攔截器
上文OkHttp解析一(分發器)提到的OkHttp中的getResponseWithInterceptorChain()中的流程就是攔截器。使用了責任鏈模式。
默認情況下有五大攔截器：
RetryAndFollowUpInterceptor
BridgeInterceptor
CacheInterceptor
ConnectInterceptor
CallServerInterceptor
RetryAndFollowUpInterceptor
重試及重定向攔截器，主要就是完成兩件事情：重試與重定向。
重試
@Override
public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Call call = realChain.call();
EventListener eventListener = realChain.eventListener();
//todo 管理類，維護了 與服務器的連接、數據流與請求三者的關係。
StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(request.url()), call, eventListener, callStackTrace);
this.streamAllocation = streamAllocation;
//重定向計數
int followUpCount = 0;
Response priorResponse = null;
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response;
boolean releaseConnection = true;
try {
//todo 請求出現了異常，那麼releaseConnection依舊爲true。
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
//todo 路由異常，連接未成功，請求還沒發出去
//The attempt to connect via a route failed. The request will not have been sent.
if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
//todo 請求發出去了，但是和服務器通信失敗了。(socket流正在讀寫數據的時候斷開連接)
// ConnectionShutdownException只對HTTP2存在。假定它就是false
//An attempt to communicate with a server failed. The request may have been sent.
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.
//todo 不是前兩種的失敗，那直接關閉清理所有資源
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
//todo 如果進過重試/重定向才成功的，則在本次響應中記錄上次響應的情況
//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();
}
//todo 處理3和4xx的一些狀態碼，如301 302重定向
Request followUp = followUpRequest(response, streamAllocation.route());
if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}
closeQuietly(response.body());
//todo 限制最大 followup 次數爲20次
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
//todo 由於requestSendStarted只在http2的io異常中爲true
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
//todo 判斷是不是可以複用同一份連接
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?");
}
request = followUp;
priorResponse = response;
}
}
StreamAllocation是用來協調Connections、Streams和Calls這三個實體的。
兩個異常都是根據recover 方法判斷是否能夠進行重試，如果返回true，則表示允許重試。
/**
* 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, StreamAllocation streamAllocation,
boolean requestSendStarted, Request userRequest) {
streamAllocation.streamFailed(e);
//todo 1、在配置OkhttpClient是設置了不允許重試（默認允許），則一旦發生請求失敗就不再重試
//The application layer has forbidden retries.
if (!client.retryOnConnectionFailure()) return false;
//todo 2、由於requestSendStarted只在http2的io異常中爲true
//We can't send the request body again.
if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody)
return false;
//todo 3、判斷是不是屬於重試的異常
//This exception is fatal.
if (!isRecoverable(e, requestSendStarted)) return false;
//todo 4、不存在更多的路由
//No more routes to attempt.
if (!streamAllocation.hasMoreRoutes()) return false;
// For failure recovery, use the same route selector with a new connection.
return true;
}
判斷是否是那些異常，是否可以重試。
private boolean isRecoverable(IOException e, boolean requestSendStarted) {
// If there was a protocol problem, don't recover.
// 協議異常，如果是那麼直接判定不能重試。
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).
// 超時異常，requestSendStarted爲false就會重試。
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.
// SSL證書異常/SSL驗證失敗異常，都不能重試。
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;
}
重定向
/**
* 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, Route route) throws IOException {
if (userResponse == null) throw new IllegalStateException();
int responseCode = userResponse.code();
final String method = userResponse.request().method();
switch (responseCode) {
// 407 客戶端使用了HTTP代理服務器，在請求頭中添加 “Proxy-Authorization”，讓代理服務器授權
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);
// 401 需要身份驗證
case HTTP_UNAUTHORIZED:
return client.authenticator().authenticate(route, userResponse);
// 308 永久重定向
// 307 臨時重定向
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
// 300 301 302 303
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.
//重定向請求中 只要不是 PROPFIND 請求，無論是POST還是其他的方法都要改爲GET請求方式，
Request.Builder requestBuilder = userResponse.request().newBuilder();
//return !(method.equals("GET") || method.equals("HEAD"));
if (HttpMethod.permitsRequestBody(method)) {
//return method.equals("PROPFIND"); // (WebDAV) redirects should also maintain the request body
final boolean maintainBody = HttpMethod.redirectsWithBody(method);
// All requests but PROPFIND should redirect to a GET request.return !method.equals("PROPFIND");
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, url)) {
requestBuilder.removeHeader("Authorization");
}
return requestBuilder.url(url).build();
// 408 客戶端請求超時
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;
}
if (userResponse.request().body() instanceof UnrepeatableRequestBody) {
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();
// 503
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;
}
}
需要注意的是，我們的followUpCount中最大次數爲20次。
/**
* How many redirects and auth challenges should we attempt? Chrome follows 21 redirects;
* Firefox,
* curl, and wget follow 20; Safari follows 16; and HTTP/1.0 recommends 5.
*/
private static final int MAX_FOLLOW_UPS = 20;
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
本攔截器是整個責任鏈中的第一個，這意味着它會是首次接觸到Request與最後接收到Response的角色，在這個攔截器中主要功能就是判斷是否需要重試與重定向。
重試的前提是出現了RouteException或者IOException。一但在後續的攔截器執行過程中出現這兩個異常，就會通過recover方法進行判斷是否進行連接重試。
重定向發生在重試的判定之後，如果不滿足重試的條件，還需要進一步調用followUpRequest根據Response 的響應碼(當如果直接請求失敗，Response都不存在就會拋出異常)。followup最大發生20次。
BridgeInterceptor
橋接攔截器，連接應用程序和服務器的橋樑，我們發出的請求將會經過它的處理才能發給服務器。其目的就是補全請求頭。在補全了請求頭後交給下一個攔截器處理，而得到響應後，主要幹兩件事情，源碼如下：
@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();
//請求體類型,如：application/x-www-form-urlencoded
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());
}
long contentLength = body.contentLength();
//請求體解析方式
if (contentLength != -1) {
requestBuilder.header("Content-Length", Long.toString(contentLength));
requestBuilder.removeHeader("Transfer-Encoding");
} else {
requestBuilder.header("Transfer-Encoding", "chunked");
requestBuilder.removeHeader("Content-Length");
}
}
//請求的主機站點
if (userRequest.header("Host") == null) {
requestBuilder.header("Host", hostHeader(userRequest.url(), false));
}
//保持長連接
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;
//接受響應支持gzip壓縮
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}
List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
//cookie身份辨別
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}
//請求的用戶信息，如:操作系統、瀏覽器等
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
Response networkResponse = chain.proceed(requestBuilder.build());
//保存cookie，在下次請求則會讀取對應的數據設置進入請求頭，默認的CookieJar不提供實現
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
//如果使用gzip返回的數據，則使用GzipSource包裝便於解析。
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();
}
對用戶構建的Request進行添加或者刪除相關頭部信息，以轉化成能夠真正進行網絡請求的Request 將符合網絡請求規範的Request交給下一個攔截器處理，並獲取Response 如果響應體經過了GZIP壓縮，那就需要解壓，再構建成用戶可用的Response並返回。