okhttp3源码解析(2)-拦截器 I
前言
上一篇博文讲到了无论发起同步请求还是异步请求，okhttp最后都通过getResponseWithInterceptorChain方法得到response，里面是一系列的拦截器通过责任链形式实现。接下来这篇文章就来详细讲讲拦截器。
okhttp3源码解析(1)-整体流程
拦截器概述
这里我们再让看下getResponseWithInterceptorChain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, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
return chain.proceed(originalRequest);
}
这里除了自定义的interceptors和networkInterceptors外，共有五个系统拦截器，分别是：
- RetryAndFollowUpInterceptor
- BridgeInterceptor
- CacheInterceptor
- ConnectInterceptor
- CallServerInterceptor
这些拦截器实现了从重试、header处理、缓存、连接、请求等，下面我们会好好介绍下着五个拦截器，不过首先我们先看下RealInterceptorChain，看下它是怎么实现责任链串行执行的。
RealInterceptorChain
RealInterceptorChain其实很简单，我们只要注意它的构造函数和proceed方法就行，其他的方法都是返回参数或修改参数。
RealInterceptorChain构造
public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
HttpCodec httpCodec, RealConnection connection, int index, Request request, Call call,
EventListener eventListener, int connectTimeout, int readTimeout, int writeTimeout) {
this.interceptors = interceptors;
this.connection = connection;
this.streamAllocation = streamAllocation;
this.httpCodec = httpCodec;
this.index = index;
this.request = request;
this.call = call;
this.eventListener = eventListener;
this.connectTimeout = connectTimeout;
this.readTimeout = readTimeout;
this.writeTimeout = writeTimeout;
}
这里的构造函数携带了很多参数，都比较好理解，这里我们需要着重看下StreamAllocation、HttpCodec以及RealConnection，这个后面详细讲讲。
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, call, eventListener, connectTimeout, readTimeout,
writeTimeout);
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");
}
if (response.body() == null) {
throw new IllegalStateException(
"interceptor " + interceptor + " returned a response with no body");
}
return response;
}
这里异常情况稍微看下，基本都是IllegalStateException加一个NullPointerException，注意这里抛出了IOException，并会在getResponseWithInterceptorChain继续往上抛，到达AsyncCall或RealCall的execute方法处理。
着重看下里面的几行有效代码：
// Call the next interceptor in the chain.
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);
实际就是根据自身参数又创建了一个RealInterceptorChain实列，修改interceptor的index，这里通过index(RealCall传入了0)拿到了第一个拦截器去执行intercept，并得到response，我开始看得是有点懵懂，接下来看下拦截器就明白了。
Interceptor接口
/**
* Observes, modifies, and potentially short-circuits requests going out and the corresponding
* responses coming back in. Typically interceptors add, remove, or transform headers on the request
* or response.
*/
public interface Interceptor {
Response intercept(Chain chain) throws IOException;
interface Chain {
Request request();
Response proceed(Request request) throws IOException;
// 省略部分代码
}
}
首先我们看下Interceptor接口，实际上它就一个方法intercept，该方法返回Response，向上抛出IOException(到达Chain的proceed方法)，这里有个Chain接口，它唯一的实现就是RealInterceptorChain，上面已经讲了。
RetryAndFollowUpInterceptor
如果我们没有设置OkHttpClient的interceptors，第一个执行的拦截器就是RetryAndFollowUpInterceptor，回想下上篇博文，RetryAndFollowUpInterceptor是在RealCall的构造函数中创建的，下面着重看下intercept，其他方法基本都是从中延展出来的。
第一步，创建streamAllocation对象
// 取了些数据后面使用
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;
看StreamAllocation简介，其是一个连接下面三者的工具(Streams为HttpCodec)，这里创建的streamAllocation会随着责任链往下传递得去，现在暂时未使用到。
This class coordinates the relationship between three entities:Connections、Streams、Calls
第二步，在循环中执行，可多次发送请求，直至获得结果
int followUpCount = 0;
Response priorResponse = null;
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
// 省略好多代码
}
这里注意下本个拦截器的名字RetryAndFollowUpInterceptor，重试和继续拦截器，所以这个循环的目的就是实现重试和继续。
第三步，获取结果，失败时进行重试
Response response;
boolean releaseConnection = true;
try {
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.
