Spring Boot Property Injection

Internal

• Spring Property Injection Concepts
• Spring Boot Concepts

Dependencies

dependencies {
    implementation('org.springframework.boot:spring-boot')
}

This declaration assumes we're using Spring Framework Dependency Management System, for Spring Framework-based libraries, or we're building a Spring Boot application.

Injecting Properties into Beans

Property injection is supported by the @ConfigurationProperties annotation. It can be used as shown below.

Configuration Property Holders

A common pattern used to handle injected configuration is to declare a configuration property holder class, whose sole purpose in the application is to be holder of configuration data. Such class bundles several configuration properties together, under a common property namespace. There is nothing special about configuration property holders. They are ordinary Spring components that have their properties injected from the Spring environment, by the virtue of @ConfigurationProperties annotations - to see how this annotation actually works, see @ConfigurationProperties below.

They can be injected into any other bean that needs those properties. One benefit of this approach is that it keeps configuration-specific details out of controllers and other application specific classes. Another benefit is that it makes it easy to share common configuration properties among several beans that may need this information. All configuration properties related to a certain piece of functionality are kept in a single place, as opposite to being scattered across several components, and if we need to add a new property or change an existing property, we do it in a single place.

Configuration property holder classes are also a good location to apply JavaBeans Validation annotations.

@Component
@ConfigurationProperties(prefix = "playground.spring.pi")
@Data
@Validated
public class MyPropertyConfiguration {

  public static final int DEFAULT_SIZE = 20;
  public static final String DEFAULT_COLOR = "blue";

  @Min(value = 5, message = "the size must be larger or equal than 5")
  @Max(value = 40, message = "the size must be smaller or equal than 40")
  private int size = DEFAULT_SIZE;
  private String color = DEFAULT_COLOR;
}

Conventionally, hardcoded defaults are specified as initialization values for the member variables.

The property values can then be set externally in a configuration file (application.properties, application.yml, etc.) as follows:

playground:
  spring:
    pi:
      size: 25
      color: "red"

The full playground project that demonstrates property holders is available here:

Playground Spring Boot @ConfigurationProperties Example

@ConfigurationProperties

This is a Spring Boot annotation, which works by default, without any additional se up, in a Spring Boot environment.

@ConfigurationProperties Implementation

The mechanism that ends with application.properties/application.yml-declared properties being injected in configuration holders consists of two steps:

1. The application.properties, application.yml and other configuration files placed in conventional locations (classpath:, file:./, classpath:config, file:./config/:) are read and parsed by the ConfigFileApplicationListener application event listener instance. The listener is triggered by ApplicationEnvironmentPreparedEvent or ApplicationPreparedEvent instances.

2. Once a @ConfigurationProperties-annotated bean is loaded, the ConfigurationPropertiesBindingPostProcessor post-processor is applied. The post-processor pulls the property values from the environment and injects them into the component. This process in called "binding".

@ConfigurationProperties Support outside Spring Boot

The following Playground example demonstrates usage of @ConfigurationProperties outside of Spring Boot. The dependencies are still needed, but we don't need a Spring Boot runtime to declare property in application.properties/application.yml and bind them to property holders. The idea behind the example is to use the ConfigFileApplicationListener code to force configuration file processing - their content will be exposed as a property source in the environment. @ConfigurationProperties will be automatically handled as long as the "org.springframework.boot:spring-boot" Spring Boot dependency is in the classpath. This article explains provides an example of how the properties can be explicitly injected with a BeanPostProcessor, if necessary. If YAML files need to be added, then "org.yaml:snakeyaml:1.23" or newer should be added to the classpath.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.context.config.ConfigFileApplicationListener;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
...
final AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.scan("playground.spring");
new ConfigFileApplicationListener().postProcessEnvironment(applicationContext.getEnvironment(), new SpringApplication());
applicationContext.refresh();

