HTTP请求的整体处理过程大体可以理解为，
- 建立TCP连接。
- 如果是HTTPS的话，完成SSL/TLS的协商。
- 发送请求。
- 获取响应。
- 结束请求，关闭连接。
然而，当为系统设置了代理的时候，整个数据流都会经过代理服务器。那么代理设置究竟是如何工作的呢？它是如何影响我们上面看到的HTTP请求的处理过程的呢？是在操作系统内核的TCP实现中的策略呢，还是HTTP stack中的机制？这里我们通过OkHttp3中的实现来一探究竟。
代理分为两种类型，一种是SOCKS代理，另一种是HTTP代理。对于SOCKS代理，在HTTP的场景下，代理服务器完成TCP数据包的转发工作。而HTTP代理服务器，在转发数据之外，还会解析HTTP的请求及响应，并根据请求及响应的内容做一些处理。这里看一下OkHttp中对代理的处理。
代理服务器的描述
在Java中，通过 java.net.Proxy 类描述一个代理服务器：
public class Proxy {
/**
* Represents the proxy type.
*
* @since 1.5
*/
public enum Type {
/**
* Represents a direct connection, or the absence of a proxy.
*/
DIRECT,
/**
* Represents proxy for high level protocols such as HTTP or FTP.
*/
HTTP,
/**
* Represents a SOCKS (V4 or V5) proxy.
*/
SOCKS
};
private Type type;
private SocketAddress sa;
/**
* A proxy setting that represents a {@code DIRECT} connection,
* basically telling the protocol handler not to use any proxying.
* Used, for instance, to create sockets bypassing any other global
* proxy settings (like SOCKS):
* <P>
* {@code Socket s = new Socket(Proxy.NO_PROXY);}
*
*/
public final static Proxy NO_PROXY = new Proxy();
// Creates the proxy that represents a {@code DIRECT} connection.
private Proxy() {
type = Type.DIRECT;
sa = null;
}
/**
* Creates an entry representing a PROXY connection.
* Certain combinations are illegal. For instance, for types Http, and
* Socks, a SocketAddress <b>must</b> be provided.
* <P>
* Use the {@code Proxy.NO_PROXY} constant
* for representing a direct connection.
*
* @param type the {@code Type} of the proxy
* @param sa the {@code SocketAddress} for that proxy
* @throws IllegalArgumentException when the type and the address are
* incompatible
*/
public Proxy(Type type, SocketAddress sa) {
if ((type == Type.DIRECT) || !(sa instanceof InetSocketAddress))
throw new IllegalArgumentException("type " + type + " is not compatible with address " + sa);
this.type = type;
this.sa = sa;
}
/**
* Returns the proxy type.
*
* @return a Type representing the proxy type
*/
public Type type() {
return type;
}
/**
* Returns the socket address of the proxy, or
* {@code null} if its a direct connection.
*
* @return a {@code SocketAddress} representing the socket end
* point of the proxy
*/
public SocketAddress address() {
return sa;
}
......
}
只用代理的类型及代理服务器地址即可描述代理服务器的全部。对于HTTP代理，代理服务器地址可以通过域名和IP地址等方式来描述。
代理选择器ProxySelector
在Java中通过ProxySelector为一个特定的URI选择代理：
public abstract class ProxySelector {
......
/**
* Selects all the applicable proxies based on the protocol to
* access the resource with and a destination address to access
* the resource at.
* The format of the URI is defined as follow:
* <UL>
* <LI>http URI for http connections</LI>
* <LI>https URI for https connections
* <LI>{@code socket://host:port}<br>
* for tcp client sockets connections</LI>
* </UL>
*
* @param uri
* The URI that a connection is required to
*
* @return a List of Proxies. Each element in the
* the List is of type
* {@link java.net.Proxy Proxy};
* when no proxy is available, the list will
* contain one element of type
* {@link java.net.Proxy Proxy}
* that represents a direct connection.
* @throws IllegalArgumentException if the argument is null
*/
public abstract List<Proxy> select(URI uri);
/**
* Called to indicate that a connection could not be established
* to a proxy/socks server. An implementation of this method can
* temporarily remove the proxies or reorder the sequence of
* proxies returned by {@link #select(URI)}, using the address
* and the IOException caught when trying to connect.
*
* @param uri
* The URI that the proxy at sa failed to serve.
* @param sa
* The socket address of the proxy/SOCKS server
*
* @param ioe
* The I/O exception thrown when the connect failed.
* @throws IllegalArgumentException if either argument is null
*/
public abstract void connectFailed(URI uri, SocketAddress sa, IOException ioe);
}
这个组件会读区系统中配置的所有代理，并根据调用者传入的URI，返回特定的代理服务器集合。由于不同系统中，配置代理服务器的方法，及相关配置的保存机制不同，该接口在不同的系统中有着不同的实现。
OkHttp3的路由
OkHttp3中抽象出Route来描述网络数据包的传输路径，最主要还是要描述直接与其建立TCP连接的目标端点。
public final class Route {
final Address address;
final Proxy proxy;
final InetSocketAddress inetSocketAddress;
public Route(Address address, Proxy proxy, InetSocketAddress inetSocketAddress) {
if (address == null) {
throw new NullPointerException("address == null");
}
if (proxy == null) {
throw new NullPointerException("proxy == null");
}
if (inetSocketAddress == null) {
throw new NullPointerException("inetSocketAddress == null");
}
this.address = address;
this.proxy = proxy;
this.inetSocketAddress = inetSocketAddress;
}
public Address address() {
return address;
}
/**
* Returns the {@link Proxy} of this route.
