【Java基础】Java注解简单入门

注解简单来说就是配置，是特别的配置，之前常用的配置文件，可以用注解替换。然后通过反射去获取注解的信息。

如何定义一个注解

你在IDE中新建一个注解定义，是这样的结构的：

package com.nicchagil.exercise.springbootexercise.annotation;

public @interface MyFirstAnnotation {

}

然后大概有4个对上述结构加上一些配置，当然，这配置是以注解的形式添加的=_=!

此注解使用在哪里

此注解会应用的哪里，可通过如下配置：

保留在什么时候

保留到什么时候：

注解体现在文档中

@Documented

子类是否继承父类的注解

@Inherited

用反射获取注解的信息

我们先定义一个注解：

package com.nicchagil.exercise.springbootexercise.annotation;

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

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface PojoPostProcessing {

	public Class targetClass();

	public String[] whiteProperties();

}

使用注解，比如这里我设置到一个方法上：

	@PojoPostProcessing(targetClass=User.class, whiteProperties={"name"})
	public List<User> selectXxx(String id) {
		......
	}

反射获取注解的信息：

	@Test
	public void test() throws NoSuchMethodException, SecurityException, ClassNotFoundException {
		Class clazz = Class.forName("com.nicchagil.exercise.springbootexercise.service.UserService");

		Method method = clazz.getMethod("selectXxx", String.class);

		boolean isAnnotationPresent = method.isAnnotationPresent(PojoPostProcessing.class);
		if (!isAnnotationPresent) {
			return;
		}

		PojoPostProcessing dpp = (PojoPostProcessing)method.getAnnotation(PojoPostProcessing.class);
		this.logger.info("dpp : {}", dpp);
	}

日志：

dpp : @com.nicchagil.exercise.springbootexercise.annotation.PojoPostProcessing(targetClass=class com.nicchagil.exercise.springbootexercise.mapper.entity.User, whiteProperties=[name])

在Spring AOP中解析注解，然后实体你注解需要做的业务

比如以下就是在Spring AOP中拦截到方法，通过JoinPoint获取反射的Method，然后看是否有指定注解，如果有注解，后面就是关于此注解要做的业务逻辑了：

package com.nicchagil.exercise.springbootexercise.aop;

import java.lang.reflect.Method;

import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.Signature;
import org.aspectj.lang.annotation.AfterReturning;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.aspectj.lang.reflect.MethodSignature;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Configuration;

import com.nicchagil.exercise.springbootexercise.annotation.PojoPostProcessing;

@Aspect
@Configuration
public class ServiceControllerLogAop {

	private Logger logger = LoggerFactory.getLogger(this.getClass());

	@Pointcut("execution(* com.nicchagil.exercise.springbootexercise.service.*Service.*(..))")
    public void myPointcut() {
    }

    @AfterReturning("myPointcut()")
    public void myBefore(JoinPoint joinPoint) {
    	/* 获取方法对象 */
        Signature signature = joinPoint.getSignature();
        if (signature == null && !(signature instanceof MethodSignature)) {
        	return;
        }

        MethodSignature methodSignature = (MethodSignature)signature;
        Method method = methodSignature.getMethod();

        /* 是否有指定注解 */
		boolean isAnnotationPresent = method.isAnnotationPresent(PojoPostProcessing.class);
		if (!isAnnotationPresent) {
			return;
		}

		/* 获取注解信息 */
		PojoPostProcessing dpp = (PojoPostProcessing)method.getAnnotation(PojoPostProcessing.class);
		this.logger.info("dpp : {}", dpp);
    }

}

过滤注解方法的返回值的指定属性

将属性置空的工具类：

package com.nicchagil.exercise.springbootexercise.util;

import java.beans.PropertyDescriptor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeanUtils;
import org.springframework.core.MethodParameter;

public class BeanClearUtils {

	private static Logger logger = LoggerFactory.getLogger(BeanClearUtils.class);

	public static void clearProperty(Object obj, String[] whiteProperty) {
		List<PropertyDescriptor> clearList = BeanClearUtils.getClearProperty(obj.getClass(), whiteProperty);

		try {
			for (PropertyDescriptor pd : clearList) {
				MethodParameter mp = BeanUtils.getWriteMethodParameter(pd);
				Method method = mp.getMethod();
				method.invoke(obj, new Object[] {null});
			}
		} catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
			throw new RuntimeException("设置空属性失败", e);
		}
	}

	/**
	 * 获取需要清除的字段列表
	 */
	public static List<PropertyDescriptor> getClearProperty(Class<?> clazz, String[] whiteProperty) {
		PropertyDescriptor[] allPropertyDescriptor = BeanUtils.getPropertyDescriptors(clazz);

		/* 需保留的字段 */
		Set<String> whitePropertySet = new HashSet<String>(Arrays.asList(whiteProperty));
		whitePropertySet.add("class"); // 此字段无须清除

		/* 需清除的字段 */
		List<PropertyDescriptor> clearList = new ArrayList<PropertyDescriptor>();
		for (PropertyDescriptor propertyDescriptor : allPropertyDescriptor) {
			if (!whitePropertySet.contains(propertyDescriptor.getName())) {
				clearList.add(propertyDescriptor);
			}
		}
		return clearList;
	}

}

定义注解：

package com.nicchagil.exercise.springbootexercise.annotation;

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

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface PojoPostProcessing {

	public String[] whiteProperties();

}

处理注解逻辑：

package com.nicchagil.exercise.springbootexercise.aop;

import java.lang.reflect.Method;

import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.Signature;
import org.aspectj.lang.annotation.AfterReturning;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.aspectj.lang.reflect.MethodSignature;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Configuration;

import com.nicchagil.exercise.springbootexercise.annotation.PojoPostProcessing;
import com.nicchagil.exercise.springbootexercise.util.BeanClearUtils;

@Aspect
@Configuration
public class BeanPropertyFilterAop {

	private Logger logger = LoggerFactory.getLogger(this.getClass());

	@Pointcut("execution(* com.nicchagil.exercise.springbootexercise.service.*Service.*(..))")
	public void myPojoPostProcessingPointcut() {
	}

	@AfterReturning(value = "myPojoPostProcessingPointcut()", returning = "returnObject")
	public void myAfter(JoinPoint joinPoint, Object returnObject) {
		/* 获取方法对象 */
		Signature signature = joinPoint.getSignature();
		if (signature == null && !(signature instanceof MethodSignature)) {
			return;
		}

		MethodSignature methodSignature = (MethodSignature) signature;
		Method method = methodSignature.getMethod();

		/* 是否有指定注解 */
		boolean isAnnotationPresent = method.isAnnotationPresent(PojoPostProcessing.class);
		if (!isAnnotationPresent) {
			return;
		}

		/* 获取注解信息 */
		PojoPostProcessing dpp = (PojoPostProcessing) method.getAnnotation(PojoPostProcessing.class);
		this.logger.info("dpp : {}", dpp);

		/* 只保留bean的指定属性值（其它属性值置空） */
		logger.info("before bean : {}", returnObject);
		BeanClearUtils.clearProperty(returnObject, dpp.whiteProperties());
		logger.info("after  bean : {}", returnObject);
	}

}

使用注解：

@PojoPostProcessing(whiteProperties={"name", "createTime"})