Annotation trước tiên được hiểu là một
dạng meta data.
Meta data là đặc tả dữ liệu cho một đối
tượng, giá trị gì đó.
VD: các tập tin mp3, ảnh, hoặc một bài viết
có thể có meta data dạng XML Format là RSS.
Đặc tả dữ liệu là một tập giá trị chứa những
thông tin gắn gọn, cơ bản mô tả về đối tượng
nào đó.
Để khai báo 1 annotation, bắt đầu bằng 1 dấu “@”,
theo sau là từ khóa interface và tên của annotation.
Có 3 kiểu annotation:
- Maker: không có phần tử, chỉ có duy nhất tên
của annotation
@interface MyAnnotation {}
- Single-element: chỉ chứa 1 dữ liệu đơn lẻ trong
annotation
@interface MyAnnotation {
Java defines seven built-in annotations.
Four are imported from java.lang.annotation:
@Retention, @Documented, @Target, and
@Inherited.
Three, @Override, @Deprecated, and
@SuppressWarnings, are included in java.lang.
@Override
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Deprecated
Deprecated is a marker annotation type that
can be applied to a method or a type
(class/interface) to indicate that the method or
type is deprecated
SuppressWarnings
import java.util.Date;
public class Main {
@SuppressWarnings(value={"deprecation"})
public static void main(String[] args) {
Date date = new Date(2009, 9, 30);
System.out.println("date = " + date);
}
}
import java.util.ArrayList;
import java.util.Iterator;
public class Main {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
ArrayList data = new ArrayList();
data.add("hello");
data.add("world");
SuppressWarnings
SuppressWarnings is used to suppress
compiler warnings. You can apply
@SuppressWarnings to types, constructors,
methods, fields, parameters, and local variables.
The following are valid parameters to
@SuppressWarnings:
unchecked. Give more detail for unchecked
conversion.
Documented
Documented is a marker annotation type used
to annotate the declaration of an annotation
type so that instances of the annotation type
will be included in the documentation.
Override annotation type is not annotated using
Documented.
Deprecated annotation type is annotated
@Documented.
Inherited
Use Inherited to annotate an annotation type,
any instance of the annotation type will be
inherited.
Use Inherited to annotate a class, the
annotation will be inherited by any subclass of
the annotated class. If the user queries the
annotation type on a class declaration, and the
class declaration has no annotation of this type,
then the class's parent class will automatically
be queried for the annotation type. This process
Retention
@Retention indicates how long annotations
whose annotated types are annotated
@Retention are to be retained.
The value of @Retention can be one of the
members of the
java.lang.annotation.RetentionPolicy enum:
SOURCE. Annotations are to be discarded by the
Java compiler.
Target
Target indicates which program element(s) can be
annotated using instances of the annotated
annotation type. The value of Target is one of the
members of the java.lang.annotation.ElementType
enum:
ANNOTATION_TYPE. The annotated annotation type
can be used to annotate annotation type declaration.
CONSTRUCTOR. The annotated annotation type can
be used to annotate constructor declaration.