*
* <strong>Warning:</strong> This may disagree with {@link Address#proxy} when it is null. When
* the address's proxy is null, the proxy selector is used.
*/
public Proxy proxy() {
return proxy;
}
public InetSocketAddress socketAddress() {
return inetSocketAddress;
}
/**
* Returns true if this route tunnels HTTPS through an HTTP proxy. See <a
* href="http://www.ietf.org/rfc/rfc2817.txt">RFC 2817, Section 5.2</a>.
*/
public boolean requiresTunnel() {
return address.sslSocketFactory != null && proxy.type() == Proxy.Type.HTTP;
}
......
}
主要通过 代理服务器的信息proxy ，及 连接的目标地址 描述路由。 连接的目标地址inetSocketAddress 根据代理类型的不同而有着不同的含义，这主要是由不同代理协议的差异而造成的。对于无需代理的情况， 连接的目标地址inetSocketAddress 中包含HTTP服务器经过了DNS域名解析的IP地址及协议端口号；对于SOCKS代理，其中包含HTTP服务器的域名及协议端口号；对于HTTP代理，其中则包含代理服务器经过域名解析的IP地址及端口号。
路由选择器RouteSelector
HTTP请求处理过程中所需的TCP连接建立过程，主要是找到一个Route，然后依据代理协议的规则与特定目标建立TCP连接。对于无代理的情况，是与HTTP服务器建立TCP连接；对于SOCKS代理及HTTP代理，是与代理服务器建立TCP连接，虽然都是与代理服务器建立TCP连接，而SOCKS代理协议与HTTP代理协议做这个动作的方式又会有一定的区别。
借助于域名解析做负载均衡已经是网络中非常常见的手法了，因而，常常会有相同域名对应不同IP地址的情况。同时相同系统也可以设置多个代理，这使Route的选择变得复杂起来。
在OkHttp中，对Route连接失败有一定的错误处理机制。OkHttp会逐个尝试找到的Route建立TCP连接，直到找到可用的那一个。这同样要求，对Route信息有良好的管理。
OkHttp3借助于 RouteSelector 类管理所有的路由信息，并帮助选择路由。 RouteSelector 主要完成3件事：
- 收集所有可用的路由。
public final class RouteSelector {
private final Address address;
private final RouteDatabase routeDatabase;
/* The most recently attempted route. */
private Proxy lastProxy;
private InetSocketAddress lastInetSocketAddress;
/* 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();
private int nextInetSocketAddressIndex;
/* State for negotiating failed routes */
private final List<Route> postponedRoutes = new ArrayList<>();
public RouteSelector(Address address, RouteDatabase routeDatabase) {
this.address = address;
this.routeDatabase = routeDatabase;
resetNextProxy(address.url(), address.proxy());
}
. . . . . .
/** Prepares the proxy servers to try. */
private void resetNextProxy(HttpUrl url, Proxy proxy) {
if (proxy != null) {
// If the user specifies a proxy, try that and only that.
proxies = Collections.singletonList(proxy);
} else {
// Try each of the ProxySelector choices until one connection succeeds. If none succeed
// then we'll try a direct connection below.
proxies = new ArrayList<>();
List<Proxy> selectedProxies = address.proxySelector().select(url.uri());
if (selectedProxies != null) proxies.addAll(selectedProxies);
// Finally try a direct connection. We only try it once!
proxies.removeAll(Collections.singleton(Proxy.NO_PROXY));
proxies.add(Proxy.NO_PROXY);
}
nextProxyIndex = 0;
}
收集路由分为两个步骤：第一步收集所有的代理；第二步则是收集特定代理服务器选择情况下的所有 连接的目标地址 。
收集代理的过程如上面的这段代码所示，有两种方式，一是外部通过address传入了代理，此时代理集合将包含这唯一的代理。address的代理最终来源于OkHttpClient，我们可以在构造OkHttpClient时设置代理，来指定由该client执行的所有请求经过特定的代理。
另一种方式是，借助于ProxySelector获取多个代理。ProxySelector最终也来源于OkHttpClient，OkHttp的用户当然也可以对此进行配置。但通常情况下，使用系统默认的ProxySelector，来获取系统中配置的代理。
收集到的所有代理保存在列表 proxies 中。
为OkHttpClient配置Proxy或ProxySelector的场景大概是，需要让连接使用代理，但不使用系统的代理配置的情况。
收集特定代理服务器选择情况下的所有路由，因代理类型的不同而有着不同的过程：
/** Returns the next proxy to try. May be PROXY.NO_PROXY but never null. */
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;
}
/** Prepares the socket addresses to attempt for the current proxy or host. */
private void resetNextInetSocketAddress(Proxy proxy) throws IOException {
// Clear the addresses. Necessary if getAllByName() below throws!
inetSocketAddresses = new ArrayList<>();
String socketHost;
int socketPort;
if (proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.SOCKS) {
socketHost = address.url().host();
socketPort = address.url().port();
} else {
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");
}
if (proxy.type() == Proxy.Type.SOCKS) {
inetSocketAddresses.add(InetSocketAddress.createUnresolved(socketHost, socketPort));
} else {
// Try each address for best behavior in mixed IPv4/IPv6 environments.
