유지 정책과 대상 유형

@Retention(RetentionPolicy.SOURCE)
public @interface GenerateBuilder {}
// APT generates a Builder class when it sees @GenerateBuilder
// The annotation itself disappears from the compiled output

@Retention(RetentionPolicy.CLASS)
public @interface ThreadSafe {}
// Visible in bytecode, invisible to Class.getAnnotation() at runtime

@Retention(RetentionPolicy.RUNTIME)
public @interface Route {
    String path();
    String method() default "GET";
}
// At runtime:
Route r = handler.getClass().getMethod("handle").getAnnotation(Route.class);
System.out.println(r.path()); // e.g. "/users"

import static java.lang.annotation.ElementType.*;
@Target({METHOD, CONSTRUCTOR})
@Retention(RetentionPolicy.RUNTIME)
public @interface Measured {}

// TYPE_USE: annotate any type reference
@NotNull String name;
List<@NonNull String> items;
public @Valid User createUser(@NotNull String name) { ... }

@Target(ElementType.TYPE_USE)
@interface NonNull {}
// Can be applied to:
List<@NonNull String> list;
@NonNull String result = (@NonNull String) obj;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Range {
    int min() default 0;
    int max() default Integer.MAX_VALUE;
}

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface Framework {}
@Framework class Base {}
class Child extends Base {} // @Framework visible on Child too

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
    String description() default "";
}

Retention r = Retry.class.getAnnotation(Retention.class);
System.out.println(r.value()); // RUNTIME, CLASS, or SOURCE