目录
一、提出问题
1.OkHttp底层也是通过Socket发送和接收请求，是如何支持http/https请求的？
2.连接池的实现原理，如何支持多路复用？怎样从连接池选择复用连接？
3.如何处理代理？
4.Route、ConnectionPool、RealConnection、steamAllocation、HttpCodec分别的作用，如何协作？
5.重定向请求或重试的处理流程？
6.如何支持http2协议？
如果刚开始学习OkHttp源码或对代理不了解的，可以先忽略代理部分的逻辑，先搞清楚直连请求的流程。OkHttp源码之所以复杂一部分原因是处理了代理和路由，但代理部分实际项目可能用不上。如果想深入了解OkHttp的代理，可以阅读：OkHttp源码解析 （三）——代理和路由（https://www.jianshu.com/p/63ba15d8877a）。
二、网络管理涉及的角色及作用
外部发起的一次请求封装为一个RealCall，一个RealCall可能对应多个Request，如初始请求及后续的重定向请求，而每一个Request会创建一个StreamAllocation来管理连接，寻找合适的RealConnection，一个Call的所有Request偏向用同一个RealConnection，对于HTTP/1.x的请求，RealConnection同时只持有一个StreamAllocation，对于HTTP/2可以同时持有多个StreamAllocation。角色的对应关系如下图。
三、各个角色的协作
1、在RetryAndFollowUpInterceptor拦截器中，为新请求创建流StreamAllocation，如果请求返回需要重定向，创建重定向Request及新的StreamAllocation，继续上面的逻辑。
2、在ConnectInterceptor拦截器中，StreamAllocation选择连接RealConnection：
第一步：优先从连接池（connectionPool）中寻找，有合适的则直接复用；
第二步：如果没有则创建新的RealConnection，并加入到连接池中，新创建的连接通过connect方法完成socket的三次握手，与服务器建立连接；
第三步：建立连接后获得网络写入流（BufferedSink，封装了InputStream）和读取流（BufferedSource，封装了outputStream）；
第四步：最后创建HttpCodec，持有BufferedSink和BufferedSource，后续写入请求和读取响应通过HttpCodec操作。
3、在CallServerInterceptor拦截器中，通过HttpCodec实现真正发送请求和读取服务器响应，最后构造Response并沿链路返回给上一级的拦截器。
各拦截器的关键代码：
RetryAndFollowUpInteceptor:
public final class RetryAndFollowUpInterceptor implements Interceptor {
@Override public Response intercept(Chain chain) throws IOException {
...
//原始请求
StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
createAddress(request.url()), call, eventListener, callStackTrace);
...
while (true) {
...
//原始请求返回
response = realChain.proceed(request, streamAllocation, null, null);
...
//创建重定向请求
Request followUp = followUpRequest(response, streamAllocation.route());
...
//重定向StreamAllocation
streamAllocation = new StreamAllocation(client.connectionPool(),createAddress(followUp.url()), call, eventListener, callStackTrace);
}
}
ConnectInterceptor：
public final class ConnectInterceptor implements Interceptor {
public Response intercept(Chain chain) throws IOException {
...
//从拦截器链里得到StreamAllocation对象
StreamAllocation streamAllocation = realChain.streamAllocation();
//寻找合适的连接并返回读写流
HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
//获取realConnetion
RealConnection connection = streamAllocation.connection();
//执行下一个拦截器
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
}
CallServerInterceptor:
public final class CallServerInterceptor implements Interceptor {
public Response intercept(Chain chain) throws IOException {
HttpCodec httpCodec = realChain.httpStream();
Request request = realChain.request();
//发送请求头和请求行（写入到缓冲区）
httpCodec.writeRequestHeaders(request);
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
...//发送body部分，post请求和"100-continue"请求
}
//flush正在发送
httpCodec.finishRequest();
//读取响应
if (responseBuilder == null) {
realChain.eventListener().responseHeadersStart(realChain.call());
//读取头部
responseBuilder = httpCodec.readResponseHeaders(false);
}
//构造响应Response
Response response = responseBuilder.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
...
//返回Response
return response;
}
}
四、详解各个角色的逻辑
（一） StreamAllocation
解释：流分配器，主要功能是管理一次连接上的流，为Request寻找合适的Realconnection，并获取网络读写流。重定向会创建新的StreamAllocation。每个Connection 有个变量allocationLimit，用于定义可以承载的并发的 streams 的数量。HTTP/1.x 的 Connection 一次只能有一个stream， HTTP/2 一般可以有多个。
public final class StreamAllocation {
public final Address address;//请求地址
private RouteSelector.Selection routeSelection;//可选路由列表
private Route route;//选中的路由
private final ConnectionPool connectionPool;//连接池
public final Call call;//请求call
// State guarded by connectionPool.
