public class Proxy {
    // 代理類型
    public enum Type {
        // 不使用代理，直連目標(biāo)服務(wù)器
        DIRECT,
        // HTTP 協(xié)議代理
        HTTP,
        // SOCKS 協(xié)議代理
        SOCKS
    };
    // 代理類型
    private Type type;
    // 代理的 IP 套接字地址(IP + 端口號)
    private SocketAddress sa;
    public final static Proxy NO_PROXY = new Proxy();
    // 默認(rèn)不使用代理
    private Proxy() {
        type = Type.DIRECT;
        sa = null;
    }
}

public abstract class ProxySelector {
    private static ProxySelector theProxySelector;
    // 創(chuàng)建 DefaultProxySelector 對象
    static {
        try {
            Class<?> c = Class.forName("sun.net.spi.DefaultProxySelector");
            if (c != null && ProxySelector.class.isAssignableFrom(c)) {
                theProxySelector = (ProxySelector) c.newInstance();
            }
        } catch (Exception e) {
            theProxySelector = null;
        }
    }
    public static ProxySelector getDefault() {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(SecurityConstants.GET_PROXYSELECTOR_PERMISSION);
        }
        return theProxySelector;
    }
    public abstract List<Proxy> select(URI uri);
    public abstract void connectFailed(URI uri, SocketAddress sa, IOException ioe);
}

public Builder() {
  proxySelector = ProxySelector.getDefault();
  if (proxySelector == null) {
    proxySelector = new NullProxySelector();
  }
}

public void domainResolution() throws UnknownHostException {
    InetAddress[] inetAddresses = InetAddress.getAllByName("baidu.com");
    for (InetAddress inetAddress : inetAddresses) {
        System.out.println(inetAddress.toString());
    }
}

public final class Route {
  // 與目標(biāo)服務(wù)器建立連接所需要的配置信息，包括目標(biāo)主機名、端口、dns 等
  final Address address;
  // 該路由的代理信息
  final Proxy proxy;
  // 代理服務(wù)器或目標(biāo)服務(wù)器的地址
  final InetSocketAddress inetSocketAddress;
  // 該路由是否使用 Http 隧道代理
  public boolean requiresTunnel() {
    return address.sslSocketFactory != null && proxy.type() == Proxy.Type.HTTP;
  }  
}

final class RouteDatabase {
  private final Set<Route> failedRoutes = new LinkedHashSet<>();
  /** Records a failure connecting to {@code failedRoute}. */
  public synchronized void failed(Route failedRoute) {
    failedRoutes.add(failedRoute);
  }
  /** Records success connecting to {@code route}. */
  public synchronized void connected(Route route) {
    failedRoutes.remove(route);
  }
  /** Returns true if {@code route} has failed recently and should be avoided. */
  public synchronized boolean shouldPostpone(Route route) {
    return failedRoutes.contains(route);
  }
}

public static final class Selection {
    // 路由列表
    private final List<Route> routes;
    // 下一個路由的索引
    private int nextRouteIndex = 0;
    Selection(List<Route> routes) {
      this.routes = routes;
    }
    // 是否有下一個路由
    public boolean hasNext() {
      return nextRouteIndex < routes.size();
    }
    // 返回下一個路由
    public Route next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      return routes.get(nextRouteIndex++);
    }
    // 返回路由列表
    public List<Route> getAll() {
      return new ArrayList<>(routes);
    }
}

private final Address address;
private final RouteDatabase routeDatabase;
private final Call call;
private final EventListener eventListener;
/* State for negotiating the next proxy to use. */
private List<Proxy> proxies = Collections.emptyList();
private int nextProxyIndex;
/* State for negotiating the next socket address to use. */
private List<InetSocketAddress> inetSocketAddresses = Collections.emptyList();
/* State for negotiating failed routes */
private final List<Route> postponedRoutes = new ArrayList<>();