List<InetAddress> addresses = address.dns().lookup(socketHost);
for (int i = 0, size = addresses.size(); i < size; i++) {
InetAddress inetAddress = addresses.get(i);
inetSocketAddresses.add(new InetSocketAddress(inetAddress, socketPort));
}
}
nextInetSocketAddressIndex = 0;
}
/**
* Obtain a "host" from an {@link InetSocketAddress}. This returns a string containing either an
* actual host name or a numeric IP address.
*/
// Visible for testing
static String getHostString(InetSocketAddress socketAddress) {
InetAddress address = socketAddress.getAddress();
if (address == null) {
// The InetSocketAddress was specified with a string (either a numeric IP or a host name). If
// it is a name, all IPs for that name should be tried. If it is an IP address, only that IP
// address should be tried.
return socketAddress.getHostName();
}
// The InetSocketAddress has a specific address: we should only try that address. Therefore we
// return the address and ignore any host name that may be available.
return address.getHostAddress();
}
收集一个特定代理服务器选择下的 连接的目标地址 因代理类型的不同而不同，这主要分为3种情况。 对于没有配置代理的情况，会对HTTP服务器的域名进行DNS域名解析，并为每个解析到的IP地址创建 连接的目标地址；对于SOCKS代理，直接以HTTP服务器的域名及协议端口号创建 连接的目标地址；而对于HTTP代理，则会对HTTP代理服务器的域名进行DNS域名解析，并为每个解析到的IP地址创建 连接的目标地址。
这里是OkHttp中发生DNS域名解析唯一的场合。对于使用代理的场景，没有对HTTP服务器的域名做DNS域名解析，也就意味着HTTP服务器的域名解析要由代理服务器完成。
代理服务器的收集是在创建 RouteSelector 完成的；而一个特定代理服务器选择下的 连接的目标地址 收集则是在选择Route时根据需要完成的。
-
RouteSelector做的第二件事情是选择可用的路由。
/**
* Returns true if there's another route to attempt. Every address has at least one route.
*/
public boolean hasNext() {
return hasNextInetSocketAddress()
|| hasNextProxy()
|| hasNextPostponed();
}
public Route next() throws IOException {
// Compute the next route to attempt.
if (!hasNextInetSocketAddress()) {
if (!hasNextProxy()) {
if (!hasNextPostponed()) {
throw new NoSuchElementException();
}
return nextPostponed();
}
lastProxy = nextProxy();
}
lastInetSocketAddress = nextInetSocketAddress();
Route route = new Route(address, lastProxy, lastInetSocketAddress);
if (routeDatabase.shouldPostpone(route)) {
postponedRoutes.add(route);
// We will only recurse in order to skip previously failed routes. They will be tried last.
return next();
}
return route;
}
/** Returns true if there's another proxy to try. */
private boolean hasNextProxy() {
return nextProxyIndex < proxies.size();
}
/** Returns true if there's another socket address to try. */
private boolean hasNextInetSocketAddress() {
return nextInetSocketAddressIndex < inetSocketAddresses.size();
}
/** Returns the next socket address to try. */
private InetSocketAddress nextInetSocketAddress() throws IOException {
if (!hasNextInetSocketAddress()) {
throw new SocketException("No route to " + address.url().host()
+ "; exhausted inet socket addresses: " + inetSocketAddresses);
}
return inetSocketAddresses.get(nextInetSocketAddressIndex++);
}
/** Returns true if there is another postponed route to try. */
private boolean hasNextPostponed() {
return !postponedRoutes.isEmpty();
}
/** Returns the next postponed route to try. */
private Route nextPostponed() {
return postponedRoutes.remove(0);
}
RouteSelector 实现了两级迭代器来提供选择路由的服务。
- 维护接失败的路由的信息，以避免浪费时间去连接一些不可用的路由。
RouteSelector借助于RouteDatabase维护失败的路由的信息。
/**
* Clients should invoke this method when they encounter a connectivity failure on a connection
* returned by this route selector.
