enable child channel plugins

api/src/main/java/io/grpc/ChannelConfigurer.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2026 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc;
+
+
+
+/**
+ * A configurer for child channels created by gRPC's internal infrastructure.
+ *
+ * <p>This interface allows users to inject configuration (such as credentials, interceptors,
+ * or flow control settings) into channels created automatically by gRPC for control plane
+ * operations. Common use cases include:
+ * <ul>
+ * <li>xDS control plane connections</li>
+ * <li>Load Balancing helper channels (OOB channels)</li>
+ * </ul>
+ *
+ * <p><strong>Usage Example:</strong>
+ * <pre>{@code
+ * // 1. Define the configurer
+ * ChannelConfigurer configurer = builder -> {
+ *   builder.maxInboundMessageSize(4 * 1024 * 1024);
+ * };
+ *
+ * // 2. Apply to parent channel - automatically used for ALL child channels
+ * ManagedChannel channel = ManagedChannelBuilder
+ *     .forTarget("xds:///my-service")
+ *     .childChannelConfigurer(configurer)
+ *     .build();
+ * }</pre>
+ *
+ * <p>Implementations must be thread-safe as the configure methods may be invoked concurrently
+ * by multiple internal components.
+ *
+ * @since 1.81.0
+ */
+@ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+public interface ChannelConfigurer {
+
+  /**
+   * Configures a builder for a new child channel.
+   *
+   * <p>This method is invoked synchronously during the creation of the child channel,
+   * before {@link ManagedChannelBuilder#build()} is called.
+   *
+   * @param builder the mutable channel builder for the new child channel
+   */
+  default void configureChannelBuilder(ManagedChannelBuilder<?> builder) {}
+
+  /**
+   * Configures a builder for a new child server.
+   *
+   * <p>This method is invoked synchronously during the creation of the child server,
+   * before {@link ServerBuilder#build()} is called.
+   *
+   * @param builder the mutable server builder for the new child server
+   */
+  default void configureServerBuilder(ServerBuilder<?> builder) {}
+}

api/src/main/java/io/grpc/ForwardingChannelBuilder.java

@@ -242,6 +242,13 @@ public T disableServiceConfigLookUp() {
     return thisT();
   }
 
+
+  @Override
+  public T childChannelConfigurer(ChannelConfigurer channelConfigurer) {
+    delegate().childChannelConfigurer(channelConfigurer);
+    return thisT();
+  }
+
   /**
    * Returns the correctly typed version of the builder.
    */

api/src/main/java/io/grpc/ForwardingChannelBuilder2.java

@@ -258,7 +258,7 @@ public T disableServiceConfigLookUp() {
   }
 
   @Override
-  protected T addMetricSink(MetricSink metricSink) {
+  public T addMetricSink(MetricSink metricSink) {
     delegate().addMetricSink(metricSink);
     return thisT();
   }
@@ -269,6 +269,13 @@ public <X> T setNameResolverArg(NameResolver.Args.Key<X> key, X value) {
     return thisT();
   }
 
+
+  @Override
+  public T childChannelConfigurer(ChannelConfigurer channelConfigurer) {
+    delegate().childChannelConfigurer(channelConfigurer);
+    return thisT();
+  }
+
   /**
    * Returns the {@link ManagedChannel} built by the delegate by default. Overriding method can
    * return different value.

api/src/main/java/io/grpc/ForwardingServerBuilder.java

@@ -192,6 +192,12 @@ public T setBinaryLog(BinaryLog binaryLog) {
     return thisT();
   }
 
+  @Override
+  public T childChannelConfigurer(ChannelConfigurer channelConfigurer) {
+    delegate().childChannelConfigurer(channelConfigurer);
+    return thisT();
+  }
+
   /**
    * Returns the {@link Server} built by the delegate by default. Overriding method can return
    * different value.

api/src/main/java/io/grpc/ManagedChannelBuilder.java

@@ -168,9 +168,7 @@ protected T interceptWithTarget(InterceptorFactory factory) {
     throw new UnsupportedOperationException();
   }
 
-  /** Internal-only. */
-  @Internal
-  protected interface InterceptorFactory {
+  public interface InterceptorFactory {
     ClientInterceptor newInterceptor(String target);
   }
 
@@ -638,8 +636,7 @@ public T disableServiceConfigLookUp() {
    * @return this
    * @since 1.64.0
    */
-  @Internal
-  protected T addMetricSink(MetricSink metricSink) {
+  public T addMetricSink(MetricSink metricSink) {
     throw new UnsupportedOperationException();
   }
 
@@ -661,6 +658,22 @@ public <X> T setNameResolverArg(NameResolver.Args.Key<X> key, X value) {
     throw new UnsupportedOperationException();
   }
 
+
+  /**
+   * Sets a configurer that will be applied to all internal child channels created by this channel.
+   *
+   * <p>This allows injecting configuration (like credentials, interceptors, or flow control)
+   * into auxiliary channels created by gRPC infrastructure, such as xDS control plane connections.
+   *
+   * @param channelConfigurer the configurer to apply.
+   * @return this
+   * @since 1.81.0
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+  public T childChannelConfigurer(ChannelConfigurer channelConfigurer) {
+    throw new UnsupportedOperationException("Not implemented");
+  }
+
   /**
    * Builds a channel using the given parameters.
    *

