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Spring Bootstrap Feature Guide

Here are some (most, hopefully all) the features of Spring Bootstrap with some commentary to help you start using them.

Commandline Arguments

Commandline arguments are passed on to any CommandLineRunner beans found in the application. Option arguments (starting with --, e.g. --server.port=9000) are converted to a PropertySource and added to the Spring Environment with first priority (they always take precedence and override values from other sources). Properties in the Environment (including System properties and OS environment variables) can always be injected into Spring components using @Value with placeholders, e.g.

@Component
public class MyService {
    @Value("${app.message:Hello World}")
    private String message;
    ...
}

The default value comes after the first colon (":").

Externalized Configuration

In addition to command line option arguments, Spring Bootstrap will pick up a file called application.properties in the root of your classpath (if there is one) and add those properties to the Spring Environment. The search path for application.properties is actually, 1) root or classpath, 2) current directory, 3) /config package in classpath, 4) /config subdir of current directory. The list is ordered by decreasing precedence (so properties can be overridden by others with the same name defined in later locations).

The values in application.properties are filtered through the existing Environment when they are used so you can refer back to previously defined values (e.g. from System properties), e.g.

app.name: MyApp
app.description: ${app.name} is a Cool New App

Spring Bootstrap also binds the properties to any bean in your application context whose type is @ConfigurationProperties. The Actuator provides some of those beans out of the box, so you can easily customize server and management properties (ports etc.), endpoint locations and logging. See below for more detail, or inspect the *Properties types in the Actuator jar.

Setting the Default Spring Profile

Spring Profiles are a way to segregate parts of the application configuration and make it only available in certain environments. Any @Component that is marked with @Profile will only be loaded in the profile specified by the latter annotation.

Spring Bootstap takes it a stage further. If you include in your application.properties a value for a property named spring.active.profiles then those profiles will be active by default. E.g.

spring.active.profiles: dev,hsqldb

Profile-dependent configuration

Spring Bootstrap loads additional properties files if there are active profiles using a naming convention application-{profile}.properties. Property values from those files override trhe default ones.

Custom Typesafe Externalized Configuration

If you want a strongly typed bean (or beans) to govern and validate the configuration of your application beyond the built in properties, all you need to do is create a @ConfigurationProperties class, e.g.

@ConfigurationProperties(name="my")
public class MyProperties {
}

and declare one either explicitly (with @Bean) or implicitly by adding

@EnableConfigurationProperties(MyProperties.class)

to one of your @Configuration (or @Component) classes. Then you can

@Autowired
private MyProperties configuration = new MyProperties();

in any of your component classes to grab that configuration and use it.

Spring Bootstrap uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there doesn't need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include underscore separated (e.g. context_path binds to contextPath), and capitalized (e.g. PORT binds to port) environment properties.

Spring will attempt 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 bean id conversionService) or custom property editors (via a CustomEditorConfigurer bean).

Spring will also validate the external configuration, by default using JSR-303 if it is on the classpath. So you can add annotations from that specification (or its implementations) to your custom properties, e.g.

@ConfigurationProperties(name="my")
public class MyProperties {
    @NotNull
    private String name;
    // .. getters and setters
}

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator.

Using Project Lombok

You can safely use Project Lombok to generate getters and setters for your @ConfigurationProperties. Refer to the documentation on the Lombok for how to enable it in your compiler or IDE.

Using YAML instead of Properties

YAML is a superset of JSON, and as such is a very convenient format for specifying hierarchical configuration data, such as that supported by Spring Actuator. If you prefer to use YAML instead of Properties files you just need to include a file called application.yml in the root of your classpath

You can if you like add profile specific YAML files (application-${profile}.yml), but a nicer alternative is to use YAML documents inside application.yml, with profile-specific documents containing a spring.profiles key. For example

server:
  port: 8080
management:
  port: 8080
  address: 0.0.0.0
---
spring:
  profiles: prod
management:
  port: 8081
  address: 10.2.68.12

Customizing the location of the External Configuration

If you don't like application.properties or application.yml as the configuration file location you can switch to another location by specifying the spring.config.name (default application) or the spring.config.location as environment properties, e.g. if launching a jar which wraps SpringApplication:

$ java -jar myproject.jar --spring.config.name=myproject

Providing Defaults for Externalized Configuration

For @ConfigurationProperties beans that are provided by the framework itself you can always change the values that are bound to it by changing application.properties. But it is sometimes also useful to change the default values imperatively in Java, so get more control over the process. You can do this by declaring a bean of the same type in your application context, e.g. for the server properties:

@Bean
public ServerProperties serverProperties() {
    ServerProperties server = new ServerProperties();
    server.setPort(8888);
    return server;
}

Server Configuration

The ServerProperties are bound to application properties, and can be used to specify

The port that the application listens on for the its endpoints (server.port defaults to 8080)
The address that the application endpoints are available on (server.address defaults to all local addresses, making it available to connections from all clients).
The context root of the application endpoints (server.context_path defaults to "/")

Tomcat Container Configuration

If you want to use Tomcat as an embedded container include at least org.apache.tomcat.embed:tomcat-embed-core and one of the org.apache.tomcat.embed:tomcat-embed-logging-* libraries (depending on the logging system you are using). Then, in addition to the generic ServerProperties, you can also bind server.tomcat.* properties in the application properties (see ServerProperties.Tomcat).

To enable the Tomcat access log valve (very common in production environments)

More fine-grained control of the Tomcat container is available if you need it. Instead of letting Spring Actuator create the container for you, just create a bean of type TomcatEmbeddedServletContainerFactory and override one of its methods, or inject some customizations, e.g.

@Configuration
public class MyContainerConfiguration {
    @Bean
    public TomcatEmbeddedServletContainerFactory tomcatEmbeddedContainerFactory() {
        TomcatEmbeddedServletContainerFactory factory = new TomcatEmbeddedServletContainerFactory();
        factory.setConnector(new Connector("AJP/1.3"));
    }
}

(the default connector uses the Http11NioProtocol so the example if overriding that behaviour).

Customizing Logging

Spring Bootstrap uses Commons Logging for logging, but leaves the implementation open. A default configuration file is provided for logback, and also for log4j and JDK logging. In each case there is console output and file output (rotating, 10MB file size).

The various logging systems can be activated by including the right libraries on the classpath, and further customized by providing a native configuration file in the root of the classpath, or in a location specified by the Spring Environment property logging.config.

Logger	Activation	Customization
JDK	slf4j-jdk14	logging.properties
Logback	logback	logback.xml
Log4j	slfj4-log4j12, log4j	log4j.properties or log4j.xml

To help with the customization some other properties are transferred from the Spring Environment to System properties:

Environment	System Property	Comments
logging.file	LOG_FILE	Used in default log configuration if defined
logging.path	LOG_PATH	Used in default log configuration if defined
PID	PID	The current process ID is discovered if possible and not already provided

All the logging systems supported can consult System properties when parsing their configuration files. See the default configurations in spring-bootstrap.jar for examples.

Application Context Initializers

To add additional application context initializers to the bootstrap startup process, add a comma-delimited list of class names to the Environment property context.initializer.classes (can be specified via application.properties).

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