上一篇中看到，在AsyncCall被执行时会先进行拦截器的操作。
回到AsyncCall的execute方法
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);
}
先进入最后面这个RealInterceptorChain查看proceed方法：
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;
}
可以看到
// 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);
这一段有点像递归，将生成的next传入中调用interceptor.intercept方法，其内部又继续调用了proceed方法，实现的是责任链设计模式。用过Scala语言等面向过程的语言的同学肯定不会觉得陌生。
既然这个realcall用来执行所有拦截器处理过程，那么就依次看一遍所有的拦截器，首先是RetryAndFollowUpInterceptor，看名字就知道是个负责断线重连的拦截器，点进类中一看，开头就一个注释：
/**
* 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;
噗。。。这老哥好逗。。。然后往下一看取了个20，可以可以，够意思了。这个MAX_FOLLOW_UPS 属性是final的，是无法改变的，最多执行20次跳转的意思。其执行的核心是intercept方法。
@Override
public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
// 从连接池根据url 新建连接
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()), callStackTrace);
// 重定向的临时变量
int followUpCount = 0;
// 上一次请求的结果，如果有重定向就不为空，但是不会有body
Response priorResponse = null;
// 处理的本体，循环直到拿到结果或者达到最大重试次数就return
while (true) {
// 如果请求取消就return掉并释放资源
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
// 没啥可说的，定义response和释放连接的开关变量
Response response = null;
boolean releaseConnection = true;
try {
// 这句话会一直往后面的拦截器依次调用，直到拿到处理结果。
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.
// 尝试通过route连接失败，调用了StreamAllocation的streamFailed方法，进行相应的释放。
// 根据连接异常看一下，如果是不可恢复的，就抛异常终止掉，否则就continue进行下一次重试。
if (!recover(e.getLastConnectException(), false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// 跟上面类似，也是判断异常类型，看是重试还是抛异常终止。
// 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();
}
// 这个方法用来控制重定向，根据http请求的返回码来确定是要结束返回请求结果还是要重定向
// 设置允许重定向并且返回码是300-303就执行重定向，重设请求头，auth那些东西
Request followUp = followUpRequest(response);
// 如果是请求成功不需要重定向，followup就是空，就释放资源返回结果。
if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}
// 需要重定向就释放掉body，准备下一次请求。
closeQuietly(response.body());
// 如果超过了最大重定向次数就释放资源抛异常。
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
// 返回结果是不可重定向的也释放资源抛异常。
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
// 如果response和重定向的url的主机、端口号、scheme任意一个不一样，释放资源抛异常。
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?");
}
// 这么多判断都做了，这下稳了吧，循环去执行下一次重定向。
request = followUp;
priorResponse = response;
}
}
这个类的核心功能就是控制重定向和失败重连。根据http请求的返回码控制重定向，根据后面几个拦截器和连接过程中抛出的异常类型控制失败重连。具体可以看上面我加的中文注释。
下面看BridgeInterceptor的intercept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());
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();
}
这个没啥可说的了，就是给请求头加各种东西，同时也处理cookie、gzip压缩。
下面看一下CacheInterceptor的intercept方法
@Override
public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
// 拿到本地时间戳
long now = System.currentTimeMillis();
// 新建一个缓存策略对象
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
/** The request to send on the network, or null if this call doesn't use the network. */
// 网络请求发出的request，如果不需要网络就是空
Request networkRequest = strategy.networkRequest;
/** The cached response to return or validate; or null if this call doesn't use a cache. */
// 网络请求返回的response，如果不需要缓存就是空
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
// 如果有cacheCandidate ，但却没有返回内容，说明这个cacheCandidate 已经过期了，关掉
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.
// 如果既不使用网络，又拿不到有效的cache，直接返回504错误，扮演了网关的角色
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.
// 如果不需要网络，那就直接返回cache
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
// 下面就是需要进行真正的网络请求的情况了
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.
