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Hibernate @Entity – Mapping Your Java Beans

Java-PodcastNow that you’ve learned how to set up your first Hibernate Java project it’s time to put that fresh new configuration to work!

What You’ll Learn

The focus of this podcast / blog post is to teach you how to create the connection between your Java objects and the SQL Database tables. Remember, the whole point of the Hibernate framework is for you to be able to write Java code that allows you to communicate with your database.

When I say that the goal is to communicate with your database, what I mean is:

  1. Create the tables (if they’re not already there) that will represent your Java objects
  2. Give Java the ability to send queries and execute scripts on your database
  3. Establish a transactional means by which to perform CRUD operations

What we’re going to be tackling in this post will the step #1, how to get Hibernate to create our tables for us.

How to get Hibernate to Create a Table

So the key to getting Hibernate to create a table for us is to understand how Hibernate makes the decision to create a table. It works by scanning a particular package that you’ve specified in your configuration file (via sessionFactory.setPackagesToScan(). Hibernate will scan that package for any Java objects annotated with the @Entity annotation. If it finds any, then it will begin the process of looking through that particular Java object to recreate it as a table in your database!

Before Hibernate will start it’s scan, not only do we need to invoke the sessionFactory.setPackagesToScan code, but we also need to enable the scanning process that the Spring framework handles. We’ll need to make a quick tweak in our applicationConfig.xml file. Take a look at mine here:

  
    
    
         

  

The magic line of code above is the <context:component-scan base-package="com.howtoprogramwithjava.example"> line. This is what Spring uses to know where to scan for annotations related to both the Spring framework and the Hibernate framework.

Unfortunately that’s not the only step (if only it were that easy!). We still need to specify a couple of things:

  1. The primary key for the table
  2. How to generate the primary keys

For both items above, we’ll use annotations.

The first annotation (@Id) is simple and it’s used to “mark” the primary key for the table, the second annotation is @GeneratedValue and it will be used to specify HOW to generate primary keys. Let’s take a look at an actual example of all these annotations in action:

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Table(name="address_book")
@Entity
public class AddressBook
{
  private Long id;
  private String name;
  private String phoneNumber;
  private String streetAddress;
  private String zipCode;
  private String city;
  private String region;
  private String country;
  
  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
	public Long getId()
	{
		return id;
	}
	public void setId(Long id)
	{
		this.id = id;
	}
	public String getName()
	{
		return name;
	}
	public void setName(String name)
	{
		this.name = name;
	}
	
	@Column(name="phone_number")
	public String getPhoneNumber()
	{
		return phoneNumber;
	}
	public void setPhoneNumber(String phoneNumber)
	{
		this.phoneNumber = phoneNumber;
	}
	
	@Column(name="street_address")
	public String getStreetAddress()
	{
		return streetAddress;
	}
	public void setStreetAddress(String streetAddress)
	{
		this.streetAddress = streetAddress;
	}
	
	@Column(name="zip_code")
	public String getZipCode()
	{
		return zipCode;
	}
	public void setZipCode(String zipCode)
	{
		this.zipCode = zipCode;
	}
	public String getCity()
	{
		return city;
	}
	public void setCity(String city)
	{
		this.city = city;
	}
	public String getRegion()
	{
		return region;
	}
	public void setRegion(String region)
	{
		this.region = region;
	}
	public String getCountry()
	{
		return country;
	}
	public void setCountry(String country)
	{
		this.country = country;
	}
}

As you can see above, the @Entity annotation is used at the class level, but the @Id and @GeneratedValue annotations are used at the method level.

It’s important to note the exact location of the @Id and @GeneratedValue annotations: just above the getter method for the id instance variable. If you were to put these annotations on the setter method, it won’t work as Hibernate only looks at the getter methods for these annotations.

Final Touches

The last thing I should mention is the naming convention for tables and columns with Hibernate. By default Hibernate doesn’t know how to insert underscores (_) into the names of tables and columns. So we’ll need to help Hibernate out by telling it how we want to name our tables and columns.

When we have any tables or columns names that are more than one word in length, we’ll need to use one of two annotations:

  1. @Table(name="")
  2. @Column(name="")

For table names, just use the @Table annotation and specify the actual name that you’d like to use for the table inside of it. This annotation should only be used at the class level.

For column names, just use the @Column annotation and specify the actual column name that you’d like to use inside of it. This annotation should only be used at method level (again, just above the getter method).

For those of you who want to see this entire process in action, just check out the video below which will take you step by step through everything you’ve just learned:

{ 9 comments… add one }
  • Andrew April 1, 2015, 8:04 pm

    Hi, Trevor, I like your blog, it’s awesome, I find it very helpful to me. Can I ask you a question? What is the main difference between field and property @Column annotation ?

  • Lars October 15, 2015, 5:13 am

    Hey.
    Great tutorials! Easily explained, short and concise, I love it! Well written.
    Where do i sign up?
    And also, do you recommend making database with annotations(code first) first as opposed to making the database first?
    Regards
    Lars

    • Trevor Page October 27, 2015, 8:50 am

      You’ll need to actually create the empty database first. By that I mean you just need to run the “create database xxxx” command and that’s it.

      You’d then write the code in Java / Hibernate to create the Entity objects, and Hibernate will then in turn create all the tables and relationships on your behalf, so you’ll never really need to write any SQL statements (DDL statements to be exact)

  • Hasan Habib July 20, 2016, 9:33 am

    Very nice, helpful and clear concept idea tutorial on hibernate annotation utilization. thanks for your tutorial.

  • Anonymous December 16, 2016, 5:35 am

    very nice ! you are doing great job
    really useful i don’t known anything before .
    thank you

  • Vusala January 16, 2017, 7:43 am

    Really helpfull.Thanks!

  • dhanraj August 23, 2017, 2:21 am

    Nyc…

  • Subelal Pal November 1, 2017, 11:58 am

    It is very helpful for me

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