Spring Boot Summary #7 - Annotations (2)

May 23, 2023

@PostConstruct

This annotation is used on methods that need to be executed after dependency injection is complete. It ensures that the method runs even if it’s not called from other resources.

@Service
public class JwtService {

    @Value("${jwt.secret}")
    private String jwtSecret;

    @PostConstruct
    protected void init() {
        jwtSecret = Base64.getEncoder()
                .encodeToString(jwtSecret.getBytes(StandardCharsets.UTF_8));
    }
    ...

In the example above, @PostConstruct is used to initialize jwtSecret after dependency injection is finished.

@Value

The @Value annotation injects values from properties files into Spring variables.

public class JwtService {

    @Value("${jwt.secret}")
    private String jwtSecret;

application.yaml

jwt:
  secret: asdfdskjfsdkjflkdjfkjsdfsdfjaas

Annotations Used to Create Custom Annotations

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface UserId {
}

@Target

Specifies where the annotation can be applied.

ElementType	Description
PACKAGE	Declaration of packages
TYPE	Declaration of types (classes, interfaces, enums)
CONSTRUCTOR	Declaration of constructors
FIELD	Declaration of enum constants and fields
METHOD	Declaration of methods
ANNOTATION_TYPE	Declaration of annotation types
LOCAL_VARIABLE	Declaration of local variables
PARAMETER	Declaration of parameters
TYPE_PARAMETER	Declaration of parameterized types

@Retention

Specifies how long annotations are to be retained.

RetentionPolicy.SOURCE: Discarded at compile time.
RetentionPolicy.CLASS: Stored in .class files, discarded at runtime (default for Java).
RetentionPolicy.RUNTIME: Retained at runtime.

Example:

import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

// Custom annotations with different retention policies
@Retention(RetentionPolicy.SOURCE)
@interface SourceRetention {
    String value() default "Source Retention";
}

@Retention(RetentionPolicy.CLASS)
@interface ClassRetention {
    String value() default "Class Retention";
}

@Retention(RetentionPolicy.RUNTIME)
@interface RuntimeRetention {
    String value() default "Runtime Retention";
}

// Example usage
@SourceRetention
class A {
}

@ClassRetention
class B {
}

@RuntimeRetention
class C {
}

// Main class
public class RetentionPolicyDemo {
    public static void main(String[] args) {
        // Getting annotations
        Annotation[] a = new A().getClass().getAnnotations();
        Annotation[] b = new B().getClass().getAnnotations();
        Annotation[] c = new C().getClass().getAnnotations();

        System.out.println("Number of annotations attached to class A: " + a.length);
        System.out.println("Number of annotations attached to class B: " + b.length);
        System.out.println("Number of annotations attached to class C: " + c.length);
        System.out.println("Annotation attached to class C: " + c[0]);
    }
}

Output:

Number of annotations attached to class A: 0
Number of annotations attached to class B: 0
Number of annotations attached to class C: 1
Annotation attached to class C: @RuntimeRetention(value="Runtime Retention")

Written by@Chaerin

Tech Blog📕

GitHub LinkedIn