Add andThen method to ConnectionPoolListener

core/src/main/java/com/linecorp/armeria/client/ConnectionPoolListener.java

@@ -15,6 +15,8 @@
  */
 package com.linecorp.armeria.client;
 
+import static java.util.Objects.requireNonNull;
+
 import java.net.InetSocketAddress;
 
 import com.linecorp.armeria.common.SessionProtocol;
@@ -136,4 +138,42 @@ default ConnectionPoolListener unwrap() {
     default void close() {
         // Do nothing by default.
     }
+
+    /**
+     * Returns a new {@link ConnectionPoolListener} which combines two {@link ConnectionPoolListener}s.
+     */
+    @UnstableApi
+    default ConnectionPoolListener andThen(ConnectionPoolListener nextConnectionPoolListener) {
+        requireNonNull(nextConnectionPoolListener, "nextConnectionPoolListener");
+        return new ConnectionPoolListener() {
+            @Override
+            public void connectionOpen(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                       InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                try {
+                    ConnectionPoolListener.this.connectionOpen(protocol, remoteAddr, localAddr, attrs);
+                } finally {
+                    nextConnectionPoolListener.connectionOpen(protocol, remoteAddr,localAddr,attrs);
+                }
+            }
+
+            @Override
+            public void connectionClosed(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                         InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                try {
+                    ConnectionPoolListener.this.connectionClosed(protocol, remoteAddr, localAddr, attrs);
+                } finally {
+                    nextConnectionPoolListener.connectionClosed(protocol, remoteAddr,localAddr,attrs);
+                }
+            }
+
+            @Override
+            public void close() {
+                try {
+                    ConnectionPoolListener.super.close();
+                } finally {
+                    nextConnectionPoolListener.close();
+                }
+            }
+        };
+    }
 }

core/src/test/java/com/linecorp/armeria/client/ConnectionPoolListenerTest.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2025 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you 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:
+ *
+ *   https://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 com.linecorp.armeria.client;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+import static org.mockito.Mockito.mock;
+
+import java.net.InetSocketAddress;
+
+import org.junit.jupiter.api.Test;
+
+import com.linecorp.armeria.common.SessionProtocol;
+
+import io.netty.util.AttributeMap;
+
+class ConnectionPoolListenerTest {
+
+    @Test
+    void andThen_shouldFinishWithoutErrors() throws Exception {
+
+        final Integer[] counterArray = new Integer[6];
+
+        final ConnectionPoolListener cpl1 = new ConnectionPoolListener() {
+            @Override
+            public void connectionOpen(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                       InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[0] = 0;
+            }
+
+            @Override
+            public void connectionClosed(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                         InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[3] = 30;
+            }
+        };
+
+        final ConnectionPoolListener cpl2 = new ConnectionPoolListener() {
+            @Override
+            public void connectionOpen(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                       InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[1] = 10;
+            }
+
+            @Override
+            public void connectionClosed(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                         InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[4] = 40;
+            }
+        };
+
+        final ConnectionPoolListener cpl3 = new ConnectionPoolListener() {
+            @Override
+            public void connectionOpen(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                       InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[2] = 20;
+            }
+
+            @Override
+            public void connectionClosed(SessionProtocol protocol, InetSocketAddress remoteAddr,
+                                         InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
+                counterArray[5] = 50;
+            }
+        };
+
+        final SessionProtocol protocol = SessionProtocol.HTTP;
+        final InetSocketAddress remoteAddr = new InetSocketAddress("localhost", 8080);
+        final InetSocketAddress localAddr = new InetSocketAddress("localhost", 9090);
+        final AttributeMap attrs = mock(AttributeMap.class);
+
+        final ConnectionPoolListener cplCombined = cpl1.andThen(cpl2).andThen(cpl3);
+
+        cplCombined.connectionOpen(protocol, remoteAddr, localAddr, attrs);
+        cplCombined.connectionClosed(protocol, remoteAddr, localAddr, attrs);
+        cplCombined.close();
+
+        assertThat(counterArray[0]).isEqualTo(0);
+        assertThat(counterArray[1]).isEqualTo(10);
+        assertThat(counterArray[2]).isEqualTo(20);
+        assertThat(counterArray[3]).isEqualTo(30);
+        assertThat(counterArray[4]).isEqualTo(40);
+        assertThat(counterArray[5]).isEqualTo(50);
+    }
+}