Inject Primitive and String Type

1. Application Properties

• Always configuring data which is commonly being used by all spring Beans.
• Called as Configuration file.
• key value pair format(key-val).
• two types of properties
  • pre-defiend(spring.application.name)
  • used defiend(db.name=alamgir)

Example application.properties file:

spring.application.name=b-dependency-injection
db.port=8080
db.url=localhost:8080
db.password=mysql
db.database=mysql,oracle,postgres

2. Using @Value Annotation

Injecting values directly:

@Value("8080")
private int port;

@Value("localhost:8080")
private String url;

Injecting from properties:

@Value("${db.password}")
private String password;

Default Values

@Value("${db.name:mysql}")
private String name;

If the property is missing, the default value mysql will be used.

Injecting Multiple Values

For list-type properties:

@Value("${db.database}")
private List<String> database;

If db.database=mysql,oracle,postgres : it becomes a list: [mysql, oracle, postgres]

How it works:

1. Spring checks application.properties (or application.yml).
2. Looks for the property.
   • If Found : proceeds.
   • If Not found : throws java.lang.IllegalArgumentException: Could not resolve placeholder 'db.password'.
3. Injects the value into the field.

3. Using @PropertySource

Add external .properties files:

@Configuration
@PropertySource("classpath:db.properties")
public class AppConfig { }

Key Points

• @PropertySource tells Spring where to load a .properties file from.
• Used on a @Configuration class.
• Works with @Value("${property.name}").
• Can load multiple files:

  @Configuration
  @PropertySource("classpath:db.properties")
  public class AppConfig { }  
  

  classpath: looks inside src/main/resources.

• If a property exists in both application.properties and a @PropertySource file:

  • Spring Boot uses the value from application.properties.
  • @PropertySource has lower priority than default Spring Boot property files.
    

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  Github Code : Inject primitives and String type : Spring Boot