Java Annotation学习笔记

  1. Annotation是什么 Annotation是类，方法或字段的一种注解或元数据，当程序被JVM编译的时候，annotation会和类编译在一起，作为一种元数据去描述被修饰的数据
  2. Annotation的定义
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package annotationApplication;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyAnnotation {
    public String name();

    public int age();
}

其中，Retention中的字段表明，该注解能在运行时通过反射得到。Target表明该注解用于何处。此处为类或者接口处可以使用。

  1. Annotation的使用（类、接口范围） annotation的定义
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package annotationDefinition;

import annotationApplication.MyAnnotation;

@MyAnnotation(name = "Mikel Arteta", age = 29)
public class MyAnnotationUsage {
}

annotation的使用

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package annotationDefinition;

import annotationApplication.MyAnnotation;

@MyAnnotation(name = "Mikel Arteta", age = 29)
public class MyAnnotationUsage {
}

下面是我定义的使用类注解的类的查找器：找到并输出注解类的annotation的value

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package annotationDefinition;

import annotationApplication.MyAnnotation;
import java.lang.annotation.Annotation;

public class MyAnnotationFinder {
    public static void main(String[] args) throws ClassNotFoundException {
        Class<?> myAnnotationUsage = Class.forName("annotationDefinition.MyAnnotationUsage");
        Annotation[] annotations = myAnnotationUsage.getAnnotations();
        for(Annotation annotation : annotations) {
            MyAnnotation annotation1 = (MyAnnotation)annotation;
            System.out.println(annotation1.name());
            System.out.println(annotation1.age());
        }
    }
}

运行结果

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annotation's name value -- > Mikel Arteta
annotation's age value -- > 29

4. Annotation的使用（方法范围） annotation的定义

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package annotationApplication;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MethodAnnotation {
    public String methodName();
    public String methodReturnType();
}

下面是我定义的使用类注解的类的查找器：找到并输出注解类的annotation的value

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package annotationDefinition;

import annotationApplication.MethodAnnotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

public class MethodAnnotationFinder {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
        Class<?> clazz = Class.forName("annotationDefinition.MethodAnnotationUsage");
        Method method = clazz.getDeclaredMethod("testMethod");
        Annotation annotation = method.getAnnotation(MethodAnnotation.class);
        MethodAnnotation annotation1 = (MethodAnnotation) annotation;
        System.out.println("annotationed method name -- > " + annotation1.methodName());
        System.out.println("annotationed method's return type -- > " + annotation1.methodReturnType());
    }
}

运行结果：

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annotationed method name -- > testMethod
  1. Annotation的使用（Parameter范围） annotation的范围
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package annotationApplication;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.PARAMETER)
public @interface ParameterAnnotation {
    public String parameterName();
    public String parameterType();
}

annotation的使用

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package annotationDefinition;

import annotationApplication.ParameterAnnotation;

public class ParameterAnnotationUsage {
    public void testMethod(@ParameterAnnotation(parameterName = "pname", parameterType = "int") int value) {
        value = 10;
    }
}

下面是我定义的使用类注解的类的查找器：找到并输出注解类的annotation的value

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package annotationDefinition;

import annotationApplication.ParameterAnnotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

public class ParameterAnnotationFinder {

    public void parameterAnnotationFinder() throws NoSuchMethodException {
        Class<?> clazz = ParameterAnnotationUsage.class;
        Method method = clazz.getMethod("testMethod", int.class);
        Annotation[][] annotations = method.getParameterAnnotations();
        //Method.getParameterAnnotations() method returns a two-dimensional Annotation array,  
        // containing an array of annotations for each method parameter.  
        Class<?>[] parameterTyeps = method.getParameterTypes();

        int i = 0;
        for (Annotation[] annotation : annotations) {
            Class<?> parameterTyep = parameterTyeps[i++];
            for (Annotation annotation1 : annotation) {
                ParameterAnnotation parameterAnnotation1 = (ParameterAnnotation) annotation1;
                System.out.println("annotation paraName : " + parameterAnnotation1.parameterName());
                System.out.println("annotation paraType : " + parameterAnnotation1.parameterType());
                System.out.println("parameter Type : " + parameterTyep);
            }
        }
    }

    public static void main(String[] args) throws NoSuchMethodException {
        new ParameterAnnotationFinder().parameterAnnotationFinder();
    }
}

运行结果：

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annotation paraName : pname
annotation paraType : int
parameter Type : int