Hibernate Introduction Tutorial

Table of Contents Introduction Suggested Audience System Requirements Introduction to Hibernate Getting Started Reverse Engineering Writing and Running Hibernate Code Conclusion FAQ Resources Feedback

1. Introduction Welcome to the MyEclipse Introduction to Hibernate tutorial. In this tutorial we are going to cover some of the basic features of using the Hibernate framework, such as OR-mapping from within MyEclipse. At it's core, Hibernate is an OR-mapping technology that is used to map database structures to Java objects at runtime. Using a persistence framework like Hibernate allows developers to focus on writing business logic instead of writing an accurate and performant persistence layer (which includes, DAOs, SQL queries, JDBC code, connection management, etc.). However, to begin using Hibernate, you normally must generate the proper persistence mappings for Hibernate to manage, set up the database connection information and code the DAOs that read or write your mapped entities. Fortunately, MyEclipse developers don't need to do any of that because MyEclipse will generate all of this code for you. And that bit of code automation frees you up for the most important task of all: writing your application logic.

2. Suggested Audience This tutorial is intended for developers who are somewhat familiar with either MyEclipse or Eclipse so you are expected to recognize navigation within the IDE and understand some of the more common concepts like "Views". Additionally, developers should be familiar with persistence in Java (JDBC, EJB, iBatis, JPA, etc.) to be able to understand the role Hibernate plays more quickly. While this tutorial will also introduce some of the basics of Hibernate, it is not intended to replace the detailed Hibernate Reference Guide found on the Hibernate site and in the Resources section below. There is a lot of functionality in Hibernate that allows it to scale to support enterprise-class applications, and those details can be learned from reading through the reference or a good Hibernate book. If either MyEclipse or Hibernate makes you feel uncomfortable, this introductory tutorial should provide you with the basics of both. If you wish to learn more about either MyEclipse or Hibernate please have a look at either our product Documentation for more material or our Resources section respectively.

3. System Requirements This tutorial was created with MyEclipse 5.1 and the bundled Hibernate 3.1 libraries. If you are using a another version of MyEclipse or Hibernate, most of these screens and instructions should still be very similar. If you are using a newer version of MyEclipse and notice portions of this tutorial looking different than the screens you are seeing, please let us know and we will make sure to resolve any inconsistencies.

4. Introduction to Hibernate In the early years of Java database and web programming, developers accessed their databases using the different classes provided by the java.sql package; you may even remember doing this yourself. Such usage consisted basically of getting a Driver from the DriverManager, creating a Connection, using the Connection, handling exceptions properly, closing the connection and so on. A common problem at this stage was forgetting to clean up database connections and getting connection exceptions in your applications after they had been running for a while. A few years later "connection pools" became a big topic since they allowed the developer to stop worrying about creating and managing (cleaning up) DB connections and instead focusing on their SQL and ResultSet parsing code. Suddenly, we had mostly solved the problem of connection exceptions to the database in long-running applications. However, it was still common to see hundreds of lines of boiler-plate code used to populate queries with values and parse the ResultSets returned from SQL queries. A few more years went by and someone had an idea to automatically map ResultSet results directly to Java objects, which mostly solved the problem of all the redundant boiler-plate parsing code. At that point in time Java database development had taken a several big steps forward and was getting much simpler. Then Hibernate came into the picture. With Hibernate came the idea that not only would it continuing doing all these automated things for you, but it would also manage the state of your objects in memory and it would worry about when and how object values would be "read from" or "written to" the database. Now all the sudden developers were dealing completely with objects (or mapped objects) and letting Hibernate handle everything else. Developers were no longer writing JDBC and SQL code at all. Instead they were using code that automatically did all that work for them. At the time Hibernate came on the scene, the other persistence technology out there was EJB 2.x. Hibernate's timing, ease of use and power all contributed to one of the fastest uptakes of a technology that the Java space that had been seen in quite awhile. In this tutorial we are going to take a look at how using MyEclipse with Hibernate can make your life even easier than using Hibernate alone. In fact, MyEclipse removes the need to write any of the Hibernate mappings or configuration files by completely generating the persistence side of your Java application in just a few seconds. We will start off with a simple database that we will reverse-engineer into a Hibernate-enabled project. Then, we will write some simple Java code to utilize the Hibernate code that MyEclipse generated automatically in order to store, retrieve and update information in our database.

5. Getting Started The project we create during this tutorial, as well as the create-table SQL script for the database table we used, can be found in our Resources section below for those of you that want to peak ahead. For the rest, we would highly encourage you to follow along with the tutorial, creating the project as we go. To get started with Hibernate in MyEclipse the first thing we need is a connection to the database that we want to build our application to use. In this particular case, it is an instance of MySQL 5 with a sample user table we've already created. We are also using the MySQL Connector/J JDBC driver to connect to our install of MySQL. So let's get started by creating a new connection, in MyEclipse, to our database: Figure 1. Creating a connection to our database Now that we have a working connection to our database, the second thing we need before we get started is a Hibernate-enabled project (Java, Web, Web Service, etc). We can create such a project by creating any of the supported types of base projects, like a Java or Web project, then adding Hibernate capabilities to that project from the MyEclipse menu, like so: Figure 2. Creating a Hibernate-enabled project