private final RouteSelector routeSelector;//路由选择器
private HttpCodec codec;//编码网络请求和响应
public StreamAllocation(ConnectionPool connectionPool, Address address, Call call,
EventListener eventListener, Object callStackTrace) {
this.connectionPool = connectionPool;
this.address = address;
this.call = call;
this.eventListener = eventListener;
this.routeSelector = new RouteSelector(address, routeDatabase(), call, eventListener);
this.callStackTrace = callStackTrace;
}
/*
* 获取流，通过findConnection得到连接，再获取读写流
*/
public HttpCodec newStream(...) {}
/*
* 寻找可复用连接及判断是否"健康"，如果不“健康”则继续循环直至找到“健康”连接
*/
private RealConnection findHealthyConnection(...) throws IOException {}
/*
* 为新stream寻找可复用连接，可能来自连接池，如果没有则新建
*/
private RealConnection findConnection(...）{}
/*
* 释放当前持有的连接，如果连接是限制分配给新流的（noNewSteam为true），则返回socket进行关闭。
* 对于HTTP/2，多个请求共享一个连接，所以对于follow-up请求期间可能被限制分配新流
*/
private Socket releaseIfNoNewStreams(){}
/*
* 请求已经完成，从连接中移除正在当前执行的流，只有移除了连接才能被复用
*/
public void streamFinished(){}
/*
* 禁止在承载此分配的连接上创建新流
*/
public void noNewStreams(){}
/*
* 取消或抛异常，释放连接
*/
public void release() {}
/*
* 释放此流所持有的资源。如果分配了足够的资源，连接将被分离或关闭。调用者必须在连接池上同步。
*/
private Socket deallocate(){}
}
（1）newStream方法
下面看下为新请求寻找连接获取读写流的逻辑。
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);
}
}
方法比较简单，第一步寻找连接RealConnection，第二步获取网络读写流HttpCodec，有点小疑问，为什么要先同步连接池再返回HttpCodec。
（2）findHealthyConnection方法
/**
* Finds a connection and returns it if it is healthy. If it is unhealthy the process is repeated
* until a healthy connection is found.
* 寻找连接，如果是“健康”的则返回，如果不是继续循环寻找。
*/
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.
//如果是新创建的连接，则不需判断是否“健康”，直接返回
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.
//检查连接是否良好，如果不是，则从连接池移除，然后继续寻找
if (!candidate.isHealthy(doExtensiveHealthChecks)) {
noNewStreams();
continue;
}
return candidate;
}
}
1、先调用findConnectoion方法返回连接对象RealConnection；
2、根据RealConnection的属性successCount=0判断连接是新创建的，新创建的连接不需要判断是否“健康”，直接返回；
3、如果successCount大于0，表示连接早已经创建，是从连接池中获取得到，这时需要判断连接是否“健康”；
4、如果非“健康”连接，则设置该连接不允许承载新的流，继续第一步；
findConnection及isHealthy的逻辑后面会分析。
（3）findConnection方法
/**
* Returns a connection to host a new stream. This prefers the existing connection if it exists,
* then the pool, finally building a new connection.
* 为新流返回连接，优先从连接池中寻找复用，如果没有最终会创建新的连接
*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
boolean foundPooledConnection = false;//是否从连接池中找到连接
RealConnection result = null;//需要返回的连接
Route selectedRoute = null;//找到路由
Connection releasedConnection;//可释放的连接
Socket toClose;//需要关闭的socket
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.
//？？没想懂什么场景这个地方已经分配了连接
releasedConnection = this.connection;
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!
releasedConnection = null;
}
if (result == null) {
// Attempt to get a connection from the pool.
//从连接池中获取一个连接，通过传入this对象，寻找到合适连接会赋值this.connection
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
//寻找到合适的连接
foundPooledConnection = true;
result = connection;
} else {
//连接池没有合适的连接，可能已经有路由信息（什么场景）
selectedRoute = route;
}
}
}
//？？什么场景
closeQuietly(toClose);
...
if (result != null) {
// If we found an already-allocated or pooled connection, we're done.
//如果已经分配连接或者从连接池中寻找到合适的，则返回
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;//需要尝试新的路由
routeSelection = routeSelector.next();//路由集合
}
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.
// 现在已经有一个ip地址集合，再次尝试重连接池中寻找可复用连接
List 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;
}
}
}
if (!foundPooledConnection) {//连接池中没有找到合适的连接
if (selectedRoute == null) {
selectedRoute = routeSelection.next();//即将根据该路由创建新的连接
}
// 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.
//创建新的连接并立即分配给当前的流，这样允许异步调用取消方法来打断即将进行的握手。
route = selectedRoute;
refusedStreamCount = 0;
result = new RealConnection(connectionPool, selectedRoute);
acquire(result, false);//分配连接给当前的流
}
}
// If we found a pooled connection on the 2nd time around, we're done.
// 如果从连接池找到合适的连接则返回。
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
return result;
}
// Do TCP + TLS handshakes. This is a blocking operation.
//新创建的连接，建立与服务器的连接，方法内会根据平台调用socket.connnect()
result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
connectionRetryEnabled, call, eventListener);
//路由库记录新连接的路由
routeDatabase().connected(result.route());
Socket socket = null;
synchronized (connectionPool) {
reportedAcquired = true;
// Pool the connection.