api/src/main/java/io/grpc/MetricRecorder.java

@@ -26,6 +26,15 @@
  */
 @Internal
 public interface MetricRecorder {
+
+  /**
+   * Returns a {@link MetricRecorder} that performs no operations.
+   * The returned instance ignores all calls and skips all validation checks.
+   */
+  static MetricRecorder noOp() {
+    return NoOpMetricRecorder.INSTANCE;
+  }
+
   /**
    * Adds a value for a double-precision counter metric instrument.
    *
@@ -176,4 +185,47 @@ interface Registration extends AutoCloseable {
     @Override
     void close();
   }
+
+  /**
+   * No-Op implementation of MetricRecorder.
+   * Overrides all default methods to skip validation checks for maximum performance.
+   */
+  final class NoOpMetricRecorder implements MetricRecorder {
+    private static final NoOpMetricRecorder INSTANCE = new NoOpMetricRecorder();
+
+    @Override
+    public void addDoubleCounter(DoubleCounterMetricInstrument metricInstrument, double value,
+                                 List<String> requiredLabelValues,
+                                 List<String> optionalLabelValues) {
+    }
+
+    @Override
+    public void addLongCounter(LongCounterMetricInstrument metricInstrument, long value,
+                               List<String> requiredLabelValues, List<String> optionalLabelValues) {
+    }
+
+    @Override
+    public void addLongUpDownCounter(LongUpDownCounterMetricInstrument metricInstrument, long value,
+                                     List<String> requiredLabelValues,
+                                     List<String> optionalLabelValues) {
+    }
+
+    @Override
+    public void recordDoubleHistogram(DoubleHistogramMetricInstrument metricInstrument,
+                                      double value, List<String> requiredLabelValues,
+                                      List<String> optionalLabelValues) {
+    }
+
+    @Override
+    public void recordLongHistogram(LongHistogramMetricInstrument metricInstrument, long value,
+                                    List<String> requiredLabelValues,
+                                    List<String> optionalLabelValues) {
+    }
+
+    @Override
+    public Registration registerBatchCallback(BatchCallback callback,
+                                              CallbackMetricInstrument... metricInstruments) {
+      return () -> { };
+    }
+  }
 }

api/src/main/java/io/grpc/NameResolver.java

@@ -358,6 +358,7 @@ public static final class Args {
     @Nullable private final MetricRecorder metricRecorder;
     @Nullable private final NameResolverRegistry nameResolverRegistry;
     @Nullable private final IdentityHashMap<Key<?>, Object> customArgs;
+    @Nullable private final ChannelConfigurer channelConfigurer;
 
     private Args(Builder builder) {
       this.defaultPort = checkNotNull(builder.defaultPort, "defaultPort not set");
@@ -372,6 +373,7 @@ private Args(Builder builder) {
       this.metricRecorder = builder.metricRecorder;
       this.nameResolverRegistry = builder.nameResolverRegistry;
       this.customArgs = cloneCustomArgs(builder.customArgs);
+      this.channelConfigurer = builder.channelConfigurer;
     }
 
     /**
@@ -470,6 +472,17 @@ public ChannelLogger getChannelLogger() {
       return channelLogger;
     }
 
+    /**
+     * Returns the configurer for child channels.
+     *
+     * @since 1.81.0
+     */
+    @Nullable
+    @Internal
+    public ChannelConfigurer getChildChannelConfigurer() {
+      return channelConfigurer;
+    }
+
     /**
      * Returns the Executor on which this resolver should execute long-running or I/O bound work.
      * Null if no Executor was set.
@@ -579,6 +592,7 @@ public static final class Builder {
       private MetricRecorder metricRecorder;
       private NameResolverRegistry nameResolverRegistry;
       private IdentityHashMap<Key<?>, Object> customArgs;
+      private ChannelConfigurer channelConfigurer = new ChannelConfigurer() {};
 
       Builder() {
       }
@@ -694,6 +708,16 @@ public Builder setNameResolverRegistry(NameResolverRegistry registry) {
         return this;
       }
 
+      /**
+       * See {@link Args#getChildChannelConfigurer()}. This is an optional field.
+       *
+       * @since 1.81.0
+       */
+      public Builder setChildChannelConfigurer(ChannelConfigurer channelConfigurer) {
+        this.channelConfigurer = channelConfigurer;
+        return this;
+      }
+
       /**
        * Builds an {@link Args}.
        *

api/src/main/java/io/grpc/Server.java

@@ -178,4 +178,5 @@ public List<ServerServiceDefinition> getMutableServices() {
    * @since 1.0.0
    */
   public abstract void awaitTermination() throws InterruptedException;
+
 }

api/src/main/java/io/grpc/ServerBuilder.java

@@ -424,6 +424,24 @@ public T setBinaryLog(BinaryLog binaryLog) {
     throw new UnsupportedOperationException();
   }
 
+
+  /**
+   * Sets a configurer that will be applied to all internal child channels created by this server.
+   *
+   * <p>This allows injecting configuration (like credentials, interceptors, or flow control)
+   * into auxiliary channels created by gRPC infrastructure, such as xDS control plane connections
+   * or OOB load balancing channels.
+   *
+   * @param channelConfigurer the configurer to apply.
+   * @return this
+   * @since 1.81.0
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+  public T childChannelConfigurer(ChannelConfigurer channelConfigurer) {
+    throw new UnsupportedOperationException("Not implemented");
+  }
+
+
   /**
    * Builds a server using the given parameters.
    *

api/src/test/java/io/grpc/ChannelConfigurerTest.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2026 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class ChannelConfigurerTest {
+
+  @Test
+  public void defaultMethods_doNothing() {
+    ChannelConfigurer configurer = new ChannelConfigurer() {};
+
+    ManagedChannelBuilder<?> mockChannelBuilder = mock(ManagedChannelBuilder.class);
+    configurer.configureChannelBuilder(mockChannelBuilder);
+    verifyNoInteractions(mockChannelBuilder);
+
+    ServerBuilder<?> mockServerBuilder = mock(ServerBuilder.class);
+    configurer.configureServerBuilder(mockServerBuilder);
+    verifyNoInteractions(mockServerBuilder);
+  }
+}