OkHttp连接拦截器篇

缓存篇

• https://blog.csdn.net/followYouself/article/details/121086502

流程图

ConnectInterceptor详解

• 连接管理拦截器。有一个连接池。可以建立http、https、http2三种连接类型。
• 默认情况下，连接池最多会持有5个连接，保活时间是5分钟。参考ConnectionPool#ConnectionPool(int, long, java.util.concurrent.TimeUnit)。
• 在这个拦截器中建立了socket、sslSocket连接。http、https连接，协商了http2协议。
• 参考资料: https://juejin.cn/post/6844904196219600910
• 参考资料：https://blog.csdn.net/hfy8971613/article/details/106541640

ExchangeFinder

• 该类主要通过findHealthyConnection函数生成一个RealConnection,然后构造一个ExchangeCodec对象用于网络请求。
• 生成RealConnection基于如下规则:

1. 如果已经有connection连接满足要求，那么直接服用当前链路。
2. 如果没有满足要求的connection连接，那么在ConnectionPool连接池中寻找可用连接，进行连接服用。
3. 如果连接池中也没有找到可用connection连接，那么根据路由表（可能会DNS请求），尝试建立一个可用连接。

/**
 * Attempts to find the connections for a sequence of exchanges. This uses the following strategies:
 *
 * 

 *   
If the current call already has a connection that can satisfy the request it is used.
 *       Using the same connection for an initial exchange and its follow-ups may improve locality.
 *
 *   
If there is a connection in the pool that can satisfy the request it is used. Note that
 *       it is possible for shared exchanges to make requests to different host names! See {@link
 *       RealConnection#isEligible} for details.
 *
 *   
If there's no existing connection, make a list of routes (which may require blocking DNS
 *       lookups) and attempt a new connection them. When failures occur, retries iterate the list
 *       of available routes.
 * 
 *
 * 
If the pool gains an eligible connection while DNS, TCP, or TLS work is in flight, this finder
 * will prefer pooled connections. Only pooled HTTP/2 connections are used for such de-duplication.
 */

ExchangeFinder#findConnection源码分析

1. 注释1处transmitter中的connection不为空，而且是可用的直接返回当前连接。
2. 注释2处说明当前transmitter中的connection为空，尝试通过transmitterAcquirePooledConnection获取ConnectionPool连接池中的可用连接。
3. 注释3，nextRouteToTry是在注释13处赋值的，它并不是记录的routes列表中的下一个路由地址。它只是记录一种异常情况:在ConnectionPool获取到可用连接，但是这个连接又马上unhealthy。
4. 重试当前的Route路由地址。
5. 返回可用连接，有可能是transmitter本身自带的，也有可能是ConnectionPool中获取的
6. 获取下一个Proxy的路由表，此处会进行DNS。返回RouteSelector.Selection对象。每个Selection包含一个路由表。
7. 根据获取的路由表，再次尝试获取ConnectionPool连接池中的可用连接。
8. ConnectionPool没有找到可用连接，获取一个Route路由对象，每个。
9. 创建一个RealConnection对象，并且传入Route路由对象。
10. 如果注释7出找到了可用连接，直接返回。
11. 真正的想服务器发起socket连接。可能进行TCP + TLS handshakes + Http/2.0协商 等步骤
12. 在注释11建立一条新的RealConnection后，在尝试在ConnectionPool获取是否有可用连接，如果找到了可用连接，优先复用在连接池中找到的连接。并且关闭注释11中建立的RealConnection，给nextRouteToTry赋值当前的Route路由对象（因为这个route对象是可用的）。注意传入的最后一个参数是true，表明是要求http2.0多路复用。
13. 参考注释3 和 注释12
14. 注释12中没有在ConnectionPool中找到可用连接，将本次新建立的连接放到连接池中。
15. 给transmitter对象中的RealConnection变量赋值。

/**
 * 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;
  RealConnection releasedConnection;
  Socket toClose;
  synchronized (connectionPool) {
    if (transmitter.isCanceled()) throw new IOException("Canceled");
    hasStreamFailure = false; // This is a fresh attempt.

    // 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 exchanges.
    releasedConnection = transmitter.connection;
    toClose = transmitter.connection != null && transmitter.connection.noNewExchanges
        ? transmitter.releaseConnectionNoEvents() : null; // 注释1

    if (transmitter.connection != null) { // 注释1
      // We had an already-allocated connection and it's good.
      result = transmitter.connection;
      releasedConnection = null;
    }

    if (result == null) { // 注释2
      // Attempt to get a connection from the pool.
      if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, null, false)) {
        foundPooledConnection = true;
        result = transmitter.connection;
      } else if (nextRouteToTry != null) {  // 注释3
        selectedRoute = nextRouteToTry;
        nextRouteToTry = null;
      } else if (retryCurrentRoute()) {  // 注释4
        selectedRoute = transmitter.connection.route();
      }
    }
  }
  // 省略代码...
  if (result != null) {
    // If we found an already-allocated or pooled connection, we're done.
    return result; // 注释5
  }

