Spring - yaml 和.properties 配置

Question

i've been working with Spring for some time and have a question regarding the very common configuration properties files (like the common application.properties that comes with every spring boot app you initialize). Recently, i also found that configurations like this can be done in yaml files. I have two questions:

1. When in a application.properties file, we write something like:

# application.properties

spring.debug = true
some-random-value = 15

does this mean that these values will be injected in the application context?

2. When we write something like:

# application.properties

spring.debug = true

does this mean, that somewhere, there is some class, that has an attribute that looks something like this? -->

@Component
class SomeClass{

@Value("spring.debug")
boolean shouldIRunInDebugMode;

...
}

2.a. If the answer to question 2 is yes, then how can I, looking at something like this:

# application.properties

spring.debug = true

find that class that is expecting that value. The same would apply to if i was looking at something like:

# application.yaml

someThidPartyLibraryName:
   shouldWeLog: true

If i see a yaml configuration file, just looking at all the configuration there usually is not enough for me to know what is happening. How can i find the class that is being affected by this configuration, so that i can better understand what this config is doing?

Thank you!

Answer 1

The response is generally yes. If you declare a property in the application.properties or application.yaml is mainly, because you would use it later in the code, for example injecting in some bean with the support of @Value annotation. However, there are also many built-in properties (let's say for example server.port ), which you usually don't have to declare and therefore you won't see explicitly in the code. Use an IDE to search the configuration properties and the manual to check the preconfigured ones in case of need.

Answer 2

Properties defined in yaml or a properties file may be accessed using the @Value annotation to inject, or using a @ConfigurationProperties class - see https://docs.spring.io/spring-boot/docs/current/reference/html/spring-boot-features.html#boot-features-external-config-typesafe-configuration-properties for complete details.

Finding the property usage is supported by some IDEs - IntelliJ allows you to click through. Otherwise it's a search through the source. For @ConfigurationProperties , once you find the class then just look for code that calls its accessor methods.

Answer 3

Your understanding regarding spring value injections from application.properties is correct. #2 - is Yes. Any property from application.properties can be injected to any java class as @Value.

Regarding #2.a - Yaml is just another format on how you organize your variable hierarchy by indentations. That's a superset to the JSON structure.

For example,

in application.properties file you can add something like this myapp.db.url=<dburl> myapp.db.username=<dbuser> myapp.db.password=<dbpassword>

the same can be represented in Yaml in a much efficient manner as below myapp: db: url:<dburl> username:<dbuser> password:<dbpassword>

And in either case, for your Jave file you can inject as

@Value("myapp.db.url" private String dbUrl;

Answer 4

Properties files and yaml files are used in Spring Boot for configurations. The main difference between the two is yaml provides structuring/grouping of configurations where as Properties are usually flat and may be repeating the same information:

For example;

Properties file

server.port = 8080
server.host = localhost

yaml file

server:
  port: 8080
  host: localhost

But in a Spring Boot AutoConfiguration class regardless of yaml or Properties used, a following looking ConfigurationProperties class will be used which will map server.port and server.host

@ConfigurationProperties(prefix = "server")
public class ServerConfiguration {

    private int port;
    private String host;

}

@Configuration
@EnableConfigurationProperties(ServerConfiguration.class)
public class ServerAutoConfiguration {

}

Hope this answers your questions.