1. Spring Cloud Context: Application Context Services

A Spring Cloud application operates by creating a “ bootstrap ” context, which is a parent context for the main application. It is responsible for loading configuration properties from the external sources and for decrypting properties in the local external configuration files. The two contexts share an Environment , which is the source of external properties for any Spring application. By default, bootstrap properties (not bootstrap.properties but properties that are loaded during the bootstrap phase) are added with high precedence, so they cannot be overridden by local configuration.

The bootstrap context uses a different convention for locating external configuration than the main application context. Instead of application.yml (or .properties ), you can use bootstrap.yml , keeping the external configuration for bootstrap and main context nicely separate. The following listing shows an example:

bootstrap.yml.

spring:
  application:
    name: foo
  cloud:
    config:
      uri: ${SPRING_CONFIG_URI:http://localhost:8888}

If your application needs any application-specific configuration from the server, it is a good idea to set the spring.application.name (in bootstrap.yml or application.yml ).

You can disable the bootstrap process completely by setting spring.cloud.bootstrap.enabled=false (for example, in system properties).

If you build an application context from SpringApplication or SpringApplicationBuilder , then the Bootstrap context is added as a parent to that context. It is a feature of Spring that child contexts inherit property sources and profiles from their parent, so the “ main ” application context contains additional property sources, compared to building the same context without Spring Cloud Config. The additional property sources are:

Because of the ordering rules of property sources, the “ bootstrap ” entries take precedence. However, note that these do not contain any data from bootstrap.yml , which has very low precedence but can be used to set defaults.

You can extend the context hierarchy by setting the parent context of any ApplicationContext you create — for example, by using its own interface or with the SpringApplicationBuilder convenience methods ( parent() , child() and sibling() ). The bootstrap context is the parent of the most senior ancestor that you create yourself. Every context in the hierarchy has its own “ bootstrap ” (possibly empty) property source to avoid promoting values inadvertently from parents down to their descendants. If there is a Config Server, every context in the hierarchy can also (in principle) have a different spring.application.name and, hence, a different remote property source. Normal Spring application context behavior rules apply to property resolution: properties from a child context override those in the parent, by name and also by property source name. (If the child has a property source with the same name as the parent, the value from the parent is not included in the child).

Note that the SpringApplicationBuilder lets you share an Environment amongst the whole hierarchy, but that is not the default. Thus, sibling contexts, in particular, do not need to have the same profiles or property sources, even though they may share common values with their parent.

The bootstrap.yml (or .properties ) location can be specified by setting spring.cloud.bootstrap.name (default: bootstrap ) or spring.cloud.bootstrap.location (default: empty) — for example, in System properties. Those properties behave like the spring.config.* variants with the same name. In fact, they are used to set up the bootstrap ApplicationContext by setting those properties in its Environment . If there is an active profile (from spring.profiles.active or through the Environment API in the context you are building), properties in that profile get loaded as well, the same as in a regular Spring Boot app — for example, from bootstrap-development.properties for a development profile.

The property sources that are added to your application by the bootstrap context are often “ remote ” (from example, from Spring Cloud Config Server). By default, they cannot be overridden locally. If you want to let your applications override the remote properties with their own System properties or config files, the remote property source has to grant it permission by setting spring.cloud.config.allowOverride=true (it does not work to set this locally). Once that flag is set, two finer-grained settings control the location of the remote properties in relation to system properties and the application’s local configuration:

The bootstrap context can be set to do anything you like by adding entries to /META-INF/spring.factories under a key named org.springframework.cloud.bootstrap.BootstrapConfiguration . This holds a comma-separated list of Spring @Configuration classes that are used to create the context. Any beans that you want to be available to the main application context for autowiring can be created here. There is a special contract for @Beans of type ApplicationContextInitializer . If you want to control the startup sequence, classes can be marked with an @Order annotation (the default order is last ).

	Warning
	When adding custom BootstrapConfiguration , be careful that the classes you add are not @ComponentScanned by mistake into your “ main ” application context, where they might not be needed. Use a separate package name for boot configuration classes and make sure that name is not already covered by your @ComponentScan or @SpringBootApplication annotated configuration classes.

The bootstrap process ends by injecting initializers into the main SpringApplication instance (which is the normal Spring Boot startup sequence, whether it is running as a standalone application or deployed in an application server). First, a bootstrap context is created from the classes found in spring.factories . Then, all @Beans of type ApplicationContextInitializer are added to the main SpringApplication before it is started.

