简介
okhttp的网络请求采用interceptors链的模式。每一级interceptor只处理自己的工作，然后将剩余的工作，交给下一级interceptor。本文将主要阅读okhttp中的RetryAndFollowUpInterceptor，了解它的作用和工作原理。
RetryAndFollowUpInterceptor
顾名思义，RetryAndFollowUpInterceptor负责okhttp的请求失败的恢复和重定向。
核心的intercept方法分两段阅读：
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Transmitter transmitter = realChain.transmitter();
int followUpCount = 0;
Response priorResponse = null;
while (true) {
transmitter.prepareToConnect(request);
if (transmitter.isCanceled()) {
throw new IOException("Canceled");
}
Response response;
boolean success = false;
try {
response = realChain.proceed(request, transmitter, null);
success = true;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
if (!recover(e.getLastConnectException(), transmitter, false, request)) {
throw e.getFirstConnectException();
}
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, transmitter, requestSendStarted, request)) throw e;
continue;
} finally {
// The network call threw an exception. Release any resources.
if (!success) {
transmitter.exchangeDoneDueToException();
}
}
...
}
}
前半段的逻辑中，RetryAndFollowUpInterceptor做了几件事：
- 通过Transmitter准备连接
- 执行请求链下一级
- 处理了下一级请求链中的RouteException和IOException。
Transmitter的实现，以后的章节再单独讲解。此处略过。我们重点看一下，RetryAndFollowUpInterceptor如何处理两个异常。
RouteException
从注释中，我们可以看到，RouteException表示客户端连接路由失败。此时会调用recover方法，如果recover方法再失败，会抛出RouteException中的FirstConnectException。
我们看一下recover方法的实现：
/**
* Report and attempt to recover from a failure to communicate with a server. Returns true if
* {@code e} is recoverable, or false if the failure is permanent. Requests with a body can only
* be recovered if the body is buffered or if the failure occurred before the request has been
* sent.
*/
private boolean recover(IOException e, Transmitter transmitter,
boolean requestSendStarted, Request userRequest) {
// The application layer has forbidden retries.
if (!client.retryOnConnectionFailure()) return false;
// We can't send the request body again.
if (requestSendStarted && requestIsOneShot(e, userRequest)) return false;
// This exception is fatal.
if (!isRecoverable(e, requestSendStarted)) return false;
// No more routes to attempt.
if (!transmitter.canRetry()) return false;
// For failure recovery, use the same route selector with a new connection.
return true;
}
首先我们调用应用层的失败回调，如果应用层返回false，就不再进行重试。
然后，我们判断请求的返回，如果请求已经开始或请求限定，只能请求一次，我们也不再进行重试。其中，只能请求一次，可能是客户端自行设定的，也可能是请求返回了404。明确告知了文件不存在，也不会再重复请求。
接下来，是okhttp认为的致命错误，不会再重复请求的，都会在isRecoverable方法中。致命错误包括：协议错误、SSL校验错误等。
private boolean isRecoverable(IOException e, boolean requestSendStarted) {
// If there was a protocol problem, don't recover.
if (e instanceof ProtocolException) {
return false;
}
// If there was an interruption don't recover, but if there was a timeout connecting to a route
// we should try the next route (if there is one).
if (e instanceof InterruptedIOException) {
return e instanceof SocketTimeoutException && !requestSendStarted;
}
// Look for known client-side or negotiation errors that are unlikely to be fixed by trying
// again with a different route.
if (e instanceof SSLHandshakeException) {
// If the problem was a CertificateException from the X509TrustManager,
// do not retry.
if (e.getCause() instanceof CertificateException) {
return false;
}
}
if (e instanceof SSLPeerUnverifiedException) {
// e.g. a certificate pinning error.
return false;
}
// An example of one we might want to retry with a different route is a problem connecting to a
// proxy and would manifest as a standard IOException. Unless it is one we know we should not
// retry, we return true and try a new route.
return true;
}
最后，在底层中寻找是否还有其他的Router可以尝试。
IOException
IOException表示连接已经建立，但读取内容时失败了。我们同样会进行recover尝试，由于代码逻辑一样，不再重复阅读。
在finally中，Transmitter会释放所有资源。
followUpRequest
接下来，我们看一下RetryAndFollowUpInterceptor中intercept后半段的实现：
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Transmitter transmitter = realChain.transmitter();
int followUpCount = 0;
Response priorResponse = null;
while (true) {
...
// 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();
}
Exchange exchange = Internal.instance.exchange(response);
Route route = exchange != null ? exchange.connection().route() : null;
Request followUp = followUpRequest(response, route);
if (followUp == null) {
if (exchange != null && exchange.isDuplex()) {
transmitter.timeoutEarlyExit();
}
return response;
}
RequestBody followUpBody = followUp.body();
if (followUpBody != null && followUpBody.isOneShot()) {
return response;
}
closeQuietly(response.body());
if (transmitter.hasExchange()) {
exchange.detachWithViolence();
}
if (++followUpCount > MAX_FOLLOW_UPS) {
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
request = followUp;
priorResponse = response;
}
}
我们拆开来看这段复杂的逻辑。大体上来说，这段逻辑主要是通过上次请求的返回，生成followUp。然后根据followUp的内容，判断是不是有效的返回。如果返回是有效的，就直接return请求的返回。如果返回无效，则request=followUp，重走while循环，重新请求。
所以这一段的核心逻辑在于followUpRequest方法。我们来看下followUpRequest的实现。
/**
* Figures out the HTTP request to make in response to receiving {@code userResponse}. This will
* either add authentication headers, follow redirects or handle a client request timeout. If a
* follow-up is either unnecessary or not applicable, this returns null.
