image.png
继续上一篇的okHttp拦截器分析（一），下一个要分析的拦截器是CacheInterceptor，听名字就知道跟缓存有关，在这之前，我们先来看看一张Http缓存的流程图，网络上找到的：
image.png
当发送相同请求的时候，先判断缓存是否过期，如果过期了，该请求会携带If-Modified-Since和If-None-Match，通过这两个值，判断本地资源是否发生变化，没有变化直接获取缓存并返回code 304。
注：
Last-Modified：服务器返回给客户端的头部信息，表示资源的最后修改时间（ETag 比较的是响应内容的特征值，而Last-Modified 比较的是响应内容的修改时间）。
很好，接下来我们来看看okHttp里面的缓存拦截器关键实现：
/** Serves requests from the cache and writes responses to the cache. */
public final class CacheInterceptor implements Interceptor {
@Override public Response intercept(Chain chain) throws IOException {
//获取到缓存
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//这个类有点叼，request和缓存的response都是CacheStrategy类返回的，也许这个CacheStrategy就是管理者吧，决定到底使用缓存还是进行网络请求
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
//缓存不能使用，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.
// 1- 如果无网络访问（请求体networkRequest为null，内部的url也是null），又无缓存，返回504错误
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.
// 2 - 如果不需要网络请求，直接返回缓存数据
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
// 3 - 进行网络请求，得到 网络返回数据
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.
//4 - HTTP_NOT_MODIFIED 标识缓存有效，网络请求返回数据和缓存数据合并，并更新缓存
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());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) { //判断是否支持缓存
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
CacheRequest cacheRequest = cache.put(response);
//5- 判断有无缓存，写入缓存
return cacheWritingResponse(cacheRequest, response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}
}
上面关键代码，我们来总结下执行流程：
1 如果无网络访问，又无缓存，返回504错误，执行2
2 如果不需要网络请求，直接返回缓存数据，否则 执行3
3 进行网络请求，得到网络返回数据，执行4
4 HTTP_NOT_MODIFIED 标识缓存有效，网络请求返回数据和缓存数据合并，并更新缓存，否则 执行5
5 判断有无缓存，写入缓存并返回response
image.png
很好，CacheInterceptor分析完了，喝杯茶压压惊。
接下来 我们来看看ConnectInterceptor，字如其名，是一个跟连接有关的拦截器，我们看看关键代码：
/** Opens a connection to the target server and proceeds to the next interceptor. */
public final class ConnectInterceptor implements Interceptor {
@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, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
}
代码看的去很少，我们只需要关注下面两行：
HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
不管是httpCodec，还是connection，都是由streamAllocation完成，我们来看看newStream()里面做了什么：
public HttpCodec newStream(
OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
int connectTimeout = chain.connectTimeoutMillis();
int readTimeout = chain.readTimeoutMillis();
int writeTimeout = chain.writeTimeoutMillis();
int pingIntervalMillis = client.pingIntervalMillis();
boolean connectionRetryEnabled = client.retryOnConnectionFailure();
try {
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);
synchronized (connectionPool) {
codec = resultCodec;
return resultCodec;
}
} catch (IOException e) {
throw new RouteException(e);
}
}
......
public HttpCodec newCodec(OkHttpClient client, Interceptor.Chain chain,
StreamAllocation streamAllocation) throws SocketException {
if (http2Connection != null) {
return new Http2Codec(client, chain, streamAllocation, http2Connection);
} else {
socket.setSoTimeout(chain.readTimeoutMillis());
source.timeout().timeout(chain.readTimeoutMillis(), MILLISECONDS);
sink.timeout().timeout(chain.writeTimeoutMillis(), MILLISECONDS);
return new Http1Codec(client, streamAllocation, source, sink);
}
}
findHealthyConnection ，是找到一个可用连接的意思，重点就在这，通过找到的可用连接newCodec(),返回了HttpCodec的实现类Http1Codec对象。我们去看看findHealthyConnection(),看看如何找到可用连接：
private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
boolean doExtensiveHealthChecks) throws IOException {
while (true) {
RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
pingIntervalMillis, connectionRetryEnabled);
// If this is a brand new connection, we can skip the extensive health checks.
synchronized (connectionPool) {
if (candidate.successCount == 0) {
return candidate;
}
}
// Do a (potentially slow) check to confirm that the pooled connection is still good. If it
// isn't, take it out of the pool and start again.
if (!candidate.isHealthy(doExtensiveHealthChecks)) {//判断连接是否可用
noNewStreams();//连接不可用，移除
continue;//不可用，就一直持续
}
return candidate;
}
}
哇，这个方法用了死循环，如果找不到可用连接，就一直卡在这里，再来看看findConnection()关键方法,有点长，分析都在源码里面的注释上：
/**
* Returns a connection to host a new stream. This prefers the existing connection if it exists,
* then the pool, finally building a new connection.
