Java_18_Features - Internet-Address Resolution SPI - Foreign Function & Memory API

The java.net.InetAddress API resolves host names to Internet Protocol (IP) addresses, and vice versa. In the current implementation, this API uses the OS’s native resolver (typically configured with a combination of a local hosts file and the DNS).

The Java 18 API defines a service-provider interface (SPI) for host name and address resolution, so that java.net.InetAddress can make use of resolvers other than the platform’s built-in resolver.

The new InetAddress API uses a Service Loader to locate a resolver provider.

The built-in implementation will be used as before when no providers are found.

To manage this SPI, you will use one of the following classes within the java.net.spi package:

• InetAddressResolverProvider — an abstract class defining the service to be located by java.util.ServiceLoader.
• InetAddressResolver — an interface that defines methods for the fundamental forward and reverse lookup operations.
• InetAddressResolver.LookupPolicy — a class whose instances describe the characteristics of a resolution request,
• InetAddressResolverProvider.Configuration — an interface describing the platform’s built-in configuration for resolution operations.

Let’s see it with a practical example. We will create a sample AddressResolver Class which extends java.net.spi.InetAddressResolver:

In Java 18, this is changed: "default" is no longer recognized; the output looks like this:

package com.sample.provider;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.net.spi.InetAddressResolver;
import java.util.stream.Stream;
public class MyAddressResolver implements InetAddressResolver {
  @Override
  public Stream<InetAddress> lookupByName(String host, LookupPolicy lookupPolicy)
      throws UnknownHostException {
      if (host.equals("fedora")) {
           return Stream.of(InetAddress.getByAddress(new byte[] {192, 168, 10, 1}));
      }
    return Stream.of(InetAddress.getByAddress(new byte[] {127, 0, 0, 1}));
  }
  @Override
  public String lookupByAddress(byte[] addr) {
    throw new UnsupportedOperationException();
  }
}

The Class contains a minimal address resolution policy to return the IP Address 192.168.10.1 for the Host “fedora”.

Next, let’s extend the class InetAddressResolverProvider to return an instance of our AddressResolver Class:

In Java 18, this is changed: "default" is no longer recognized; the output looks like this:

package com.sample.provider;
import java.net.spi.InetAddressResolver;
import java.net.spi.InetAddressResolverProvider;
public class MyAddressResolverProvider extends InetAddressResolverProvider {
  @Override
  public InetAddressResolver get(Configuration configuration) {
    return new MyAddressResolver();
  }
  @Override
  public String name() {
    return "MyAddressResolverProvider Internet Address Resolver Provider";
  }
}

Finally, to register the Class as a Service Provider Interface, we need to add to the Service Provider Configuration Class in a file META-INF/services/java.net.spi.InetAddressResolverProvider

com.sample.provider.MyAddressResolverProvider

That’s all. You can test the InetAddress resolution as follows:

In Java 18, this is changed: "default" is no longer recognized; the output looks like this:

public class App {
  public static void main(String[] args) throws UnknownHostException {
    InetAddress[] addresses = InetAddress.getAllByName("fedora");
    System.out.println("addresses = " + Arrays.toString(addresses));
  }
}