Java通过反射注解赋值的方法详解

目录

• 问题描述
• 最终解决
• if/else 普通解法
• 通过反射注解赋值属性
• 解题思路
• 汇总某些字段的和
• 总结
• 源码

前段时间，领导分配一个统计销售区域汇总的数据，解决方案使用到了反射获取注解，通过注解获取属性或者设置字段属性。

问题描述

查询公司列表，分别是公司id、区域id、区域名称:

创建公司类Company：

public class Company {

    public Company(Integer id,  Integer areaId, String areaName) {
        this.id = id;
        this.areaId = areaId;
        this.areaName = areaName;
    }

    /**
     * 公司id
     */
    private Integer id;

    /**
     * 区域id
     */
    private Integer areaId;

    /**
     * 区域名称
     */
    private String areaName;

    // 省略get/set方法 

}

最终解决

要求汇总各个区域公司数量,得到如下汇总：

最终区域实体AreaStatistic:

public class AreaStatistic {

    @ColumnProperty("华东大区")
    private Integer eastChina = 0;

    @ColumnProperty("华东id")
    private Integer eastChinaId;

    @ColumnProperty("华南大区")
    private Integer southChina = 0;

    @ColumnProperty("华南id")
    private Integer southChinaId;

    @ColumnProperty("华北大区")
    private Integer northChina = 0;

    @ColumnProperty("华北id")
    private Integer northChinaId;

    @Override
    public String toString() {
        return "AreaStatistic{\n" +
                "华东Id=" + eastChinaId +
                ",华东=" + eastChina +
                ", \n华南Id=" + southChinaId +
                ", 华南=" + southChina +
                ", \n华北Id=" + northChinaId +
                ", 华北=" + northChina +
                '}';
    }
    // 省略get/set方法
}

if/else 普通解法

AreaStatistic areaStatistic = new AreaStatistic();
for (Company company:companyList) {
    String areaName = company.getAreaName();
    if ("华南".equals(areaName)) {
        areaStatistic.setSouthChina(areaStatistic.getSouthChina()+1);
        areaStatistic.setSouthChinaId(company.getAreaId());
    } else if ("华北".equals(areaName)) {
        areaStatistic.setNorthChina(areaStatistic.getNorthChina()+1);
        areaStatistic.setNorthChinaId(company.getAreaId());
    } else if ("华东".equals(areaName)) {
        areaStatistic.setEastChina(areaStatistic.getEastChina()+1);
        areaStatistic.setEastChinaId(company.getAreaId());
    }
}

输出：

华东Id=3,华东=2, 
华南Id=1, 华南=1, 
华北Id=2, 华北=2

这种做法的缺点：

• 要写大量的条件判断语句，非常的繁琐。
• 增加和减少统计区域，都要修改代码。

针对上面的缺点，使用反射获取注解，通过注解获取属性赋值。

通过反射注解赋值属性

解题思路

1.遍历公司列表，获取到区域id和区域名称。

2.创建自定义注解@ColumnProperty:

@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface ColumnProperty {

    String value() default "";

}

3.通过反射获取属性，然后遍历字段属性获取注解。

在AreaStatistic字段属性上添加注解：

@ColumnProperty("华东大区")
private Integer eastChina = 0;

@ColumnProperty("华东id")
private Integer eastChinaId;

@ColumnProperty("华南大区")
private Integer southChina = 0;

@ColumnProperty("华南id")
private Integer southChinaId;

@ColumnProperty("华北大区")
private Integer northChina = 0;

@ColumnProperty("华北id")
private Integer northChinaId;

4.通过反射获取属性，然后遍历字段属性获取注解。

Class staticClass = areaStatistic.getClass();
Field[] fields = staticClass.getDeclaredFields();
for (Field field : fields) {
    ColumnProperty property = field.getAnnotation(ColumnProperty.class);
    String value = property.value();
}

5.匹配区域名称和字段属性，比如遍历公司区域是华东，就遍历到华东大区注解对应的字段，并赋值或者获取字段值。

if (value != null) {
    int indexOf = value.indexOf("大区");
    if (indexOf != -1 && value.length() == 4) {
        if (areaName.equals(value.substring(0,2))) {
            field.setAccessible(true);
            field.set(areaStatistic,(Integer) field.get(areaStatistic) + 1);
        }
     }
}