*/
private Request followUpRequest(Response userResponse, @Nullable Route route) throws IOException {
if (userResponse == null) throw new IllegalStateException();
int responseCode = userResponse.code();
final String method = userResponse.request().method();
switch (responseCode) {
case HTTP_PROXY_AUTH:
Proxy selectedProxy = route != null
? route.proxy()
: client.proxy();
if (selectedProxy.type() != Proxy.Type.HTTP) {
throw new ProtocolException("Received HTTP_PROXY_AUTH (407) code while not using proxy");
}
return client.proxyAuthenticator().authenticate(route, userResponse);
case HTTP_UNAUTHORIZED:
return client.authenticator().authenticate(route, userResponse);
case HTTP_PERM_REDIRECT:
case HTTP_TEMP_REDIRECT:
// "If the 307 or 308 status code is received in response to a request other than GET
// or HEAD, the user agent MUST NOT automatically redirect the request"
if (!method.equals("GET") && !method.equals("HEAD")) {
return null;
}
// fall-through
case HTTP_MULT_CHOICE:
case HTTP_MOVED_PERM:
case HTTP_MOVED_TEMP:
case HTTP_SEE_OTHER:
// Does the client allow redirects?
if (!client.followRedirects()) return null;
String location = userResponse.header("Location");
if (location == null) return null;
HttpUrl url = userResponse.request().url().resolve(location);
// Don't follow redirects to unsupported protocols.
if (url == null) return null;
// If configured, don't follow redirects between SSL and non-SSL.
boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
if (!sameScheme && !client.followSslRedirects()) return null;
// Most redirects don't include a request body.
Request.Builder requestBuilder = userResponse.request().newBuilder();
if (HttpMethod.permitsRequestBody(method)) {
final boolean maintainBody = HttpMethod.redirectsWithBody(method);
if (HttpMethod.redirectsToGet(method)) {
requestBuilder.method("GET", null);
} else {
RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
requestBuilder.method(method, requestBody);
}
if (!maintainBody) {
requestBuilder.removeHeader("Transfer-Encoding");
requestBuilder.removeHeader("Content-Length");
requestBuilder.removeHeader("Content-Type");
}
}
// When redirecting across hosts, drop all authentication headers. This
// is potentially annoying to the application layer since they have no
// way to retain them.
if (!sameConnection(userResponse.request().url(), url)) {
requestBuilder.removeHeader("Authorization");
}
return requestBuilder.url(url).build();
case HTTP_CLIENT_TIMEOUT:
// 408's are rare in practice, but some servers like HAProxy use this response code. The
// spec says that we may repeat the request without modifications. Modern browsers also
// repeat the request (even non-idempotent ones.)
if (!client.retryOnConnectionFailure()) {
// The application layer has directed us not to retry the request.
return null;
}
RequestBody requestBody = userResponse.request().body();
if (requestBody != null && requestBody.isOneShot()) {
return null;
}
if (userResponse.priorResponse() != null
&& userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
// We attempted to retry and got another timeout. Give up.
return null;
}
if (retryAfter(userResponse, 0) > 0) {
return null;
}
return userResponse.request();
case HTTP_UNAVAILABLE:
if (userResponse.priorResponse() != null
&& userResponse.priorResponse().code() == HTTP_UNAVAILABLE) {
// We attempted to retry and got another timeout. Give up.
return null;
}
if (retryAfter(userResponse, Integer.MAX_VALUE) == 0) {
// specifically received an instruction to retry without delay
return userResponse.request();
}
return null;
default:
return null;
}
}
这段代码非常长，大部分是switch/case的各种返回码处理。followUpRequest方法从宏观上来讲，是输入response，生成新的requests。如果response的内容不需要重试，则直接返回null。如果需要重试，则根据response的内容，生成重试策略，返回重试发出的request。
其中，重定向和超时是最主要的重试情况。在处理重定向和超时时，okhttp进行了很多判断，排除了一些不必要重试的情况。如，location不存在，或者重定向的url协议头不一致等情况。
而followUpCount则是为了限制okhttp的重试次数。
总结
RetryAndFollowUpInterceptor在okhttp中承担了重试和重定向的逻辑。其中包括了，建立连接、读取内容失败的重试 和 完整读取请求返回后的重定向。针对各种返回码，okhttp对无需重试的一些场景进行了裁剪，减少了无效重试的概率。同时，对不规范的重定向返回进行的过滤和校验。
网络请求的场景复杂，在设计网络框架时，对于各种未知情况的处理，是一项比较有挑战的工作。okhttp作为一个高可用的网络框架，在RetryAndFollowUpInterceptor这一拦截器中，提供了一个异常处理的优秀范本。
当读者需要自己设计网络库时，可以参考okhttp中RetryAndFollowUpInterceptor对于异常处理的做法，避免一些难以预测和重现的问题。
如有问题，欢迎指正。
