Every entity in Hibernate has to have a field which MAPS to the primary key of a table in a database. Such a field is identified by the @Id annotation. Such a field is also known as the identifier field. Identifiers are of two types: Simple Identifiers and Generated Identifiers. 

Simple Identifiers: This is the most straightforward way of mapping. The mapping is done by using the @Id annotation. Simple identifiers map to a single property of the below types: 

• Any Java primitive type 
• Any primitive WRAPPER class type 
• java.lang.String 
• java.util.Date 
• java.sql.Date 
• java.math.BigDecimal 
• java.math.BigInteger 

Let us look at a small example: 

Generated Identifiers: When the primary key values have to be generated automatically, @GeneratedValue annotation is added. Generated identifiers are of 4 types: 

• Auto: This is the default generation type. Values are DETERMINED based on the types of the primary key value. 
• Identity: This type is dependent on the IdentityGenerator which expects auto-incremented or auto generated identity column values from the database 
• Sequence: A class named SequenceStyleGenerator is PROVIDED by Hibernate to work with sequence – based id. This generator USES sequences if they are supported by the database. 
• Table: This type of generator generates values by using the underlying database table.