A Spring Cloud application operates by creating a "bootstrap" context, which is a parent context for the main application. Out of the box it is responsible for loading configuration properties from the external sources, and also 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. Bootstrap properties are added with high precedence, so they cannot be overridden by local configuration, by default.

The bootstrap context uses a different convention for locating external configuration than the main application context, so instead of application.yml (or .properties) you use bootstrap.yml, keeping the external configuration for bootstrap and main context nicely separate. Example:

bootstrap.yml. 

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

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

You can disable the bootstrap process completely by setting spring.cloud.bootstrap.enabled=false (e.g. 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 will contain 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, but 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 simply setting the parent context of any ApplicationContext you create, e.g. using its own interface, or with the SpringApplicationBuilder convenience methods (parent(), child() and sibling()). The bootstrap context will be the parent of the most senior ancestor that you create yourself. Every context in the hierarchy will have its own "bootstrap" property source (possibly empty) to avoid promoting values inadvertently from parents down to their descendants. Every context in the hierarchy can also (in principle) have a different spring.application.name and hence a different remote property source if there is a Config Server. Normal Spring application context behaviour 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 one from the parent is not included in the child).

Note that the SpringApplicationBuilder allows you to 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 will share common things with their parent.

The bootstrap.yml (or .properties) location can be specified using spring.cloud.bootstrap.name (default "bootstrap") or spring.cloud.bootstrap.location (default empty), e.g. 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) then properties in that profile will be loaded as well, just like in a regular Spring Boot app, e.g. from bootstrap-development.properties for a "development" profile.

The property sources that are added to you application by the bootstrap context are often "remote" (e.g. from a Config Server), and by default they cannot be overridden locally, except on the command line. If you want to allow your applications to 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 doesn’t work to set this locally). Once that flag is set there are some finer grained settings to control the location of the remote properties in relation to System properties and the application’s local configuration: spring.cloud.config.overrideNone=true to override with any local property source, and spring.cloud.config.overrideSystemProperties=false if only System properties and env vars should override the remote settings, but not the local config files.

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

	Warning
	Be careful when adding custom BootstrapConfiguration 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 that is not already covered by your @ComponentScan or @SpringBootApplication annotated configuration classes.

The bootstrap process ends by injecting initializers into the main SpringApplication instance (i.e. the normal Spring Boot startup sequence, whether it is running as a standalone app or deployed in an application server). First a bootstrap context is created from the classes found in spring.factories and 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 Config Server, but you can add additional sources by adding beans of type PropertySourceLocator to the bootstrap context (via spring.factories). You could use this to insert additional properties from a different server, or from a database, for instance.

As an example, consider the following trivial 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, i.e. the one that we are supplying additional property sources for. It will already have its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this Environment (e.g. by keying it on the spring.application.name, as is done in the default Config Server property source locator).

If you create a jar with this class in it and then add a META-INF/spring.factories containing:

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

then the "customProperty" PropertySource will show up in any application that includes that jar on its classpath.

The application will listen for an EnvironmentChangeEvent and react 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 will have a list of key values that have changed, and the application will use those to:

Note that the Config Client does not by default poll for changes in the Environment, and 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 then it is better to broadcast the EnvironmentChangeEvent to all the instances instead of having them polling for changes (e.g. 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 (those APIs are public and part of core Spring). You can verify the changes are bound to @ConfigurationProperties beans by visiting the /configprops endpoint (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.

A Spring @Bean that is marked as @RefreshScope will get special treatment when there is a configuration change. This 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, we probably want the holders of those connections to be able to complete what they are doing. Then the next time someone borrows a connection from the pool he gets one with the new URL.

Refresh scope beans are lazy proxies that initialize when they are used (i.e. 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 just need to invalidate its cache entry.

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

Note

@RefreshScope works (technically) on an @Configuration class, but it might lead to surprising behaviour: e.g. it does not mean that all the @Beans defined in that class are themselves @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 which it will be rebuilt on a refresh and its dependencies re-injected, at which point they will be re-initialized from the refreshed @Configuration).

Spring Cloud has an Environment pre-processor for decrypting property values locally. It follows the same rules as the Config Server, and has the same external configuration via encrypt.*. Thus you can use encrypted values in the form {cipher}* and as long as there is a valid key then they will be decrypted before the main application context gets the Environment. To use the encryption features in an application you need to include Spring Security RSA in your classpath (Maven co-ordinates "org.springframework.security:spring-security-rsa") and you also need the full strength JCE extensions in your JVM.

If you are getting an exception due to "Illegal key size" and you are using Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information:

Extract files into JDK/jre/lib/security folder (whichever version of JRE/JDK x64/x86 you are using).

For a Spring Boot Actuator application there are some additional management endpoints:

Commons provides the @EnableDiscoveryClient annotation. This looks for implementations of the DiscoveryClient interface via META-INF/spring.factories. Implementations of Discovery Client will add a configuration class to spring.factories under the org.springframework.cloud.client.discovery.EnableDiscoveryClient key. Examples of DiscoveryClient implementations: are Spring Cloud Netflix Eureka, Spring Cloud Consul Discovery and Spring Cloud Zookeeper Discovery.

By default, implementations of DiscoveryClient will auto-register the local Spring Boot server with the remote discovery server. This can be disabled by setting autoRegister=false in @EnableDiscoveryClient.

	Note
	The use of @EnableDiscoveryClient is no longer required. It is enough to just have a DiscoveryClient implementation on the classpath to cause the Spring Boot application to register with the service discovery server.