The default property source for external configuration added by the bootstrap process is the Spring Cloud Config Server, but you can add additional sources by adding beans of type PropertySourceLocator to the bootstrap context (through spring.factories ). For instance, you can insert additional properties from a different server or from a database.

As an example, consider the following custom locator:

@Configuration
public class CustomPropertySourceLocator implements PropertySourceLocator {
    @Override
    public PropertySource<?> locate(Environment environment) {
        return new MapPropertySource("customProperty",
                Collections.<String, Object>singletonMap("property.from.sample.custom.source", "worked as intended"));
}

The Environment that is passed in is the one for the ApplicationContext about to be created — in other words, the one for which we supply additional property sources for. It already has its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this Environment (for example, by keying it on spring.application.name , as is done in the default Spring Cloud Config Server property source locator).

If you create a jar with this class in it and then add a META-INF/spring.factories containing the following, the customProperty PropertySource appears in any application that includes that jar on its classpath:

org.springframework.cloud.bootstrap.BootstrapConfiguration=sample.custom.CustomPropertySourceLocator

If you are going to use Spring Boot to configure log settings than you should place this configuration in `bootstrap.[yml | properties] if you would like it to apply to all events.

	Note
	For Spring Cloud to initialize logging configuration properly you cannot use a custom prefix. For example, using custom.loggin.logpath will not be recognized by Spring Cloud when initializing the logging system.

The application listens for an EnvironmentChangeEvent and reacts to the change in a couple of standard ways (additional ApplicationListeners can be added as @Beans by the user in the normal way). When an EnvironmentChangeEvent is observed, it has a list of key values that have changed, and the application uses those to:

Note that the Config Client does not, by default, poll for changes in the Environment . Generally, we would not recommend that approach for detecting changes (although you could set it up with a @Scheduled annotation). If you have a scaled-out client application, it is better to broadcast the EnvironmentChangeEvent to all the instances instead of having them polling for changes (for example, by using the Spring Cloud Bus ).

The EnvironmentChangeEvent covers a large class of refresh use cases, as long as you can actually make a change to the Environment and publish the event. Note that those APIs are public and part of core Spring). You can verify that the changes are bound to @ConfigurationProperties beans by visiting the /configprops endpoint (a normal Spring Boot Actuator feature). For instance, a DataSource can have its maxPoolSize changed at runtime (the default DataSource created by Spring Boot is an @ConfigurationProperties bean) and grow capacity dynamically. Re-binding @ConfigurationProperties does not cover another large class of use cases, where you need more control over the refresh and where you need a change to be atomic over the whole ApplicationContext . To address those concerns, we have @RefreshScope .

When there is a configuration change, a Spring @Bean that is marked as @RefreshScope gets special treatment. This feature addresses the problem of stateful beans that only get their configuration injected when they are initialized. For instance, if a DataSource has open connections when the database URL is changed via the Environment , you probably want the holders of those connections to be able to complete what they are doing. Then, the next time something borrows a connection from the pool, it gets one with the new URL.

Sometimes, it might even be mandatory to apply the @RefreshScope annotation on some beans which can be only initialized once. If a bean is "immutable", you will have to either annotate the bean with @RefreshScope or specify the classname under the property key spring.cloud.refresh.extra-refreshable .

	Important
	If you create a DataSource bean yourself and the implementation is a HikariDataSource , return the most specific type, in this case HikariDataSource . Otherwise, you will need to set spring.cloud.refresh.extra-refreshable=javax.sql.DataSource .

Refresh scope beans are lazy proxies that initialize when they are used (that is, when a method is called), and the scope acts as a cache of initialized values. To force a bean to re-initialize on the next method call, you must invalidate its cache entry.

The RefreshScope is a bean in the context and has a public refreshAll() method to refresh all beans in the scope by clearing the target cache. The /refresh endpoint exposes this functionality (over HTTP or JMX). To refresh an individual bean by name, there is also a refresh(String) method.

To expose the /refresh endpoint, you need to add following configuration to your application:

management:
  endpoints:
    web:
      exposure:
        include: refresh

Note

@RefreshScope works (technically) on an @Configuration class, but it might lead to surprising behavior. For example, it does not mean that all the @Beans defined in that class are themselves in @RefreshScope . Specifically, anything that depends on those beans cannot rely on them being updated when a refresh is initiated, unless it is itself in @RefreshScope . In that case, it is rebuilt on a refresh and its dependencies are re-injected. At that point, they are re-initialized from the refreshed @Configuration ).

Spring Cloud has an Environment pre-processor for decrypting property values locally.