器又是另一个非常重要的环节，后面重点提到。

1.2.7总结同步请求
同步请求逻辑相对简单，通过Call# execute()最终会调用 RealCall# execute()，然后通过分发器Dispatcher将任务添加到同步队列中，然后通过一系列拦截器操作后进行请求，最后返回Response，全程都在主线程中运行，是阻塞式的。

1.3 异步请求源码分析
1.3.1 Call#enqueue
异步请求，会调用Call#enqueue方法，因为是异步，所以需要传递一个Callback回调，Call#enqueue中调用了RealCall#enqueue

1.3.2 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));
}
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RealCall#enqueue中同样会调用到Dispatcher中，只是调用enqueue方法，同时new AsyncCall将Callback 包一层.AsyncCall是RealCall的内部类，从中可以获取RealCall的Request等成员。

1.3.2 Dispatcher#enqueue

synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
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/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
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主要做了几件事：
1.判断正在执行的异步任务队列中任务数是否小于maxRequests，且正在执行的任务的host小于 maxRequestsPerHost，这两个值的大小为：

private int maxRequests = 64;
private int maxRequestsPerHost = 5;
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同时满足条件，则将AsyncCall 添加到runningAsyncCalls队列中，runningAsyncCalls是异步任务的队列，否则添加到readyAsyncCalls等待队列中。

2.调用Dispatcher#executorService方法，获取Android系统提供的线程池

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;
}
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**注意：**这里设置线程池的最大容量为 Integer.MAX_VALUE，但其实受限于maxRequests，所以最多容量也就64而已。

3.通过ExecutorService执行AsyncCall任务，可想而知AsyncCall一定是实现了Runnable接口。

1.3.3 NamedRunnable#run
AsyncCall继承自NamedRunnable，所以当AsyncCall任务执行时，会执行NamedRunnable#run

@Override public final void run() {
String oldName = Thread.currentThread().getName();
Thread.currentThread().setName(name);
try {
execute();
} finally {
Thread.currentThread().setName(oldName);
}
}
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主要逻辑在execute()方法

1.3.4 AsyncCall#execute

@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);
}
}
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可以看到还是会和同步请求一样，调用getResponseWithInterceptorChain()执行各种拦截器，返回Response，无论是同步还是异步最后都会执行Dispatcher#inished()方法，这个后面会提到。

1.3.5 总结异步请求

Call#enqueue会调用到 Dispatcher#enqueue，然后判断是否符合最大请求数maxRequests（64），最大请求Host数maxRequestsPerHost （5），符合条件的添加到异步任务队列runningAsyncCalls，通过线程池执行任务，否则添加到等待队列readyAsyncCalls。

2 Dispatcher分析
前面分析同步和异步请求的时候，都提到Dispatcher，我们这里重新总结一下：
1.维护了3个队列，同步请求执行队列runningSyncCalls，异步请求执行队列runningAsyncCalls，异步请求等待队列readyAsyncCalls，3个队列的添加逻辑前面已经提过。

/** 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<>();

/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
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2.维护了异步请求的线程池，异步执行任务通过线程池进行任务执行

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;
}
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3.RealCall中请求任务执行完后，进行回收，我们知道Call会被封装成RealCall，但无论同步还是异步执行完成后，都会调用以下代码

finally {
client.dispatcher().finished(this);
}
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2.1 Dispatcher#finished
分为同步和异步的finished，我们先看同步的代码：
2.1.1 同步finished

/** Used by {@code Call#execute} to signal completion. */
void finished(RealCall call) {
finished(runningSyncCalls, call, false);
}
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注意这里传入的第三个参数为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();
}
}
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1.首先将队列中call进行移除
2.如果promoteCalls为true还会调用 promoteCalls()方法
3.计算runningCallsCount，即为同步和异步执行队列的size总和

public synchronized int runningCallsCount() {
return runningAsyncCalls.size() + runningSyncCalls.size();
}
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4.当runningCallsCount为0时说明已经没有任务了，进行回调

if (runningCallsCount == 0 && idleCallback != null) {
idleCallback.run();
}
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void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
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2.1.2 异步finished
异步代码如下：

void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
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和同步的区别是，传入的promoteCalls为true，所以当执行finished时会比同步多执行一个promoteCalls()方法
Dispatcher#promoteCalls