*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
boolean foundPooledConnection = false;
RealConnection result = null;
Route selectedRoute = null;
Connection releasedConnection;
Socket toClose;
//异常情况，直接抛出
synchronized (connectionPool) {
if (released) throw new IllegalStateException("released");
if (codec != null) throw new IllegalStateException("codec != null");
if (canceled) throw new IOException("Canceled");
// Attempt to use an already-allocated connection. We need to be careful here because our
// already-allocated connection may have been restricted from creating new streams.
releasedConnection = this.connection;
toClose = releaseIfNoNewStreams();
if (this.connection != null) { //
//经过releaseIfNoNewStreams，connection不为null，则连接是可用的
// We had an already-allocated connection and it's good.
result = this.connection;
releasedConnection = null;
}
if (!reportedAcquired) {
// If the connection was never reported acquired, don't report it as released!
releasedConnection = null;
}
//无可用连接，去连接池connectionPool中获取
if (result == null) {
// Attempt to get a connection from the pool.
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
foundPooledConnection = true;
result = connection;
} else {
selectedRoute = route;
}
}
}
closeQuietly(toClose);
if (releasedConnection != null) {
eventListener.connectionReleased(call, releasedConnection);
}
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
}
if (result != null) {//上面通过去连接池中找，如果result不为null，说明找到了可用连接
// If we found an already-allocated or pooled connection, we're done.
return result;
}
// If we need a route selection, make one. This is a blocking operation.
//如果在连接池中也没找到可用连接， 就需要一个路由信息，这是一个阻塞操作
boolean newRouteSelection = false;
if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
newRouteSelection = true;
routeSelection = routeSelector.next();
}
synchronized (connectionPool) {
if (canceled) throw new IOException("Canceled");
if (newRouteSelection) {
// Now that we have a set of IP addresses, make another attempt at getting a connection from
// the pool. This could match due to connection coalescing.
//提供address，再次从连接池中获取连接
List<Route> routes = routeSelection.getAll();
for (int i = 0, size = routes.size(); i < size; i++) {
Route route = routes.get(i);
Internal.instance.get(connectionPool, address, this, route);
if (connection != null) {
foundPooledConnection = true;
result = connection;
this.route = route;
break;
}
}
}
//提供路路由信息，然后进行查找可用链接，还是没有找到可用链接，就需要生成一个新的连接
if (!foundPooledConnection) {
if (selectedRoute == null) {
selectedRoute = routeSelection.next();
}
// Create a connection and assign it to this allocation immediately. This makes it possible
// for an asynchronous cancel() to interrupt the handshake we're about to do.
route = selectedRoute;
refusedStreamCount = 0;
result = new RealConnection(connectionPool, selectedRoute);
acquire(result, false);
}
}
// If we found a pooled connection on the 2nd time around, we're done.
//如果连接是从连接池中找到的，直接拿出来使用
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
return result;
}
// Do TCP + TLS handshakes. This is a blocking operation.
result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
connectionRetryEnabled, call, eventListener);
routeDatabase().connected(result.route());
Socket socket = null;
synchronized (connectionPool) {
reportedAcquired = true;
// Pool the connection.
//将新生成的连接放入连接池中
Internal.instance.put(connectionPool, result);
// If another multiplexed connection to the same address was created concurrently, then
// release this connection and acquire that one.
//如果是一个http2连接，http2连接应具有多路复用特性，
if (result.isMultiplexed()) {
socket = Internal.instance.deduplicate(connectionPool, address, this);
result = connection;
}
}
closeQuietly(socket);
eventListener.connectionAcquired(call, result);
return result;
}
我们来回顾下上面的获取可用连接的流程：
先检测链接是否可用:
a 可用的话直接返回，结束流程。
b 不可用，先去 连接池中查找：
连接池找到：结束流程。
未找到：提供address，再次去连接池中查找。如果找到了直接结束流程，如果没找到：生成一个新的连接，并且将这个新的链接加入到连接池，然后返回这个新链接。
ConnectInterceptor 分析结束。
接下来是CallServerInterceptor，这个拦截器用来完成最终的请求执行，这里面涉及到OKio这个库，假装它就是一个httpUrlConnection就行了，由于是另外一个库，这里就不分析了。
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