org.hibernate.AnnotationException: No identifier specified for entity

When you are writing a piece of code from scratch a lot of time you do silly mistakes and still wonder why its not working. Well same thing happened the other day when I added a Hibernate Entity class in one project and was struggling to make it work.
The exception was:

org.hibernate.AnnotationException: No identifier specified for entity: net.viralpatel.hibernate.Employee
 at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:650)
 at org.hibernate.cfg.AnnotationConfiguration.processArtifactsOfType(AnnotationConfiguration.java:498)
 at org.hibernate.cfg.AnnotationConfiguration.secondPassCompile(AnnotationConfiguration.java:277)
 at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1112)
 at org.hibernate.ejb.Ejb3Configuration.buildMappings(Ejb3Configuration.java:1269)
 at org.hibernate.ejb.EventListenerConfigurator.configure(EventListenerConfigurator.java:150)
 at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:888)
 at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:416)
 at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:126)
 at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:227)
 at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:273)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1367)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1333)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:471)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory$1.run(AbstractAutowireCapableBeanFactory.java:409)
 at java.security.AccessController.doPrivileged(Native Method)
Code language: Java (java)

The error here is that in your Entity class, you have not defined a primary key. Thus specify either @Id annotation or an @EmbeddedId annotation.
So if you have an entity class Employee like below:

package net.viralpatel.hibernate;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name="EMPLOYEE")
public class Employee {
    @Column(name="employee_id")
    private Long employeeId;
    @Column(name="firstname")
    private String firstname;
    @Column(name="lastname")
    private String lastname;
    //Getter and Setter methods
}Code language: Java (java)

And if you try to execute this, it will generate exception org.hibernate.AnnotationException: No identifier specified for entity: net.viralpatel.hibernate.Employee
So the solution is just add @Id to appropriate primary key column.

package net.viralpatel.hibernate;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
import javax.persistence.Id;
@Entity
@Table(name="EMPLOYEE")
public class Employee {
    @Id
    @Column(name="employee_id")
    private Long employeeId;
    @Column(name="firstname")
    private String firstname;
    @Column(name="lastname")
    private String lastname;
    //Getter and Setter methods
}Code language: Java (java)

Thus every class defined as Entity with @Entity annotation, needs an @Id or @EmbeddedId property.

Hope that helps and reduce your debugging effort.