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.
}
}

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逻辑很清晰，就是当异步执行队列readyAsyncCalls有空闲位置时，遍历等待队列，将readyAsyncCalls的任务取出加入readyAsyncCalls，然后线程池对任务进行执行。

3.拦截器解析
3.1 拦截器执行顺序
在之前分析同步和异步任务的时候，分析过getResponseWithInterceptorChain()方法执行后就会返回请求结果Response

return chain.proceed(originalRequest);
}
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1.首先将调用者自定义的拦截器都放入interceptors集合的最前面，然后是分别添加okhttp中必须的几个拦截器，后面我们会一一分析
2.创建拦截器的链RealInterceptorChain,将interceptors传入

从上述代码可以看出拦截器调用的先后顺序依次是
client.interceptors()–>RetryAndFollowUpInterceptor–>BridgeInterceptor–>CacheInterceptor–>ConnectInterceptor–>client.networkInterceptors()–>CallServerInterceptor

这里使用到了非常经典的设计模式，就是责任链模式，reques自上而下下传递执行，然后Response至下而上返回

这里3个参数是为 null的

3.2 RealInterceptorChain#proceed
@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}
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关键部分代码如下：

// 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);
Response response = interceptor.intercept(next);

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又创建了一个RealInterceptorChain，然后获取interceptors中第一位的拦截器开始执行，这里index=0。然后就会按照顺序执行各拦截器。

3.3 RetryAndFollowUpInterceptor
如果没有自定义拦截器情况下，首先会走到RetryAndFollowUpInterceptor的intercept方法

@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) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}

Response response;
boolean releaseConnection = true;
try {
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
//省略
} catch (IOException e) {
//省略
} finally {
// We're throwing an unchecked exception. Release any resources.
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();
}

Request followUp = followUpRequest(response, streamAllocation.route());

if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}

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());
}

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?");
}

request = followUp;
priorResponse = response;
}
}

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1.创建StreamAllocation

StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(request.url()), call, eventListener, callStackTrace);
this.streamAllocation = streamAllocation;
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主要传入OkHttpClient中的ConnectionPool，还有通过请求request.url()创建出Address对象，主要是HTTP请求中一些SSLSocket，host认证，Dns等

realChain.proceed
try {
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
}
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realChain执行proceed方法，此时streamAllocation已经有值传入，此时再次进入RealInterceptorChain#proceed方法中

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();

calls++;

//省略
// 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);
Response response = interceptor.intercept(next);

//省略
return response;
}
}
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这里再次进来index已经变为1，然后又再新创建一个RealInterceptorChain，从interceptors中取出下一个拦截器，执行下一个拦截器的逻辑

**总结一下：**这里责任链模式，每一个拦截器执行时都会创建一个拦截器链RealInterceptorChain，index也会随之增加1，这样在 interceptors.get(index)中就会取出下一个拦截器，一直向下执行到没有拦截器为止，同时每一个拦截的Response是下一个拦截器执行的返回的结果

RetryAndFollowUpInterceptor最重要的是创建了StreamAllocation

3.4 BridgeInterceptor拦截器
BridgeInterceptor#intercept

@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();

RequestBody body = userRequest.body();
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.
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}

List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}

if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}

Response networkResponse = chain.proceed(requestBuilder.build());

HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);

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();
}
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BridgeInterceptor拦截器的作用主要是添加一些网络请求的必备参数，例如Content-Type，Content-Length，Host，ConnectionAccept-Encoding，Cookie，User-Agent等，如果有使用gzip的话，还会进行gzip的处理

3.5 CacheInterceptor拦截器
3.5.1 CacheInterceptor#intercept
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;

long now = System.currentTimeMillis();

CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;

if (cache != null) {
cache.trackResponse(strategy);
}

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.
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.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}

Response networkResponse = null;
try {
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());
}
}