// 如果既进行了网络请求，又存在cache，说明我们是在做一个conditional get请求
// 如果请求的资源服务器没有过修改，就在请求头中告知客户端。客户端直接使用cache就可以了。
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());
}
}
// 最正常的情况应该是这个，拿到返回后，放到cache中，供以后使用
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;
}
再看一下ConnectInterceptor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);
}
这个类的方法就比较短了，就是检查一下是不是get，是的话就要检查一下连接，以防止conditional get的情况。
调用steamAllocate在连接池中获取连接方法，里面有一些连接管理策略啊DNS获取ip地址啊什么鬼的，这个下一篇有分析。
然后是networkInterceptors, 这个默认是没有的，我们可以自己设置拦截器，进行我们想要的操作，比如说打印log啊什么的，下面是自己设置拦截器的代码
okClient.networkInterceptors().add(new Interceptor() {
@Override
public Response intercept(Chain chain) throws IOException {
return null;
}
});
具体我也没试过，应该仿照其他拦截器的写法就好了。
最后是CallServerInterceptor
@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();
// 这里会根据http协议的版本1.x 或者2.0 来执行不同的处理方法，默认肯定是1.x，
// 调用okhttp3.internal.http1.Http1Codec#writeRequestHeaders方法
// 根据url和http协议版本拼接字符串，得到类似这种的字符串 “GET /system!getSystemTime.htm HTTP/1.1”
// 然后将请求一行一行的sink进去，都是在Http1Codec类中调用的，sink的实现在okio的库中，有个RealBufferSink来具体实现。
// okio就是更底层处理tcp连接的库了，里面都是一些字符串操作和位运算。
httpCodec.writeRequestHeaders(request);
Response.Builder responseBuilder = null;
// 如果有post、put、patch请求的那种请求体，或者OPTIONS、DELETE那种可能有请求体的请求，就把请求体也写进去。说
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();
}
}
// 请求内容写入完毕，调用sink.flush方法
httpCodec.finishRequest();
// 解析服务器返回的内容，构建responseBuilder
if (responseBuilder == null) {
responseBuilder = httpCodec.readResponseHeaders(false);
}
// 再根据builder构造出response对象，内部还是各种属性赋值
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
// 根据返回码判断
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;
}
看到这个类可以知道，这是一个实际负责网络请求，和服务器交互的类，调用okio发出请求拿到响应之后，返回给之前的那些拦截器。
以上，最后再复习一下，各个拦截器的功能分别是：
RetryAndFollowUpInterceptor：
战场指挥：负责失败重连以及重定向的
台词：“我说打哪儿就打哪儿，说打几枪就打几枪”
BridgeInterceptor：
后勤部门：主要负责构造请求，如请求头，以及构造请求结果和处理如是否有gzip压缩等
台词：“我也不是谦虚，就做了一点微小的工作，构造http请求也要按照基本法，一个http传输的数据啊，当然要看用户的设置，但是也要考虑到历史的cookie...”
CacheInterceptor：
情报部门：缓存处理，读取缓存，缓存更新等
台词：“别请求了用我存的吧？”，“我这存了上一分钟的数据，真没过期，不信你问服务器过期没”，“哎哎大哥，请求到新的数据了吗存我一份呗”
ConnectInterceptor：
战斗在网络请求一线的：负责和服务器建立连接，这个的执行时间是很长的，是实际建立连接的拦截器
台词：“男人嘛，不能太快。。。”（RouteSelector、Dns.SYSTEM、JDK 赞了此条说说）
networkInterceptors：
打杂人员：我们代码中自定义的网络拦截器
台词：“大哥们让我先打个log可不可以....”
CallServerInterceptor：
战斗在网络请求一线的：负责跟服务器交互，发送请求，获取响应
台词：“所以说啊，掌握二进制这门外语还是非常重要的！”（okio：喵喵喵？）
RealInterceptorChain：
司令官：责任链设计模式搞的最外层的类，负责执行上面的这些拦截器
台词：“楼上都是SB”
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