//添加到连接池，添加前需要加锁
Internal.instance.put(connectionPool, result);
// If another multiplexed connection to the same address was created concurrently, then
// release this connection and acquire that one.
//
if (result.isMultiplexed()) {
socket = Internal.instance.deduplicate(connectionPool, address, this);
result = connection;
}
}
closeQuietly(socket);
eventListener.connectionAcquired(call, result);
return result;
}
总结findConnection的流程如下：
1、先判断是否已经分配了连接，有则返回（没想懂是什么场景）；
2、没有则根据address从连接池中找可重用的连接Internal.intance.get(connectionPool,address,this,null)，找到则返回；
3、如果没有确定路由，则需要尝试新的路由，通过路由选择器返回路由集合，这时得到一个ip地址集合；
4、遍历路由集合再次从连接池中寻找可复用的连接，有则设为待返回的连接；
5、如最终从连接池中没有找到合适的连接，则新建连接new RealConnection(connectionPool,selectedRoute)，并马上分配给当前的流；
6、新建立的连接，建立与服务器连接result.connect(即sockect.connect)，并把路由记录到路由库中，把创建的连接添加到连接池Internal.intance.put(connectionPool ,result).
（二） RealConnection
解析：建立在Socket之上的物理通信信道，持有StreamAllocation队列。
public final class RealConnection extends Http2Connection.Listenerimplements Connection {
private final ConnectionPool connectionPool;//连接池
private final Route route; //当前连接到路由
private Socket rawSocket; //底层socket
private Socket socket; //应用层socket
private Handshake handshake; //https的握手
private Protocol protocol; //协议
private Http2Connection http2Connection; //HTTP/2的链接
private BufferedSource source; //网络读取流
private BufferedSink sink; //网络写入流
public boolean noNewStreams; //标识是否能继续添加流，一但设为true，则一直为true，不能再添加流
public int allocationLimit =1; //承载流(allocationStream)的最大的并发数
public final List> allocations =new ArrayList<>(); //当前承载流的集合
public RealConnection(ConnectionPool connectionPool, Route route){}
/*
* 连接，中创建新RealConnection对象后调用，完成socket连接
*/
public void connect(int connectTimeout, int readTimeout, int writeTimeout,int pingIntervalMillis, boolean connectionRetryEnabled, Call call,EventListener eventListener){}
/*
* 连接隧道
*/
private void connectTunnel(int connectTimeout, int readTimeout, int writeTimeout, Call call,EventListener eventListener)throws IOException{}
/*
* socket连接，完成tcp三次握手
*/
private void connectSocket(int connectTimeout, int readTimeout, Call call,EventListener eventListener)throws IOException{}
/*
* 建立协议
*/
private void establishProtocol(ConnectionSpecSelector connectionSpecSelector,int pingIntervalMillis, Call call, EventListener eventListener)throws IOException{}
/*
*https请求，建立tls连接
*/
private void connectTls(ConnectionSpecSelector connectionSpecSelector)throws IOException{}
/*
* 创建通道
*/
private Request createTunnel(int readTimeout, int writeTimeout, Request tunnelRequest,HttpUrl url)throws IOException{}
/*
* 构造创建通道的请求
*/
private Request createTunnelRequest(){}
/*
* 是否符合条件，如果能分配新流则返回true，
* /
public boolean isEligible(Address address, @Nullable Route route){}
/*
* 将io流BufferedSource，BufferedSink封装为HttpCodec
*/
public HttpCodec newCodec(OkHttpClient client, Interceptor.Chain chain,StreamAllocation streamAllocation)throws SocketException{}
/*
* 是否“健康”，如果准备好建立新流则返回true
*/
public boolean isHealthy(boolean doExtensiveChecks){}
}
（1）connect方法
新创建RealConnection后，通过connect方法服务器建立连接，完成tcp三次握手，下面介绍下connect的方法。
public void connect(int connectTimeout, int readTimeout, int writeTimeout,int pingIntervalMillis, boolean connectionRetryEnabled, Call call,EventListener eventListener) {
if (protocol != null) throw new IllegalStateException("already connected");//只能调用一次
RouteException routeException = null;
List connectionSpecs = route.address().connectionSpecs();
ConnectionSpecSelector connectionSpecSelector = new ConnectionSpecSelector(connectionSpecs);
if (route.address().sslSocketFactory() == null) {
if (!connectionSpecs.contains(ConnectionSpec.CLEARTEXT)) {
throw new RouteException(new UnknownServiceException(
"CLEARTEXT communication not enabled for client"));
}
String host = route.address().url().host();
if (!Platform.get().isCleartextTrafficPermitted(host)) {
throw new RouteException(new UnknownServiceException(
"CLEARTEXT communication to " + host + " not permitted by network security policy"));
}
}
while (true) { //对应SSLHandshakeException/SSLProtocolException的抛错，会重试
try {
//判断是否需要隧道，如果是通过HTTP代理完成https请求，则返回true
//true的条件：address.sslSocketFactory != null && proxy.type() == Proxy.Type.HTTP
if (route.requiresTunnel()) {
//建立隧道，与http代理之间建立socket连接
connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener);
if (rawSocket == null) {
// We were unable to connect the tunnel but properly closed down our resources.
