Add composite pattern to GrpcChannelBuilderCustomizer

spring-grpc-core/src/main/java/org/springframework/grpc/client/ChannelBuilderOptions.java

@@ -142,10 +142,11 @@ public final class ChannelBuilderOptions {
 	 * @return a new immutable options instance populated with the specified
 	 * {@code customizer} and the settings of this current options instance.
 	 */
+	@SuppressWarnings("unchecked")
 	public <T extends ManagedChannelBuilder<T>> ChannelBuilderOptions withCustomizer(
 			GrpcChannelBuilderCustomizer<T> customizer) {
 		return new ChannelBuilderOptions(this.interceptors, this.mergeWithGlobalInterceptors, this.shutdownGracePeriod,
-				customizer);
+				this.customizer.then(customizer));
 	}
 
 }

spring-grpc-core/src/main/java/org/springframework/grpc/client/GrpcChannelBuilderCustomizer.java

@@ -37,6 +37,13 @@ public interface GrpcChannelBuilderCustomizer<T extends ManagedChannelBuilder<T>
 	 */
 	void customize(String authority, T builder);
 
+	default GrpcChannelBuilderCustomizer<T> then(GrpcChannelBuilderCustomizer<T> other) {
+		return (authority, builder) -> {
+			customize(authority, builder);
+			other.customize(authority, builder);
+		};
+	}
+
 	/**
 	 * Used to indicate no customizations should be made to the builder.
 	 * @param <T> type of channel builder

spring-grpc-core/src/test/java/org/springframework/grpc/client/ChannelBuilderOptionsTests.java

@@ -17,6 +17,7 @@ package org.springframework.grpc.client;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoInteractions;
 
 import java.time.Duration;
@@ -26,6 +27,7 @@ import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
 import io.grpc.ClientInterceptor;
+import io.grpc.ManagedChannelBuilder;
 import io.grpc.netty.NettyChannelBuilder;
 
 /**
@@ -56,7 +58,18 @@ class ChannelBuilderOptionsTests {
 		assertThat(options.interceptors()).containsExactly(interceptor1, interceptor2);
 		assertThat(options.mergeWithGlobalInterceptors()).isTrue();
 		assertThat(options.shutdownGracePeriod()).isEqualTo(Duration.ofMinutes(1));
-		assertThat(options.customizer()).isSameAs(customizer);
+		assertThat(options.customizer()).isNotEqualTo(GrpcChannelBuilderCustomizer.defaults());
+	}
+
+	@SuppressWarnings({ "rawtypes", "unchecked" })
+	@Test
+	void customizerApplied() {
+		ManagedChannelBuilder builder = mock();
+		GrpcChannelBuilderCustomizer customizer = mock();
+		var options = ChannelBuilderOptions.defaults().withCustomizer(customizer);
+		var applied = options.<ManagedChannelBuilder>customizer();
+		applied.customize("localhost", builder);
+		verify(customizer).customize("localhost", builder);
 	}
 
 	@Test