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Camel JPA support
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/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.camel.component.jpa;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;

import org.apache.camel.Exchange;
import org.springframework.orm.jpa.SharedEntityManagerCreator;

/**
 * Helper for JPA.
 */
public final class JpaHelper {

    private JpaHelper() {
    }

    /**
     * Gets or creates an {@link javax.persistence.EntityManager} to use.
     *
     * @param exchange                 the current exchange, or null if no exchange
     * @param entityManagerFactory     the entity manager factory (mandatory)
     * @param usePassedInEntityManager whether to use an existing {@link javax.persistence.EntityManager} which has been stored
     *                                 on the exchange in the header with key {@link org.apache.camel.component.jpa.JpaConstants#ENTITY_MANAGER}
     * @param useSharedEntityManager   whether to use SharedEntityManagerCreator if not already passed in                             
     * @return the entity manager (is never null)
     */
    public static EntityManager getTargetEntityManager(Exchange exchange, EntityManagerFactory entityManagerFactory,
                                                       boolean usePassedInEntityManager, boolean useSharedEntityManager, boolean allowRecreate) {
        EntityManager em = null;

        // favor using entity manager provided as a header from the end user
        if (exchange != null && usePassedInEntityManager) {
            em = exchange.getIn().getHeader(JpaConstants.ENTITY_MANAGER, EntityManager.class);
        }

        // then try reuse any entity manager which has been previously created and stored on the exchange
        if (em == null && exchange != null) {
            em = exchange.getProperty(JpaConstants.ENTITY_MANAGER, EntityManager.class);
        }

        if (em == null && useSharedEntityManager) {
            em = SharedEntityManagerCreator.createSharedEntityManager(entityManagerFactory);
        }
        
        if (em == null) {
            // create a new entity manager
            em = entityManagerFactory.createEntityManager();
            if (exchange != null) {
                // we want to reuse the EM so store as property and make sure we close it when done with the exchange
                exchange.setProperty(JpaConstants.ENTITY_MANAGER, em);
                exchange.addOnCompletion(new JpaCloseEntityManagerOnCompletion(em));
            }
        }

        if (allowRecreate && em == null || !em.isOpen()) {
            // create a new entity manager
            em = entityManagerFactory.createEntityManager();
            if (exchange != null) {
                // we want to reuse the EM so store as property and make sure we close it when done with the exchange
                exchange.setProperty(JpaConstants.ENTITY_MANAGER, em);
                exchange.addOnCompletion(new JpaCloseEntityManagerOnCompletion(em));
            }
        }

        return em;
    }

}