//无法与代理建立连接
break;
}
} else {
//建立socket连接，不需要代理
connectSocket(connectTimeout, readTimeout, call, eventListener);
}
//创建协议，完成连接
establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener);
eventListener.connectEnd(call, route.socketAddress(), route.proxy(), protocol);
break;
} catch (IOException e) {
...
//connectionRetryEnabled :是否运行重试连接，在okhttpclient的builder设置，默认true
//connectionSpecSelector.connectionFailed(e)针对不同的报错有不同的策略，返回true则重试
if (!connectionRetryEnabled || !connectionSpecSelector.connectionFailed(e)) {
throw routeException;
}
}
}
...
if (http2Connection != null) { //http2运行一个RealConnection建立多个流
synchronized (connectionPool) {
allocationLimit = http2Connection.maxConcurrentStreams();
}
}
总结connect的方法如下：
1、判断是否需要隧道（隧道代理），如果需要则建立与代理服务器的sockect连接；
2、不需要隧道则直接建立与服务器的sockect连接；
3、确定网络协议，如果是https请求则进行tls握手；
4、如果是HTTP/2，则新建http2Connection来处理请求，完成HTTP/2的协议协商。
（2）connectSocket方法
解析了连接的整理流程，下面对其中调用的方法进行分析，首先看下connectSocket方法：
private void connectSocket(int connectTimeout, int readTimeout, Call call,EventListener eventListener) throws IOException {
Proxy proxy = route.proxy();
Address address = route.address();
//如果是直连或者HTTP代理，则通过socketFactory创建socket，如果是socket代理，则直接new Socket
rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP
? address.socketFactory().createSocket(): new Socket(proxy);
eventListener.connectStart(call, route.socketAddress(), proxy);
rawSocket.setSoTimeout(readTimeout);
try {
//根据不同的平台，完成socket连接，实际是socket.connect(address, connectTimeout);
Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);
} catch (ConnectException e) {
ConnectException ce = new ConnectException("Failed to connect to " + route.socketAddress());
ce.initCause(e);
throw ce;
}
try {
//okio封装sockect读写流
source = Okio.buffer(Okio.source(rawSocket));
sink = Okio.buffer(Okio.sink(rawSocket));
} catch (NullPointerException npe) {
if (NPE_THROW_WITH_NULL.equals(npe.getMessage())) {
throw new IOException(npe);
}
}
}
流程比较简单，总结如下：
1、对于直连及http代理请求，通过SocketFactory创建socket，对于SOCKET代理传入proxy创建socket；
2、设置socket超时时间；
3、完成特定平台的socket连接，实际是socket.connect(address, connectTimeout)；
4、创建用于I/O的读写流source 、sink
这里可以发现，代理请求处理的不同：
· SOCKET代理：传入代理对象proxy手动创建socket，其他没有什么特别的处理，都交由java标准库的socket去处理，route的socketAddress包含目标http服务器的域名，对外界而言，不需要做处理。
· HTTP代理：对于明文的HTTP代理， 也不需要特别的处理，route的socketAddress包含着代理服务器的IP地址，会自动建立与代理服务器的连接，代理服务器解析后再转发请求内容。
（3）connectTunnel方法
通过HTTP代理发送https请求需要用到隧道代理，也是一种协定方式，总结建立隧道的流程：
1、客户端发送CONNECT请求到代理服务器，请求建立通道；请求会包含目标服务器的主机名和端口
2、代理服务器与目标服务器建立TCP连接；
3、代理服务器回应客户端；
4、客户端向代理服务器发送请求，代理服务器原封不动转发客户端请求（原生TCP packet）；
5、响应过程同请求过程。
了解隧道代理的基本流程，就好理解OkHttp中有关隧道代理的代码逻辑了。下面介绍connectTunnel方法：
private void connectTunnel(int connectTimeout, int readTimeout, int writeTimeout, Call call,EventListener eventListener) throws IOException {
//构建连接请求
Request tunnelRequest = createTunnelRequest();
HttpUrl url = tunnelRequest.url();
for (int i = 0; i < MAX_TUNNEL_ATTEMPTS; i++) {
//建立与代理服务器的socket连接
connectSocket(connectTimeout, readTimeout, call, eventListener);
//建立隧道，发送不加密的代理请求并获取返回结果
tunnelRequest = createTunnel(readTimeout, writeTimeout, tunnelRequest, url);
if (tunnelRequest == null) break; // 返回null表示已经与代理服务器建立隧道，退出循环
// The proxy decided to close the connection after an auth challenge. We need to create a new
// connection, but this time with the auth credentials.
