【Java SE】Annotation

1# Annotation

Provides library support for the Java programming language annotation facility.

2# Key Class

2.1# java.lang

• @Override
  Indicates that a method declaration is intended to override a method declaration in a supertype.
• @Deprecated
  A program element annotated @Deprecated is one that programmers are discouraged from using, typically because it is dangerous, or because a better alternative exists.
  values：deprecation，unchecked，fallthrough，path，serial，finally，all
• @SuppressWarnings
  Indicates that the named compiler warnings should be suppressed in the annotated element (and in all program elements contained in the annotated element).
• @SafeVarargs (since 1.7)
  A programmer assertion that the body of the annotated method or constructor does not perform potentially unsafe operations on its varargs parameter.
• @FunctionalInterface (since 1.8)
  An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification.

2.2# java.lang.Annotation | Annotation Types

• @Target
  Indicates the contexts in which an annotation type is applicable.
• @Relention
  Indicates how long annotations with the annotated type are to be retained.
• @Documented
  Indicates that annotations with a type are to be documented by javadoc and similar tools by default.
• @Inherited
  Indicates that an annotation type is automatically inherited.
• @Native (since 1.8)
  Indicates that a field defining a constant value may be referenced from native code.
• @Repeatable (since 1.8)
  The annotation type java.lang.annotation.Repeatable is used to indicate that the annotation type whose declaration it (meta-)annotates is repeatable.

2.2# java.lang.Annotation | Enum

• ElementType
  • TYPE：Class, interface (including annotation type), or enum declaration
  • FIELD：Field declaration (includes enum constants)
  • METHOD：Method declaration
  • PARAMETER：Formal parameter declaration
  • CONSTRUCTOR：Constructor declaration
  • LOCAL_VARIABLE：Local variable declaration
  • ANNOTATION_TYPE：Annotation type declaration
  • PACKAGE：Package declaration
  • TYPE_PARAMETER：Type parameter declaration
  • TYPE_USE：Use of a type
• RetentionPolicy
  表示需要在什么级别保存该注释信息，用于描述注解的生命周期（即：被描述的注解在什么范围内有效）
  • SOURCE：Annotations are to be discarded by the compiler.在源文件中有效（即源文件保留）
  • CLASS：Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.在class文件中有效（即class保留）
  • RUNTIME：Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.在运行时有效（即运行时保留）

3# Common Annotation Class

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    ElementType[] value();
}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    RetentionPolicy value();
}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}

4# Annotation Elements Returns

• int,float,boolean,byte,double,char,long,short
• String
• Class
• enum
• Annotation
• Array type all of above
  PS. You can use 'default ' to statement the default value the element returns

5# Custom Annotation

5.1# Annotation Class

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Robot {
    public enum RobotSize {BIG, MED, MIN;}
    int robotcount() default 1;
    String robotName() default "ET";
    RobotSize robotSize() default RobotSize.MED;
}

5.2# applied to a method

@Robot(robotcount = 5, robotName = "XMAN", robotSize = Robot.RobotSize.BIG)
    public void buildRobot() {}

5.3# get the annotation info by reflection

public class RobotBuilder {
    public static void main(String[] args) {
        getAnnotationInfo(RobotBuilder.class);
    }

    private static void getAnnotationInfo(Class<RobotBuilder> robotBuilderClass) {
        Method[] methods = robotBuilderClass.getDeclaredMethods();
        for (Method method : methods) {
            Robot annotation = method.getAnnotation(Robot.class);
            if (annotation != null) {
                int robotcount = annotation.robotcount();
                String robotName = annotation.robotName();
                Robot.RobotSize robotSize = annotation.robotSize();
                System.out.println(">>>robotcount:"+robotcount);
                System.out.println(">>>robotName:"+robotName);
                System.out.println(">>>robotSize:"+robotSize);
            }
        }
    }

    @Robot(robotcount = 5, robotName = "XMAN", robotSize = Robot.RobotSize.BIG)
    public void buildRobot() {}
}

output

>>>robotcount:5
>>>robotName:XMAN
>>>robotSize:BIG