Techniques For Integrating Hibernate Into Legacy Java Code Part 1
Below is a MRR and PLR article in category Computers Technology -> subcategory Web Development.
Techniques for Integrating Hibernate into Legacy Java Code: Part 1
Summary:
If you often work with legacy code, you may find yourself managing Java projects with vast lines of code but lacking modern libraries. A common example is the outdated implementation of the data access layer. Today, using Hibernate and DAOs, typically managed by Spring, is the standard approach.
Keywords: Java, Hibernate, Spring, Eclipse
Article:
If you’re like me, you spend a lot of time dealing with legacy code that doesn’t utilize modern methodologies and libraries. I've handled Java projects with hundreds of thousands of lines of code where the only third-party library is a JDBC driver. A frequent case is the old-fashioned implementation of the data access layer. Nowadays, Hibernate and DAOs, usually managed by Spring, are the norm.
This article describes the steps I took to convert a large application from custom data access to Hibernate and Spring using Eclipse’s refactoring tools. The goal was to make existing business logic, like Struts Actions, JSPs, Delegate classes, and Business Services, access the datastore using Hibernate and Spring, without direct manual changes to that code. Part 1 covers creating Hibernate data object classes and DAOs, and refactoring existing code to utilize these new types. Part 2 will complete the project by integrating Hibernate DAOs and configuring everything with Spring.
First, we need to create Hibernate model and DAO classes. Given the legacy nature of the application and its data structure, we’ll use a bottom-up approach to build our data access layer. This means generating Java code and Hibernate configuration files from the existing database. There are many free tools available to simplify this process. I recommend using an Eclipse Plugin for creating and maintaining Hibernate artifacts (search for "Hibernate Eclipse Plugin" to get started). The specifics of creating Hibernate classes and config files are well-documented elsewhere, so I won’t dive into details here. However, in this project, the Hibernate DAO lifecycles are managed by Spring, so the DAO classes should extend `HibernateDAOSupport`.
Now, we have Java classes (POJOs) mapped to our database tables, but none of the existing code uses these new data object classes. This is where Eclipse’s refactoring tools are invaluable. For example, if we have a legacy class called `AccountInfo` that corresponds to the `ACCOUNT` database table, right-click the class and select Refactor -> Extract Interface. Name the new interface `IAccount` and select "Use the extracted interface type where possible." Adjust other options based on your preferences. Click OK, and Eclipse will change all occurrences of `AccountInfo` references to `IAccount` and recompile. Repeat this for each object model class.
If you’ve never appreciated OOP languages, you’re about to discover why they’re great. Now, refactor the code so all existing legacy elements connect to the new Hibernate model classes instead of the old ones. Continuing with the `AccountInfo` example, create a new class?"consider creating a new package for this step?"called `Account`, which extends the Hibernate POJO for `Account` and implements the `IAccount` interface.
This next part is the most time-consuming but manageable. The new class will likely contain many empty methods marked with TODO comments. This is because the `IAccount` interface probably defines many methods not implemented in the Hibernate `Account` POJO. To handle these, ensure the new `Account` class delegates to its superclass when necessary to fulfill its contract as an `IAccount` type. For instance, if the legacy `AccountInfo` class had a getter/setter for a property called `username`, but the `ACCOUNT` table column is `LOGIN_NAME`, simply implement the `get/setUsername` methods in `Account` to delegate to `get/setLoginName` (from the superclass). You might also need to translate data types, like changing Strings to INTs or TIMESTAMPs when necessary. Do this for each object model class.
To complete the data model layer, update the appropriate Hibernate and Spring configuration files to reference these new object model classes. Now, the application can map database records to Java objects via Hibernate, without manually editing the legacy code. To finalize this refactoring project, you need to integrate the Spring-supported Hibernate DAOs similarly. In Part 2 of this article, I will cover how to refactor the legacy code to read, write, and update data using Hibernate and Spring.
You can find the original non-AI version of this article here: Techniques For Integrating Hibernate Into Legacy Java Code Part 1.
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