//如果代理服务器因身份认证问题关闭了连接，需要创建新的连接，带上身份凭证
closeQuietly(rawSocket);
rawSocket = null;
sink = null;
source = null;
eventListener.connectEnd(call, route.socketAddress(), route.proxy(), null);
}
}
先创建与代理服务器的socket连接，然后再发送代理请求建立隧道（按照隧道代理的协议方式）。
（4）createTunnelRequest方法
建立隧道的需要构建代理请求， 那代理的请求发了什么，下面介绍createTunnelRequest方法：
private Request createTunnelRequest() {
return new Request.Builder()
.url(route.address().url())
.header("Host", Util.hostHeader(route.address().url(), true))
.header("Proxy-Connection", "Keep-Alive") // For HTTP/1.0 proxies like Squid.
.header("User-Agent", Version.userAgent())
.build();
}
从代码可以看到通道请求包括很少的头部信息，是因为与服务器代理之间的连接是不加密的，避免发生如cookies等敏感信息到代理服务器。
（5）createTunnel方法
下面看下再看下如何创建隧道：
private Request createTunnel(int readTimeout, int writeTimeout, Request tunnelRequest,
HttpUrl url) throws IOException {
// Make an SSL Tunnel on the first message pair of each SSL + proxy connection.
// 建立隧道的协议请求内容
String requestLine = "CONNECT " + Util.hostHeader(url, true) + " HTTP/1.1";
while (true) {//？？swtich各种分支都会退出循环，没想懂循环的场景
Http1Codec tunnelConnection = new Http1Codec(null, null, source, sink);
source.timeout().timeout(readTimeout, MILLISECONDS);
sink.timeout().timeout(writeTimeout, MILLISECONDS);
// 发送代理请求
tunnelConnection.writeRequest(tunnelRequest.headers(), requestLine);
tunnelConnection.finishRequest();
// 读取响应
Response response = tunnelConnection.readResponseHeaders(false)
.request(tunnelRequest)
.build();
// The response body from a CONNECT should be empty, but if it is not then we should consume
// it before proceeding.
long contentLength = HttpHeaders.contentLength(response);
if (contentLength == -1L) {
contentLength = 0L;
}
Source body = tunnelConnection.newFixedLengthSource(contentLength);
Util.skipAll(body, Integer.MAX_VALUE, TimeUnit.MILLISECONDS);
body.close();
switch (response.code()) {
case HTTP_OK:
// Assume the server won't send a TLS ServerHello until we send a TLS ClientHello. If
// that happens, then we will have buffered bytes that are needed by the SSLSocket!
// This check is imperfect: it doesn't tell us whether a handshake will succeed, just
// that it will almost certainly fail because the proxy has sent unexpected data.
if (!source.buffer().exhausted() || !sink.buffer().exhausted()) {
throw new IOException("TLS tunnel buffered too many bytes!");
}
return null;
case HTTP_PROXY_AUTH:
tunnelRequest = route.address().proxyAuthenticator().authenticate(route, response);
if (tunnelRequest == null) throw new IOException("Failed to authenticate with proxy");
if ("close".equalsIgnoreCase(response.header("Connection"))) {
return tunnelRequest;
}
break;
default:
throw new IOException(
"Unexpected response code for CONNECT: " + response.code());
}
}
}
（6）establishProtocol方法
上面的方法只是建立了socket连接，无论是与目标服务器的连接，还是与代理服务器的，下面的方法是确定网络协议，如果是http请求，协议为http/1.1，可以直接返回，如果是https请求， 则建立ssl连接，如果HTTP/2则需要进行协议协商。这里就解析了OkHttp如何在socket连接之上实现http、https、HTTP/2等协议。
private void establishProtocol(ConnectionSpecSelector connectionSpecSelector,
int pingIntervalMillis, Call call, EventListener eventListener) throws IOException {
if (route.address().sslSocketFactory() == null) {
protocol = Protocol.HTTP_1_1;
socket = rawSocket;
return;
}
eventListener.secureConnectStart(call);
connectTls(connectionSpecSelector);
eventListener.secureConnectEnd(call, handshake);
if (protocol == Protocol.HTTP_2) {
socket.setSoTimeout(0); // HTTP/2 connection timeouts are set per-stream.
http2Connection = new Http2Connection.Builder(true)
.socket(socket, route.address().url().host(), source, sink)
.listener(this)
.pingIntervalMillis(pingIntervalMillis)
.build();
http2Connection.start();
}
}
从代码可以看出：
1、如果sslSocketFactory为空，说明是http请求，协议为HTTP_1_1，返回；
2、如果sslSocketFactory非空，需要进行TLS握手；
3、如果是协议是HTTP_2，则构建Http2Connection，完成与服务器的协商。
（7）connectTls方法
再看下如何建立TLS连接：
private void connectTls(ConnectionSpecSelector connectionSpecSelector) throws IOException {
Address address = route.address();
SSLSocketFactory sslSocketFactory = address.sslSocketFactory();
boolean success = false;
SSLSocket sslSocket = null;
try {
// Create the wrapper over the connected socket.
