The Why and How Behind Listener-Based Dependency Injection

he Spring Framework introduced many people to the idea of dependency injection, the notion that instead of looking up objects you need to collaborate with, you simply have them injected into your class via JavaBean-style getters and setters. This article introduces a new, complimentary form of dependency injection that is based not on properties but on the idea of channels and callback listeners. Much like classic dependency injection (which I consider property-based), this listener-based injection has the ability to reduce boilerplate code, promote looser coupling, and help make code more testable and reusable.

In the same way that the JavaBean introspection used in dependency injection can infer properties by the presence of methods such as setPropertyName/getPropertyName, the listener introspection used in listener injection can infer communication channels on an object from methods such as addChannelName/removeChannelName, as the method signature defines the listener or callback interface. This article demonstrates how to use listener injection to clearly communicate relationships between POJOs using the Spring framework.