*/
public void connectFailed(Route failedRoute, IOException failure) {
if (failedRoute.proxy().type() != Proxy.Type.DIRECT && address.proxySelector() != null) {
// Tell the proxy selector when we fail to connect on a fresh connection.
address.proxySelector().connectFailed(
address.url().uri(), failedRoute.proxy().address(), failure);
}
routeDatabase.failed(failedRoute);
}
RouteDatabase是一个简单的容器：
public 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 failedRoute}. */
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);
}
}
代理选择器ProxySelector的实现
在OkHttp3中，ProxySelector对象由OkHttpClient维护。
public class OkHttpClient implements Cloneable, Call.Factory {
. . . . . .
final ProxySelector proxySelector;
private OkHttpClient(Builder builder) {
this.dispatcher = builder.dispatcher;
this.proxy = builder.proxy;
this.protocols = builder.protocols;
this.connectionSpecs = builder.connectionSpecs;
this.interceptors = Util.immutableList(builder.interceptors);
this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
this.proxySelector = builder.proxySelector;
. . . . . .
public ProxySelector proxySelector() {
return proxySelector;
}
. . . . . .
public Builder() {
dispatcher = new Dispatcher();
protocols = DEFAULT_PROTOCOLS;
connectionSpecs = DEFAULT_CONNECTION_SPECS;
proxySelector = ProxySelector.getDefault();
. . . . . .
Builder(OkHttpClient okHttpClient) {
this.dispatcher = okHttpClient.dispatcher;
this.proxy = okHttpClient.proxy;
this.protocols = okHttpClient.protocols;
this.connectionSpecs = okHttpClient.connectionSpecs;
this.interceptors.addAll(okHttpClient.interceptors);
this.networkInterceptors.addAll(okHttpClient.networkInterceptors);
this.proxySelector = okHttpClient.proxySelector;
在创建OkHttpClient时，可以通过为OkHttpClient.Builder设置ProxySelector来定制ProxySelector。若没有指定，则使用系统默认的ProxySelector。OpenJDK 1.8版默认的ProxySelector为sun.net.spi.DefaultProxySelector：
public abstract class ProxySelector {
/**
* The system wide proxy selector that selects the proxy server to
* use, if any, when connecting to a remote object referenced by
* an URL.
*
* @see #setDefault(ProxySelector)
*/
private static ProxySelector theProxySelector;
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;
}
}
/**
* Gets the system-wide proxy selector.
*
* @throws SecurityException
* If a security manager has been installed and it denies
* {@link NetPermission}{@code ("getProxySelector")}
* @see #setDefault(ProxySelector)
* @return the system-wide {@code ProxySelector}
* @since 1.5
*/
public static ProxySelector getDefault() {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(SecurityConstants.GET_PROXYSELECTOR_PERMISSION);
}
return theProxySelector;
}
在Android平台上，默认ProxySelector所用的则是另外的实现：
public abstract class ProxySelector {
private static ProxySelector defaultSelector = new ProxySelectorImpl();
/**
* Returns the default proxy selector, or null if none exists.
*/
public static ProxySelector getDefault() {
return defaultSelector;
}
/**
* Sets the default proxy selector. If {@code selector} is null, the current
* proxy selector will be removed.
*/
public static void setDefault(ProxySelector selector) {
defaultSelector = selector;
}
Android平台下，默认的ProxySelector ProxySelectorImpl，其实现 (不同Android版本实现不同，这里以android-6.0.1_r61为例) 如下：
package java.net;
import java.io.IOException;
import java.util.Collections;
import java.util.List;
final class ProxySelectorImpl extends ProxySelector {
@Override public void connectFailed(URI uri, SocketAddress sa, IOException ioe) {
if (uri == null || sa == null || ioe == null) {
throw new IllegalArgumentException();
}
}
@Override public List<Proxy> select(URI uri) {
return Collections.singletonList(selectOneProxy(uri));
}
private Proxy selectOneProxy(URI uri) {
if (uri == null) {
throw new IllegalArgumentException("uri == null");
}
String scheme = uri.getScheme();
if (scheme == null) {
throw new IllegalArgumentException("scheme == null");
}
int port = -1;
Proxy proxy = null;
String nonProxyHostsKey = null;
boolean httpProxyOkay = true;
if ("http".equalsIgnoreCase(scheme)) {
port = 80;
nonProxyHostsKey = "http.nonProxyHosts";
proxy = lookupProxy("http.proxyHost", "http.proxyPort", Proxy.Type.HTTP, port);
} else if ("https".equalsIgnoreCase(scheme)) {
port = 443;
nonProxyHostsKey = "https.nonProxyHosts"; // RI doesn't support this
proxy = lookupProxy("https.proxyHost", "https.proxyPort", Proxy.Type.HTTP, port);
} else if ("ftp".equalsIgnoreCase(scheme)) {
port = 80; // not 21 as you might guess
nonProxyHostsKey = "ftp.nonProxyHosts";
proxy = lookupProxy("ftp.proxyHost", "ftp.proxyPort", Proxy.Type.HTTP, port);
} else if ("socket".equalsIgnoreCase(scheme)) {
httpProxyOkay = false;
} else {
return Proxy.NO_PROXY;
}
if (nonProxyHostsKey != null
&& isNonProxyHost(uri.getHost(), System.getProperty(nonProxyHostsKey))) {
return Proxy.NO_PROXY;
}
if (proxy != null) {
return proxy;
}
if (httpProxyOkay) {
proxy = lookupProxy("proxyHost", "proxyPort", Proxy.Type.HTTP, port);
if (proxy != null) {
return proxy;
}
}
proxy = lookupProxy("socksProxyHost", "socksProxyPort", Proxy.Type.SOCKS, 1080);
if (proxy != null) {
return proxy;
}
return Proxy.NO_PROXY;
}
/**
* Returns the proxy identified by the {@code hostKey} system property, or
* null.