//在原来的已经 建立连接的socket上加一层ssl，java中传入原始socket构造SSLSocket
sslSocket = (SSLSocket) sslSocketFactory.createSocket(
rawSocket, address.url().host(), address.url().port(), true /* autoClose */);
// Configure the socket's ciphers, TLS versions, and extensions.
//配置socket的加解密器 ，TLS版本及扩展内容
ConnectionSpec connectionSpec = connectionSpecSelector.configureSecureSocket(sslSocket);
if (connectionSpec.supportsTlsExtensions()) {
Platform.get().configureTlsExtensions(
sslSocket, address.url().host(), address.protocols());
}
// Force handshake. This can throw!
//ssl握手
sslSocket.startHandshake();
// block for session establishment
SSLSession sslSocketSession = sslSocket.getSession();
if (!isValid(sslSocketSession)) {
throw new IOException("a valid ssl session was not established");
}
Handshake unverifiedHandshake = Handshake.get(sslSocketSession);
// Verify that the socket's certificates are acceptable for the target host.
//验证socket的证书是否被服务器接受
if (!address.hostnameVerifier().verify(address.url().host(), sslSocketSession)) {
//获取X509Certificate证书对象
X509Certificate cert = (X509Certificate) unverifiedHandshake.peerCertificates().get(0);
throw new SSLPeerUnverifiedException("Hostname " + address.url().host() + " not verified:"
+ "\n certificate: " + CertificatePinner.pin(cert)
+ "\n DN: " + cert.getSubjectDN().getName()
+ "\n subjectAltNames: " + OkHostnameVerifier.allSubjectAltNames(cert));
}
// Check that the certificate pinner is satisfied by the certificates presented.
address.certificatePinner().check(address.url().host(),
unverifiedHandshake.peerCertificates());
// Success! Save the handshake and the ALPN protocol.
String maybeProtocol = connectionSpec.supportsTlsExtensions()
? Platform.get().getSelectedProtocol(sslSocket)
: null;
socket = sslSocket;
source = Okio.buffer(Okio.source(socket));
sink = Okio.buffer(Okio.sink(socket));
handshake = unverifiedHandshake;
protocol = maybeProtocol != null
? Protocol.get(maybeProtocol)
: Protocol.HTTP_1_1;
success = true;
} catch (AssertionError e) {
if (Util.isAndroidGetsocknameError(e)) throw new IOException(e);
throw e;
} finally {
if (sslSocket != null) {
Platform.get().afterHandshake(sslSocket);
}
if (!success) {
closeQuietly(sslSocket);
}
}
}
TLS连接是对原始的TCP连接的一个封装，以提供TLS握手，及数据收发过程中的加密解密等功能。在Java中，用SSLSocket来描述。建立TLS连接的大致流程可总结为：
1、在原始已经建立连接的socket的基础上，用SSLSocketFactory构建SSLSocket；
2、配置SSLSocket，包括加解密器，TLS协议版本，如果ConnectionSpec支持TLS扩展参数，配置TLS扩展参数；
3、开始TLS握手sslSocket.startHandshake（）；
4、握手完后，获取服务器返回的证书信息SSLSession；
5、对握手过程返回证书新息SSLSession进行验证hostnameVerifier().verify（）；
6、验证远程主机证书；
7、如果ConnectionSpec支持TLS扩展参数，获取握手过程完成的协议协商所选择的协议，主要用于http2的ALPN扩展；
8、获取I/O操作的读写流，okio的BufferedSource和BufferedSink，保存协议及握手信息。
（8）isEligible方法
对于一个请求，优先从连接池寻找可复用的连接，如何判断连接是否能否被复用，下面解释判断能否复用的方法：
/**
* Returns true if this connection can carry a stream allocation to {@code address}. If non-null
* {@code route} is the resolved route for a connection.
*/
public boolean isEligible(Address address, @Nullable Route route) {
// If this connection is not accepting new streams, we're done.
// 当前分配的流已达到上限或者已经设为不允许再分配流
if (allocations.size() >= allocationLimit || noNewStreams) return false;
// If the non-host fields of the address don't overlap, we're done.
// host之外的配置要匹配，包括协议版本、代理、ssl、端口等
if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;
// If the host exactly matches, we're done: this connection can carry the address.
// 如果host也完全匹配，则可放心复用
if (address.url().host().equals(this.route().address().url().host())) {
return true; // This connection is a perfect match.
}
// At this point we don't have a hostname match. But we still be able to carry the request if
// our connection coalescing requirements are met. See also:
// https://hpbn.co/optimizing-application-delivery/#eliminate-domain-sharding
// https://daniel.haxx.se/blog/2016/08/18/http2-connection-coalescing/
// 1. This connection must be HTTP/2.
