chore: Introduce ConnectionInfoCache interface (part of #992).

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

@@ -32,7 +32,7 @@
  * SQL Admin API. The operations to retrieve information with the API are largely done
  * asynchronously, and this class should be considered threadsafe.
  */
-class BaseConnectionInfoCache {
+class BaseConnectionInfoCache implements ConnectionInfoCache {
   private final AccessTokenSupplier accessTokenSupplier;
   private final CloudSqlInstanceName instanceName;
   private final RefreshStrategy refreshStrategy;
@@ -96,7 +96,8 @@ private ConnectionInfo getConnectionInfo(long timeoutMs) {
    * Returns an unconnected {@link SSLSocket} using the SSLContext associated with the instance. May
    * block until required instance data is available.
    */
-  SSLSocket createSslSocket(long timeoutMs) throws IOException {
+  @Override
+  public SSLSocket createSslSocket(long timeoutMs) throws IOException {
     return (SSLSocket)
         getConnectionInfo(timeoutMs).getSslContext().getSocketFactory().createSocket();
   }
@@ -108,7 +109,8 @@ SSLSocket createSslSocket(long timeoutMs) throws IOException {
    * @throws IllegalArgumentException If the instance has no IP addresses matching the provided
    *     preferences.
    */
-  ConnectionMetadata getConnectionMetadata(long timeoutMs) {
+  @Override
+  public ConnectionMetadata getConnectionMetadata(long timeoutMs) {
     ConnectionInfo info = getConnectionInfo(timeoutMs);
     String preferredIp = null;
 
@@ -132,23 +134,28 @@ ConnectionMetadata getConnectionMetadata(long timeoutMs) {
         info.getSslData().getTrustManagerFactory());
   }
 
-  void forceRefresh() {
+  @Override
+  public void forceRefresh() {
     this.refreshStrategy.forceRefresh();
   }
 
-  void refreshIfExpired() {
+  @Override
+  public void refreshIfExpired() {
     this.refreshStrategy.refreshIfExpired();
   }
 
-  RefreshStrategy getRefresher() {
+  @Override
+  public RefreshStrategy getRefresher() {
     return this.refreshStrategy;
   }
 
+  @Override
   public CloudSqlInstanceName getInstanceName() {
     return instanceName;
   }
 
-  void close() {
+  @Override
+  public void close() {
     refreshStrategy.close();
   }
 }

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

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2024 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 java.io.IOException;
+import javax.net.ssl.SSLSocket;
+
+/** ConnectionInfoCache is the contract for a caching strategy for ConnectionInfo. */
+public interface ConnectionInfoCache {
+
+  /**
+   * Returns an unconnected {@link SSLSocket} using the SSLContext associated with the instance. May
+   * block until required instance data is available.
+   */
+  SSLSocket createSslSocket(long timeoutMs) throws IOException;
+
+  /**
+   * Returns metadata needed to create a connection to the instance.
+   *
+   * @return returns ConnectionMetadata containing the preferred IP and SSL connection data.
+   * @throws IllegalArgumentException If the instance has no IP addresses matching the provided
+   *     preferences.
+   */
+  ConnectionMetadata getConnectionMetadata(long timeoutMs);
+
+  void forceRefresh();
+
+  void refreshIfExpired();
+
+  RefreshStrategy getRefresher();
+
+  CloudSqlInstanceName getInstanceName();
+
+  void close();
+}

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

@@ -41,7 +41,7 @@ class Connector {
   private final ListenableFuture<KeyPair> localKeyPair;
   private final long minRefreshDelayMs;
 
-  private final ConcurrentHashMap<ConnectionConfig, BaseConnectionInfoCache> instances =
+  private final ConcurrentHashMap<ConnectionConfig, ConnectionInfoCache> instances =
       new ConcurrentHashMap<>();
   private final int serverProxyPort;
   private final ConnectorConfig config;
@@ -107,7 +107,7 @@ Socket connect(ConnectionConfig config, long timeoutMs) throws IOException {
       return UnixSocketChannel.open(socketAddress).socket();
     }
 
-    BaseConnectionInfoCache instance = getConnection(config);
+    ConnectionInfoCache instance = getConnection(config);
     try {
 
       String instanceIp = instance.getConnectionMetadata(timeoutMs).getPreferredIpAddress();
@@ -137,8 +137,8 @@ Socket connect(ConnectionConfig config, long timeoutMs) throws IOException {
     }
   }
 
-  BaseConnectionInfoCache getConnection(ConnectionConfig config) {
-    BaseConnectionInfoCache instance =
+  ConnectionInfoCache getConnection(ConnectionConfig config) {
+    ConnectionInfoCache instance =
         instances.computeIfAbsent(config, k -> createConnectionInfo(config));
 
     // If the client certificate has expired (as when the computer goes to
@@ -151,7 +151,7 @@ BaseConnectionInfoCache getConnection(ConnectionConfig config) {
     return instance;
   }
 
-  private BaseConnectionInfoCache createConnectionInfo(ConnectionConfig config) {
+  private ConnectionInfoCache createConnectionInfo(ConnectionConfig config) {
     logger.debug(
         String.format("[%s] Connection info added to cache.", config.getCloudSqlInstance()));
     return new BaseConnectionInfoCache(