*/
private Proxy lookupProxy(String hostKey, String portKey, Proxy.Type type, int defaultPort) {
String host = System.getProperty(hostKey);
if (host == null || host.isEmpty()) {
return null;
}
int port = getSystemPropertyInt(portKey, defaultPort);
return new Proxy(type, InetSocketAddress.createUnresolved(host, port));
}
private int getSystemPropertyInt(String key, int defaultValue) {
String string = System.getProperty(key);
if (string != null) {
try {
return Integer.parseInt(string);
} catch (NumberFormatException ignored) {
}
}
return defaultValue;
}
/**
* Returns true if the {@code nonProxyHosts} system property pattern exists
* and matches {@code host}.
*/
private boolean isNonProxyHost(String host, String nonProxyHosts) {
if (host == null || nonProxyHosts == null) {
return false;
}
// construct pattern
StringBuilder patternBuilder = new StringBuilder();
for (int i = 0; i < nonProxyHosts.length(); i++) {
char c = nonProxyHosts.charAt(i);
switch (c) {
case '.':
patternBuilder.append("\\.");
break;
case '*':
patternBuilder.append(".*");
break;
default:
patternBuilder.append(c);
}
}
// check whether the host is the nonProxyHosts.
String pattern = patternBuilder.toString();
return host.matches(pattern);
}
}
在Android平台上，主要是从系统属性System properties中获取代理服务器的配置信息，这里会过滤掉不能进行代理的主机的访问。
前面我们看到 RouteSelector 通过 Address 提供的Proxy和ProxySelector来收集Proxy信息及连接的目标地址信息。OkHttp3中用 Address 描述建立连接所需的配置信息，包括HTTP服务器的地址，DNS，SocketFactory，Proxy，ProxySelector及TLS所需的一些设施等等：
public final class Address {
final HttpUrl url;
final Dns dns;
final SocketFactory socketFactory;
final Authenticator proxyAuthenticator;
final List<Protocol> protocols;
final List<ConnectionSpec> connectionSpecs;
final ProxySelector proxySelector;
final Proxy proxy;
final SSLSocketFactory sslSocketFactory;
final HostnameVerifier hostnameVerifier;
final CertificatePinner certificatePinner;
public Address(String uriHost, int uriPort, Dns dns, SocketFactory socketFactory,
SSLSocketFactory sslSocketFactory, HostnameVerifier hostnameVerifier,
CertificatePinner certificatePinner, Authenticator proxyAuthenticator, Proxy proxy,
List<Protocol> protocols, List<ConnectionSpec> connectionSpecs, ProxySelector proxySelector) {
this.url = new HttpUrl.Builder()
.scheme(sslSocketFactory != null ? "https" : "http")
.host(uriHost)
.port(uriPort)
.build();
if (dns == null) throw new NullPointerException("dns == null");
this.dns = dns;
if (socketFactory == null) throw new NullPointerException("socketFactory == null");
this.socketFactory = socketFactory;
if (proxyAuthenticator == null) {
throw new NullPointerException("proxyAuthenticator == null");
}
this.proxyAuthenticator = proxyAuthenticator;
if (protocols == null) throw new NullPointerException("protocols == null");
this.protocols = Util.immutableList(protocols);
if (connectionSpecs == null) throw new NullPointerException("connectionSpecs == null");
this.connectionSpecs = Util.immutableList(connectionSpecs);
if (proxySelector == null) throw new NullPointerException("proxySelector == null");
this.proxySelector = proxySelector;
this.proxy = proxy;
this.sslSocketFactory = sslSocketFactory;
this.hostnameVerifier = hostnameVerifier;
this.certificatePinner = certificatePinner;
}
/**
* Returns a URL with the hostname and port of the origin server. The path, query, and fragment of
* this URL are always empty, since they are not significant for planning a route.
*/
public HttpUrl url() {
return url;
}
/** Returns the service that will be used to resolve IP addresses for hostnames. */
public Dns dns() {
return dns;
}
/** Returns the socket factory for new connections. */
public SocketFactory socketFactory() {
return socketFactory;
}
/** Returns the client's proxy authenticator. */
public Authenticator proxyAuthenticator() {
return proxyAuthenticator;
}
/**
* Returns the protocols the client supports. This method always returns a non-null list that
* contains minimally {@link Protocol#HTTP_1_1}.
*/
public List<Protocol> protocols() {
return protocols;
}
public List<ConnectionSpec> connectionSpecs() {
return connectionSpecs;
}
/**
* Returns this address's proxy selector. Only used if the proxy is null. If none of this
* selector's proxies are reachable, a direct connection will be attempted.
