What is the difference between @Retention and @Target annotations?

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Difference Between @Retention and @Target Annotations

@Retention and @Target are both meta-annotations in Java used to define the characteristics of custom annotations. However, they serve different purposes:

1. @Retention Annotation

  • Purpose: Specifies how long the annotation should be retained and available for use.
  • Values:
  • RetentionPolicy.SOURCE: The annotation is discarded by the compiler and not included in the bytecode. It is only available in the source code.
  • RetentionPolicy.CLASS: The annotation is included in the bytecode but not available at runtime. It can be read by the compiler but not used by reflection.
  • RetentionPolicy.RUNTIME: The annotation is retained in the bytecode and is available at runtime. It can be accessed via reflection.

Example:

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
@Retention(RetentionPolicy.RUNTIME) // Available at runtime  

public @interface MyAnnotation {


String value();


}

2. @Target Annotation

  • Purpose: Specifies where the annotation can be applied (its target).
  • Values:
  • ElementType.TYPE: Can be applied to classes, interfaces, or enums.
  • ElementType.FIELD: Can be applied to fields (variables).
  • ElementType.METHOD: Can be applied to methods.
  • ElementType.PARAMETER: Can be applied to method parameters.
  • ElementType.CONSTRUCTOR: Can be applied to constructors.
  • ElementType.LOCAL_VARIABLE: Can be applied to local variables.
  • ElementType.ANNOTATION_TYPE: Can be applied to other annotations.

Example:

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
@Target(ElementType.METHOD) // Can only be applied to methods  

public @interface MyMethodAnnotation {


String description();


}

Conclusion

  • Use @Retention to control the lifecycle and availability of your annotation.
  • Use @Target to specify the contexts in which your annotation can be applied.