okhttp基本调用过程(源码解析)

1.先写一个okhttp的请求（同步）

new Thread(new Runnable() {
@Override
public void run() {
OkHttpClient okHttpClient = new OkHttpClient();
Request request = new Request.Builder()
.url("https://www.baidu.com")
.build();
try {
Call call = okHttpClient.newCall(request);
call.execute();
} catch (IOException e) {
e.printStackTrace();
}
}
}).start();

首先 通过newCall来返回一个call，在这里我们先看看newcall这个方法

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

这是他实际上返回的是一个realcall，这个realcall是一个实现了call接口的类，此类就是我们真实call的实现，接线来看看call.execute()方法；

@Override public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
try {
client.dispatcher().executed(this);
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} finally {
client.dispatcher().finished(this);
}
}

这个方法最重要的地方就是下面这一段代码

try {
client.dispatcher().executed(this);
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} finally {
client.dispatcher().finished(this);
}

首先调用了dispatcher的executed方法，这个方法是干什么呢，看源码

synchronized void executed(RealCall call) {
runningSyncCalls.add(call);
}

所以，executed方法就是将这个call加入到了同步队列中，并没有其他操作了。

回到上一步，看到getResponseWithInterceptorChain()方法，重要滴地方来了

我们先看看getResponseWithInterceptorChain()干了什么

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);
return chain.proceed(originalRequest);
}

当当当当，你没有看错，一系列的拦截器，然后通过责任链模式，依次执行每个拦截器，稍后会对这些拦截器进行说明。okhttp正是通过这一系列的拦截器完成了对请求的封装，服务器的交互，数据的返回等等。

这里我们先看到chain.proceed(originalRequest)方法，chain是什么，chain是RealInterceptorChain，即拦截器链，这里我们先看到proceed方法的实现

@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}

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.
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);

// Confirm that the next interceptor made its required call to 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");
}

return response;
}

这么长的代码 其实主要看这里就好了

// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);

首先找到下一个拦截器，这个拦截器是按照加入list列表的顺序来的，第一个是Retry拦截器，第二个是Bridge拦截器。。。。其他几个去看源码加入过程,第一次进来index是0，所以Interceptor interceptor = interceptors.get(index);得到的是Retry拦截器， next其实是下一个拦截器，因为index+1，然后利用责任链模式，把所有的拦截器链起来。

先回到加入拦截器哪里，其中interceptors.addAll(client.interceptors());是我们自定义的拦截器，然后retryAndFollowUpInterceptor，这个拦截器是用来在网络请求失败后进行重试或者重定向时发起新的请求等，下面我们看看这个拦截器的intercept方法，这个方法是所有拦截的核心，所有拦截器都会调用这个方法

先看下面的代码

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

streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()), callStackTrace);

int followUpCount = 0;
Response priorResponse = null;
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}

Response response = null;
boolean releaseConnection = true;
try {
response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);

这里只贴了一半的代码，后面会讲为什么，首先拿到了我们请求的request，然后创建了一个streamAllocation，这个streamAllocation的作用主要是用来重连滴，对于这个东西还没深入看，就知道是这么个东西，然后重点来了，看到 response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null)， 上一步我们讲到了chain是下一个拦截器，我们调用下一个拦截的proceed方法，其实就是Bridge的proceed方法

@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());

这里的代码依然没有全，我们先看这个代码，其他是在构造header，然后我们来到最后一行，那么这个chain.proceed依然会执行下一个拦截器的intercept方法，这一次执行的是CacheInterceptor的intercept方法，我们看到这个方法

@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);

这个拦截器一看名字就清楚了，是缓存拦截器，首先判断缓存是不是空的，然后拿到缓存策略，如果缓存不是null，就进行缓存策略，然后巴拉巴拉，细节我也不明白，反正就是判断是否直接可以用缓存的数据，最后来到了proceed，这里就不用详细说了，直接讲下一个拦截器ConnectInterceptor

我们看这个拦截器的intercept方法

@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, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();

return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

先拿到HttpCodec，就是通过一些判断拿到HttpCodec对象，这个对象主要是来说明用HTTP1. 1还是HTTP2.0协议来进行访问的（如果是http则直接用HTTP1.1， 如果是HTTPS，则需要与服务器进行判断是否支持2.0）

RealConnection connection = streamAllocation.connection();

