Roman Böhm

Spring Boot’s externalized configuration is a powerful feature. But with great power comes, ahem, at least sometimes, a bunch of unused properties polluting your environment. That what=where-does-this-get-used in application-doineedthem.properties? You might be able to remove it.

Solution

First, create an EnvironmentPostProcessor, which “[a]llows for customization of the application’s Environment prior to the application context being refreshed.” This should be the correct time to snapshot your properties and inject property-usage tracking capabilities.

public class UnusedPropertiesEnvironmentPostProcessor implements EnvironmentPostProcessor {

    private static Set<String> getAllPropertyNames(ConfigurableEnvironment environment) {
        var result = new HashSet<String>();
        for (var ps : environment.getPropertySources()) {
            // One can pick one's own heuristic on what to track. 
            // `instance of OriginTrackedMapPropertySource` should cover externalized config files
            // such as application.properties.
            if (ps instanceof OriginTrackedMapPropertySource otps) {
                result.addAll(Arrays.asList(otps.getPropertyNames()));
            }
        }
        return result;
    }

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        var propertyNames = getAllPropertyNames(environment);
        environment.getPropertySources().addFirst(new PropertySource<>("Usage tracking property source adapter") {
            @Override
            public Object getProperty(String name) {
                if ("unused.properties".equals(name)) {
                    return propertyNames;
                }
                propertyNames.remove(name);
                return null;
            }
        });
    }
}

Essentially, we’re adding a dummy PropertySource instance that gets consulted before all the others. For every getProperty(String name) invocation, it will simply mark that property as used, i.e. remove it from the set of all previously detected properties, and, by returning null, will let the next PropertySource instance in line handle the call.

Whenever one retrieves the value of the unused.properties property, one gets the set of the properties that haven’t been used anywhere until this point in time.

Next, create a spring.factories file under src/test/resources/META-INF with the following content to bootstrap the above EnvironmentPostProcessor instance:

org.springframework.boot.env.EnvironmentPostProcessor=\
  replace.with.your.package.UnusedPropertiesEnvironmentPostProcessor

@SpringBootTest
class UnusedPropertiesTest {

  @Value("${unused.properties}")
  Set<String> unusedProperties;

  @Test
  void containsNoUnusedProperties() {
    assertThat(unusedProperties).isEmpty();
  }
}

If you have e.g. a property foo=bar in your application.properties that you do not @Value("${foo}") anywhere, this test is going to fail.

Programmatically find unused properties in a Spring Boot app

2024-06-20

Problem

Solution