Use Hibernate Annotation on databean: Cannot add constraint to MySQL table - mysql

Question

I try to add constraints to MySQL table via databeans using hibernate annotation:
package databean;
import javax.persistence.*;
#Entity
#Table(name = "ADMIN",
uniqueConstraints = {
#UniqueConstraint(columnNames = "userName"),
#UniqueConstraint(columnNames = "email")
})
public class AdminBean {
// Private fields
#Id #GeneratedValue
#Column(name = "adminId")
private int adminId; // PK
#Column(name = "userName")
private String userName;
I also have 2 xml files for mapping: hibernate.cfg.xml and Admin.hbm.xml
1）
mapping resource="hibernate/Admin.hbm.xml"/
2）
<hibernate-mapping>
<class name="databean.AdminBean" table="ADMIN">
<meta attribute="class-description">
This class contains the admin detail.
</meta>
<id name="adminId" type="int" column="adminId">
<generator class="native"/>
</id>
<property name="userName" column="userName" type="string"/>
<property name="firstName" column="firstName" type="string"/>
<property name="lastName" column="lastName" type="string"/>
<property name="email" column="email" type="string"/>
<property name="phone" column="phone" type="string"/>
<property name="password" column="password" type="string"/>
<property name="city" column="city" type="string"/>
<property name="zipCode" column="zipCode" type="string"/>
</class>
</hibernate-mapping>
According to what I've found online:
"So far you have seen how Hibernate uses XML mapping file for the transformation of data from POJO to database tables and vice versa. Hibernate annotations is the newest way to define mappings without a use of XML file. You can use annotations in addition to or as a replacement of XML mapping metadata. Hibernate Annotations is the powerful way to provide the metadata for the Object and Relational Table mapping. All the metadata is clubbed into the POJO java file along with the code this helps the user to understand the table structure and POJO simultaneously during the development." (http://www.tutorialspoint.com/hibernate/hibernate_annotations.htm)
If I comment out the xml files mapping, I will get the exception of "Unknown entity: databean.AdminBean". Cannot even find the databean now.
Question:
1) Shouldn't the databean with hibernate annotations work even if there is no mapping in xml file? (So it does not even work now...)
2) If there is no hibernate annotation with databean, should I add constraints directly into the xml mapping file?
3) What is the best way to add constraints to MySQL tables? Since they are all automatically created when I run the application, it doesn't seem to make much sense to add constraints by building tables manually in mySQL first?
4) Is it really necessary to add constraints to both databean and (maybe) xml files?
Update:
Even when I try to set the property in xml file, mySQL database seems not to be able to "catch the constraints" and tables can still add duplicates items.
Now I doubt it might not be a problem with the way I add those constraints, but more like a "disconnection" with mySQL table and my code...though other parts can function pretty well. Anyone has faced similar situations before?

Answer 1

I find the reason: I did not drop the previous table when there is no constraint added when it is first created automatically by my app. Hope no one makes the same mistake. :)