Created some classes and a generic get method

src/io/github/norbipeti/chat/server/db/DataProvider.java

@@ -29,19 +29,23 @@ public class DataProvider implements AutoCloseable {
 	}
 
 	public List<User> getUsers() {
-		EntityManager em = emf.createEntityManager();
-		TypedQuery<User> query = em.createQuery("SELECT u FROM User u", User.class);
-		List<User> users = query.getResultList();
-		em.close();
-		return users;
+		return get(User.class);
 	}
 
-	public User getUser(Long id) {
+	public List<Message> getMessages() {
+		return get(Message.class);
+	}
+
+	public List<Conversation> getConversations() {
+		return get(Conversation.class);
+	}
+
+	private <T> List<T> get(Class<T> cl) {
 		EntityManager em = emf.createEntityManager();
-		TypedQuery<User> query = em.createQuery("SELECT u FROM User u WHERE u.id = " + id, User.class);
-		User user = query.getResultList().get(0);
+		TypedQuery<T> query = em.createQuery("SELECT x FROM " + cl.getSimpleName() + " x", cl);
+		List<T> results = query.getResultList();
 		em.close();
-		return user;
+		return results;
 	}
 
 	public void removeUser(User user) {
@@ -53,17 +57,6 @@ public class DataProvider implements AutoCloseable {
 		em.close();
 	}
 
-	public EntityManager startTransaction() {
-		EntityManager em = emf.createEntityManager();
-		em.getTransaction().begin();
-		return em;
-	}
-
-	public void endTransaction(EntityManager em) {
-		em.getTransaction().commit();
-		em.close();
-	}
-
 	@Override
 	public void close() {
 		if (emf != null)

src/io/github/norbipeti/chat/server/db/domain/Conversation.java

@@ -0,0 +1,27 @@
+package io.github.norbipeti.chat.server.db.domain;
+
+import java.util.List;
+
+import javax.persistence.Entity;
+
+@Entity
+public class Conversation {
+	private List<Message> messsages;
+	private List<User> users;
+
+	public List<Message> getMesssages() {
+		return messsages;
+	}
+
+	public void setMesssages(List<Message> messsages) {
+		this.messsages = messsages;
+	}
+
+	public List<User> getUsers() {
+		return users;
+	}
+
+	public void setUsers(List<User> users) {
+		this.users = users;
+	}
+}

src/io/github/norbipeti/chat/server/db/domain/Message.java

@@ -0,0 +1,45 @@
+package io.github.norbipeti.chat.server.db.domain;
+
+import java.util.Date;
+
+import javax.persistence.Entity;
+
+@Entity
+public class Message {
+	private User sender;
+	private Date time;
+	private String message;
+	private Conversation conversation;
+
+	public User getSender() {
+		return sender;
+	}
+
+	public void setSender(User sender) {
+		this.sender = sender;
+	}
+
+	public Date getTime() {
+		return time;
+	}
+
+	public void setTime(Date time) {
+		this.time = time;
+	}
+
+	public String getMessage() {
+		return message;
+	}
+
+	public void setMessage(String message) {
+		this.message = message;
+	}
+
+	public Conversation getConversation() {
+		return conversation;
+	}
+
+	public void setConversation(Conversation conversation) {
+		this.conversation = conversation;
+	}
+}