#558: Only close connection pool from batch-module, if batch-module h…

…as been included in the first place

modules/batch-module/src/main/java/org/simplejavamail/internal/batchsupport/BatchSupport.java

@@ -131,11 +131,11 @@ private void checkConfigureOAuth2Token(Session session) {
 	 */
 	@NotNull
 	@Override
-	public Future<?> shutdownConnectionPools(@NotNull Session session) {
+	public Future<Void> shutdownConnectionPools(@NotNull Session session) {
 		if (smtpConnectionPool == null) {
 			LOGGER.warn("user requested connection pool shutdown, but there is no connection pool to shut down (yet)");
 			return completedFuture(null);
 		}
 		return smtpConnectionPool.shutdownPool(session);
 	}
-}
+}

modules/core-module/src/main/java/org/simplejavamail/api/mailer/Mailer.java

@@ -120,7 +120,7 @@ public interface Mailer extends AutoCloseable {
 	 * <p>
 	 * <strong>Note:</strong> this is only works in combination with the {@value org.simplejavamail.internal.modules.BatchModule#NAME}.
 	 */
-	Future<?> shutdownConnectionPool();
+	Future<Void> shutdownConnectionPool();
 
 	/**
 	 * @return The server connection details. Will be {@code null} in case a custom fixed {@link Session} instance is used.

modules/core-module/src/main/java/org/simplejavamail/internal/modules/BatchModule.java

@@ -57,5 +57,5 @@ public interface BatchModule {
 	 * Shuts down connection pool(s) and closes remaining open connections. Waits until all connections still in use become available again to deallocate them as well.
 	 */
 	@NotNull
-	Future<?> shutdownConnectionPools(@NotNull Session session);
-}
+	Future<Void> shutdownConnectionPools(@NotNull Session session);
+}

modules/simple-java-mail/src/main/java/org/simplejavamail/mailer/internal/MailerImpl.java

@@ -373,11 +373,14 @@ public boolean validate(@NotNull final Email email)
 	 * @see Mailer#shutdownConnectionPool()
 	 */
 	@Override
-	public Future<?> shutdownConnectionPool() {
+	public Future<Void> shutdownConnectionPool() {
 		if (!operationalConfig.isExecutorServiceIsUserProvided()) {
 			operationalConfig.getExecutorService().shutdown();
 		}
-		return ModuleLoader.loadBatchModule().shutdownConnectionPools(session);
+		if (ModuleLoader.batchModuleAvailable()) {
+			return ModuleLoader.loadBatchModule().shutdownConnectionPools(session);
+		}
+		return CompletableFuture.completedFuture(null);
 	}
 
 	@Override
@@ -458,4 +461,4 @@ public EmailGovernance getEmailGovernance() {
 	public void close() throws ExecutionException, InterruptedException {
 		shutdownConnectionPool().get();
 	}
-}
+}