// If we have a cache response too, then we're doing a conditional get.
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();

Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();

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;
}
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1.先从cache中以chain.request()为key获取缓存的Response，这里的request就是外部调用时创建的，这里的cache是InternalCache，实现类为okhttp3.Cache,查看get方法

3.5.2 okhttp3.Cache#get
@Nullable
Response get(Request request) {
String key = key(request.url());

Snapshot snapshot;
try {
snapshot = this.cache.get(key);
if (snapshot == null) {
return null;
}
} catch (IOException var7) {
return null;
}

Cache.Entry entry;
try {
entry = new Cache.Entry(snapshot.getSource(0));
} catch (IOException var6) {
Util.closeQuietly(snapshot);
return null;
}

Response response = entry.response(snapshot);
if (!entry.matches(request, response)) {
Util.closeQuietly(response.body());
return null;
} else {
return response;
}
}
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1.根据请求的url，进行计算获得一个key
2.在内部cache中通过key看有没保存的快照Snapshot。这里cache是采用了DiskLruCache的算法
3.如果Snapshot不为空，通过Snapshot创建出Cache.Entry，查看一下Cache.Entry的组成

其实就是存储了一些请求返回的信息

4.通过entry.response方法获取缓存中的Response

public Response response(Snapshot snapshot) {
String contentType = this.responseHeaders.get("Content-Type");
String contentLength = this.responseHeaders.get("Content-Length");
Request cacheRequest = (new okhttp3.Request.Builder()).url(this.url).method(this.requestMethod, (RequestBody)null).headers(this.varyHeaders).build();
return (new okhttp3.Response.Builder()).request(cacheRequest).protocol(this.protocol).code(this.code).message(this.message).headers(this.responseHeaders).body(new Cache.CacheResponseBody(snapshot, contentType, contentLength)).handshake(this.handshake).sentRequestAtMillis(this.sentRequestMillis).receivedResponseAtMillis(this.receivedResponseMillis).build();
}
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通过缓存得参数构造Request ，然后通过Request再创建出Response
5.校验缓存中的请求和相应是否和传入的Request所关联的一致

entry.matches(request, response)
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6.将请求链chain中的request和缓存Response构造出一个CacheStrategy

CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
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3.5.3 CacheStrategy#Factory方法

其实就是从缓存的cacheResponse中取出一些值进行赋值

3.5.4 CacheStrategy.Factory#get方法
public CacheStrategy get() {
CacheStrategy candidate = getCandidate();

if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// We're forbidden from using the network and the cache is insufficient.
return new CacheStrategy(null, null);
}

return candidate;
}
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主要逻辑在getCandidate方法中

CacheStrategy#getCandidate
private CacheStrategy getCandidate() {
// No cached response.
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}

// Drop the cached response if it's missing a required handshake.
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}

// If this response shouldn't have been stored, it should never be used
// as a response source. This check should be redundant as long as the
// persistence store is well-behaved and the rules are constant.
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}

CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}

CacheControl responseCaching = cacheResponse.cacheControl();
if (responseCaching.immutable()) {
return new CacheStrategy(null, cacheResponse);
}

long ageMillis = cacheResponseAge();
long freshMillis = computeFreshnessLifetime();

if (requestCaching.maxAgeSeconds() != -1) {
freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}

long minFreshMillis = 0;
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}

long maxStaleMillis = 0;
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}

if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
Response.Builder builder = cacheResponse.newBuilder();
if (ageMillis + minFreshMillis >= freshMillis) {
builder.addHeader("Warning", "110 HttpURLConnection "Response is stale"");
}
long oneDayMillis = 24 * 60 * 60 * 1000L;
if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
builder.addHeader("Warning", "113 HttpURLConnection "Heuristic expiration"");
}
return new CacheStrategy(null, builder.build());
}