*/
public ProxySelector proxySelector() {
return proxySelector;
}
/**
* Returns this address's explicitly-specified HTTP proxy, or null to delegate to the {@linkplain
* #proxySelector proxy selector}.
*/
public Proxy proxy() {
return proxy;
}
/** Returns the SSL socket factory, or null if this is not an HTTPS address. */
public SSLSocketFactory sslSocketFactory() {
return sslSocketFactory;
}
/** Returns the hostname verifier, or null if this is not an HTTPS address. */
public HostnameVerifier hostnameVerifier() {
return hostnameVerifier;
}
/** Returns this address's certificate pinner, or null if this is not an HTTPS address. */
public CertificatePinner certificatePinner() {
return certificatePinner;
}
. . . . . .
}
OkHttp3中通过职责链执行HTTP请求。在其中的RetryAndFollowUpInterceptor里创建Address对象时，从OkHttpClient对象获取ProxySelector。Address对象会被用于创建StreamAllocation对象。StreamAllocation在建立连接时，从Address对象中获取ProxySelector以选择路由。
public final class RetryAndFollowUpInterceptor implements Interceptor {
. . . . . .
private Address createAddress(HttpUrl url) {
SSLSocketFactory sslSocketFactory = null;
HostnameVerifier hostnameVerifier = null;
CertificatePinner certificatePinner = null;
if (url.isHttps()) {
sslSocketFactory = client.sslSocketFactory();
hostnameVerifier = client.hostnameVerifier();
certificatePinner = client.certificatePinner();
}
return new Address(url.host(), url.port(), client.dns(), client.socketFactory(),
sslSocketFactory, hostnameVerifier, certificatePinner, client.proxyAuthenticator(),
client.proxy(), client.protocols(), client.connectionSpecs(), client.proxySelector());
}
在StreamAllocation中，Address对象会被用于创建 RouteSelector 对象：
public final class StreamAllocation {
. . . . . .
public StreamAllocation(ConnectionPool connectionPool, Address address) {
this.connectionPool = connectionPool;
this.address = address;
this.routeSelector = new RouteSelector(address, routeDatabase());
}
代理协议
如我们在 OkHttp3 HTTP请求执行流程分析 中看到的，OkHttp3对HTTP请求是通过Interceptor链来处理的。
RetryAndFollowUpInterceptor创建StreamAllocation对象，处理http的重定向及出错重试。对后续Interceptor的执行的影响为修改Request并创建StreamAllocation对象。
BridgeInterceptor补全缺失的一些http header。对后续Interceptor的执行的影响主要为修改了Request。
CacheInterceptor处理http缓存。对后续Interceptor的执行的影响为，若缓存中有所需请求的响应，则后续Interceptor不再执行。
ConnectInterceptor借助于前面分配的StreamAllocation对象建立与服务器之间的连接，并选定交互所用的协议是HTTP 1.1还是HTTP 2。对后续Interceptor的执行的影响为，创建了HttpStream和connection。
CallServerInterceptor作为Interceptor链中的最后一个Interceptor，用于处理IO，与服务器进行数据交换。
在OkHttp3中，收集的路由信息，是在ConnectInterceptor中建立连接时用到的。ConnectInterceptor 借助于 StreamAllocation 完成整个连接的建立，包括TCP连接建立，代理协议所要求的协商，以及SSL/TLS协议的协商，如ALPN等。我们暂时略过整个连接建立的完整过程，主要关注TCP连接建立及代理协议的协商过程的部分。
StreamAllocation 的findConnection()用来为某次特定的网络请求寻找一个可用的连接。
/**
* 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,
boolean connectionRetryEnabled) throws IOException {
Route selectedRoute;
synchronized (connectionPool) {
if (released) throw new IllegalStateException("released");
if (codec != null) throw new IllegalStateException("codec != null");
if (canceled) throw new IOException("Canceled");
RealConnection allocatedConnection = this.connection;
if (allocatedConnection != null && !allocatedConnection.noNewStreams) {
return allocatedConnection;
}
// Attempt to get a connection from the pool.
RealConnection pooledConnection = Internal.instance.get(connectionPool, address, this);
if (pooledConnection != null) {
this.connection = pooledConnection;
return pooledConnection;
}
selectedRoute = route;
}
if (selectedRoute == null) {
selectedRoute = routeSelector.next();
synchronized (connectionPool) {
route = selectedRoute;
refusedStreamCount = 0;
}
}
RealConnection newConnection = new RealConnection(selectedRoute);
acquire(newConnection);
synchronized (connectionPool) {
Internal.instance.put(connectionPool, newConnection);
this.connection = newConnection;
if (canceled) throw new IOException("Canceled");
}
newConnection.connect(connectTimeout, readTimeout, writeTimeout, address.connectionSpecs(),
connectionRetryEnabled);
routeDatabase().connected(newConnection.route());
return newConnection;
}
OkHttp3中有一套连接池的机制，这里先尝试从连接池中寻找可用的连接，找不到时才会新建连接。新建连接的过程是：
- 选择一个Route；
- 创建
RealConnection连接对象。 - 将连接对象保存进连接池中。
- 建立连接。
RealConnection 中建立连接的过程是这样的：
public final class RealConnection extends Http2Connection.Listener implements Connection {
private final Route route;
/** The low-level TCP socket. */
private Socket rawSocket;
/**
* The application layer socket. Either an {@link SSLSocket} layered over {@link #rawSocket}, or
* {@link #rawSocket} itself if this connection does not use SSL.
