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

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。

3. Annotation的使用（类、接口范围）

annotation的定义

package annotationDefinition;

import annotationApplication.MyAnnotation;

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

annotation的使用

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

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：

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

4. Annotation的使用（方法范围）

annotation的定义

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();
}

annotation的使用

package annotationDefinition;

import annotationApplication.MethodAnnotation;

public class MethodAnnotationUsage {

@MethodAnnotation(methodName = "testMethod", methodReturnType = "void")
public void testMethod(){
}
}

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

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：

annotationed method name -- > testMethod
annotationed method's return type -- > void

5. Annotation的使用（field范围）

annotation的定义

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.FIELD)
public @interface FieldAnnotation {
public String fieldType();
public String fieldModifier();
}

annotation的使用

package annotationDefinition;

import annotationApplication.FieldAnnotation;

public class FieldAnnotationUsage {
@FieldAnnotation(fieldType = "int", fieldModifier = "public")
public double i = 0.01;

@FieldAnnotation(fieldType = "String", fieldModifier = "private")
private String testString;
}

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

package annotationDefinition;

import annotationApplication.FieldAnnotation;

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

public class FieldAnnotationFinder {
public static void main(String args[]) {
Class<?> clazz = FieldAnnotationUsage.class;
Field[] fields = clazz.getDeclaredFields();
Annotation[] annotation = null;

for (int i = 0; i < fields.length; i++) {
annotation = fields[i].getDeclaredAnnotations();
for (int j = 0; j < annotation.length; j++) {
FieldAnnotation fieldAnnotation = (FieldAnnotation) annotation[j];
System.out.println(" field's annotation type value -- > " + fieldAnnotation.fieldType());
System.out.println(" field's annotation modifier value -- > " + fieldAnnotation.fieldModifier());
}
System.out.println(" annotationed field's name -- > " + fields[i].getName());
}
}
}

运行结果：

field's annotation type value -- > int
field's annotation modifier value -- > public
annotationed field's name -- > i
field's annotation type value -- > String
field's annotation modifier value -- > private
annotationed field's name -- > testString

5. Annotation的使用（Parameter范围）

annotation的范围

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

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

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：

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