Make transport.Bootstrap usable with no netty-resolver on classpath

transport/src/main/java/io/netty/bootstrap/Bootstrap.java

@@ -47,20 +47,18 @@ public class Bootstrap extends AbstractBootstrap<Bootstrap, Channel> {
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(Bootstrap.class);
 
-    private static final AddressResolverGroup<?> DEFAULT_RESOLVER = DefaultAddressResolverGroup.INSTANCE;
-
     private final BootstrapConfig config = new BootstrapConfig(this);
 
-    @SuppressWarnings("unchecked")
-    private volatile AddressResolverGroup<SocketAddress> resolver =
-            (AddressResolverGroup<SocketAddress>) DEFAULT_RESOLVER;
+    private ExternalAddressResolver externalResolver;
+    private volatile boolean disableResolver;
     private volatile SocketAddress remoteAddress;
 
     public Bootstrap() { }
 
     private Bootstrap(Bootstrap bootstrap) {
         super(bootstrap);
-        resolver = bootstrap.resolver;
+        externalResolver = bootstrap.externalResolver;
+        disableResolver = bootstrap.disableResolver;
         remoteAddress = bootstrap.remoteAddress;
     }
 
@@ -74,7 +72,18 @@ private Bootstrap(Bootstrap bootstrap) {
      */
     @SuppressWarnings("unchecked")
     public Bootstrap resolver(AddressResolverGroup<?> resolver) {
-        this.resolver = (AddressResolverGroup<SocketAddress>) (resolver == null ? DEFAULT_RESOLVER : resolver);
+        this.externalResolver = resolver == null ? null : new ExternalAddressResolver(resolver);
+        disableResolver = false;
+        return this;
+    }
+
+    /**
+     * Disables address name resolution. Name resolution may be re-enabled with
+     * {@link Bootstrap#resolver(AddressResolverGroup)}
+     */
+    public Bootstrap disableResolver() {
+        externalResolver = null;
+        disableResolver = true;
         return this;
     }
 
@@ -188,10 +197,15 @@ public void operationComplete(ChannelFuture future) throws Exception {
     private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress,
                                                final SocketAddress localAddress, final ChannelPromise promise) {
         try {
+            if (disableResolver) {
+                doConnect(remoteAddress, localAddress, promise);
+                return promise;
+            }
+
             final EventLoop eventLoop = channel.eventLoop();
             AddressResolver<SocketAddress> resolver;
             try {
-                resolver = this.resolver.getResolver(eventLoop);
+                resolver = ExternalAddressResolver.getOrDefault(externalResolver).getResolver(eventLoop);
             } catch (Throwable cause) {
                 channel.close();
                 return promise.setFailure(cause);
@@ -301,6 +315,29 @@ final SocketAddress remoteAddress() {
     }
 
     final AddressResolverGroup<?> resolver() {
-        return resolver;
+        if (disableResolver) {
+            return null;
+        }
+        return ExternalAddressResolver.getOrDefault(externalResolver);
+    }
+
+    /* Holder to avoid NoClassDefFoundError in case netty-resolver dependency is excluded
+       (e.g. some address families do not need name resolution) */
+    static final class ExternalAddressResolver {
+        final AddressResolverGroup<SocketAddress> resolverGroup;
+
+        @SuppressWarnings("unchecked")
+        ExternalAddressResolver(AddressResolverGroup<?> resolverGroup) {
+            this.resolverGroup = (AddressResolverGroup<SocketAddress>) resolverGroup;
+        }
+
+        @SuppressWarnings("unchecked")
+        static AddressResolverGroup<SocketAddress> getOrDefault(ExternalAddressResolver externalResolver) {
+            if (externalResolver == null) {
+                AddressResolverGroup<?> defaultResolverGroup = DefaultAddressResolverGroup.INSTANCE;
+                return (AddressResolverGroup<SocketAddress>) defaultResolverGroup;
+            }
+            return externalResolver.resolverGroup;
+        }
     }
 }

transport/src/main/java/io/netty/bootstrap/BootstrapConfig.java

@@ -37,7 +37,8 @@ public SocketAddress remoteAddress() {
     }
 
     /**
-     * Returns the configured {@link AddressResolverGroup} or the default if non is configured yet.
+     * Returns the configured {@link AddressResolverGroup}, {@code null} if resolver was disabled
+     * with {@link Bootstrap#disableResolver()}, or the default if non is configured yet.
      */
     public AddressResolverGroup<?> resolver() {
         return bootstrap.resolver();
@@ -47,7 +48,11 @@ public AddressResolverGroup<?> resolver() {
     public String toString() {
         StringBuilder buf = new StringBuilder(super.toString());
         buf.setLength(buf.length() - 1);
-        buf.append(", resolver: ").append(resolver());
+        AddressResolverGroup<?> resolver = resolver();
+        if (resolver != null) {
+            buf.append(", resolver: ")
+                    .append(resolver);
+        }
         SocketAddress remoteAddress = remoteAddress();
         if (remoteAddress != null) {
             buf.append(", remoteAddress: ")

transport/src/test/java/io/netty/bootstrap/BootstrapTest.java

@@ -38,6 +38,7 @@
 import io.netty.resolver.AddressResolver;
 import io.netty.resolver.AddressResolverGroup;
 import io.netty.resolver.AbstractAddressResolver;
+import io.netty.resolver.DefaultAddressResolverGroup;
 import io.netty.util.AttributeKey;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.Future;
@@ -69,6 +70,8 @@
 import static org.hamcrest.Matchers.sameInstance;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -299,6 +302,24 @@ public void execute() {
         }
     }
 
+    @Test
+    void testResolverDefault() throws Exception {
+        Bootstrap bootstrap = new Bootstrap();
+
+        assertTrue(bootstrap.config().toString().contains("resolver:"));
+        assertNotNull(bootstrap.config().resolver());
+        assertEquals(DefaultAddressResolverGroup.class, bootstrap.config().resolver().getClass());
+    }
+
+    @Test
+    void testResolverDisabled() throws Exception {
+        Bootstrap bootstrap = new Bootstrap();
+        bootstrap.disableResolver();
+
+        assertFalse(bootstrap.config().toString().contains("resolver:"));
+        assertNull(bootstrap.config().resolver());
+    }
+
     @Test
     public void testAsyncResolutionSuccess() throws Exception {
         final Bootstrap bootstrapA = new Bootstrap();
@@ -311,6 +332,10 @@ public void testAsyncResolutionSuccess() throws Exception {
         bootstrapB.group(groupB);
         bootstrapB.channel(LocalServerChannel.class);
         bootstrapB.childHandler(dummyHandler);
+
+        assertTrue(bootstrapA.config().toString().contains("resolver:"));
+        assertThat(bootstrapA.resolver(), is(instanceOf(TestAddressResolverGroup.class)));
+
         SocketAddress localAddress = bootstrapB.bind(LocalAddress.ANY).sync().channel().localAddress();
 
         // Connect to the server using the asynchronous resolver.