*/
public Socket socket;
private Handshake handshake;
private Protocol protocol;
public volatile Http2Connection http2Connection;
public int successCount;
public BufferedSource source;
public BufferedSink sink;
public int allocationLimit;
public final List<Reference<StreamAllocation>> allocations = new ArrayList<>();
public boolean noNewStreams;
public long idleAtNanos = Long.MAX_VALUE;
public RealConnection(Route route) {
this.route = route;
}
public void connect(int connectTimeout, int readTimeout, int writeTimeout,
List<ConnectionSpec> connectionSpecs, boolean connectionRetryEnabled) {
if (protocol != null) throw new IllegalStateException("already connected");
RouteException routeException = null;
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 (protocol == null) {
try {
if (route.requiresTunnel()) {
buildTunneledConnection(connectTimeout, readTimeout, writeTimeout,
connectionSpecSelector);
} else {
buildConnection(connectTimeout, readTimeout, writeTimeout, connectionSpecSelector);
}
} catch (IOException e) {
closeQuietly(socket);
closeQuietly(rawSocket);
socket = null;
rawSocket = null;
source = null;
sink = null;
handshake = null;
protocol = null;
if (routeException == null) {
routeException = new RouteException(e);
} else {
routeException.addConnectException(e);
}
if (!connectionRetryEnabled || !connectionSpecSelector.connectionFailed(e)) {
throw routeException;
}
}
}
}
在这个方法中，SSLSocketFactory为空，也就是要求请求/响应明文传输时，先做安全性检查，以确认系统允许明文传输，允许以请求的域名做明文传输。
然后根据路由的具体情况，执行不同的连接建立过程。对于需要创建隧道连接的路由，执行buildTunneledConnection()，对于其它情况，则执行buildConnection()。
判断是否要建立隧道连接的依据是代理的类型，以及连接的类型：
/**
* Returns true if this route tunnels HTTPS through an HTTP proxy. See <a
* href="http://www.ietf.org/rfc/rfc2817.txt">RFC 2817, Section 5.2</a>.
*/
public boolean requiresTunnel() {
return address.sslSocketFactory != null && proxy.type() == Proxy.Type.HTTP;
}
如果是HTTP代理，且请求建立SSL/TLS加密通道 (http/1.1的https和http2) ，则需要建立隧道连接。其它情形不需要建立隧道连接。
非隧道连接的建立
非隧道连接的建立过程为：
/** Does all the work necessary to build a full HTTP or HTTPS connection on a raw socket. */
private void buildConnection(int connectTimeout, int readTimeout, int writeTimeout,
ConnectionSpecSelector connectionSpecSelector) throws IOException {
connectSocket(connectTimeout, readTimeout);
establishProtocol(readTimeout, writeTimeout, connectionSpecSelector);
}
private void connectSocket(int connectTimeout, int readTimeout) throws IOException {
Proxy proxy = route.proxy();
Address address = route.address();
rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP
? address.socketFactory().createSocket()
: new Socket(proxy);
rawSocket.setSoTimeout(readTimeout);
try {
Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);
} catch (ConnectException e) {
throw new ConnectException("Failed to connect to " + route.socketAddress());
}
source = Okio.buffer(Okio.source(rawSocket));
sink = Okio.buffer(Okio.sink(rawSocket));
}
有 3 种情况需要建立非隧道连接：
- 无代理。
- 明文的HTTP代理。
- SOCKS代理。
非隧道连接的建立过程为，建立TCP连接，然后在需要时完成SSL/TLS的握手及HTTP/2的握手建立Protocol。建立TCP连接的过程为：
- 创建Socket。非SOCKS代理的情况下，通过SocketFactory创建；在SOCKS代理则传入proxy手动new一个出来。
- 为Socket设置读超时。
- 完成特定于平台的连接建立。
- 创建用语IO的source和sink。
AndroidPlatform 的 connectSocket() 是这样的：
@Override public void connectSocket(Socket socket, InetSocketAddress address,
int connectTimeout) throws IOException {
try {
socket.connect(address, connectTimeout);
} catch (AssertionError e) {
if (Util.isAndroidGetsocknameError(e)) throw new IOException(e);
throw e;
} catch (SecurityException e) {
// Before android 4.3, socket.connect could throw a SecurityException
// if opening a socket resulted in an EACCES error.
