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The Spring Framework

by Benoy Jose  

Introduction

Officially the spring season might be over but spring still seems to be lingering around. If you did not get the smell of the Spring Framework this summer then you might be missing something. You might be having your fingers crossed skeptically about another run of the mill Framework system that we have been seeing for the last couple of years. Hundreds of open source and propriety frameworks have sprung all over the place making it difficult to identify what each framework does, so the skepticism is understandable, but before we write off spring like one of those others it might be worth briefly peeking through it. Spring proposes a new paradigm, a pluggable, non- intrusive and robust framework.

Summary

The "Struts" framework is no doubt a good framework to enhance the ability of the web tier, but the biggest drawback is the fact that it caters only to the web tier and leaves most of the Enterprise tier or middle tier to the fancy of the application architects. The Application architects need to provide an additional framework to deal with the enterprise tier and make sure that the new framework integrates well with the Struts framework. Spring tries to alleviate this problem by providing a comprehensive framework, which includes an MVC framework, an AOP integration framework, a JDBC integration framework, and an EJB integration framework. It also provides integration modules for major O/R mapping tools like Hibernate and JDO. Spring provides all these in a modular fashion without imposing any layer on to the user. Spring is not a take-it-or- leave-it kind of framework. It tries to seamlessly blend into the existing framework users have without hindrances. Spring also provides transaction management support using Java classes, email support packages using framework classes, web services support through proxies and many more features like the above. As mentioned earlier all these packages are optional and spring does not make any of them mandatory. Spring can seamlessly integrate with existing applications and provide specific functionality that you intend to provide with minimal demands for customization. A user can continue to use Struts for the web tier and toplink O/R for the database and meanwhile hook spring to provide e-mail services and web services support. Spring is based on the Inversion of Control/Dependency Injection pattern that has been making rounds in message boards all over the Internet.

Inversion of Control/Dependency Injection

Dependency injection is kind of an Inversion of Control pattern. It is proposed as an alternative for the Service Locator. Martin flower discusses the Dependency Injection pattern, Inversion of Control pattern and the differences it has with the service locator in an article at his site (See resources below).

Dependency Injection proposes separating the implementation of an object and the construction of objects that depend on them. The job of coordinating the implementation and construction is left to the Assembler code. The object that needs to be implemented does not need to instantiate the dependent objects and can rely on the assembler to do the job. The assembler will gather and instantiate, if necessary, all the dependent objects and make them available to the implemented object. Since the assembler does not depend on the code directly changes can be made to the assembler without any changes to the code. The assembler gathers the required classes through configuration files so a change in the assembler only needs changes to the configuration file. In this case the Assembler code would be the spring framework.

Since the classes are independent and are integrated through the assembler, independent testing of each class can be done without affecting other application codes.

There are three types of Dependency Injections.