RetryAndFollowUpInterceptor源码分析
本文基于okhttp3.10.0
近段时间准备陆陆续续把okhttp拦截器分析一遍，既然是本系列开篇那先来回顾下拦截器是干嘛的，然后在开始今天的主题。
1. 拦截器简介
先简单看下拦截器是干嘛的，okhttp默认给我们提供了5个拦截器
- RetryAndFollowUpInterceptor
- BridgeInterceptor
- CacheInterceptor
- ConnectInterceptor
- CallServerInterceptor
当我们通过okhttp发送请求的时候是将请求封装为一个Request对象，然后包装成一个即将请求的Call对象，无论是异步或者同步请求最终都是通过RealCall#getResponseWithInterceptorChain()方法获取请求结果，那么我们看下方法体
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);
}
可以看到是将拦截器组合起来创建一个RealInterceptorChain对象并将第五个参数赋值为0，而第五个参数是赋值给了RealInterceptorChain中的index成员变量，然后通过chain#proceed(originalRequest)获取请求结果。
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
...
// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
writeTimeout);//index+1链条的下一个节点
Interceptor interceptor = interceptors.get(index);//拿到第一个拦截器
Response response = interceptor.intercept(next);
...
return response;
}
第一次进来的时候index为0所以拿到的是第一个拦截器，而interceptor#intercept()传入的RealInterceptorChain的index为index+1即为1，所以当拦截器中调用chain#proceed(originalRequest)则会调到下个拦截器形成一个递归关系，完成整个拦截器链的调用。
//拦截器伪代码
public class Interceptor {
Response intercept(Chain chain) throws IOException{
Request request = chain.request();
//对Request做操作
Response response = chain.process()
//对Response做操作
}
}
那么可以简单的理解拦截器就是处理请求的，输入一个请求返回一个响应，按照http协议约定的代码实现。
2. RetryAndFollowUpInterceptor做了啥
- 创建StreamAllocation它会在 ConnectInterceptor 中真正被使用到，主要就是用于获取连接服务端的 Connection 和用于进行跟服务端进行数据传输的输入输出流 HttpStream，具体的操作后面用到再说
- 出错重试
- 继续请求（主要是重定向）
2.1 出错重试
@Override public Response intercept(Chain chain) throws IOException {
while (true) {
if (canceled) {//请求取消了释放连接
streamAllocation.release();
throw new IOException("Canceled");
}
Response response;
boolean releaseConnection = true;//默认释放连接
try {
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;//请求成功暂不释放连接
} catch (RouteException e) {
//出现异常判断能不能进行重试
if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;//可以重试不释放连接
continue;//通过continue继续执行while循环重试
} catch (IOException e) {
//出现异常判断能不能进行重试
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
releaseConnection = false;//可以重试不释放连接
continue;//通过continue继续执行while循环重试
} finally {
//不能重试则释放资源
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
}
}
请求取消了释放连接，请求成功暂不释放连接，请求抛出RouteException和IOException通过recover()判断能不能进行重试，如果可以也暂不释放连接，并通过continue继续进行while循环重新请求。
private boolean recover(IOException e, StreamAllocation streamAllocation,
boolean requestSendStarted, Request userRequest) {
streamAllocation.streamFailed(e);
// The application layer has forbidden retries.
if (!client.retryOnConnectionFailure()) return false;//应用层可以配置拒绝重试
// We can't send the request body again.
if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;//如果请求已经发送并且body实现了UnrepeatableRequestBody则不重试
// This exception is fatal.
if (!isRecoverable(e, requestSendStarted)) return false;//请求发生致命错误
// No more routes to attempt.
if (!streamAllocation.hasMoreRoutes()) return false;//没有更多的路线
// For failure recovery, use the same route selector with a new connection.
return true;
}
第一个if是调用client#retryOnConnectionFailure()判断能不能进行重试
public boolean retryOnConnectionFailure() {
return retryOnConnectionFailure;
}
而它实际是读取的OkHttpClient的retryOnConnectionFailure属性默认为true支持重试
第二个if是判断如果请求已经发送，并且body实现了UnrepeatableRequestBody则不再重试，UnrepeatableRequestBody在这里只是做一个标记的作用
第三个if判断是不是致命异常，如果是则不再重试
private boolean isRecoverable(IOException e, boolean requestSendStarted) {
// If there was a protocol problem, don't recover.