Commons now provides a ServiceRegistry interface which provides methods like register(Registration) and deregister(Registration) which allow you to provide custom registered services. Registration is a marker interface.

@Configuration
@EnableDiscoveryClient(autoRegister=false)
public class MyConfiguration {
    private ServiceRegistry registry;

    public MyConfiguration(ServiceRegistry registry) {
        this.registry = registry;
    }

    // called via some external process, such as an event or a custom actuator endpoint
    public void register() {
        Registration registration = constructRegistration();
        this.registry.register(registration);
    }
}

Each ServiceRegistry implementation has its own Registry implementation.

RestTemplate can be automatically configured to use ribbon. To create a load balanced RestTemplate create a RestTemplate @Bean and use the @LoadBalanced qualifier.

	Warning
	A RestTemplate bean is no longer created via auto configuration. It must be created by individual applications.

@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
    @Autowired
    private RestTemplate restTemplate;

    public String doOtherStuff() {
        String results = restTemplate.getForObject("http://stores/stores", String.class);
        return results;
    }
}

The URI needs to use a virtual host name (ie. service name, not a host name). The Ribbon client is used to create a full physical address. See RibbonAutoConfiguration for details of how the RestTemplate is set up.

2.3.1 Retrying Failed Requests

A load balanced RestTemplate can be configured to retry failed requests. By default this logic is disabled, you can enable it by adding Spring Retry to your application’s classpath. The load balanced RestTemplate will honor some of the Ribbon configuration values related to retrying failed requests. If you would like to disable the retry logic with Spring Retry on the classpath you can set spring.cloud.loadbalancer.retry.enabled=false. The properties you can use are client.ribbon.MaxAutoRetries, client.ribbon.MaxAutoRetriesNextServer, and client.ribbon.OkToRetryOnAllOperations. See the Ribbon documentation for a description of what there properties do.

If you would like to implement a BackOffPolicy in your retries you will need to create a bean of type LoadBalancedBackOffPolicyFactory, and return the BackOffPolicy you would like to use for a given service.

@Configuration
public class MyConfiguration {
    @Bean
    LoadBalancedBackOffPolicyFactory backOffPolciyFactory() {
        return new LoadBalancedBackOffPolicyFactory() {
            @Override
            public BackOffPolicy createBackOffPolicy(String service) {
        		return new ExponentialBackOffPolicy();
        	}
        };
    }
}

	Note
	client in the above examples should be replaced with your Ribbon client’s name.

2.3.2 Multiple RestTemplate objects

If you want a RestTemplate that is not load balanced, create a RestTemplate bean and inject it as normal. To access the load balanced RestTemplate use the @LoadBalanced qualifier when you create your @Bean.

	Important
	Notice the @Primary annotation on the plain RestTemplate declaration in the example below, to disambiguate the unqualified @Autowired injection.

@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate loadBalanced() {
        return new RestTemplate();
    }

    @Primary
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
    @Autowired
    private RestTemplate restTemplate;

    @Autowired
    @LoadBalanced
    private RestTemplate loadBalanced;

    public String doOtherStuff() {
        return loadBalanced.getForObject("http://stores/stores", String.class);
    }

    public String doStuff() {
        return restTemplate.getForObject("http://example.com", String.class);
    }
}

	Tip
	If you see errors like java.lang.IllegalArgumentException: Can not set org.springframework.web.client.RestTemplate field com.my.app.Foo.restTemplate to com.sun.proxy.$Proxy89 try injecting RestOperations instead or setting spring.aop.proxyTargetClass=true.

WebClient can be configured to use the LoadBalancerClient. A `LoadBalancerExchangeFilterFunction is auto-configured if spring-webflux is on the classpath.

public class MyClass {
    @Autowired
    private LoadBalancerExchangeFilterFunction lbFunction;

    public Mono<String> doOtherStuff() {
        return WebClient.builder().baseUrl("http://stores")
            .filter(lbFunction)
            .build()
            .get()
            .uri("/stores")
            .retrieve()
            .bodyToMono(String.class);
    }
}

The URI needs to use a virtual host name (ie. service name, not a host name). The LoadBalancerClient is used to create a full physical address.

Sometimes it is useful to ignore certain named network interfaces so they can be excluded from Service Discovery registration (eg. running in a Docker container). A list of regular expressions can be set that will cause the desired network interfaces to be ignored. The following configuration will ignore the "docker0" interface and all interfaces that start with "veth".

application.yml. 

spring:
  cloud:
    inetutils:
      ignoredInterfaces:
        - docker0
        - veth.*

You can also force to use only specified network addresses using list of regular expressions:

application.yml. 

spring:
  cloud:
    inetutils:
      preferredNetworks:
        - 192.168
        - 10.0

You can also force to use only site local addresses. See Inet4Address.html.isSiteLocalAddress() for more details what is site local address.

application.yml. 

spring:
  cloud:
    inetutils:
      useOnlySiteLocalInterfaces: true

Spring Cloud Commons provides beans for creating both Apache HTTP clients (ApacheHttpClientFactory) as well as OK HTTP clients (OkHttpClientFactory). The OkHttpClientFactory bean will only be created if the OK HTTP jar is on the classpath. In addition, Spring Cloud Commons provides beans for creating the connection managers used by both clients, ApacheHttpClientConnectionManagerFactory for the Apache HTTP client and OkHttpClientConnectionPoolFactory for the OK HTTP client. You can provide your own implementation of these beans if you would like to customize how the HTTP clients are created in downstream projects. You can also disable the creation of these beans by setting spring.cloud.httpclientfactories.apache.enabled or spring.cloud.httpclientfactories.ok.enabled to false.