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The @Target annotation is a meta-annotation in Java used to restrict the syntactic contexts where a custom annotation can be applied. By specifying one or more ElementType enum constants, it dictates the exact Java elements that are legally permitted to be decorated by the defined annotation, enforcing these restrictions at compile time.

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Syntax and Declaration

The @Target annotation accepts a single element named value, which is an array of java.lang.annotation.ElementType. 
import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

// Single element target
@Target(ElementType.METHOD)
public @interface SingleTargetAnnotation {}

// Multiple element targets require array initializer syntax
@Target({ElementType.FIELD, ElementType.CONSTRUCTOR, ElementType.METHOD})
public @interface MultiTargetAnnotation {}
If the @Target meta-annotation is omitted from an annotation declaration, the resulting annotation can be applied to any declaration context (classes, methods, fields, etc.), but it cannot be applied to type parameter or type use contexts.

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The ElementType Enum

The scope of @Target is strictly defined by the constants within the ElementType enum. Each constant maps to a specific structural component of the Java programming language:
  • TYPE: Class, interface (including annotation interfaces), enum, or record declaration.
  • FIELD: Field declaration, including enum constants.
  • METHOD: Method declaration.
  • PARAMETER: Formal parameter declaration.
  • CONSTRUCTOR: Constructor declaration.
  • LOCAL_VARIABLE: Local variable declaration.
  • ANNOTATION_TYPE: Annotation interface declaration (used to create other meta-annotations).
  • PACKAGE: Package declaration (applied within package-info.java).
  • TYPE_PARAMETER: Type variable declaration (introduced in Java 8).
  • TYPE_USE: Any use of a type (introduced in Java 8).
  • MODULE: Module declaration (introduced in Java 9, applied within module-info.java).
  • RECORD_COMPONENT: Record component declaration (introduced in Java 14).

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Compilation Mechanics

The Java compiler reads the @Target annotation during the annotation processing phase. If a developer attempts to apply an annotation to a structural element not explicitly declared in the annotation’s @Target array, the compiler throws an annotation type not applicable to this kind of declaration error. Internally, @Target is defined as follows, demonstrating that it is restricted to annotating other annotations: 
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    ElementType[] value();
}
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