@Override protected void execute() {
boolean signalledCallback = false;
timeout.enter();
try {
// 跟同步执行一样，最后都会调用到这里
Response response = getResponseWithInterceptorChain();
if (retryAndFollowUpInterceptor.isCanceled()) {
signalledCallback = true;
responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
} else {
signalledCallback = true;
responseCallback.onResponse(RealCall.this, response);
}
} catch (IOException e) {
e = timeoutExit(e);
if (signalledCallback) {
// Do not signal the callback twice!
Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
} else {
eventListener.callFailed(RealCall.this, e);
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}}
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从上面的源码可以知道，拦截链的处理OKHttp帮我们默认做了五步拦截处理，其中RetryAndFollowUpInterceptor、BridgeInterceptor、CallServerInterceptor内部的源码很简洁易懂，此处不再多说，下面将对OKHttp最为核心的两部分：缓存处理和连接处理（连接池）进行讲解。
### []( )二、网络请求缓存处理之CacheInterceptor@Override public Response intercept(Chain chain) throws IOException {
// 根据request得到cache中缓存的response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
// request判断缓存的策略，是否要使用了网络，缓存或两者都使用
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
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.
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.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
// 调用下一个拦截器，决定从网络上来得到response
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.
// 如果本地已经存在cacheResponse，那么让它和网络得到的networkResponse做比较，决定是否来更新缓存的cacheResponse
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) {
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
// 缓存未经缓存过的response
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;}
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缓存拦截器会根据请求的信息和缓存的响应的信息来判断是否存在缓存可用，如果有可以使用的缓存，那么就返回该缓存给用户，否则就继续使用责任链模式来从服务器中获取响应。当获取到响应的时候，又会把响应缓存到磁盘上面。
### []( )三、ConnectInterceptor之连接池@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是对 HTTP 协议操作的抽象，有两个实现：Http1Codec和Http2Codec，顾名思义，它们分别对应 HTTP/1.1 和 HTTP/2 版本的实现。在这个方法的内部实现连接池的复用处理
HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);}
// Returns a connection to host a new stream. This // prefers the existing connection if it exists,
// then the pool, finally building a new connection.
// 调用 streamAllocation 的 newStream() 方法的时候，最终会经过一系列
// 的判断到达 StreamAllocation 中的 findConnection() 方法
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
...
// 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.
// 尝试使用已分配的连接，已经分配的连接可能已经被限制创建新的流
releasedConnection = this.connection;
// 释放当前连接的资源，如果该连接已经被限制创建新的流，就返回一个Socket以关闭连接
toClose = releaseIfNoNewStreams();
if (this.connection != null) {
// 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!
// 如果该连接从未被标记为获得，不要标记为发布状态，reportedAcquired 通过 acquire() 方法修改
releasedConnection = null;
}
if (result == null) {
// Attempt to get a connection from the pool.
// 尝试供连接池中获取一个连接
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
foundPooledConnection = true;
result = connection;
} else {
selectedRoute = route;
}
}
}
// 关闭连接
closeQuietly(toClose);
if (releasedConnection != null) {
eventListener.connectionReleased(call, releasedConnection);
}
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
}
if (result != null) {
// If we found an already-allocated or pooled connection, we're done.
// 如果已经从连接池中获取到了一个连接，就将其返回
return result;
}
// If we need a route selection, make one. This is a blocking operation.
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.
// 根据一系列的 IP地址从连接池中获取一个链接
List<Route> routes = routeSelection.getAll();
for (int i = 0, size = routes.size(); i < size;i++) {
Route route = routes.get(i);
// 从连接池中获取一个连接
Internal.instance.get(connectionPool, address, this, route);
if (connection != null) {
foundPooledConnection = true;
result = connection;
this.route = route;
break;
}
}
}
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.
// 在连接池中如果没有该连接，则创建一个新的连接，并将其分配，这样我们就可以在握手之前进行终端
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.
if (foundPooledConnection) {
// 如果我们在第二次的时候发现了一个池连接，那么我们就将其返回
eventListener.connectionAcquired(call, result);
return result;
}
// Do TCP + TLS handshakes. This is a blocking operation.
// 进行 TCP 和 TLS 握手
result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
connectionRetryEnabled, call, eventListener);
routeDatabase().connected(result.route());
Socket socket = null;
synchronized (connectionPool) {
reportedAcquired = true;
// Pool the connection.
// 将该连接放进连接池中
Internal.instance.put(connectionPool, result);
// If another multiplexed connection to the same address was created concurrently, then
// release this connection and acquire that one.
// 如果同时创建了另一个到同一地址的多路复用连接，释放这个连接并获取那个连接
if (result.isMultiplexed()) {
socket = Internal.instance.deduplicate(connectionPool, address, this);
result = connection;
}
}
closeQuietly(socket);
eventListener.connectionAcquired(call, result);
return result;}
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从以上的源码分析可知：
* 判断当前的连接是否可以使用：流是否已经被关闭，并且已经被限制创建新的流；
* 如果当前的连接无法使用，就从连接池中获取一个连接；
* 连接池中也没有发现可用的连接，创建一个新的连接，并进行握手，然后将其放到连接池中。
在从连接池中获取一个连接的时候，使用了 Internal 的 get() 方法。Internal 有一个静态的实例，会在 OkHttpClient 的静态代码快中被初始化。我们会在 Internal 的 get() 中调用连接池的 get() 方法来得到一个连接。并且，从中我们明白了连接复用的一个好处就是省去了进行 TCP 和 TLS 握手的一个过程。因为建立连接本身也是需要消耗一些时间的，连接被复用之后可以提升我们网络访问的效率。
接下来，我们来详细分析下ConnectionPool是如何实现连接管理的。
OkHttp 的缓存管理分成两个步骤，一边当我们创建了一个新的连接的时候，我们要把它放进缓存里面；另一边，我们还要来对缓存进行清理。在 ConnectionPool 中，当我们向连接池中缓存一个连接的时候，只要调用双端队列的 add() 方法，将其加入到双端队列即可，而清理连接缓存的操作则交给线程池来定时执行。private final Deque connections = new ArrayDeque<>();
void put(RealConnection connection) {
assert (Thread.holdsLock(this));
if (!cleanupRunning) {
cleanupRunning = true;
// 使用线程池执行清理任务
executor.execute(cleanupRunnable);
}
// 将新建的连接插入到双端队列中
connections.add(connection);}
private final Runnable cleanupRunnable = new Runnable() {
@Override public void run() {
while (true) {
// 内部调用 cleanup() 方法来清理无效的连接
long waitNanos = cleanup(System.nanoTime());
if (waitNanos == -1) return;
if (waitNanos > 0) {
long waitMillis = waitNanos / 1000000L;
waitNanos -= (waitMillis * 1000000L);
synchronized (ConnectionPool.this) {
try {
ConnectionPool.this.wait(waitMillis, (int) waitNanos);
} catch (InterruptedException ignored) {
}
}
}
}};
long cleanup(long now) {
int inUseConnectionCount = 0;
int idleConnectionCount = 0;
RealConnection longestIdleConnection = null;
long longestIdleDurationNs = Long.MIN_VALUE;
// Find either a connection to evict, or the time that the next eviction is due.
synchronized (this) {
// 遍历所有的连接
for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
RealConnection connection = i.next();
// If the connection is in use, keep searching.
// 遍历所有的连接
if (pruneAndGetAllocationCount(connection, now) > 0) {
inUseConnectionCount++;
continue;
}
idleConnectionCount++;
// If the connection is ready to be evicted, we're done.
// 如果找到了一个可以被清理的连接，会尝试去寻找闲置时间最久的连接来释放
long idleDurationNs = now - connection.idleAtNanos;
if (idleDurationNs > longestIdleDurationNs) {
longestIdleDurationNs = idleDurationNs;
longestIdleConnection = connection;
}
}
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