Introduce initialize() in AbstractRouting[DataSource|ConnectionFactory]

This commit introduces initialize() methods in
AbstractRoutingDataSource and AbstractRoutingConnectionFactory as an
alternative to invoking afterPropertiesSet().

Closes gh-31248

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSource.java

@@ -116,6 +116,15 @@ public abstract class AbstractRoutingDataSource extends AbstractDataSource imple
 
 	@Override
 	public void afterPropertiesSet() {
+		initialize();
+	}
+
+	/**
+	 * Synchronizes targetDataSources to resolvedDataSources
+	 * and defaultTargetDataSource to resolvedDefaultDataSource.
+	 * @throws IllegalArgumentException in case of targetDataSources is null
+	 */
+	public void initialize() {
 		if (this.targetDataSources == null) {
 			throw new IllegalArgumentException("Property 'targetDataSources' is required");
 		}

spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSourceTests.java

@@ -146,6 +146,31 @@ class AbstractRoutingDataSourceTests {
 		assertThat(routingDataSource.determineTargetDataSource()).isSameAs(ds);
 	}
 
+	@Test
+	void testInitialize_synchronizeTargetDataSourcesToResolvedDataSources() {
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return null;
+			}
+		};
+
+		DataSource ds1 = new StubDataSource();
+		DataSource ds2 = new StubDataSource();
+
+		Map<Object, Object> targetDataSources = new HashMap<>();
+		targetDataSources.put("ds1", ds1);
+		targetDataSources.put("ds2", ds2);
+		routingDataSource.setTargetDataSources(targetDataSources);
+
+		routingDataSource.initialize();
+
+		Map<Object, DataSource> resolvedDataSources = routingDataSource.getResolvedDataSources();
+		assertThat(resolvedDataSources).hasSize(2);
+		assertThat(resolvedDataSources.get("ds1")).isSameAs(ds1);
+		assertThat(resolvedDataSources.get("ds2")).isSameAs(ds2);
+	}
+
 	@Test
 	public void notInitialized() {
 		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {