前言
本章分析访问网络的逻辑,即RealCall.getResponseWithInterceptorChain()这个方法.这里okHttp框架使用了拦截链来对网络访问进行干预处理.每一条拦截器代表了一部分的业务逻辑.
上代码:
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
//先构建一个拦截链条
List<Interceptor> interceptors = new ArrayList<>();
//将我们自定义的拦截器加入
interceptors.addAll(client.interceptors());
//重试和重定向拦截器
interceptors.add(retryAndFollowUpInterceptor);
//桥接拦截器,将request转化为网络可用的请求,将网络返回的response转化为对用户的response
interceptors.add(new BridgeInterceptor(client.cookieJar()));
//缓存拦截器
interceptors.add(new CacheInterceptor(client.internalCache()));
//连接拦截器
interceptors.add(new ConnectInterceptor(client));
//如果不是socket,添加自定义的网络拦截器
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
//连接服务器拦截器,真正请求数据的业务
interceptors.add(new CallServerInterceptor(forWebSocket));
//下面开始,会按照添加到连接器链条的顺序,依次执行拦截器
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
return chain.proceed(originalRequest);
}
从代码可以看到,在将所有拦截器添加到链条之后,new了一个RealInterceptorChain实例,并执行proceed方法.下面分析这个方法.
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
// Call the next interceptor in the chain.
//新建一个chain,调用下一个拦截器的逻辑
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);
return response;
}
依靠每个拦截其中调用的chain.processd方法,即实现拦截器的依次执行.
RetryAndFollowUpInterceptor
除了我们自定义的拦截器,这是框架执行的第一个拦截器.再这个拦截器中,我们拿到网络请求的结果后,会进行失败重试或重定向.
@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;
//在一个循环中不断重试,直到达成指定条件
while (true) {
//取消
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response;
boolean releaseConnection = true;
try {
//调用获取网络返回结果
response = realChain.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.
//连接路由失败,请求尚未发送,recover放回true表示可恢复的请求,false表不可,如果false则throw异常,终端重试
if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
throw e.getFirstConnectException();
}
//可恢复请求,继续
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
//如果出现io异常,判断是否请求可恢复
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.
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
//priorResponse 是上次循环中的response
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
//重定向request
Request followUp;
try {
//这里会根据response的返回码,处理不同的情况.比如返回码是重定向,即重新封装一个到重定向地址的request返回
followUp = followUpRequest(response, streamAllocation.route());
} catch (IOException e) {
streamAllocation.release();
throw e;
}
if (followUp == null) {
streamAllocation.release();
return response;
}
closeQuietly(response.body());
//重定向次数超过上限,则退出
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
//不可重复的请求体
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
//比较重定向url和当前url是否相同,如不同,则新建streamAllocation
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;
}
}
可以看到,重试和重定向拦截器是在一个循环中不断重试,知道不在满足重试的条件.
BridgeInterceptor
这是一个将应用层request转化为网络岑request,比如添加一些请求头,如Content-Type,User-Agent,Accept-Encoding;并将网络层response转化为应用层response的拦截器.代码如下,不再赘述.
@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();
}
CacheInterceptor
okHttp的缓存策略和http的缓存策略是一致的.http缓存机制请参考连接.下面分析代码:
@Override public Response intercept(Chain chain) throws IOException {
//取出缓存的response,cache是一个本地缓存方案,以文件方式缓存到本地,
//可以根据request取出相应的response,类似map
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//缓存策略,这个类解析了request的缓存设置`get`方法根据不同的策略设置缓存
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
//统计网络请求,和缓存response计数,有则加一
if (cache != null) {
cache.trackResponse(strategy);
}
//本地缓存不为空,但是根据缓存策略为空,所以本地缓存不可用,关闭
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.
//当不能进行网络请求,并且缓存response已经过期时,返回504.
//从CacheStrategy.Factory.get方法可知,当request设置为OnlyIfCached时达成此条件
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.
//缓存有效时,直接返回缓存
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
//网络获取数据
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.
//当网络数据和缓存数据都存在时,
if (cacheResponse != null) {
//当网络返回码是内容未改变,组装缓存数据返回
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());
}
}
//网络返回数据
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
//网络数据有body并且可缓存时,缓存数据
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
CacheRequest cacheRequest = cache.put(response);
return cacheWritingResponse(cacheRequest, response);
}
//此请求不可缓存时删除
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}
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