fix: only keep track of sockets opened using domain name

core/src/main/java/com/google/cloud/sql/core/MonitoredCache.java

@@ -16,15 +16,16 @@
 
 package com.google.cloud.sql.core;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Strings;
 import java.io.IOException;
 import java.net.Socket;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Iterator;
-import java.util.List;
+import java.util.Set;
 import java.util.Timer;
 import java.util.TimerTask;
+import java.util.WeakHashMap;
 import java.util.function.Function;
 import javax.net.ssl.SSLSocket;
 import org.slf4j.Logger;
@@ -38,7 +39,11 @@
 class MonitoredCache implements ConnectionInfoCache {
   private static final Logger logger = LoggerFactory.getLogger(Connector.class);
   private final ConnectionInfoCache cache;
-  private final List<Socket> sockets = Collections.synchronizedList(new ArrayList<>());
+  // Use weak references to hold the open sockets. If a socket is no longer in
+  // use by the application, the garabage collector will automatically remove
+  // it from this set.
+  private final Set<Socket> sockets =
+      Collections.synchronizedSet(Collections.newSetFromMap(new WeakHashMap<>()));
   private final Function<ConnectionConfig, CloudSqlInstanceName> resolve;
   private final TimerTask task;
 
@@ -49,6 +54,8 @@ class MonitoredCache implements ConnectionInfoCache {
     this.cache = cache;
     this.resolve = resolve;
 
+    // If this was configured with a domain name, start the domain name check
+    // and socket cleanup periodic task.
     if (!Strings.isNullOrEmpty(cache.getConfig().getDomainName())) {
       long failoverPeriod = cache.getConfig().getConnectorConfig().getFailoverPeriod().toMillis();
       this.task =
@@ -64,6 +71,11 @@ public void run() {
     }
   }
 
+  @VisibleForTesting
+  int getOpenSocketCount() {
+    return sockets.size();
+  }
+
   private void checkDomainName() {
     // Resolve the domain name again. If it changed, close the sockets
     try {
@@ -149,6 +161,10 @@ public synchronized boolean isClosed() {
   }
 
   synchronized void addSocket(SSLSocket socket) {
-    sockets.add(socket);
+    // Only add the socket if this was configured using a domain name,
+    // and therefore the background socket cleanup task is running.
+    if (!Strings.isNullOrEmpty(cache.getConfig().getDomainName())) {
+      sockets.add(socket);
+    }
   }
 }

