注解Annotation

JDK内置的三个基本注解

在编译时进行格式检查

• @override：限定重写父类方法，该注解只能用于方法
• @Deprecated：用于表示所修饰的元素（类，方法等）已经过时。
• @SuppressWarnings：抑制编译器警告（例如定义了一个变量但是没有使用，编译就会警告没有使用（idea中变量会呈灰色），加上这个注解后就不会警告了）

		@SuppressWarnings("unused")
        int num = 10; 
        @SuppressWarnings({"unused","rawtypes"})
        ArrayList list = new ArrayList();

自定义注解Annotation

可以参照@SuppressWarnings定义

• 注解声明为：@interfacce
• 内部定义成员，通常使用value表示
• 可以定义成员的默认值，使用default定义
• 如果自定义注解没有成员，表明是一个标识作用。

如果注解有成员，在使用注解时，需要指明成员的值(无默认值时)。

public @interface MyAnnotation {
    String value();
}

or

public @interface MyAnnotation {
    String value() dafault "hello";
}

JDK提供的四种元注解

元注解：对现有的注解进行解释说明的注解

• Retention：指定所修饰的Annotation的生命周期：SOURCE/CLASS(默认)/RUNTIME(只有声明为RUNTIME的注解才能通过反射获取)
  ①SOURCE：Annotations are to be discarded by the compiler.
  ②CLASS：Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time. This is the default behavior.
  ③RUNTIME：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.

//@Retention(RetentionPolicy.SOURCE)
//@Retention(RetentionPolicy.RUNTIME)
@Retention(RetentionPolicy.CLASS)
public @interface MyAnnotation {
    String value() default "hello";
}

• Target：用于修饰Annotation定义，用于指定被修饰的Annotation能用于修饰那些程序元素。@Target也包含一个名为value的成员变量。
  ① TYPE：Class, interface (including annotation type), or enum declaration
  ② FIELD：Field declaration (includes enum constants)
  ③ METHOD：Method declaration
  ④ PARAMETER：Formal parameter declaration
  ⑤ CONSTRUCTOR：Constructor declaration
  ⑥ LOCAL_VARIABLE：Local variable declaration
  ⑦ ANNOTATION_TYPE：Annotation type declaration
  ⑧ PACKAGE：Package declaration
• Documented：
• Inherited：被他修饰的Annotation将具有继承性，如果某个类使用了被@Inherited修饰的Annotation，则其子类将自动具有该注解。
  MyAnnotation.java：

package com.pan;

import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

//@Retention(RetentionPolicy.SOURCE)
//@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    String value() default "hello";
}

AnnotationTest.java

package com.pan;

import java.lang.annotation.Annotation;

public class AnnotationTest {
    public static void main(String[] args) {
        Class clazz = Student.class;
        Annotation[] annotations = clazz.getAnnotations();
        for(int i = 0; i < annotations.length;i++){
            System.out.println(annotations[i]);//@com.pan.MyAnnotation(value=hello)
        }
    }
}

@MyAnnotation("hello")
class Person{
    private String name;
    private int age;

    public Person(){}

    @MyAnnotation()
    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }
}
class Student extends Person{
    public Student(){
        super();
    }
}

如果不加Inherited：

MyAnnotation.java：

package com.pan;

import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

//@Retention(RetentionPolicy.SOURCE)
//@Retention(RetentionPolicy.RUNTIME)
//@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    String value() default "hello";
}

AnnotationTest.java

package com.pan;

import java.lang.annotation.Annotation;

public class AnnotationTest {
    public static void main(String[] args) {
        Class clazz = Student.class;
        Annotation[] annotations = clazz.getAnnotations();
        for(int i = 0; i < annotations.length;i++){
            System.out.println(annotations[i]);//啥也没有
        }
    }
}

@MyAnnotation("hello")
class Person{
    private String name;
    private int age;

    public Person(){}

    @MyAnnotation()
    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }
}
class Student extends Person{
    public Student(){
        super();
    }
}

可重复注解(JDK8新特性)

①：在MyAnnotation上声明@Repeatable，成员值为MyAnnotations.class
②：MyAnnotation的Target和Retention和MyAnnotations相同
MyAnnotations.java

package com.pan;

import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotations {
    MyAnnotation[] value();
}

MyAnnotation.java

package com.pan;
import java.lang.annotation.Inherited;
import java.lang.annotation.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

//@Retention(RetentionPolicy.SOURCE)
//@Retention(RetentionPolicy.RUNTIME)
@Repeatable(MyAnnotations.class)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    String value() default "hello";
}

AnnotationTest.java

package com.pan;
import java.lang.annotation.Annotation;
public class AnnotationTest {
    public static void main(String[] args) {
        Class clazz = Student.class;
        Annotation[] annotations = clazz.getAnnotations();
        for(int i = 0; i < annotations.length;i++){
            System.out.println(annotations[i]);
            //@com.pan.MyAnnotations(value=[@com.pan.MyAnnotation(value=hello), @com.pan.MyAnnotation(value=Hi)])
        }
    }
}

@MyAnnotation("hello")
@MyAnnotation("Hi")
class Person{
    private String name;
    private int age;

    public Person(){}

    @MyAnnotation()
    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }
}
class Student extends Person{
    public Student(){
        super();
    }
}

类型注解(JDK8新特性)

• ElementType.TYPE_PARAMETER:表示该注解能写在类型变量的声明语句中（如：泛型声明）
• ElementType.TYPE_USE:表示该注解能写在使用类型的任何语句中

package com.pan;
import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
//@Retention(RetentionPolicy.SOURCE)
//@Retention(RetentionPolicy.RUNTIME)
@Repeatable(MyAnnotations.class)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target({TYPE,FIELD,METHOD,PARAMETER,CONSTRUCTOR,LOCAL_VARIABLE,TYPE_PARAMETER,TYPE_USE})
public @interface MyAnnotation {
    String value() default "hello";
}

class Generic<@MyAnnotation T>{
    public void show() throws @MyAnnotation RuntimeException{
        ArrayList<@MyAnnotation String> list = new ArrayList<>();
        int num = (@MyAnnotation int)10L;
    }
}

视频地址