Externalized Configuration

By default, when a specified config data location does not exist, Spring Boot will throw a ConfigDataLocationNotFoundException and your application will not start.

If you want to specify a location, but you do not mind if it does not always exist, you can use the optional: prefix. You can use this prefix with the spring.config.location and spring.config.additional-location properties, as well as with spring.config.import declarations.

For example, a spring.config.import value of optional:file:./myconfig.properties allows your application to start, even if the myconfig.properties file is missing.

If you want to ignore all ConfigDataLocationNotFoundExceptions and always continue to start your application, you can use the spring.config.on-not-found property. Set the value to ignore using SpringApplication.setDefaultProperties(…​) or with a system/environment variable.

If a config file location includes the * character for the last path segment, it is considered a wildcard location. Wildcards are expanded when the config is loaded so that immediate subdirectories are also checked. Wildcard locations are particularly useful in an environment such as Kubernetes when there are multiple sources of config properties.

For example, if you have some Redis configuration and some MySQL configuration, you might want to keep those two pieces of configuration separate, while requiring that both those are present in an application.properties file. This might result in two separate application.properties files mounted at different locations such as /config/redis/application.properties and /config/mysql/application.properties. In such a case, having a wildcard location of config/*/, will result in both files being processed.

By default, Spring Boot includes config/*/ in the default search locations. It means that all subdirectories of the /config directory outside of your jar will be searched.

You can use wildcard locations yourself with the spring.config.location and spring.config.additional-location properties.

As well as application property files, Spring Boot will also attempt to load profile-specific files using the naming convention application-{profile}. For example, if your application activates a profile named prod and uses YAML files, then both application.yaml and application-prod.yaml will be considered.

Profile-specific properties are loaded from the same locations as standard application.properties, with profile-specific files always overriding the non-specific ones. If several profiles are specified, a last-wins strategy applies. For example, if profiles prod,live are specified by the spring.profiles.active property, values in application-prod.properties can be overridden by those in application-live.properties.

The Environment has a set of default profiles (by default, [default]) that are used if no active profiles are set. In other words, if no profiles are explicitly activated, then properties from application-default are considered.

Application properties may import further config data from other locations using the spring.config.import property. Imports are processed as they are discovered, and are treated as additional documents inserted immediately below the one that declares the import.

For example, you might have the following in your classpath application.properties file:

This will trigger the import of a dev.properties file in current directory (if such a file exists). Values from the imported dev.properties will take precedence over the file that triggered the import. In the above example, the dev.properties could redefine spring.application.name to a different value.

An import will only be imported once no matter how many times it is declared. The order an import is defined inside a single document within the properties/yaml file does not matter. For instance, the two examples below produce the same result:

In both of the above examples, the values from the my.properties file will take precedence over the file that triggered its import.

Several locations can be specified under a single spring.config.import key. Locations will be processed in the order that they are defined, with later imports taking precedence.

Some cloud platforms cannot add a file extension to volume mounted files. To import these extensionless files, you need to give Spring Boot a hint so that it knows how to load them. You can do this by putting an extension hint in square brackets.

For example, suppose you have a /etc/config/myconfig file that you wish to import as yaml. You can import it from your application.properties using the following:

When running applications on a cloud platform (such as Kubernetes) you often need to read config values that the platform supplies. It is not uncommon to use environment variables for such purposes, but this can have drawbacks, especially if the value is supposed to be kept secret.

As an alternative to environment variables, many cloud platforms now allow you to map configuration into mounted data volumes. For example, Kubernetes can volume mount both ConfigMaps and Secrets.

There are two common volume mount patterns that can be used:

For the first case, you can import the YAML or Properties file directly using spring.config.import as described above. For the second case, you need to use the configtree: prefix so that Spring Boot knows it needs to expose all the files as properties.

As an example, let’s imagine that Kubernetes has mounted the following volume:

The contents of the username file would be a config value, and the contents of password would be a secret.

To import these properties, you can add the following to your application.properties or application.yaml file:

You can then access or inject myapp.username and myapp.password properties from the Environment in the usual way.

If you have multiple config trees to import from the same parent folder you can use a wildcard shortcut. Any configtree: location that ends with /*/ will import all immediate children as config trees. As with a non-wildcard import, the names of the folders and files under each config tree form the property name.

