How to Read System Environment Variables Using SpEL in Spring ApplicationContext?

Reading System Environment Variables in Spring ApplicationContext

In the realm of Java application development, reading system environment variables in the Spring application context is a common need. With Spring 3.0 and its introduction of Spring Expression Language (SpEL), this task becomes considerably easier.

To read a system environment variable, you can employ SpEL within your Spring configuration. Here's an example:

<code class="xml"><util:properties id="dbProperties"
    location="classpath:config_#{systemEnvironment['ENV_VARIABLE_NAME']}/db.properties" /></code>

In this example, the ENV_VARIABLE_NAME placeholder refers to the actual value of the system environment variable you want to access.

To set the value of this environment variable, you can use the -D command-line argument when launching your Java application:

<code class="bash">$> java -DENV_VARIABLE_NAME=QA ...</code>

By setting the environment variable accordingly, you can dynamically load different configuration files based on the system environment. For instance, if ENV_VARIABLE_NAME is set to DEV, the application would load properties from the classpath:config_DEV/db.properties file.

In addition to accessing system environment variables, SpEL provides a number of other powerful features. To learn more, refer to the official Spring documentation on SpEL: [https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/html/expressions.html](https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/html/expressions.html).

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