InetAddressResolver (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: java.net.spi, interface: InetAddressResolver
Keywords: java.net.spi.InetAddressResolver interface

# Interface InetAddressResolver

public interface InetAddressResolver
This interface defines operations for looking up host names and IP addresses. `InetAddress` delegates all lookup operations to the system-wide resolver.

The system-wide resolver can be customized by deploying an implementation of `InetAddressResolverProvider`.

Since:
  18

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Interface
  Description
  `static final class`
  `InetAddressResolver.LookupPolicy`
  A `LookupPolicy` object describes characteristics that can be applied to a lookup operation.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `String`
  `lookupByAddress(byte[] addr)`
  Lookup the host name corresponding to the raw IP address provided.
  `Stream<InetAddress>`
  `lookupByName(String host, InetAddressResolver.LookupPolicy lookupPolicy)`
  Given the name of a host, returns a stream of IP addresses of the requested address family associated with a provided hostname.

- ## Method Details

  - ### lookupByName

    Stream<InetAddress> lookupByName(String host, InetAddressResolver.LookupPolicy lookupPolicy) throws UnknownHostException
    Given the name of a host, returns a stream of IP addresses of the requested address family associated with a provided hostname.

    `host` should be a machine name, such as "`www.example.com`", not a textual representation of its IP address. No validation is performed on the given `host` name: if a textual representation is supplied, the name resolution is likely to fail and `UnknownHostException` may be thrown.

    The address family type and addresses order are specified by the `LookupPolicy` instance. Lookup operation characteristics could be acquired with `InetAddressResolver.LookupPolicy.characteristics()`. If `InetAddressResolver.LookupPolicy.IPV4` and `InetAddressResolver.LookupPolicy.IPV6` characteristics provided then this method returns addresses of both IPV4 and IPV6 families.

    Parameters:
      `host` - the specified hostname
      `lookupPolicy` - the address lookup policy
    Returns:
      a stream of IP addresses for the requested host
    Throws:
      `NullPointerException` - if either parameter is `null`
      `UnknownHostException` - if no IP address for the `host` could be found
    See Also:
      - `InetAddressResolver.LookupPolicy`

  - ### lookupByAddress

    String lookupByAddress(byte[] addr) throws UnknownHostException
    Lookup the host name corresponding to the raw IP address provided.

    `addr` argument is in network byte order: the highest order byte of the address is in `addr[0]`.

    IPv4 address byte array must be 4 bytes long and IPv6 byte array must be 16 bytes long.

    Parameters:
      `addr` - byte array representing a raw IP address
    Returns:
      `String` representing the host name mapping
    Throws:
      `UnknownHostException` - if no host name is found for the specified IP address
      `IllegalArgumentException` - if the length of the provided byte array doesn't correspond to a valid IP address length
      `NullPointerException` - if addr is `null`