if (e instanceof ProtocolException) {//出现协议错误
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) {//中断异常
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) {//ssl握手异常
// If the problem was a CertificateException from the X509TrustManager,
// do not retry.
if (e.getCause() instanceof CertificateException) {
return false;
}
}
if (e instanceof SSLPeerUnverifiedException) {//验证异常
// 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;
}
第四个if如果没有更多的线路则不进行重试，其他情况则进行重试
2.2 继续请求（主要是重定向）
@Override public Response intercept(Chain chain) throws IOException {
int followUpCount = 0;//继续请求次数
Response priorResponse = null;//上次的响应
while (true) {
//...省略请求重试代码
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {//如果有前一个响应则附加到responses中
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
Request followUp = followUpRequest(response, streamAllocation.route());//判断需不需要继续请求
if (followUp == null) {//不需要继续请求
if (!forWebSocket) {
streamAllocation.release();
}
return response;//直接返回响应
}
//下面则是需要继续请求
closeQuietly(response.body());//去掉body
if (++followUpCount > MAX_FOLLOW_UPS) {//如果继续请求次数大于20次报错终止
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
if (followUp.body() instanceof UnrepeatableRequestBody) {//如果body实现了UnrepeatableRequestBody接口不进行继续请求
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
if (!sameConnection(response, followUp.url())) {//不能重用连接
streamAllocation.release();//释放旧的连接
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;//记录继续请求的request
priorResponse = response;//上次的response
}
}
代码注释已经很全了，直接看到对于继续请求的判断followUpRequest()
private Request followUpRequest(Response userResponse, Route route) throws IOException {
if (userResponse == null) throw new IllegalStateException();
int responseCode = userResponse.code();//拿到http状态码
final String method = userResponse.request().method();//请求方法
switch (responseCode) {
case HTTP_PROXY_AUTH://407代理服务器验证
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);//返回一个附带验证信息的Request
case HTTP_UNAUTHORIZED://401和407类似，只不过401是服务器验证失败
return client.authenticator().authenticate(route, userResponse);//返回一个附带验证信息的Request
case HTTP_PERM_REDIRECT://308
case HTTP_TEMP_REDIRECT://307
// "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")) {//307 308如果不是get或者head的话则不在进行重定向
return null;
}
// fall-through
case HTTP_MULT_CHOICE://300
case HTTP_MOVED_PERM://301
case HTTP_MOVED_TEMP://302
case HTTP_SEE_OTHER://303
// Does the client allow redirects?
if (!client.followRedirects()) return null;//客户端是否允许重定向 默认是允许
String location = userResponse.header("Location");//获取响应头中重定向地址
if (location == null) return null;
HttpUrl url = userResponse.request().url().resolve(location);//解析成HttpUrl对象
// 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;//是否允许http到https重定向
// Most redirects don't include a request body.
Request.Builder requestBuilder = userResponse.request().newBuilder();
if (HttpMethod.permitsRequestBody(method)) {//如果请求方法不是get或者head
final boolean maintainBody = HttpMethod.redirectsWithBody(method);//请求方法为PROPFIND需要有body
if (HttpMethod.redirectsToGet(method)) {//如果请求方法不是PROPFIND重定向为get请求并去除请求体
requestBuilder.method("GET", null);
} else {//请求方法是PROPFIND，加入请求体并保持原有请求方法
RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
requestBuilder.method(method, requestBody);
}
if (!maintainBody) {//如果没有请求体则移除对应头部字段
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, url)) {//不是同一个连接移除Authorization字段
requestBuilder.removeHeader("Authorization");
}
return requestBuilder.url(url).build();
case HTTP_CLIENT_TIMEOUT://408 进行重试
// 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()) {//判断是否允许重试
// The application layer has directed us not to retry the request.