For example, given the following volume:

You can use configtree:/etc/config/*/ as the import location:

This will add db.username, db.password, mq.username and mq.password properties.

Configuration trees can also be used for Docker secrets. When a Docker swarm service is granted access to a secret, the secret gets mounted into the container. For example, if a secret named db.password is mounted at location /run/secrets/, you can make db.password available to the Spring environment using the following:

The values in application.properties and application.yaml are filtered through the existing Environment when they are used, so you can refer back to previously defined values (for example, from System properties or environment variables). The standard ${name} property-placeholder syntax can be used anywhere within a value. Property placeholders can also specify a default value using a : to separate the default value from the property name, for example ${name:default}.

The use of placeholders with and without defaults is shown in the following example:

Assuming that the username property has not been set elsewhere, app.description will have the value MyApp is a Spring Boot application written by Unknown.

Spring Boot allows you to split a single physical file into multiple logical documents which are each added independently. Documents are processed in order, from top to bottom. Later documents can override the properties defined in earlier ones.

For application.yaml files, the standard YAML multi-document syntax is used. Three consecutive hyphens represent the end of one document, and the start of the next.

For example, the following file has two logical documents:

For application.properties files a special #--- or !--- comment is used to mark the document splits:

It is sometimes useful to only activate a given set of properties when certain conditions are met. For example, you might have properties that are only relevant when a specific profile is active.

You can conditionally activate a properties document using spring.config.activate.*.

The following activation properties are available:

For example, the following specifies that the second document is only active when running on Kubernetes, and only when either the “prod” or “staging” profiles are active:

YAML documents need to be converted from their hierarchical format to a flat structure that can be used with the Spring Environment. For example, consider the following YAML document:

In order to access these properties from the Environment, they would be flattened as follows:

Likewise, YAML lists also need to be flattened. They are represented as property keys with [index] dereferencers. For example, consider the following YAML:

The preceding example would be transformed into these properties:

Spring Framework provides two convenient classes that can be used to load YAML documents. The YamlPropertiesFactoryBean loads YAML as Properties and the YamlMapFactoryBean loads YAML as a Map.

You can also use the YamlPropertySourceLoader class if you want to load YAML as a Spring PropertySource.

It is possible to bind a bean declaring standard JavaBean properties as shown in the following example:

The preceding POJO defines the following properties:

The example in the previous section can be rewritten in an immutable fashion as shown in the following example:

In this setup, the presence of a single parameterized constructor implies that constructor binding should be used. This means that the binder will find a constructor with the parameters that you wish to have bound. If your class has multiple constructors, the @ConstructorBinding annotation can be used to specify which constructor to use for constructor binding. To opt out of constructor binding for a class with a single parameterized constructor, the constructor must be annotated with @Autowired. Constructor binding can be used with records. Unless your record has multiple constructors, there is no need to use @ConstructorBinding.

Nested members of a constructor bound class (such as Security in the example above) will also be bound through their constructor.

Default values can be specified using @DefaultValue on constructor parameters and record components. The conversion service will be applied to coerce the annotation’s String value to the target type of a missing property.

Referring to the previous example, if no properties are bound to Security, the MyProperties instance will contain a null value for security. To make it contain a non-null instance of Security even when no properties are bound to it (when using Kotlin, this will require the username and password parameters of Security to be declared as nullable as they do not have default values), use an empty @DefaultValue annotation:

Spring Boot provides infrastructure to bind @ConfigurationProperties types and register them as beans. You can either enable configuration properties on a class-by-class basis or enable configuration property scanning that works in a similar manner to component scanning.

Sometimes, classes annotated with @ConfigurationProperties might not be suitable for scanning, for example, if you’re developing your own auto-configuration or you want to enable them conditionally. In these cases, specify the list of types to process using the @EnableConfigurationProperties annotation. This can be done on any @Configuration class, as shown in the following example:

To use configuration property scanning, add the @ConfigurationPropertiesScan annotation to your application. Typically, it is added to the main application class that is annotated with @SpringBootApplication but it can be added to any @Configuration class. By default, scanning will occur from the package of the class that declares the annotation. If you want to define specific packages to scan, you can do so as shown in the following example:

We recommend that @ConfigurationProperties only deal with the environment and, in particular, does not inject other beans from the context. For corner cases, setter injection can be used or any of the *Aware interfaces provided by the framework (such as EnvironmentAware if you need access to the Environment). If you still want to inject other beans using the constructor, the configuration properties bean must be annotated with @Component and use JavaBean-based property binding.