Playground @ConfigurationProperties Support outside Spring Boot

Using Property Configuration Holders with Plain Spring TestContext Framework

Property configuration holders are a Spring Boot pattern. They require Spring Boot dependencies and runtime to function. However, they're useful, so this is a method to get them to work with plain TestContext Framework tests:

Using Property Configuration Holders with Plain Spring TestContext Framework

Exposing Individual Configuration Properties on Components

Individual properties can also be injected directly into established components, as shown:

@Component
@ConfigurationProperties(prefix = "playground.spring.pi")
public class MyComponent {

  public static final int DEFAULT_WEIGHT = 50;

  private int weight = DEFAULT_WEIGHT;

  public void setWeight(int weight) {

    this.weight = weight;
  }
  ...
}

The property values can then be set externally in a configuration file as follows:

playground:
  spring:
    pi:
      weight: 30

However, if you have the option, bundling configuration properties into configuration property holders is generally a better approach, for the reasons presented in that section.

@EnableConfigurationProperties

@EnableConfigurationProperties

Property Injection and Auto-Configuration

The beans that are automatically configured by the Spring Boot autoconfiguration. process are all configurable by properties drawn from the Spring environment.

Spring Boot and Profiles

Also see:

Spring Property Injection

Defining Profile-Specific Properties in application.yml|application.properties

There are two ways of defining profile-specific properties in application.yml: using a file per profile, or using different sections in the same file. In case of an application.properties, profile-specific properties can only be defined if we use a file per profile, the different-sections-in-the-same-file method is not available.

Use a File per Profile

Create an YAML or a properties file that contains only the properties for a specific profile. The name of the file should follow the following convention:

application-<profile-name>.yml|properties

Use Different Sections in the Same File

This method only works for application.yml files. It involves placing profile-specific properties alongside non-profile-specific properties, separated by three hyphens and the "spring.profiles" property set to the name of the profile:

playground.color: "red"

---
spring.profiles: "san_francisco"

playground.color: "yellow"

---
spring.profiles: "new_york"

playground.color: "green"

bootstrap.yml

bootstrap.yml

Configuration Property Placeholders

When setting properties, you aren’t limited to declaring their values as string and numeric values. Instead, property values can be derived from from other configuration properties, using the "${...}" (placeholder marker) syntax:

some: 
  attribute: "bright"

playground:
  spring:
    pi:
      color: "${some.attribute} red"

Also see:

Property Placeholders

Default Values for Property Placeholders

It is possible to specify in-line default values for property placeholders, with the ":<default-value>" syntax. These values are used when the property the placeholder is relying on does not exist in the environment:

some.color: "${some.other.color:blue}"

Configuration Property Metadata

Some IDEs attempt to find metadata about configuration properties, and display a warning around them if such metadata is not found.

Configuration property metadata is completely optional and doesn’t prevent configuration properties from working, but the metadata can be useful for providing some minimal documentation around the configuration properties in the IDE. A way to provide configuration property metadata is to provide a ./src/main/resources/META-INF/additional-spring-configuration-metadata.json file:

{
  "properties": [
    {
      "name": "some.attribute",
      "type": "java.lang.String",
      "description": "this is description for 'some.attribute'."
    },
    {
      "name": "playground.spring.pi.size",
      "type": "java.lang.Integer",
      "description": "this is description for 'playground.spring.pi.size'."
    },
    {
      "name": "playground.spring.pi.color",
      "type": "java.lang.String",
      "description": "this is description for 'playground.spring.pi.color'."
    },
    {
      "name": "playground.spring.pi.weight",
      "type": "java.lang.Integer",
      "description": "this is description for 'playground.spring.pi.weight'."
    }
   ]
}

In IntelliJ IDEA, you will also need to re-run Spring Boot Configuration Annotation Processor to update generated metadata.

TODO: https://docs.spring.io/spring-boot/docs/2.0.6.RELEASE/reference/html/configuration-metadata.html#configuration-metadata-annotation-processor.