return null;
}
if (userResponse.request().body() instanceof UnrepeatableRequestBody) {
return null;
}
if (userResponse.priorResponse() != null
&& userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {//发生两次408则不在重试
// We attempted to retry and got another timeout. Give up.
return null;
}
if (retryAfter(userResponse, 0) > 0) {//response中Retry-After有值则不再重试
return null;
}
return userResponse.request();
case HTTP_UNAVAILABLE://503 默认不重试
if (userResponse.priorResponse() != null
&& userResponse.priorResponse().code() == HTTP_UNAVAILABLE) {//出现两次503则不再重试
// We attempted to retry and got another timeout. Give up.
return null;
}
if (retryAfter(userResponse, Integer.MAX_VALUE) == 0) {//response中包含Retry-After并且值为0进行重试
// specifically received an instruction to retry without delay
return userResponse.request();
}
return null;
default:
return null;
}
}
这里就是对http响应码做区分看是否需要再次请求，先来科普下http响应码
- 401 & 407 身份验证
- 300 & 301 & 302 & 304 & 307 & 308 重定向
- 408 客户端超时不常用
- 503 服务器不可用
返回码为401&407的话，获取带验证信息的Request重新请求，408这个不常用默认是会重试的，503为服务不可用默认不重试，接下来是重头戏3xx重定向。
case HTTP_PERM_REDIRECT://308
case HTTP_TEMP_REDIRECT://307
if (!method.equals("GET") && !method.equals("HEAD")) {//307 308如果不是get或者head的话则不在进行重定向
return null;
}
case HTTP_MULT_CHOICE://300
case HTTP_MOVED_PERM://301
case HTTP_MOVED_TEMP://302
case HTTP_SEE_OTHER://303
if (!client.followRedirects()) return null;//客户端是否允许重定向 默认是允许
String location = userResponse.header("Location");//获取响应头中重定向地址
if (location == null) return null;
HttpUrl url = userResponse.request().url().resolve(location);//解析成HttpUrl对象
if (url == null) return null;
boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
if (!sameScheme && !client.followSslRedirects()) return null;//是否允许http到https重定向
Request.Builder requestBuilder = userResponse.request().newBuilder();
if (HttpMethod.permitsRequestBody(method)) {//如果请求方法不是get或者head
final boolean maintainBody = HttpMethod.redirectsWithBody(method);//请求方法为PROPFIND需要有body
if (HttpMethod.redirectsToGet(method)) {//如果请求方法不是PROPFIND重定向为get请求并去除请求体
requestBuilder.method("GET", null);
} else {//请求方法是PROPFIND，加入请求体并保持原有请求方法
RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
requestBuilder.method(method, requestBody);
}
if (!maintainBody) {//如果没有请求体则移除对应头部字段
requestBuilder.removeHeader("Transfer-Encoding");
requestBuilder.removeHeader("Content-Length");
requestBuilder.removeHeader("Content-Type");
}
}
if (!sameConnection(userResponse, url)) {//不是同一个连接移除Authorization字段
requestBuilder.removeHeader("Authorization");
}
return requestBuilder.url(url).build();
可以看到在返回码307或308的时候，只有请求方法不是get不是head才会重定向，300、301、302、303都会重定向，先判断客户端是否允许重定向client#followRedirects()，默认是允许的，然后拿到response中头部location的值作为重定向地址，然后对url做了一些判断，最关键的来了如果你的请求不是get不是head，我们就假设是post则他会重定向为get请求并且把请求体丢了，对你没有看错基本等于是重定向一律转为get请求并且把请求体丢了。 其实http协议中规定的是301、302禁止将post转为get的，但是由于大部分浏览器都没有遵守都是转为了get，所以okhttp也沿用了这个做法。
3. 总结
简单总结下RetryAndFollowUpInterceptor做了3件大事
- 创建StreamAllocation（这个后面细说）
- 请求发生错误，判断能不能恢复，如果可以进行重试
- 根据响应码进行区分处理，我们最常用的重定向就是在这里处理，需要注意的是重定向一律转为get请求并丢弃了请求体