This style of configuration works particularly well with the SpringApplication external YAML configuration, as shown in the following example:

To work with @ConfigurationProperties beans, you can inject them in the same way as any other bean, as shown in the following example:

As well as using @ConfigurationProperties to annotate a class, you can also use it on public @Bean methods. Doing so can be particularly useful when you want to bind properties to third-party components that are outside of your control.

To configure a bean from the Environment properties, add @ConfigurationProperties to its bean registration, as shown in the following example:

Any JavaBean property defined with the another prefix is mapped onto that AnotherComponent bean in manner similar to the preceding SomeProperties example.

Spring Boot uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there does not need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include dash-separated environment properties (for example, context-path binds to contextPath), and capitalized environment properties (for example, PORT binds to port).

As an example, consider the following @ConfigurationProperties class:

With the preceding code, the following properties names can all be used:

When lists are configured in more than one place, overriding works by replacing the entire list.

For example, assume a MyPojo object with name and description attributes that are null by default. The following example exposes a list of MyPojo objects from MyProperties:

Consider the following configuration:

If the dev profile is not active, MyProperties.list contains one MyPojo entry, as previously defined. If the dev profile is enabled, however, the list still contains only one entry (with a name of my another name and a description of null). This configuration does not add a second MyPojo instance to the list, and it does not merge the items.

When a List is specified in multiple profiles, the one with the highest priority (and only that one) is used. Consider the following example:

In the preceding example, if the dev profile is active, MyProperties.list contains one MyPojo entry (with a name of my another name and a description of null). For YAML, both comma-separated lists and YAML lists can be used for completely overriding the contents of the list.

For Map properties, you can bind with property values drawn from multiple sources. However, for the same property in multiple sources, the one with the highest priority is used. The following example exposes a Map<String, MyPojo> from MyProperties:

Consider the following configuration:

If the dev profile is not active, MyProperties.map contains one entry with key key1 (with a name of my name 1 and a description of my description 1). If the dev profile is enabled, however, map contains two entries with keys key1 (with a name of dev name 1 and a description of my description 1) and key2 (with a name of dev name 2 and a description of dev description 2).

Spring Boot attempts to coerce the external application properties to the right type when it binds to the @ConfigurationProperties beans. If you need custom type conversion, you can provide a ConversionService bean (with a bean named conversionService) or custom property editors (through a CustomEditorConfigurer bean) or custom Converters (with bean definitions annotated as @ConfigurationPropertiesBinding).

Spring Boot attempts to validate @ConfigurationProperties classes whenever they are annotated with Spring’s @Validated annotation. You can use JSR-303 jakarta.validation constraint annotations directly on your configuration class. To do so, ensure that a compliant JSR-303 implementation is on your classpath and then add constraint annotations to your fields, as shown in the following example:

To ensure that validation is always triggered for nested properties, even when no properties are found, the associated field must be annotated with @Valid. The following example builds on the preceding MyProperties example:

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator. The @Bean method should be declared static. The configuration properties validator is created very early in the application’s lifecycle, and declaring the @Bean method as static lets the bean be created without having to instantiate the @Configuration class. Doing so avoids any problems that may be caused by early instantiation.

The @Value annotation is a core container feature, and it does not provide the same features as type-safe configuration properties. The following table summarizes the features that are supported by @ConfigurationProperties and @Value:

If you define a set of configuration keys for your own components, we recommend you group them in a POJO annotated with @ConfigurationProperties. Doing so will provide you with structured, type-safe object that you can inject into your own beans.

SpEL expressions from application property files are not processed at time of parsing these files and populating the environment. However, it is possible to write a SpEL expression in @Value. If the value of a property from an application property file is a SpEL expression, it will be evaluated when consumed through @Value.