How to Inject Properties from External Sources into Annotated Spring Beans?

Injecting External Properties into Annotated Spring Beans

When configuring Spring beans through annotations, it becomes necessary to inject property values from external sources. This article addresses the question of how to inject a property value into a bean that was configured using annotations.

The provided code shows a Spring bean, PersonDaoImpl, configured through annotations without an explicit definition in the Spring XML file. However, the need arises to inject a property from app.properties into this bean.

Traditionally, this would be achieved by defining the property in the XML file and using a PropertyPlaceholderConfigurer to read the value from the external file. However, with the bean being defined through annotations, this approach is not possible.

To address this, Spring offers a solution using EL support in Spring 3. The following code demonstrates the idea:

In this example, systemProperties is an implicit object that allows access to system properties. dbName is then injected with the value of the system property databaseName.

Similarly, other beans can be injected using EL expressions. For example:

Here, strategyBean is assumed to be a Spring bean, and its databaseKeyGenerator property will be injected into the current bean.

Spring also allows properties to be injected from a Properties object:

In this example, a property from the myProperties object is injected into the field githubOauthClientId.

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