即为当前请求找到合适的连接，可能复用已有连接也可能是重新创建的连接，返回的连接由连接池负责决定。

接下就是proceed，这次调用的就是最后一个拦截器了CallServerInterceptor

看到这个拦截器的方法

@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();
httpCodec.writeRequestHeaders(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();
responseBuilder = httpCodec.readResponseHeaders(true);
}

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

if (responseBuilder == null) {
responseBuilder = httpCodec.readResponseHeaders(false);
}

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

int code = response.code();
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;
}

与服务器交互，拿到数据，进行各种判断，然后返回response，这个就是我们从服务器拿到的数据啦，不过还没完，这次返回后，并没有把这个response直接返回给我们，而是还有其他操作，当return后，会回到

RealInterceptorChain的proceed方法的后面一段

// Confirm that the next interceptor made its required call to 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");
}

return response;

到了这里执行完return后，最后一个拦截器 CallServerInterceptor 的职责才结束了，这个时候，return response后会回到倒数第二个拦截器，也就是ConnectInterceptor，不过此拦截器由于proceed方法后没有其他操作，直接return了，我们就又回到了上一段代码，再次return后 回到倒数第三个拦截器的proceed方法后面的方法，即CacheInterceptor 拦截器得到返回的response。然后对response做了一些操作处理。

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();

// 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();

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;

这里return后，回到chain的proceed方法，然后返回response，回到Bridge拦截器的

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);
responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}

return responseBuilder.build();
}

这就是之前没有把代码写完的原因，当责任一层一层执行完之后，才会回到这里，跟递归差不多滴样子，这里又是对response处理，然后返回，最后会回到第一个拦截器也就是RetryAndFollowUpInterceptor 的

try {
response = ((RealInterceptorChain) chain).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.
if (!recover(e.getLastConnectException(), false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, requestSendStarted, request)) throw e;
releaseConnection = false;
continue;
} 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);

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()), callStackTrace);
} 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;
}

然后在 这个判断中

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

将结果返回，这里返回之后，回到RealCall的exceute方法的result那里，得到最后的结果result，

@Override public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
try {
client.dispatcher().executed(this);
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} finally {
client.dispatcher().finished(this);
}
}

执行client.dispatcher().finished(this)，返回result到我们最开始的调用处。这就是一个同步请求的流程。

异步的流程

OkHttpClient okHttpClient = new OkHttpClient();
Request request = new Request.Builder()
.url("https://www.baidu.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 {

}
});

进入 enqueue方法

``java

@Override public void enqueue(Callback responseCallback) {

synchronized (this) {

if (executed) throw new IllegalStateException(“Already Executed”);

executed = true;

}

captureCallStackTrace();

client.dispatcher().enqueue(new AsyncCall(responseCallback));

}

主要代码是 client.dispatcher().enqueue(new AsyncCall(responseCallback)); 生成一个AsyncCall,关于这个call，大家可以去看看源码就好了，然后调用enqueue
```java
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}

<div class="se-preview-section-delimiter"></div>

这里先进行了判断，如果正在请求的call小于maxrequests 并且 当前call的host的连接数小于maxRequestsPerHost，则加入runningAsyncCalls，并执行execute方法.

java

这里先进行了判断，如果正在请求的call小于maxrequests 并且 当前call的host的连接数小于maxRequestsPerHost，则加入runningAsyncCalls，并执行execute方法.
```java
@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 {
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
}

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到了这里 就跟之前同步请求是一样的了，onResponse和onFailure 就是我们的异步回调咯。

当然，如果不满足条件的话 机会进入readyAsyncCalls这个队列，

当一个请求执行完之后会调用finish方法，我们找到这个finish方法的实现处

``java

到了这里  就跟之前同步请求是一样的了，onResponse和onFailure 就是我们的异步回调咯。
当然，如果不满足条件的话  机会进入readyAsyncCalls这个队列，
当一个请求执行完之后会调用finish方法，我们找到这个finish方法的实现处
```java
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();
}
}

<div class="se-preview-section-delimiter"></div>

这里看到 promoteCalls()

java

这里看到 promoteCalls()
```java
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.
}
}

这里就是从readyAsyncCalls里面取出call去进行请求的实现啦。

综上就是okhttp的调用过程，如果有不对的地方，欢迎留言，我也会去纠正，小白敬上!!!