Demonstrate address resolution SPI

Author: Nicolai Parlog

README.md

@@ -89,6 +89,7 @@ Check out the [jpms](http://blog.codefx.org/tag/jpms/) tag on my blog, [this dem
 
 ## New APIs
 
+* ⑱ [address resolution SPI](src/main/java/org/codefx/demo/java18/api/ip_resolution) ([JEP 418](http://openjdk.java.net/jeps/418))
 * ⑰ [random generator](src/main/java/org/codefx/demo/java17/api/random) ([article](https://nipafx.dev/java-random-generator/), [JEP 356](https://openjdk.java.net/jeps/356), [very good Javadoc](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/random/package-summary.html))
 * ⑯ [Unix domain sockets](src/main/java/org/codefx/demo/java16/api/unix_sockets) ([article](https://nipafx.dev/java-unix-domain-sockets/), [JEP-380](https://openjdk.java.net/jeps/380))
 * ⑪ HTTP/2 client: [simple](src/main/java/org/codefx/demo/java11/api/http2/Http2Api.java), [more formal](src/main/java/org/codefx/demo/java11/api/http2/formalized) ([tutorial](https://blog.codefx.org/java/http-2-api-tutorial/), [reactive request/response bodies](https://blog.codefx.org/java/reactive-http-2-requests-responses/))

src/main/java/module-info.java

@@ -1,5 +1,7 @@
 import org.codefx.demo.java17.api.random.Xkcd;
+import org.codefx.demo.java18.api.ip_resolution.ForwardingInetAddressResolverProvider;
 
+import java.net.spi.InetAddressResolverProvider;
 import java.util.random.RandomGenerator;
 
 /**
@@ -12,4 +14,5 @@
 	requires java.net.http;
 
 	provides RandomGenerator with Xkcd;
+	provides InetAddressResolverProvider with ForwardingInetAddressResolverProvider;
 }

src/main/java/org/codefx/demo/java18/api/ip_resolution/ForwardingInetAddressResolver.java

@@ -0,0 +1,33 @@
+package org.codefx.demo.java18.api.ip_resolution;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.net.spi.InetAddressResolver;
+import java.util.Arrays;
+import java.util.stream.Stream;
+
+/**
+ * An {@link InetAddressResolver} that simply forwards to the builtin resolver.
+ * It acts as a starting point for adding custom behavior to host and address resolution.
+ */
+public class ForwardingInetAddressResolver implements InetAddressResolver {
+
+	private InetAddressResolver builtinResolver;
+
+	public ForwardingInetAddressResolver(InetAddressResolver builtinResolver) {
+		this.builtinResolver = builtinResolver;
+	}
+
+	@Override
+	public Stream<InetAddress> lookupByName(String host, LookupPolicy lookupPolicy) throws UnknownHostException {
+		System.out.printf("Looking up '%s'.%n", host);
+		return builtinResolver.lookupByName(host, lookupPolicy);
+	}
+
+	@Override
+	public String lookupByAddress(byte[] addr) throws UnknownHostException {
+		System.out.printf("Looking up '%s'.%n", Arrays.toString(addr));
+		return builtinResolver.lookupByAddress(addr);
+	}
+
+}

src/main/java/org/codefx/demo/java18/api/ip_resolution/ForwardingInetAddressResolverProvider.java

@@ -0,0 +1,18 @@
+package org.codefx.demo.java18.api.ip_resolution;
+
+import java.net.spi.InetAddressResolver;
+import java.net.spi.InetAddressResolverProvider;
+
+public class ForwardingInetAddressResolverProvider extends InetAddressResolverProvider {
+
+	@Override
+	public InetAddressResolver get(Configuration configuration) {
+		return new ForwardingInetAddressResolver(configuration.builtinResolver());
+	}
+
+	@Override
+	public String name() {
+		return "Injectable";
+	}
+
+}

src/main/java/org/codefx/demo/java18/api/ip_resolution/ResolverDemo.java

@@ -0,0 +1,30 @@
+package org.codefx.demo.java18.api.ip_resolution;
+
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+
+import static java.net.http.HttpResponse.BodyHandlers.ofLines;
+
+/**
+ * This demo sends a simple request to a website. If the project is wired up
+ * correctly, the address resolution machinery picks up the other two classes
+ * in this package and uses them for resolution, in which case you should see
+ * a log message like "Looking up ..." on the command line for each request.
+ */
+public class ResolverDemo {
+
+	public static void main(String[] args) throws Exception {
+		HttpClient client = HttpClient.newHttpClient();
+		HttpRequest request = HttpRequest
+				.newBuilder(new URI("https://nipafx.dev"))
+				.GET()
+				.build();
+		client
+				.send(request, ofLines())
+				.body()
+				.limit(5)
+				.forEach(System.out::println);
+	}
+
+}