Introduction to Java Annotations with Real-World Examples

What are Java Annotations?

Java Annotations are a form of metadata introduced in JDK 5 that provide additional information about the code.

Annotations do not directly affect program semantics but can be used by compilers, tools, and frameworks to perform various operations.

Common Use Cases

1. Compiler Instructions
   Example: @Override tells the compiler that a method overrides a superclass method.
2. Compile-Time Processing (Code Generation, Validation)
   Example: @Getter from Lombok.
3. Runtime Processing (via Reflection)
   Example: Spring annotations like @Autowired, @RequestMapping.

Built-in Annotations in Java

| Annotation | Description | |———————-|———————————————| | @Override | Indicates a method overrides a superclass method | | @Deprecated | Marks a method/class as deprecated | | @SuppressWarnings | Suppresses compiler warnings |

Example:

public class MyClass {
    @Override
    public String toString() {
        return "MyClass instance";
    }

    @Deprecated
    public void oldMethod() {
        System.out.println("Old method");
    }

    @SuppressWarnings("unchecked")
    public void example() {
        List list = new ArrayList();  // Suppress warning for raw type
    }
}

Creating My Own Annotations

1. Define MyLog Annotation

First the Annotation @MyLog is created using @interface, targeted at methods (@Target(ElementType.METHOD)) and retained at runtime (@Retention(RetentionPolicy.RUNTIME)). It has a value() field with a default string.

import java.lang.annotation.*;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyLog {
    String value() default "default log";
}

2. Use the Annotation

The @MyLog("Calling sayHello") annotation is applied to the sayHello() method of a Service class.

public class Service {
    @MyLog("Calling sayHello")
    public void sayHello() {
        System.out.println("Hello!");
    }
}

3. Read the Annotation via Reflection

At last, reflection is used to check if sayHello() has the @MyLog annotation.

import java.lang.reflect.Method;

public class Main {
    public static void main(String[] args) throws Exception {
        Method method = Service.class.getMethod("sayHello");
        if (method.isAnnotationPresent(MyLog.class)) {
            MyLog log = method.getAnnotation(MyLog.class);
            System.out.println("Log value: " + log.value());
        }

        new Service().sayHello();
    }
}

Annotations in Frameworks (Spring Example)

Example:

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello Spring Boot";
    }
}