core/src/test/java/com/google/cloud/sql/core/MonitoredCacheTest.java

@@ -0,0 +1,208 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * 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 com.google.cloud.sql.core;
+
+import com.google.cloud.sql.ConnectorConfig;
+import java.io.IOException;
+import java.time.Duration;
+import java.util.Timer;
+import javax.net.ssl.HandshakeCompletedListener;
+import javax.net.ssl.SSLSession;
+import javax.net.ssl.SSLSocket;
+import org.junit.AfterClass;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class MonitoredCacheTest {
+  private static final Timer timer = new Timer(true);
+
+  @AfterClass
+  public static void afterClass() {
+    timer.cancel();
+  }
+
+  @Test
+  public void testMonitoredCacheHoldsSocketsWithDomainName() {
+    CloudSqlInstanceName name = new CloudSqlInstanceName("proj:reg:inst", "db.example.com");
+    ConnectionConfig config =
+        new ConnectionConfig.Builder()
+            .withCloudSqlInstance("proj:reg:inst")
+            .withDomainName("db.example.com")
+            .build();
+    MockCache mockCache = new MockCache(config);
+
+    MonitoredCache cache = new MonitoredCache(mockCache, timer, connectionConfig -> name);
+    MockSslSocket socket = new MockSslSocket();
+    cache.addSocket(socket);
+    Assert.assertEquals("1 socket in cache", 1, cache.getOpenSocketCount());
+    cache.close();
+    Assert.assertTrue("socket closed", socket.closed);
+  }
+
+  @Test
+  public void testMonitoredCachePurgesClosedSockets() throws InterruptedException {
+    CloudSqlInstanceName name = new CloudSqlInstanceName("proj:reg:inst", "db.example.com");
+    // Purge sockets every 10ms.
+    ConnectionConfig config =
+        new ConnectionConfig.Builder()
+            .withCloudSqlInstance("proj:reg:inst")
+            .withDomainName("db.example.com")
+            .withConnectorConfig(
+                new ConnectorConfig.Builder().withFailoverPeriod(Duration.ofMillis(10)).build())
+            .build();
+    MockCache mockCache = new MockCache(config);
+
+    MonitoredCache cache = new MonitoredCache(mockCache, timer, connectionConfig -> name);
+    MockSslSocket socket = new MockSslSocket();
+    cache.addSocket(socket);
+    Assert.assertEquals("1 socket in cache", 1, cache.getOpenSocketCount());
+    socket.close();
+    Thread.sleep(20);
+    Assert.assertEquals("0 socket in cache", 0, cache.getOpenSocketCount());
+  }
+
+  @Test
+  public void testMonitoredCacheWithoutDomainNameIgnoresSockets() {
+    CloudSqlInstanceName name = new CloudSqlInstanceName("proj:reg:inst");
+    ConnectionConfig config =
+        new ConnectionConfig.Builder().withCloudSqlInstance("proj:reg:inst").build();
+    MockCache mockCache = new MockCache(config);
+
+    MonitoredCache cache = new MonitoredCache(mockCache, timer, connectionConfig -> name);
+    MockSslSocket socket = new MockSslSocket();
+    cache.addSocket(socket);
+    Assert.assertEquals("0 socket in cache", 0, cache.getOpenSocketCount());
+  }
+
+  private static class MockSslSocket extends SSLSocket {
+    boolean closed;
+
+    @Override
+    public synchronized boolean isClosed() {
+      return closed;
+    }
+
+    @Override
+    public synchronized void close() {
+      this.closed = true;
+    }
+
+    @Override
+    public String[] getSupportedCipherSuites() {
+      return new String[0];
+    }
+
+    @Override
+    public String[] getEnabledCipherSuites() {
+      return new String[0];
+    }
+
+    @Override
+    public void setEnabledCipherSuites(String[] suites) {}
+
+    @Override
+    public String[] getSupportedProtocols() {
+      return new String[0];
+    }
+
+    @Override
+    public String[] getEnabledProtocols() {
+      return new String[0];
+    }
+
+    @Override
+    public void setEnabledProtocols(String[] protocols) {}
+
+    @Override
+    public SSLSession getSession() {
+      return null;
+    }
+
+    @Override
+    public void addHandshakeCompletedListener(HandshakeCompletedListener listener) {}
+
+    @Override
+    public void removeHandshakeCompletedListener(HandshakeCompletedListener listener) {}
+
+    @Override
+    public void startHandshake() throws IOException {}
+
+    @Override
+    public void setUseClientMode(boolean mode) {}
+
+    @Override
+    public boolean getUseClientMode() {
+      return false;
+    }
+
+    @Override
+    public void setNeedClientAuth(boolean need) {}
+
+    @Override
+    public boolean getNeedClientAuth() {
+      return false;
+    }
+
+    @Override
+    public void setWantClientAuth(boolean want) {}
+
+    @Override
+    public boolean getWantClientAuth() {
+      return false;
+    }
+
+    @Override
+    public void setEnableSessionCreation(boolean flag) {}
+
+    @Override
+    public boolean getEnableSessionCreation() {
+      return false;
+    }
+  }
+
+  private static class MockCache implements ConnectionInfoCache {
+    private final ConnectionConfig config;
+
+    MockCache(ConnectionConfig config) {
+      this.config = config;
+    }
+
+    @Override
+    public ConnectionMetadata getConnectionMetadata(long timeoutMs) {
+      return null;
+    }
+
+    @Override
+    public void forceRefresh() {}
+
+    @Override
+    public void refreshIfExpired() {}
+
+    @Override
+    public void close() {}
+
+    @Override
+    public boolean isClosed() {
+      return false;
+    }
+
+    @Override
+    public ConnectionConfig getConfig() {
+      return config;
+    }
+  }
+}