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本文主要研究一下HttpClient的DnsResolver
DnsResolver
org/apache/http/conn/DnsResolver.java
代码语言：javascript复制/**
* 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
代码语言：javascript复制/**
* 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}
*/
@Override
public 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
代码语言：javascript复制/**
* 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();
@Override
public 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
代码语言：javascript复制@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;
}
@Override
public 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。