// 连接时的失败情况，请求还未发送，选择是否掩盖异常进行重试，不掩盖则为false，跳出循环
if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
throw e.getFirstConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
// 和服务器通信失败，请求可能已发送
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();
}
}
这里篇幅有点长了，实际就做了两件事，一是调用realChain的proceed方法拿到response，二是对各个异常进行recover：
/**
* 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);
// The application layer has forbidden retries.
if (!client.retryOnConnectionFailure()) return false;
// We can't send the request body again.
if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;
// This exception is fatal.
if (!isRecoverable(e, requestSendStarted)) return false;
// No more routes to attempt.
if (!streamAllocation.hasMoreRoutes()) return false;
// For failure recovery, use the same route selector with a new connection.
return true;
}
比较有意思的就是，如果对异常进行recover，那么就不会抛出异常，而是由continue进入下一次循环，也即是重试了。在finally里面则是通过releaseConnection判断是否需要释放连接，抛出异常的时候releaseConnection没有赋值，就会释放连接。
第四步，根据返回码判断是否需要再发起请求(验证、重定向、重试等)，不需要重新请求则返回response
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {
// 上一个循环已经获得了回复，根据上一个回复信息创建response，储存上一个回复
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
Request followUp;
try {
followUp = followUpRequest(response, streamAllocation.route());
} catch (IOException e) {
streamAllocation.release();
throw e;
}
if (followUp == null) {
streamAllocation.release();
return response;
}
// 关闭流
closeQuietly(response.body());
// 超出followUp次数
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
// 不可抵达
if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}
上面一步体现了重试的思想，那么这一步就是followUp了。大致意思就是通过followUpRequest函数判断下是否要发起下一个请求，如果需要会得到一个followUp的Request，并在下一次循环执行请求。
/**
* 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) {
case HTTP_PROXY_AUTH:
case HTTP_UNAUTHORIZED:
case HTTP_PERM_REDIRECT:
case HTTP_TEMP_REDIRECT:
// ...省略部分代码
}
}
followUpRequest这个函数比较长，大致意思就是根据返回码做出一些处理，并构建下一次的Request，大致就是验证、重定向、重试等，读者有兴趣可以详细看下。
第五步，判断是否是相同链接(重定向了)，不同链接则重新创建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?");
}
// 下一个循环
request = followUp;
priorResponse = response;
这里就是判断下followUp的url是否变换了，如果变换了streamAllocation也要进行更新，因为它事处理连接、流、请求的工具。
到这就进入下一次循环了，同时RetryAndFollowUpInterceptor的大致功能也了解的差不多了，最终是在下面这行代码进入到了下一个拦截器：
response = realChain.proceed(request, streamAllocation, null, null);
注意streamAllocation在这里传递下去了，会到达RealInterceptorChain的proceed方法，里面会使用它创建个新的RealInterceptorChain，并进入到下一个拦截器，后续拦截器都能使用到这个实列。
BridgeInterceptor
上面讲解了下RetryAndFollowUpInterceptor，比较复杂，一开始也看得我头疼，不过万事开头难，你不去尝试，没有经过锻炼，那永远都只是一个搬砖工，只有去试才能创造可能，才能有机会，才能有成长。
话又说多了，BridgeInterceptor这个拦截器相对RetryAndFollowUpInterceptor来说是简单多了，接下来我也分步骤讲解下intercept内的内容。
第一步，添加各种请求头
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
// 根据已有请求头添加header(来自RetryAndFollowUpInterceptor的followUp)
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");
}
}
// 添加其他header
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");
}
// 添加cookies
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());
}
这里就是向requestBuilder里面写入header，唯一需要注意的就是cookieJar，它是从OkHttpClient.Builder里面传过来的(如果没主动设置)：
// 默认值
cookieJar = CookieJar.NO_COOKIES;
CookieJar NO_COOKIES = new CookieJar() {
@Override public void saveFromResponse(HttpUrl url, List<Cookie> cookies) {
}
@Override public List<Cookie> loadForRequest(HttpUrl url) {
return Collections.emptyList();
}
};
这里cookieJar的默认值就是NO_COOKIES，它在保存和读取cookies时都是不操作的。
第二步，通过责任链发起请求，进入下一个责任链，request的header填充完毕
Response networkResponse = chain.proceed(requestBuilder.build());
这里和RetryAndFollowUpInterceptor类似，就是调用RealInterceptorChain的proceed方法进入下一个责任链获得结果。
第三步，保存cookies
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
这里是保存cookies，如果没对OkHttpClient.Builder传入cookieJar，就是默认的NO_COOKIES，不会对cookies操作。
第四步，根据回复进行处理(解压)，得到最终的response
// 向networkResponse里写入userRequest并创建新的Response
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
// 对应上面transparentGzip=true的解压操作
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();
以第二步前后做对比，就会发起其实这些操作是对称的，header操作、cookie的操作以及Gzip操作。
小结
写到这里篇幅也比较长了，但是拦截器的大致功能已经十分清楚了，剩下三个系统的拦截器，我们在下篇博文再讲解。
okhttp3源码解析(3)-拦截器 II
