Hibernate Primary Key Generator Annotation

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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 are the newest way to define mappings without the 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.

If you going to make your application portable to other EJB 3 compliant ORM applications, you must use annotations to represent the mapping information, but still if you want greater flexibility, then you should go with XML-based mappings.

  • If you are using Hibernate as JPA Provider then avoid using the IDENTITY Strategy. Hibernate requires a primary key value for each managed entity and therefore has to perform the insert statement immediately. The GenerationType.SEQUENCE is the most preferred way to generate primary key values. This uses the underlying.
  • In Part1 of your question, the code snippet indicates that a generator named increment will be used to obtain the primary key value. Increment is then defined in the next annotation @GenericGenerator. @GenericGenerator is a hibernate annotation used to denote a custom generator, which can be a class or shortcut to a generator supplied by Hibernate.

Environment Setup for Hibernate Annotation

Primary Key Generation Using Table Generator. At this example, you would be seeing a database table for generating a primary keys. Anatomy of @TableGenerator. The @TableGenerator annotation defines a primary key generator that may be referenced by name when a generator element is specified for the @GeneratedValue. I am trying to create a Privilege class with Annotations whose Primary Key is a String. I will assign them manually while inserting. Therefore no need for hibernate to generate a value for it. I'm trying to do something like that: @Id @GeneratedValue(generator = 'assigned') @Column(name = 'ROLENAME', nullable = false) private String roleName. The JPA specification supports 4 different primary key generation strategies that generate the primary key values programmatically or use database features, like auto-incremented columns or sequences. The only thing you have to do is to add the @GeneratedValue annotation to your primary key attribute and choose a generation strategy.

JPA and Hibernate support different strategies to generate primary key values. One of them is the identity strategy which uses an auto-incremented database column. If you want to use this strategy, you have to annotate the primary key attribute @Id and with the @GeneratedValue annotation and set the strategy attribute to GenerationType.IDENTITY. The GeneratedValue annotation may be applied to a primary key property or field of an entity or mapped superclass in conjunction with the Id annotation. The use of the GeneratedValue annotation is only required to be supported for simple primary keys. Use of the GeneratedValue annotation is not supported for derived primary keys.

First of all you would have to make sure that you are using JDK 5.0 otherwise you need to upgrade your JDK to JDK 5.0 to take advantage of the native support for annotations.

Second, you will need to install the Hibernate 3.x annotations distribution package, available from the sourceforge: (Download Hibernate Annotation) and copy hibernate-annotations.jar, lib/hibernate-comons-annotations.jar and lib/ejb3-persistence.jar from the Hibernate Annotations distribution to your CLASSPATH.

Annotated Class Example

As I mentioned above while working with Hibernate Annotation, 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.

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Consider we are going to use the following EMPLOYEE table to store our objects −

Following is the mapping of Employee class with annotations to map objects with the defined EMPLOYEE table −

Hibernate detects that the @Id annotation is on a field and assumes that it should access properties of an object directly through fields at runtime. If you placed the @Id annotation on the getId() method, you would enable access to properties through getter and setter methods by default. Hence, all other annotations are also placed on either fields or getter methods, following the selected strategy.

Following section will explain the annotations used in the above class.

@Entity Annotation

The EJB 3 standard annotations are contained in the javax.persistence package, so we import this package as the first step. Second, we used the @Entity annotation to the Employee class, which marks this class as an entity bean, so it must have a no-argument constructor that is visible with at least protected scope.

@Table Annotation

The @Table annotation allows you to specify the details of the table that will be used to persist the entity in the database.

The @Table annotation provides four attributes, allowing you to override the name of the table, its catalogue, and its schema, and enforce unique constraints on columns in the table. For now, we are using just table name, which is EMPLOYEE.

@Id and @GeneratedValue Annotations

Each entity bean will have a primary key, which you annotate on the class with the @Id annotation. The primary key can be a single field or a combination of multiple fields depending on your table structure.

By default, the @Id annotation will automatically determine the most appropriate primary key generation strategy to be used but you can override this by applying the @GeneratedValue annotation, which takes two parameters strategy and generator that I'm not going to discuss here, so let us use only the default key generation strategy. Letting Hibernate determine which generator type to use makes your code portable between different databases.

@Column Annotation

The @Column annotation is used to specify the details of the column to which a field or property will be mapped. You can use column annotation with the following most commonly used attributes −

  • name attribute permits the name of the column to be explicitly specified.

  • length attribute permits the size of the column used to map a value particularly for a String value.

  • nullable attribute permits the column to be marked NOT NULL when the schema is generated.

  • unique attribute permits the column to be marked as containing only unique values.

Create Application Class

Hibernate Annotations One To Many

Finally, we will create our application class with the main() method to run the application. We will use this application to save few Employee's records and then we will apply CRUD operations on those records.

Database Configuration

Now let us create hibernate.cfg.xml configuration file to define database related parameters.

Compilation and Execution

Here are the steps to compile and run the above mentioned application. Make sure, you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.

Pengertian Primary Key

  • Win 7 product key generator. Delete Employee.hbm.xml mapping file from the path.

  • Create Employee.java source file as shown above and compile it.

  • Create ManageEmployee.java source file as shown above and compile it.

  • Execute ManageEmployee binary to run the program.

You would get the following result, and records would be created in EMPLOYEE table.

Hibernate Annotations Tutorial

If you check your EMPLOYEE table, it should have the following records −