// 运行到这里说明多个host指向一个ip的特殊情况，只允许HTTP/2复用，条件比较严格，要满足后续4点
if (http2Connection == null) return false;
// 2. The routes must share an IP address. This requires us to have a DNS address for both
// hosts, which only happens after route planning. We can't coalesce connections that use a
// proxy, since proxies don't tell us the origin server's IP address.
if (route == null) return false;
if (route.proxy().type() != Proxy.Type.DIRECT) return false;
if (this.route.proxy().type() != Proxy.Type.DIRECT) return false;
if (!this.route.socketAddress().equals(route.socketAddress())) return false;
// 3. This connection's server certificate's must cover the new host.
if (route.address().hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
if (!supportsUrl(address.url())) return false;
// 4. Certificate pinning must match the host.
try {
address.certificatePinner().check(address.url().host(), handshake().peerCertificates());
} catch (SSLPeerUnverifiedException e) {
return false;
}
return true; // The caller's address can be carried by this connection.
}
判断连接是否可复用条件：
1、先要满足流分配上限数（HTTP/1.x 1个，HTTP/2 多个）；
2、Address的配置完全相同，如SSL、代理、端口、主机名都要匹配；
3、如果Address不匹配也可能有复用，主要是同一个主机配置了多个域名，且新请求已经选择的路由，条件必须同时满足：HTTP/2请求，新请求不是代理请求、当前连接也不是代理连接，路由ip、端口匹配，证书匹配。
（三） ConnectionPool
解析：我们都知道，在复杂的网络环境下，频繁创建和断开Socket连接是非常浪费资源和耗时的（需要3次握手4次挥手），如果是https连接还要进行ssl握手，http协议的keepalive对于解决这一问题有重要的作用。
连接空闲后存活一段时间及连接复用需就要对连接进行管理，这里引入了连接池的概念。okhttp支持单个地址最多5个空闲连接（keepalive状态），保活时间是5分钟，超出时间的连接会被回收。okhttp用ConnectionPool实现连接池的功能，对连接进行管理和回收。
ConnectionPool内部维护一个队列存放连接，一个线程池清理连接。
/*
* 连接池，实现连接的复用。通过一个队列维护当前所有的连接（RealConnection）
* 最多同时持有5个空闲连接，保活时间为5分钟
*/
public final class ConnectionPool {
/**
* Background threads are used to cleanup expired connections. There will be at most a single
* thread running per connection pool. The thread pool executor permits the pool itself to be
* garbage collected.用于清理失效连接的后台线程池。线程池允许自身进行垃圾回收。
*/
private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
new SynchronousQueue(), Util.threadFactory("OkHttp ConnectionPool", true));
/** The maximum number of idle connections for each address. */
private final int maxIdleConnections;//每个地址最多空闲连接数
private final long keepAliveDurationNs;
//清理连接线程，在线程池executor中调用
private final Runnable cleanupRunnable = new Runnable() {
@Override public void run() {
while (true) {
//执行清理，并返回下次清理的时间
long waitNanos = cleanup(System.nanoTime());
if (waitNanos == -1) return;
if (waitNanos>0) {
long waitMillis = waitNanos / 1000000L;
waitNanos -= (waitMillis * 1000000L);
synchronized (ConnectionPool.this) {
try {
//阻塞等待，等时间到后继续循环清理
ConnectionPool.this.wait(waitMillis, (int) waitNanos);
} catch (InterruptedException ignored) {
}
}
}
}
}
};
private final Deque connections = new ArrayDeque<>();//存放连接的队列
final RouteDatabase routeDatabase = new RouteDatabase();//路由库
boolean cleanupRunning;
/**
* Create a new connection pool with tuning parameters appropriate for a single-user application.
* The tuning parameters in this pool are subject to change in future OkHttp releases. Currently
* this pool holds up to 5 idle connections which will be evicted after 5 minutes of inactivity.
*/
public ConnectionPool() {
this(5, 5, TimeUnit.MINUTES);
}
public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
this.maxIdleConnections = maxIdleConnections;
this.keepAliveDurationNs = timeUnit.toNanos(keepAliveDuration);
// Put a floor on the keep alive duration, otherwise cleanup will spin loop.
if (keepAliveDuration<= 0) {
throw new IllegalArgumentException("keepAliveDuration <= 0: " + keepAliveDuration);
}
}
/*
* 获取可复用连接，如果没有则返回null
*/
@Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {}
/*
* 将新创建的连接加入连接池
*/
void put(RealConnection connection){}
}
（1）get方法
/**
* Returns a recycled connection to {@code address}, or null if no such connection exists. The
* route is null if the address has not yet been routed.
