SmallRye Config 1.8 Released

SmallRye Config version 1.8 was released a few days ago.

The latest version of SmallRye Config introduced a few experimental changes to enhance the API and to make SmallRye Config more extensible. One of the experimental changes is the new interceptor model, to intercept the lookup of a configuration value.

The ConfigSourceInterceptor

The ConfigSourceInterceptor allows to intercept the resolution of a configuration name before the Config resolves the configuration value and before any conversion taking place. This is useful to implement features like property expansion or configuration profiles.

Implementations of ConfigSourceInterceptor are loaded via the java.util.ServiceLoader mechanism and can be registered by providing a resource named META-INF/services/io.smallrye.config.ConfigSourceInterceptor, which contains the fully qualified ConfigSourceInterceptor implementation class name as its content.

A single method must be implemented, ConfigValue getValue(ConfigSourceInterceptorContext context, String name). The ConfigSourceInterceptorContext is used to proceed with the interceptor chain. The chain can be short-circuited by returning an instance of ConfigValue.

public interface ConfigSourceInterceptor extends Serializable {
    ConfigValue getValue(ConfigSourceInterceptorContext context, String name);

The ConfigValue objects hold information about the key name, value, config source origin, and ordinal.

Additionally, the ConfigSourceInterceptor may also intercept resolution of configuration names or configuration values with the methods Iterator<String> iterateNames(ConfigSourceInterceptorContext context) and Iterator<ConfigValue> iterateValues(ConfigSourceInterceptorContext context).

The Interceptor chain applies before any conversion takes place.


With the Interceptor API, it becomes trivial to implement a logging mechanism and to find out which ConfigSource loaded the resolved config:

public class LoggingConfigSourceInterceptor implements ConfigSourceInterceptor {
    private static final Logger LOG = Logger.getLogger("io.smallrye.config");

    public ConfigValue getValue(final ConfigSourceInterceptorContext context, final String name) {
        final ConfigValue configValue = context.proceed(name);
        final String key = configValue.getName();
        final String value = configValue.getValue();
        final String configSource = configValue.getConfigSourceName();

        LOG.infov("The key {0} was loaded from {1} with the value {2}", key, configSource, value);

        return configValue;

SmallRye Config does ship with its own logging interceptor, so you don’t need to implement one.

Built-In Interceptors

SmallRye Config provides the following built-in interceptors to expand its feature set:

  • RelocateConfigSourceInterceptor

  • ProfileConfigSourceInterceptor

  • ExpressionConfigSourceInterceptor

  • FallbackConfigSourceInterceptor

  • LoggingConfigSourceInterceptor

  • SecretKeyConfigSourceInterceptor

Not every interceptor is registered by default. Only the ProfileConfigSourceInterceptor, ExpressionConfigSourceInterceptor and SecretKeyConfigSourceInterceptor are added on new instances of Config. Other interceptors require manual registration via the ServiceLoader mechanism.

To learn more about all the built-in interceptors, please refer to the documentation page.

Expression Expansion

The ExpressionConfigSourceInterceptor provides expression expansion on configuration values. An expression string is a mix of plain strings and expression segments, which are wrapped into ${ …​ }.

For instance, consider the following configuration properties file:


Then the expression configuration will be resolved and expanded to the value 1234.

Additionally, the expression expansion engine supports the following segments:

  • ${expression:value} - Provides a default value after the : if the expansion doesn’t find a value.

  • ${my.prop${compose}} - Composed expressions. Inner expressions are resolved first.

  • ${my.prop}${my.prop} - Multiple expressions.

Configuration Profiles

The ProfileConfigSourceInterceptor allows multiple configurations with the same name and selects them via a profile property.

To be able to set properties with the same name, each property needs to be prefixed with % followed by the profile name and a dot:


Lookup is always performed with the my.prop property name. To use the profile dev, the configuration smallrye.config.profile=dev has to be set in any valid ConfigSource.

When looking up the property my.prop with the dev profile active, the value is 5678.

Only one profile can be active at any given time.

API Enhancements

Aside from interceptors, a few additional API enhancements shipped with the latest version of SmallRye Config:

  • ConfigValue - The ConfigValue is a metadata object that holds additional information after the lookup of a configuration.

  • ConfigValueConfigSource - Extends the original ConfigSource to expose methods that return a ConfigValue.

  • SmallRyeConfigBuilder#withSecretKeys - Hide configuration properties that contain passwords or other kinds of secrets to prevent accidental exposure of such values.


All the new features detailed here are experimental. The team is happy with them, and they had careful consideration when designed. We cannot guarantee that they won’t suffer any changes in the next few releases, especially considering that the SmallRye team is also pushing to have these added to the MicroProfile Config specification, which may require some changes.

This shouldn’t discourage developers to use these new features. We believe these add a lot of utility and improve the developer experience regarding configuration of applications.

Please, feel free to drop us any feedback in the SmallRye Mailing List.

Additional Resources