IOException ioException = new IOException("Exception in connect");
ioException.initCause(e);
throw ioException;
}
}
设置了SOCKS代理的情况下，仅有的特别之处在于，是通过传入proxy手动创建的Socket。route的socketAddress包含着目标HTTP服务器的域名。由此可见SOCKS协议的处理，主要是在Java标准库的 java.net.Socket 中处理的。对于外界而言，就好像是于HTTP服务器直接建立连接一样，因为连接时传入的地址都是HTTP服务器的域名。
而对于明文HTTP代理的情况下，这里没有任何特殊的处理。route的socketAddress包含着代理服务器的IP地址。HTTP代理自身会根据请求及响应的实际内容，建立与HTTP服务器的TCP连接，并转发数据。猜测HTTP代理服务器是根据HTTP请求中的”Host”等header内容来确认HTTP服务器地址的。
暂时先略过对建立协议过程的分析。
HTTP代理的隧道连接
buildTunneledConnection()用于建立隧道连接：
/**
* Does all the work to build an HTTPS connection over a proxy tunnel. The catch here is that a
* proxy server can issue an auth challenge and then close the connection.
*/
private void buildTunneledConnection(int connectTimeout, int readTimeout, int writeTimeout,
ConnectionSpecSelector connectionSpecSelector) throws IOException {
Request tunnelRequest = createTunnelRequest();
HttpUrl url = tunnelRequest.url();
int attemptedConnections = 0;
int maxAttempts = 21;
while (true) {
if (++attemptedConnections > maxAttempts) {
throw new ProtocolException("Too many tunnel connections attempted: " + maxAttempts);
}
connectSocket(connectTimeout, readTimeout);
tunnelRequest = createTunnel(readTimeout, writeTimeout, tunnelRequest, url);
if (tunnelRequest == null) break; // Tunnel successfully created.
// 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.
closeQuietly(rawSocket);
rawSocket = null;
sink = null;
source = null;
}
establishProtocol(readTimeout, writeTimeout, connectionSpecSelector);
}
这里主要是两个过程：
- 建立隧道连接。
- 建立Protocol。
建立隧道连接的过程又分为几个步骤：
- 创建隧道请求
- 建立Socket连接
- 发送请求建立隧道
隧道请求是一个常规的HTTP请求，只是请求的内容有点特殊。最初创建的隧道请求如：
/**
* Returns a request that creates a TLS tunnel via an HTTP proxy. Everything in the tunnel request
* is sent unencrypted to the proxy server, so tunnels include only the minimum set of headers.
* This avoids sending potentially sensitive data like HTTP cookies to the proxy unencrypted.
*/
private Request createTunnelRequest() {
return new Request.Builder()
.url(route.address().url())
.header("Host", Util.hostHeader(route.address().url(), true))
.header("Proxy-Connection", "Keep-Alive")
.header("User-Agent", Version.userAgent()) // For HTTP/1.0 proxies like Squid.
.build();
}
一个隧道请求的例子如下：
Tunnel Request
请求的”Host” header中包含了目标HTTP服务器的域名。建立socket连接的过程这里不再赘述。
创建隧道的过程是这样子的：
/**
* To make an HTTPS connection over an HTTP proxy, send an unencrypted CONNECT request to create
* the proxy connection. This may need to be retried if the proxy requires authorization.
*/
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.
String requestLine = "CONNECT " + Util.hostHeader(url, true) + " HTTP/1.1";
while (true) {
Http1Codec tunnelConnection = new Http1Codec(null, null, source, sink);
source.timeout().timeout(readTimeout, MILLISECONDS);
sink.timeout().timeout(writeTimeout, MILLISECONDS);
tunnelConnection.writeRequest(tunnelRequest.headers(), requestLine);
tunnelConnection.finishRequest();
Response response = tunnelConnection.readResponse().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());
}
}
}
在前面创建的TCP连接之上，完成与代理服务器的HTTP请求/响应交互。请求的内容类似下面这样：
"CONNECT m.taobao.com:443 HTTP/1.1"
这里可能会根据HTTP代理是否需要认证而有多次HTTP请求/响应交互。
总结一下OkHttp3中代理相关的处理：
- 没有设置代理的情况下，直接与HTTP服务器建立TCP连接，然后进行HTTP请求/响应的交互。
- 设置了SOCKS代理的情况下，创建Socket时，为其传入proxy，连接时还是以HTTP服务器为目标地址。在标准库的Socket中完成SOCKS协议相关的处理。此时基本上感知不到代理的存在。
- 设置了HTTP代理时的HTTP请求，与HTTP代理服务器建立TCP连接。HTTP代理服务器解析HTTP请求/响应的内容，并根据其中的信息来完成数据的转发。也就是说，如果HTTP请求中不包含”Host” header，则有可能在设置了HTTP代理的情况下无法与HTTP服务器建立连接。
- 设置了HTTP代理时的HTTPS/HTTP2请求，与HTTP服务器建立通过HTTP代理的隧道连接。HTTP代理不再解析传输的数据，仅仅完成数据转发的功能。此时HTTP代理的功能退化为如同SOCKS代理类似。
- 设置了代理时，HTTP服务器的域名解析会被交给代理服务器执行。其中设置了HTTP代理时，会对HTTP代理的域名做域名解析。
关于HTTP代理的更多内容，可以参考HTTP 代理原理及实现（一）。
OkHttp3中代理相关的处理大体如此。
