Reconnect JPA (Hibernate) EntityManager to JDBC connection

package com.foo.bar;

import org.hibernate.jpa.internal.EntityManagerFactoryAdapter;

import javax.annotation.Nonnull;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.sql.Connection;
import java.util.HashMap;
import java.util.Map;

/**
 * <p>
 *      Holds one {@link EntityManager} for every Thread to allow lazy-loading.
 * </p>
 * <p>
 *      This is an implementation of the <em>persistence context per conversation</em> strategy. See
 *      "Java Persistence with Hibernate, Second Edition" (ISBN 9781617290459), chapter 11 for details.
 *
 * </p>
 * @see <a href="http://stackoverflow.com/questions/24060782/jpa-2-hibernate-persistence-context-per-conversation-closed-connection">JPA 2 (Hibernate) persistence context per conversation - closed connection</a>
 * @see <a href="http://www.manning.com/bauer3/">Java Persistence with Hibernate, Second Edition</a>
 */
public class EntityManagerHolder {
    private static final ThreadLocal<EntityManager> entityManager = new ThreadLocal<EntityManager>();
    private static EntityManagerFactoryAdapter entityManagerFactoryAdapter;

    /**
     * @return The {@link EntityManager} linked to this thread
     */
    public static EntityManager getEntityManager() {
        return entityManager.get();
    }

    /**
     * If the invoking thread does not have an associated {@link EntityManager} yet,
     * creates a new one and associates it with the thread. Otherwise, reconnects
     * the existing EntityManager to a JDBC connection (if the connection has been closed).
     *
     * @param connection An active JDBC connection to use for the EntityManager
     */
    public static synchronized void initializeEntityManager(@Nonnull Connection connection) {
        EntityManager currentEntityManager = entityManager.get();

        if (currentEntityManager == null || !currentEntityManager.isOpen()) {

            if (entityManagerFactoryAdapter == null) {
                // First invocation ever; create a factory to produce EntityManagers from a JDBC connection
                EntityManagerFactory entityManagerFactory =
                        Persistence.createEntityManagerFactory("myPersistenceUnit");
                entityManagerFactoryAdapter = new EntityManagerFactoryAdapter(entityManagerFactory);
            }

            // Start the conversation by creating the EntityManager for this thread
            EntityManager newEntityManager = entityManagerFactoryAdapter.createEntityManager(connection);
            entityManager.set(newEntityManager);
        } else {
            // Continuing conversation; ensure the connection of the EntityManager is alive
            entityManagerFactoryAdapter.reconnect(currentEntityManager, connection);
        }
    }

    /**
     * Conversation has been finished; close the EntityManager linked to this thread.
     */
    public static synchronized void closeEntityManager() {
        if (entityManager.get() != null && entityManager.get().isOpen()) {
            entityManager.get().close();
        }
    }
}