Added functionality for non-autoincremented id fields.

src/main/java/org/omnifaces/persistence/model/BaseAutoIdEntity.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2018 OmniFaces.
+ *
+ * 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 org.omnifaces.persistence.model;
+
+import static javax.persistence.GenerationType.IDENTITY;
+
+import java.io.Serializable;
+
+import javax.persistence.GeneratedValue;
+import javax.persistence.Id;
+import javax.persistence.MappedSuperclass;
+
+import org.omnifaces.persistence.service.BaseEntityService;
+
+/**
+ * <p>
+ * Let all your entities that autogenerate their Ids extend from this. Then you can make use of {@link BaseEntityService}. This mapped superclass
+ * already automatically takes care of the <code>id</code> column.
+ * <p>
+ * There are two more mapped superclasses which may also be of interest.
+ * <ul>
+ * <li>{@link TimestampedEntity} - extends {@link BaseAutoIdEntity} with <code>created</code> and <code>lastModified</code>
+ * columns and automatically takes care of them.
+ * <li>{@link VersionedEntity} - extends {@link TimestampedEntity} with a <code>@Version</code> column.
+ * </ul>
+ *
+ * @param <I> The generic ID type, usually {@link Long}.
+ * @author Bauke Scholtz
+ */
+
+@MappedSuperclass
+public abstract class BaseAutoIdEntity<I extends Comparable<I> & Serializable> extends BaseEntity<I> {
+
+	private static final long serialVersionUID = 1L;
+
+	@Id @GeneratedValue(strategy = IDENTITY)
+	private I id;
+
+	@Override
+	public I getId() {
+		return id;
+	}
+
+	@Override
+	public void setId(I id) {
+		this.id = id;
+	}
+
+}

src/main/java/org/omnifaces/persistence/model/BaseEntity.java

@@ -1,31 +1,38 @@
+/*
+ * Copyright 2018 OmniFaces.
+ *
+ * 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 org.omnifaces.persistence.model;
 
-import static javax.persistence.GenerationType.IDENTITY;
-
 import java.io.Serializable;
 import java.util.Objects;
 
 import javax.persistence.EntityListeners;
-import javax.persistence.GeneratedValue;
-import javax.persistence.Id;
 import javax.persistence.MappedSuperclass;
 
 import org.omnifaces.persistence.listener.BaseEntityListener;
 import org.omnifaces.persistence.service.BaseEntityService;
 
 /**
  * <p>
- * Let all your entities extend from this. Then you can make use of {@link BaseEntityService}. This mapped superclass
- * already automatically takes care of the <code>id</code> column.
+ * Let all your entities that don't auto-generate ids extend from this. Then you can make use of {@link BaseEntityService}.
+ * Entity Id field and its accessors must be implemented in subclasses. It can also be used as a base class to provide for
+ * <code>@GeneratedValue(strategy = SEQUENCE)</code> (i.e. other than IDENTITY functionality).
  * <p>
- * There are two more mapped superclasses which may also be of interest.
- * <ul>
- * <li>{@link TimestampedEntity} - extends {@link BaseEntity} with <code>created</code> and <code>lastModified</code>
- * columns and automatically takes care of them.
- * <li>{@link VersionedEntity} - extends {@link TimestampedEntity} with a <code>@Version</code> column.
- * </ul>
+ * See {@link BaseAutoIdEntity} class for a base to extend the entities with Id to be set automatically.
  *
- * @param <I> The generic ID type, usually {@link Long}.
+ * @param <I> The generic ID type, usually {@link String}.
  * @author Bauke Scholtz
  */
 @MappedSuperclass
@@ -34,18 +41,11 @@ public abstract class BaseEntity<I extends Comparable<I> & Serializable> impleme
 
 	private static final long serialVersionUID = 1L;
 
-	@Id @GeneratedValue(strategy = IDENTITY)
-	private I id;
+        @Override
+	public abstract I getId();
 
-	@Override
-	public I getId() {
-		return id;
-	}
-
-	@Override
-	public void setId(I id) {
-		this.id = id;
-	}
+        @Override
+	public abstract void setId(I id);
 