// 初始化代理列表
private void resetNextProxy(HttpUrl url, Proxy proxy);
// 是否有下一個代理
private boolean hasNextProxy();
// 是否含有路由可以嘗試連接
public boolean hasNext();
// 初始化連接代理或目標(biāo)服務(wù)器的地址列表
private void resetNextInetSocketAddress(Proxy proxy) throws IOException;
// 返回代理列表中下一個代理
private Proxy nextProxy() throws IOException;
// 返回路由集合
public Selection next() throws IOException;

private void resetNextProxy(HttpUrl url, Proxy proxy) {
    // 若設(shè)置了代理，僅使用這一個代理
    if (proxy != null) {
      // If the user specifies a proxy, try that and only that.
      proxies = Collections.singletonList(proxy);
    } else {
      // 若沒有設(shè)置代理，則調(diào)用代理選擇器的 select 方法獲取代理列表
      // Try each of the ProxySelector choices until one connection succeeds.
      List<Proxy> proxiesOrNull = address.proxySelector().select(url.uri());
      // 若 select 返回的代理列表為空，認(rèn)為不使用代理，以 Proxy.NO_PROXY 初始化
      proxies = proxiesOrNull != null && !proxiesOrNull.isEmpty()
          ? Util.immutableList(proxiesOrNull)
          : Util.immutableList(Proxy.NO_PROXY);
    }
    nextProxyIndex = 0;
}

private boolean hasNextProxy() {
	return nextProxyIndex < proxies.size();
}

public boolean hasNext() {
	return hasNextProxy() || !postponedRoutes.isEmpty();
}

private void resetNextInetSocketAddress(Proxy proxy) throws IOException {
    // Clear the addresses. Necessary if getAllByName() below throws!
    inetSocketAddresses = new ArrayList<>();
    // 主機名
    String socketHost;
    // 端口號
    int socketPort;
    // 若代理類型為直連或 SOCKS，則使用目標(biāo)服務(wù)器的主機名和端口號
    if (proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.SOCKS) {
      socketHost = address.url().host();
      socketPort = address.url().port();
    } else {
      // 若代理類型為 HTTP，則使用代理服務(wù)器的主機名和端口號
      SocketAddress proxyAddress = proxy.address();
      if (!(proxyAddress instanceof InetSocketAddress)) {
        throw new IllegalArgumentException(
            "Proxy.address() is not an " + "InetSocketAddress: " + proxyAddress.getClass());
      }
      InetSocketAddress proxySocketAddress = (InetSocketAddress) proxyAddress;
      socketHost = getHostString(proxySocketAddress);
      socketPort = proxySocketAddress.getPort();
    }
    if (socketPort < 1 || socketPort > 65535) {
      throw new SocketException("No route to " + socketHost + ":" + socketPort
          + "; port is out of range");
    }
    // SOCKS 類型的代理只會生成一個通往目標(biāo)服務(wù)器的地址
    if (proxy.type() == Proxy.Type.SOCKS) {
      inetSocketAddresses.add(InetSocketAddress.createUnresolved(socketHost, socketPort));
    } else {
      eventListener.dnsStart(call, socketHost);
      // Try each address for best behavior in mixed IPv4/IPv6 environments.
      List<InetAddress> addresses = address.dns().lookup(socketHost);
      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));
      }
    }
}

private Proxy nextProxy() throws IOException {
    if (!hasNextProxy()) {
      throw new SocketException("No route to " + address.url().host()
          + "; exhausted proxy configurations: " + proxies);
    }
    Proxy result = proxies.get(nextProxyIndex++);
    resetNextInetSocketAddress(result);
    return result;
}

public Selection next() throws IOException {
    // 若沒有路由集合了，拋出異常
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    // Compute the next set of routes to attempt.
    List<Route> routes = new ArrayList<>();
	// 循環(huán)直到?jīng)]有代理可用
    while (hasNextProxy()) {
      // Postponed routes are always tried last. For example, if we have 2 proxies and all the
      // routes for proxy1 should be postponed, we'll move to proxy2. Only after we've exhausted
      // all the good routes will we attempt the postponed routes.
      // 從代理列表中取出一個代理
      Proxy proxy = nextProxy();
      // 遍歷該代理對應(yīng)的地址列表
      for (int i = 0, size = inetSocketAddresses.size(); i < size; i++) {
        // 創(chuàng)建該地址對應(yīng)的路由
        Route route = new Route(address, proxy, inetSocketAddresses.get(i));
        // 若該路由在黑名單，則添加到 postponedRoutes
        if (routeDatabase.shouldPostpone(route)) {
          postponedRoutes.add(route);
        } else {
        // 否則添加到 routes
          routes.add(route);
        }
      }
      // 若該代理對應(yīng)的地址列表不為空，退出循環(huán)
      if (!routes.isEmpty()) {
        break;
      }
    }
	// 若所有代理的地址列表均為空，則嘗試使用黑名單中的路由
    if (routes.isEmpty()) {
      // We've exhausted all Proxies so fallback to the postponed routes.
      routes.addAll(postponedRoutes);
      postponedRoutes.clear();
    }
	// 返回路由集合
    return new Selection(routes);
}