聊聊HttpClient的DnsResolver

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本文主要研究一下HttpClient的DnsResolver

DnsResolver

org/apache/http/conn/DnsResolver.java

/*** Users may implement this interface to override the normal DNS lookup offered* by the OS.** @since 4.2*/
public interface DnsResolver {/*** Returns the IP address for the specified host name, or null if the given* host is not recognized or the associated IP address cannot be used to* build an InetAddress instance.** @see InetAddress** @param host*            The host name to be resolved by this resolver.* @return The IP address associated to the given host name, or null if the*         host name is not known by the implementation class.*/InetAddress[] resolve(String host) throws UnknownHostException;}

DnsResolver定义了resolve方法，可用于替换OS提供的DNS lookup

InMemoryDnsResolver

org/apache/http/impl/conn/InMemoryDnsResolver.java

/*** In-memory {@link DnsResolver} implementation.** @since 4.2*/
public class InMemoryDnsResolver implements DnsResolver {/** Logger associated to this class. */private final Log log = LogFactory.getLog(InMemoryDnsResolver.class);/*** In-memory collection that will hold the associations between a host name* and an array of InetAddress instances.*/private final Map<String, InetAddress[]> dnsMap;/*** Builds a DNS resolver that will resolve the host names against a* collection held in-memory.*/public InMemoryDnsResolver() {dnsMap = new ConcurrentHashMap<String, InetAddress[]>();}/*** Associates the given array of IP addresses to the given host in this DNS overrider.* The IP addresses are assumed to be already resolved.** @param host*            The host name to be associated with the given IP.* @param ips*            array of IP addresses to be resolved by this DNS overrider to the given*            host name.*/public void add(final String host, final InetAddress... ips) {Args.notNull(host, "Host name");Args.notNull(ips, "Array of IP addresses");dnsMap.put(host, ips);}/*** {@inheritDoc}*/@Overridepublic InetAddress[] resolve(final String host) throws UnknownHostException {final InetAddress[] resolvedAddresses = dnsMap.get(host);if (log.isInfoEnabled()) {log.info("Resolving " + host + " to " + Arrays.deepToString(resolvedAddresses));}if(resolvedAddresses == null){throw new UnknownHostException(host + " cannot be resolved");}return resolvedAddresses;}}

InMemoryDnsResolver实现了DnsResolver接口，它用一个ConcurrentHashMap来存放dns信息，提供add方法往map添加host及对应的ip地址，然后其resolve就是从这个map来读取对应的ip地址信息

SystemDefaultDnsResolver

org/apache/http/impl/conn/SystemDefaultDnsResolver.java

/*** DNS resolver that uses the default OS implementation for resolving host names.** @since 4.2*/
public class SystemDefaultDnsResolver implements DnsResolver {public static final SystemDefaultDnsResolver INSTANCE = new SystemDefaultDnsResolver();@Overridepublic InetAddress[] resolve(final String host) throws UnknownHostException {return InetAddress.getAllByName(host);}}

SystemDefaultDnsResolver实现了DnsResolver，它用的就是jdk提供的InetAddress.getAllByName，默认是走的OS的DNS，可以通过sun.net.spi.nameservice.provider.<n>去自定义

DefaultHttpClientConnectionOperator

org/apache/http/impl/conn/DefaultHttpClientConnectionOperator.java

@Contract(threading = ThreadingBehavior.IMMUTABLE_CONDITIONAL)
public class DefaultHttpClientConnectionOperator implements HttpClientConnectionOperator {static final String SOCKET_FACTORY_REGISTRY = "http.socket-factory-registry";private final Log log = LogFactory.getLog(getClass());private final Lookup<ConnectionSocketFactory> socketFactoryRegistry;private final SchemePortResolver schemePortResolver;private final DnsResolver dnsResolver;public DefaultHttpClientConnectionOperator(final Lookup<ConnectionSocketFactory> socketFactoryRegistry,final SchemePortResolver schemePortResolver,final DnsResolver dnsResolver) {super();Args.notNull(socketFactoryRegistry, "Socket factory registry");this.socketFactoryRegistry = socketFactoryRegistry;this.schemePortResolver = schemePortResolver != null ? schemePortResolver :DefaultSchemePortResolver.INSTANCE;this.dnsResolver = dnsResolver != null ? dnsResolver :SystemDefaultDnsResolver.INSTANCE;}@Overridepublic void connect(final ManagedHttpClientConnection conn,final HttpHost host,final InetSocketAddress localAddress,final int connectTimeout,final SocketConfig socketConfig,final HttpContext context) throws IOException {final Lookup<ConnectionSocketFactory> registry = getSocketFactoryRegistry(context);final ConnectionSocketFactory sf = registry.lookup(host.getSchemeName());if (sf == null) {throw new UnsupportedSchemeException(host.getSchemeName() +" protocol is not supported");}final InetAddress[] addresses = host.getAddress() != null ?new InetAddress[] { host.getAddress() } : this.dnsResolver.resolve(host.getHostName());final int port = this.schemePortResolver.resolve(host);for (int i = 0; i < addresses.length; i++) {final InetAddress address = addresses[i];final boolean last = i == addresses.length - 1;Socket sock = sf.createSocket(context);sock.setSoTimeout(socketConfig.getSoTimeout());sock.setReuseAddress(socketConfig.isSoReuseAddress());sock.setTcpNoDelay(socketConfig.isTcpNoDelay());sock.setKeepAlive(socketConfig.isSoKeepAlive());if (socketConfig.getRcvBufSize() > 0) {sock.setReceiveBufferSize(socketConfig.getRcvBufSize());}if (socketConfig.getSndBufSize() > 0) {sock.setSendBufferSize(socketConfig.getSndBufSize());}final int linger = socketConfig.getSoLinger();if (linger >= 0) {sock.setSoLinger(true, linger);}conn.bind(sock);final InetSocketAddress remoteAddress = new InetSocketAddress(address, port);if (this.log.isDebugEnabled()) {this.log.debug("Connecting to " + remoteAddress);}try {sock = sf.connectSocket(connectTimeout, sock, host, remoteAddress, localAddress, context);conn.bind(sock);if (this.log.isDebugEnabled()) {this.log.debug("Connection established " + conn);}return;} catch (final SocketTimeoutException ex) {if (last) {throw new ConnectTimeoutException(ex, host, addresses);}} catch (final ConnectException ex) {if (last) {final String msg = ex.getMessage();throw "Connection timed out".equals(msg)? new ConnectTimeoutException(ex, host, addresses): new HttpHostConnectException(ex, host, addresses);}} catch (final NoRouteToHostException ex) {if (last) {throw ex;}}if (this.log.isDebugEnabled()) {this.log.debug("Connect to " + remoteAddress + " timed out. " +"Connection will be retried using another IP address");}}}//......
}    

DefaultHttpClientConnectionOperator的connect方法会通过dnsResolver.resolve解析host

小结

HttpClient提供了DnsResolver接口，可以用于自定义DNS解析，是除了使用sun.net.spi.nameservice.provider.<n>去自定义JDK全局dns解析外的另外一种方案。