  // 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(); // 注释6
  }

  List<Route> routes = null;
  synchronized (connectionPool) {
    if (transmitter.isCanceled()) 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.
      routes = routeSelection.getAll();
      if (connectionPool.transmitterAcquirePooledConnection(
          address, transmitter, routes, false)) { // 注释7
        foundPooledConnection = true;
        result = transmitter.connection;
      }
    }

    if (!foundPooledConnection) {
      if (selectedRoute == null) {
        selectedRoute = routeSelection.next(); // 注释8
      }

      // 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.
      result = new RealConnection(connectionPool, selectedRoute); // 注释9
      connectingConnection = result;
    }
  }

  // If we found a pooled connection on the 2nd time around, we're done.
  if (foundPooledConnection) {  // 注释10
    eventListener.connectionAcquired(call, result);
    return result;
  }

  // Do TCP + TLS handshakes. This is a blocking operation.
  result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
      connectionRetryEnabled, call, eventListener);  // 注释11
  connectionPool.routeDatabase.connected(result.route());

  Socket socket = null;
  synchronized (connectionPool) {
    connectingConnection = null;
    // Last attempt at connection coalescing, which only occurs if we attempted multiple
    // concurrent connections to the same host.
    // 注释12
    if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, routes, true)) {
      // We lost the race! Close the connection we created and return the pooled connection.
      result.noNewExchanges = true;
      socket = result.socket();
      result = transmitter.connection;

      // It's possible for us to obtain a coalesced connection that is immediately unhealthy. In
      // that case we will retry the route we just successfully connected with.
      nextRouteToTry = selectedRoute; // 注释13
    } else {
      connectionPool.put(result); // 注释14
      transmitter.acquireConnectionNoEvents(result); // 注释15
    }
  }
  closeQuietly(socket);

  eventListener.connectionAcquired(call, result);
  return result;
}

RealConnection

• 表示一个真实的socket连接。
• RealConnection#connect是一个重要的函数，这个函数建立了TCP socket、sslSocket连接、进行https TLS安全协商过程、HTTP 2.0协议协商。
• socket的连接动作都是在这个函数完成的。几个关键函数RealConnection#connect、connectSocket、createTunnel、establishProtocol、connectTls、startHttp2。
• createTunnel中发送的CONNECT消息，参考资料https://www.jianshu.com/p/54357cdd4736。

http协议版本	连接流程
http/1.1	1. DNS 2. TCP Socket(未加密)
https/1.1	1. DNS 2. TCP Socket(未加密) 3. TLS安全隧道。"CONNECT"消息、TLS协商
http/2.0	1. DNS 2. TCP Socket(未加密) 3. TLS安全隧道。"CONNECT"消息、TLS协商 4. Http2协商。sendConnectionPreface、settings

连接池ConnectionPool

• transmitterAcquirePooledConnection获取缓存池中的可用连接。
• put函数，向缓存池中增加可用连接。

Transmitter

代理和DNS

Java定义的代理类型三种DIRECT、HTTP、SOCKS

• 需要注意的是HTTP代理和HTTP协议是两个不同的概念。
• SOCKS代理是不需要在本地DNS解析的，可以直接将域名发送到SOCKS代理服务器上。DIRECT、HTTP代理需要进行DNS解析。参考方法RouteSelector#resetNextInetSocketAddress。
• HTTP代理分为普通代理和隧道代理。普通代理是明文传输，即http请求；隧道代理是加密传输，仅仅转发TCP包，即https和http2.0。隧道代理在建立时，会要求先发送connect请求建立隧道，进行SSL握手，因为数据是加密的，代理服务器没有办法解析数据，只能转发数据。
• 默认的ProxySelector类是sun.net.spi.DefaultProxySelector，在OkHttpClient.Builder#Builder()初始化默认值。参考方法java.net.ProxySelector#getDefault。
• 代理和路由相关的参考资料:https://www.jianshu.com/p/63ba15d8877a

DNS解析

• 提供一个域名，返回一个IP地址列表。
• URL通过DNS解析成为具体的服务器IP地址。java提供的DNS解析方法java.net.InetAddress#getAllByName。可以解析IPV4 和 IPV6地址。
• Dns接口定义了lookup方法寻找IP地址。
• Dns#SYSTEM变量实现了该接口。匿名内部类。
• DNS寻址动作在 RouteSelector#resetNextInetSocketAddress方法中实现

//okhttp3.internal.connection.RouteSelector#resetNextInetSocketAddress
if (proxy.type() == Proxy.Type.SOCKS) { // 如果是socks代理，是不需要DNS解析的，可以直接将域名发送到服务器进行解析。
  inetSocketAddresses.add(InetSocketAddress.createUnresolved(socketHost, socketPort));
} else { // DIRECT、HTTP代理需要进行DNS解析
  eventListener.dnsStart(call, socketHost);

  // Try each address for best behavior in mixed IPv4/IPv6 environments.
  List<InetAddress> addresses = address.dns().lookup(socketHost); // DNS解析
  if (addresses.isEmpty()) {
    throw new UnknownHostException(address.dns() + " returned no addresses for " + socketHost);
  }

  eventListener.dnsEnd(call, socketHost, addresses);

  for (int i = 0, size = addresses.size(); i < size; i++) {
    InetAddress inetAddress = addresses.get(i);
    inetSocketAddresses.add(new InetSocketAddress(inetAddress, socketPort));
  }
}