Made PersistenceManager less error prone (and reduced amount of code)

server/src/main/java/envoy/server/data/PersistenceManager.java

@@ -28,15 +28,13 @@ public final class PersistenceManager {
 	 * @since Envoy Server Standalone v0.1-alpha
 	 */
 	private PersistenceManager() {
-		Runtime.getRuntime().addShutdownHook(new Thread(() -> {
-			transaction.begin();
+		Runtime.getRuntime().addShutdownHook(new Thread(() -> transaction(() -> {
 			ConnectionManager.getInstance()
 				.getOnlineUsers()
 				.stream()
 				.map(this::getUserByID)
 				.forEach(user -> { user.setStatus(UserStatus.OFFLINE); user.setLastSeen(Instant.now()); entityManager.merge(user); });
-			transaction.commit();
-		}));
+		})));
 	}
 
 	/**
@@ -239,10 +237,7 @@ public final class PersistenceManager {
 		c2.getContacts().add(c1);
 
 		// Synchronize changes with the database
-		transaction.begin();
-		entityManager.merge(c1);
-		entityManager.merge(c2);
-		transaction.commit();
+		transaction(() -> { entityManager.merge(c1); entityManager.merge(c2); });
 	}
 
 	/**
@@ -254,47 +249,33 @@ public final class PersistenceManager {
 		return entityManager.createNamedQuery(User.findContacts).setParameter("user", user).getResultList();
 	}
 
-	private void persist(Object obj) {
+	private void persist(Object obj) { transaction(() -> entityManager.persist(obj)); }
+
+	private void merge(Object obj) { transaction(() -> entityManager.merge(obj)); }
+
+	private void remove(Object obj) { transaction(() -> entityManager.remove(obj)); }
+
+	/**
+	 * Performs a transaction with the given Runnable, that should somewhere call
+	 * {@link EntityManager}.
+	 *
+	 * @param entityManagerRelatedAction the action that changes something in the
+	 *                                   database
+	 * @since Envoy Server v0.3-beta
+	 */
+	private void transaction(Runnable entityManagerRelatedAction) {
 		try {
 			transaction.begin();
-			entityManager.persist(obj);
+			entityManagerRelatedAction.run();
 			transaction.commit();
 
 			// Last transaction threw an error resulting in the transaction not being closed
 		} catch (final IllegalStateException e) {
 			if (transaction.isActive()) {
 				transaction.rollback();
-				persist(obj);
-			}
-		}
-	}
-
-	private void merge(Object obj) {
-		try {
-			transaction.begin();
-			entityManager.merge(obj);
-			transaction.commit();
-
-			// Last transaction threw an error resulting in the transaction not being closed
-		} catch (final IllegalStateException e) {
-			if (transaction.isActive()) {
-				transaction.rollback();
-				merge(obj);
-			}
-		}
-	}
-
-	private void remove(Object obj) {
-		try {
-			transaction.begin();
-			entityManager.remove(obj);
-			transaction.commit();
-
-			// Last transaction threw an error resulting in the transaction not being closed
-		} catch (final IllegalStateException e) {
-			if (transaction.isActive()) {
-				transaction.rollback();
-				remove(obj);
+				transaction.begin();
+				entityManagerRelatedAction.run();
+				transaction.commit();
 			}
 		}
 	}