* 根据address获取可重用的连接，如果没有返回null。
* 如果没有选择路由，入参route是null，StreamAllocation的findConnection方法，第一次调用get方法
* 的入参route就是null
*/
@Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {
assert (Thread.holdsLock(this));
for (RealConnection connection : connections) {
if (connection.isEligible(address, route)) {
streamAllocation.acquire(connection, true);
return connection;
}
}
return null;
}
方法比较简单，遍历队列中每个连接，调用isEligible方法判断是否适合复用，能则分配给streamAllocation，判断是否适合的方法isEligible前面有分析。
（2）put方法
void put(RealConnection connection) {
assert (Thread.holdsLock(this));
if (!cleanupRunning) {
cleanupRunning = true;
executor.execute(cleanupRunnable);
}
connections.add(connection);
}
}
新创建的连接需要通过put方法加入到连接池，先执行清理，再添加到队列。
（3）cleanUp方法
/**
* Performs maintenance on this pool, evicting the connection that has been idle the longest if
* either it has exceeded the keep alive limit or the idle connections limit.
*
* Returns the duration in nanos to sleep until the next scheduled call to this method. Returns
* -1 if no further cleanups are required.
*/
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) {
for (Iterator i = connections.iterator(); i.hasNext(); ) {
RealConnection connection = i.next();
// If the connection is in use, keep searching.
if (pruneAndGetAllocationCount(connection, now) > 0) {
inUseConnectionCount++;
continue;
}
idleConnectionCount++;
// If the connection is ready to be evicted, we're done.
long idleDurationNs = now - connection.idleAtNanos;
if (idleDurationNs > longestIdleDurationNs) {
longestIdleDurationNs = idleDurationNs;
longestIdleConnection = connection;
}
}
// 找出空闲时间最长的连接
if (longestIdleDurationNs >= this.keepAliveDurationNs
|| idleConnectionCount > this.maxIdleConnections) {
// We've found a connection to evict. Remove it from the list, then close it below (outside
// of the synchronized block).
connections.remove(longestIdleConnection);
} else if (idleConnectionCount > 0) {
// A connection will be ready to evict soon.
return keepAliveDurationNs - longestIdleDurationNs;
} else if (inUseConnectionCount > 0) {
// All connections are in use. It'll be at least the keep alive duration 'til we run again.
return keepAliveDurationNs;
} else {
// No connections, idle or in use.
cleanupRunning = false;
return -1;
}
}
closeQuietly(longestIdleConnection.socket());
// Cleanup again immediately.
return 0;
}
清理的逻辑不复杂，就是遍历队列中的连接，调用pruneAndGetAllocationCount方法返回引用数，判断当前连接是否空闲，跳过正在被用的连接，对于空闲的连接，更新空闲持续的时间，通过遍历得到空闲时间最长的连接，如果超过了设定的保活时间或者空闲连接超过最大数量，则移除并关闭该连接，继续执行清除，如果没有需要移除的，返回下次清理时间，即最快达到设定保活的时间。
（4）pruneAndGetAllocationCount方法
* Prunes any leaked allocations and then returns the number of remaining live allocations on
* {@code connection}. Allocations are leaked if the connection is tracking them but the
* application code has abandoned them. Leak detection is imprecise and relies on garbage
* collection.
*/
private int pruneAndGetAllocationCount(RealConnection connection, long now) {
List> references = connection.allocations;//弱引用列表
for (int i = 0; i < references.size(); ) {
Reference reference = references.get(i);
if (reference.get() != null) {
i++;
continue;
}
// We've discovered a leaked allocation. This is an application bug.
StreamAllocation.StreamAllocationReference streamAllocRef =
(StreamAllocation.StreamAllocationReference) reference;
String message = "A connection to " + connection.route().address().url()
+ " was leaked. Did you forget to close a response body?";
Platform.get().logCloseableLeak(message, streamAllocRef.callStackTrace);
references.remove(i);
connection.noNewStreams = true;
// If this was the last allocation, the connection is eligible for immediate eviction.
if (references.isEmpty()) {
connection.idleAtNanos = now - keepAliveDurationNs;
return 0;
}
}
return references.size();
}
RealConnection通过一个列表记录当前建立的流List
（4）小结
由上面的分析可总结连接池复用的原理：
· OkHttp通过ConnectionPool维护线程池；
· ConnectionPool通过队列Deque
· 新创建的连接通过put方法加入到队列，加入队列前先执行一遍清理；
· get方法会根据传入的Address和Route遍历连接队列，返回可以复用的连接，复用的条件既要满足分配流的上限原则，也需protocol、ssl、host等配置匹配；
· ConnectionPool通过一个专门的线程清理失效的连接，该线程每执行完一次清理都会根据返回的等待时间阻塞等待；
· 清理的逻辑即遍历每个连接，通过连接对StreamAlloction的弱引用计数器来判断是否空闲（计数为0则说明空闲），通过遍历队列，找出空闲时长最长的连接，再根据已到保活的时长（keepalive）或空闲连接数的上限进行清理回收。
五、总结
至此基本解析了OkHttp网络连接管理的流程，由于篇幅及时间有限的原因，中间有些细节没有展开细分析。在分析的过程也解答了文章开头提出的疑问。
参考
https://www.jianshu.com/p/6166d28983a2
https://blog.csdn.net/yueaini10000/article/details/83305787
https://blog.csdn.net/FrancisHe/article/details/84667562#_HTTP__2