6. Reverse Engineering Now that we have a database connection and a project properly configured, the next thing for us to do is tell MyEclipse to reverse-engineer our database table into Hibernate (Java) objects and put them into our project. In the example below we use the simplest form of reverse-engineering, letting the wizard take all the default values. However, for maximum control you could optionally use the Next button and step through the wizard to select details like primary key generation strategy, object names, types and more. Let's reverse-engineer our table now: Figure 3. Reverse-engineering our user table into Hibernate (Java) objects Now that our table has been reverse-engineered into our project, there are all sorts of Hibernate tools we can use in MyEclipse to work with those objects (even without writing code!). The first tool we will look at is the HQL Editor. The HQL Editor, and other HQL views in the Hibernate perspective, assist in the development, evaluation or testing of HQL queries. HQL is a SQL-like language called "Hibernate Query Language". It can sometimes look like simplified SQL and uses object names and references instead of table and column names. A great place to learn about HQL is the Hibernate Reference Document in our Resources section. With the HQL Editor you can actually write HQL on the fly into the editor, then run it. The editor, utilizing the objects that MyEclipse has reverse-engineered from the database, will actually translate the query to SQL (shown in the bottom right) and then run it. The result is returned in Java objects and is shown in the bottom left corner. Let's have a look at how this works now: Figure 4. Using the HQL Editor and HQL Views

7. Writing and Running Hibernate Code It is important to be familiar with the tools that MyEclipse provides, but since we've just taken a look at some of the nicer tools included, it's time to start writing our own code! As mentioned before, one of the nicest parts of using MyEclipse to work with Hibernate is the fact that it generates all the boiler-plate Hibernate mapping and even DAO code for you. This means that after you are done reverse-engineering a database you are ready to start writing your application to read, write and update objects in your database. In this tutorial we will write a series of simple methods that do 3 things in the following order: Create a new User and add him to the database Load a User from the database, using his primary key, and print out its information Change the User's values, update that record in the database and print the changed values to verify The three methods ( addUser, listUser, changeUser) are all called from the main method in our new class. To better understand how this Hibernate code is written, let's create that class, put those methods in the class (using copy & paste) and then review them line-by-line: Figure 5. Creating a new class that uses our generated Hibernate code After looking at that code we can see how straight forward everything is. We use the DAO classes that MyEclipse generated for us to get, update and save our objects to the database and MyEclipse/Hibernate take care of all the other details for us. It couldn't get much easier than this. Now to the fun part, let's run the code and see if it actually does the right thing: Figure 6. Running our example Hibernate code Very nice, it worked correctly just as we had hoped! We used the HQL Editor to query our database and make sure that our user was saved to it. Additionally, we could have just as easily switched to the Database Perspective and queried the database from there to see that the user record was in the table.

8. Conclusion While the application in this tutorial may seem simple, the techniques and information provided is critical to understanding both Hibernate and MyEclipse. The fundamentals presented in this tutorial apply to pretty much any Hibernate-enabled application you would want to develop: the core idea of object mapping to the database. After getting these basics working, you are free to enhance, extend or change your application in any way you need to and be assured that MyEclipse will continue to help you develop and extend it. We hope you have found this tutorial helpful. If you had comments about this tutorial or suggestions/questions for us, please let us know. We always value our user's feedback especially on educational materials such as these.

9. FAQ How does Hibernate compare to EJB 3 / JPA? Hibernate 3.2 is actually JPA-compatible, implementing all the new annotations that make JPA so automatic and painless to use. So instead of using a commercial implementation of JPA, you can use Hibernate and still keep all the standard JPA annotations in your classes without any changes. Can Hibernate scale to very large applications? Yes. Actually the roots of Hibernate come from the two founders own experiences working as consultants on large enterprise applications. Hibernate is their vision of how persistence should function in an application. Also a few years ago, Gavin King, put out a challenge to the community to find hand-tweaked JDBC SQL that executed a magnitude faster than the generated SQL from Hibernate to make a point that the framework is very focused on being functional, flexible and performant. Do I need all the Hibernate libraries in my application? There are a lot of them! Not necessarily. Hibernate is a very complex framework and makes use of a lot of other 3rd party frameworks. Depending on what you are doing with Hibernate it's possible that you don't need a lot of the JARs that are in your build path. There is a README that ships with Hibernate in the /lib directory explaining which each library does if you really want to trim back your deployment footprint. What is the "Session" in Hibernate? Why do I need to bother with it? Hibernate does a lot of "magic" under the covers. Part of that magic is to monitor mapped object's state and see if they have changed (e.g. a setter has been called) an then persist the changed values to the database in a timely fashion. In a small system you could imagine Hibernate managing all the objects at a time, but in bigger enterprise systems where there may be millions of mapped objects loaded at a time, a computer simply wouldn't have enough memory or CPU cycles to process so many entities. This is where the "session" comes into play. A Hibernate session represents a container of all the objects processed since that session has been opened. Most folks find that making a session the length of a transaction works for them, although for performance-critical applications that might be too short of a span of time for the session to exist when weighed against the cost of creating one. Another Hibernate session design is to use a ServletFilter to open a Hibernate session when a new HTTP session is created and then close the Hibernate session when the HTTP session is destroyed. For most small to mid-sized web applications running on a server with a decent sized heap this is a great balance between performance and memory requirements. Although you should always be aware of what objects you are loading and saving to a database, because if the session is left open, it's possible those objects could be hanging around in memory. MyEclipse FAQ Support Forum FAQ

10. Resources Below are links to resources that we hope will help answer most of the questions you could have while working your way through this tutorial pertaining to Hibernate: Files Introduction to Hibernate Project (Used in this Tutorial) Create Table SQL Script for user table Reference Hibernate 3.2 Reference Document ( HTML) ( PDF) Hibernate 3.x Javadoc Hibernate Annotations 3.2 Reference Document ( HTML) ( PDF) EJB3 / JPA Javadoc (Hibernate is JPA Compatible) Hibernate Wiki Basics Hibernate Tutorial - The Road to Hibernate Complete Hibernate Tutorial Official External Hibernate Links