// Find a condition to add to the request. If the condition is satisfied, the response body
// will not be transmitted.
String conditionName;
String conditionValue;
if (etag != null) {
conditionName = "If-None-Match";
conditionValue = etag;
} else if (lastModified != null) {
conditionName = "If-Modified-Since";
conditionValue = lastModifiedString;
} else if (servedDate != null) {
conditionName = "If-Modified-Since";
conditionValue = servedDateString;
} else {
return new CacheStrategy(request, null); // No condition! Make a regular request.
}

Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);

Request conditionalRequest = request.newBuilder()
.headers(conditionalRequestHeaders.build())
.build();
return new CacheStrategy(conditionalRequest, cacheResponse);
}
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getCandidate方法决定CacheStrategy的构成，一般会有如下一些情况：
noCache ：不使用缓存，全部走网络
noStore ：不使用缓存，也不存储缓存
onlyIfCached ：只使用缓存
maxAge ：设置最大失效时间，失效则不使用
maxStale ：设置最大失效时间，失效则不使用
minFresh ：设置最小有效时间，失效则不使用
FORCE_NETWORK ：强制走网络
FORCE_CACHE ：强制走缓存

可以发现CacheStrategy中cacheResponse为null空有几种情况
1）没有缓存的response
2）如果这个请求是https的，但上次缓存的cacheResponse没有TLS handshake
3.通过isCacheable判断，一些请求返回值不符合要求的不缓存，还有就是请求头中有配置no-store参数时
4.请求头中声明了“no-cache”，或者“If-Modified-Since”，“If-None-Match”（服务器缓存）
5.请求头中没有添加任何条件时候

继续看 get()方法

if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// We're forbidden from using the network and the cache is insufficient.
return new CacheStrategy(null, null);
}
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如果外部设置了onlyIfCached（只读缓存），但缓存又无效，那就构造的CacheStrategy中既没有request也没有request

3.5.5 再次回到 CacheInterceptor#intercept
// If we're forbidden from using the network and the cache is insufficient, fail.
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();
}
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1.如果设置了only-if-cached，只读缓存，但又没有缓存的Response，那就返回504

// If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
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2.networkRequest == null这里表示只用缓存，不用网络请求，那就将缓存返回

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();

// 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());
}
}
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4.如果旧的cacheResponse不为null，又通过网络请求返回操作码304，则将新的response更新

if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
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5.新的请求进行缓存，然后过期缓存进行移除

3.6 ConnectInterceptor拦截器
ConnectInterceptor#intercept

@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);
}
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从链中获取出StreamAllocation，通过streamAllocation.newStream方法返回一个HttpCodec，HttpCodec的作用是对请求进行编码，然后对响应进行解码

3.6.1 StreamAllocation#newStream
public HttpCodec newStream(
OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
int connectTimeout = chain.connectTimeoutMillis();
int readTimeout = chain.readTimeoutMillis();
int writeTimeout = chain.writeTimeoutMillis();
int pingIntervalMillis = client.pingIntervalMillis();
boolean connectionRetryEnabled = client.retryOnConnectionFailure();

try {
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);

synchronized (connectionPool) {
codec = resultCodec;
return resultCodec;
}
} catch (IOException e) {
throw new RouteException(e);
}
}

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通过findHealthyConnection方法创建一个RealConnection，此类负责https连接的主要工作，RealConnection#newCodec方法可以创建出HttpCodec，并返回。

3.6.2 StreamAllocation#findHealthyConnection
通过findConnection方法创建RealConnection，如果这个RealConnection是全新的连接，就跳过连接健康检查，如果是之前已经连接过的RealConnection，则判断是不是一个健康的连接，如果否的话就将其从连接池connectionPool中进行回收。接下来看findConnection方法做了什么

3.6.3 StreamAllocation#findConnection
......
if (result == null) {
// Attempt to get a connection from the pool.
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
foundPooledConnection = true;
result = connection;
} else {
selectedRoute = route;
}
}
......
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Internal.instance.get方法从连接池中获取是否有复用的连接，Internal是一个接口，它的实现在OkHttpClient的内部类中

1.OkHttpClient.Internal.instance#get

@Override public RealConnection get(ConnectionPool pool, Address address,
StreamAllocation streamAllocation, Route route) {
return pool.get(address, streamAllocation, route);
}
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2.ConnectionPool#get

@Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {
assert (Thread.holdsLock(this));
for (RealConnection connection : connections) {
if (connection.isEligible(address, route)) {
streamAllocation.acquire(connection, true);
return connection;
}
}
return null;
}
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在连接池中找出能匹配Address 的连接，注意这里的route传进来为null,connection.isEligible方法判断连接池中connection是否可复用，主要通过判断请求连接的host是否一致，具体逻辑在RealConnection#isEligible中
3.RealConnection#isEligible

public boolean isEligible(Address address, @Nullable Route route) {
// If this connection is not accepting new streams, we're done.
if (allocations.size() >= allocationLimit || noNewStreams) return false;

// If the non-host fields of the address don't overlap, we're done.
if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;

// If the host exactly matches, we're done: this connection can carry the address.
if (address.url().host().equals(this.route().address().url().host())) {
return true; // This connection is a perfect match.
}

// At this point we don't have a hostname match. But we still be able to carry the request if
// our connection coalescing requirements are met. See also:
// https://hpbn.co/optimizing-application-delivery/#eliminate-domain-sharding
// https://daniel.haxx.se/blog/2016/08/18/http2-connection-coalescing/

// 1. This connection must be HTTP/2.
if (http2Connection == null) return false;

// 2. The routes must share an IP address. This requires us to have a DNS address for both
// hosts, which only happens after route planning. We can't coalesce connections that use a
// proxy, since proxies don't tell us the origin server's IP address.
if (route == null) return false;
if (route.proxy().type() != Proxy.Type.DIRECT) return false;
if (this.route.proxy().type() != Proxy.Type.DIRECT) return false;
if (!this.route.socketAddress().equals(route.socketAddress())) return false;

// 3. This connection's server certificate's must cover the new host.
if (route.address().hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
if (!supportsUrl(address.url())) return false;

// 4. Certificate pinning must match the host.
try {
address.certificatePinner().check(address.url().host(), handshake().peerCertificates());
} catch (SSLPeerUnverifiedException e) {
return false;
}

return true; // The caller's address can be carried by this connection.
}
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通过streamAllocation.acquire(connection, true)，将connection和StreamAllocation相关联

4.StreamAllocation#acquire

public void acquire(RealConnection connection, boolean reportedAcquired) {
assert (Thread.holdsLock(connectionPool));
if (this.connection != null) throw new IllegalStateException();

this.connection = connection;
this.reportedAcquired = reportedAcquired;
connection.allocations.add(new StreamAllocationReference(this, callStackTrace));
}
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在这里将connection赋值给了StreamAllocation，然后connection有一个集合存放与其关联的StreamAllocation，这里StreamAllocationReference是一个弱引用。

3.6.4 再次回到ConnectInterceptor#intercept
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
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通过streamAllocation获取出connection，继续传递给下一个拦截器

4.CallServerInterceptor 拦截器
这个是Okhttp中自带的最后一个拦截器

4.1CallServerInterceptor#intercept
@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();

realChain.eventListener().requestHeadersStart(realChain.call());
httpCodec.writeRequestHeaders(request);
realChain.eventListener().requestHeadersEnd(realChain.call(), request);

Response.Builder responseBuilder = null;
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 ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
realChain.eventListener().responseHeadersStart(realChain.call());
responseBuilder = httpCodec.readResponseHeaders(true);
}

if (responseBuilder == null) {
// Write the request body if the "Expect: 100-continue" expectation was met.
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);
}

Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();

int code = response.code();
if (code == 100) {
// server sent a 100-continue even though we did not request one.
// try again to read the actual response
responseBuilder = httpCodec.readResponseHeaders(false);

response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();

code = response.code();
}

realChain.eventListener()
.responseHeadersEnd(realChain.call(), response);

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 {
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}

if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}

if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}

return response;
}
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这个拦截器主要的任务如下：
1.写入请求头
2.写入请求体
3.读取响应头
4.读取响应体

这样所有的Okhttp流程基本分析完毕，再往深入网络连接流这一块能力有限无法进行分析
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