 	/**
 	 * Hashes by default the classname and ID.

src/main/java/org/omnifaces/persistence/model/TimestampedEntity.java

@@ -20,7 +20,7 @@
  * @author Bauke Scholtz
  */
 @MappedSuperclass
-public abstract class TimestampedEntity<I extends Comparable<I> & Serializable> extends BaseEntity<I> implements Timestamped {
+public abstract class TimestampedEntity<I extends Comparable<I> & Serializable> extends BaseAutoIdEntity<I> implements Timestamped {
 
 	private static final long serialVersionUID = 1L;
 

src/main/java/org/omnifaces/persistence/service/BaseEntityService.java

@@ -30,6 +30,7 @@
 import static org.omnifaces.utils.stream.Streams.stream;
 
 import java.io.Serializable;
+import java.lang.reflect.Field;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.lang.reflect.TypeVariable;
@@ -61,6 +62,7 @@
 import javax.persistence.ElementCollection;
 import javax.persistence.EntityManager;
 import javax.persistence.EntityNotFoundException;
+import javax.persistence.GeneratedValue;
 import javax.persistence.ManyToOne;
 import javax.persistence.NamedQuery;
 import javax.persistence.OneToMany;
@@ -182,6 +184,7 @@ public abstract class BaseEntityService<I extends Comparable<I> & Serializable,
 
 	private final Class<I> identifierType;
 	private final Class<E> entityType;
+        private boolean shouldSetAutoId;
 
 	private Provider provider;
 	private Database database;
@@ -205,7 +208,24 @@ public BaseEntityService() {
 		Entry<Class<?>, Class<?>> typeMapping = TYPE_MAPPINGS.computeIfAbsent(getClass(), BaseEntityService::computeTypeMapping);
 		identifierType = (Class<I>) typeMapping.getKey();
 		entityType = (Class<E>) typeMapping.getValue();
-	}
+
+                shouldSetAutoId = autoIdPresent();
+	}
+
+        private boolean autoIdPresent() {
+            Class<?> tmp = entityType;
+            Field fld = null;
+            do {
+                try {
+                    Field f = tmp.getDeclaredField("id");
+                    fld = f;
+                    break;
+                } catch (NoSuchFieldException e) {
+                    tmp = tmp.getSuperclass();
+                }
+            } while (tmp != null);
+            return fld == null ? false : fld.isAnnotationPresent(GeneratedValue.class);
+        }
 
 	/**
 	 * The postconstruct initializes the element and one-to-many collections.
@@ -419,12 +439,14 @@ public List<E> getAll() {
 	 * Persist given entity. Any bean validation constraint violation will be logged separately.
 	 * @param entity Entity to persist.
 	 * @return Entity ID.
-	 * @throws IllegalEntityStateException When entity has an ID.
+	 * @throws IllegalEntityStateException When entity has an ID and it should be autogenerated.
 	 */
 	public I persist(E entity) {
-		if (entity.getId() != null) {
-			throw new IllegalEntityStateException(entity, "Entity has an ID. Use update() instead.");
-		}
+                if (shouldSetAutoId) {
+                        if (entity.getId() != null) {
+                                throw new IllegalEntityStateException(entity, "Entity has an ID. Use update() instead.");
+                        }
+                }
 
 		try {
 			getEntityManager().persist(entity);
@@ -445,12 +467,14 @@ public I persist(E entity) {
 	 * beyond the scope of this method.
 	 * @param entity Entity to update.
 	 * @return Updated entity.
-	 * @throws IllegalEntityStateException When entity has no ID.
+	 * @throws IllegalEntityStateException When entity has no ID and it should be autogenerated.
 	 */
 	public E update(E entity) {
-		if (entity.getId() == null) {
-			throw new IllegalEntityStateException(entity, "Entity has no ID. Use persist() instead.");
-		}
+                if (shouldSetAutoId) {
+                        if (entity.getId() == null) {
+                                throw new IllegalEntityStateException(entity, "Entity has no ID. Use persist() instead.");
+                        }
+                }
 
 		if (validator != null) {
 			// EntityManager#merge() doesn't directly throw ConstraintViolationException without performing flush, so we can't put it in a