6.区域id赋值也是相同的解题思路。

根据上面的思路，有如下代码汇总：

// 遍历公司
for (Company company:companyList) {
  setAreaProperty(areaStatistic2,company.getAreaName(),company.getAreaId());
}

private void setAreaProperty(AreaStatistic areaStatistic,String areaName,Integer areaId) throws IllegalAccessException {
    // 反射获取注解
    Class staticClass = areaStatistic.getClass();
    Field[] fields = staticClass.getDeclaredFields();
    for (Field field : fields) {
        ColumnProperty property = field.getAnnotation(ColumnProperty.class);
        String value = property.value();
        if (value != null) {
            int indexOf = value.indexOf("大区");
            if (indexOf != -1 && value.length() == 4) {
                // 匹配到注解属性并赋值
                if (areaName.equals(value.substring(0,2))) {
                    field.setAccessible(true);
                    field.set(areaStatistic,(Integer) field.get(areaStatistic) + 1);
                    for (Field idField : fields) {
                        ColumnProperty idProperty = idField.getAnnotation(ColumnProperty.class);
                        String idValue = idProperty.value();
                        if (idValue.equals(areaName+"id")) {
                            idField.setAccessible(true);
                            idField.set(areaStatistic,areaId);
                            break;
                        }
                    }
                    break;
                }
            }
        }
    }
}

输出：

华东Id=3,华东=2, 
华南Id=1, 华南=1, 
华北Id=2, 华北=2

汇总某些字段的和

上面算出各个区域的汇总之后，还要算出全部区域的总和，这里还是使用到注解，把属性字段包含大区都累加起来:

AreaStatistic statistic = new AreaStatistic();
statistic.setEastChina(2);
statistic.setNorthChina(3);
statistic.setSouthChina(1);
int sum = 0;
Class staticClass = statistic.getClass();
Field[] fields = staticClass.getDeclaredFields();
for (Field field : fields) {
    ColumnProperty property = field.getAnnotation(ColumnProperty.class);
    String value = property.value();
    if (value.indexOf("大区") != -1) {
        field.setAccessible(true);
        sum += field.get(statistic) == null ? 0 : (Integer) field.get(statistic);
    }
}
System.out.println(sum);

输出结果：

6

总结

1.自定义注解，通过反射获取注解

2.通过匹配注解值，获取或者复制对应的字段属性。

赋值主要代码为:

field.setAccessible(true);
field.set(Model,value);

源码

package reflect;

import org.junit.Test;
import reflect.annotation.ColumnProperty;
import reflect.model.AreaStatistic;
import reflect.model.Company;

import javax.print.DocFlavor;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

/**
 * @Author: laizc
 * @Date: created in 2022-07-21
 * @desc: 通过注解设值
 */
public class SetValueByAnnotation {

    /**
     * 通过属性赋值
     */
    @Test
    public void test() throws IllegalAccessException {
        // 添加数据
        Company company5 = new Company(1,1,"华南");
        Company company1 = new Company(2,2,"华北");
        Company company2 = new Company(3,2,"华北");
        Company company3 = new Company(4,3,"华东");
        Company company4 = new Company(5,3,"华东");
        List<Company> companyList = new ArrayList<>();
        companyList.add(company5);
        companyList.add(company1);
        companyList.add(company2);
        companyList.add(company3);
        companyList.add(company4);

        // 解法1
        AreaStatistic areaStatistic = new AreaStatistic();
        for (Company company:companyList) {
            String areaName = company.getAreaName();
            if ("华南".equals(areaName)) {
                areaStatistic.setSouthChina(areaStatistic.getSouthChina()+1);
                areaStatistic.setSouthChinaId(company.getAreaId());
            } else if ("华北".equals(areaName)) {
                areaStatistic.setNorthChina(areaStatistic.getNorthChina()+1);
                areaStatistic.setNorthChinaId(company.getAreaId());
            } else if ("华东".equals(areaName)) {
                areaStatistic.setEastChina(areaStatistic.getEastChina()+1);
                areaStatistic.setEastChinaId(company.getAreaId());
            }
        }
        System.out.println(areaStatistic);
        // 解法二
        AreaStatistic areaStatistic2 = new AreaStatistic();
        for (Company company:companyList) {
            setAreaProperty(areaStatistic2,company.getAreaName(),company.getAreaId());
        }
        System.out.println(areaStatistic2);

    }

    private void setAreaProperty(AreaStatistic areaStatistic,String areaName,Integer areaId) throws IllegalAccessException {
        Class staticClass = areaStatistic.getClass();
        Field[] fields = staticClass.getDeclaredFields();
        for (Field field : fields) {
            ColumnProperty property = field.getAnnotation(ColumnProperty.class);
            String value = property.value();
            if (value != null) {
                int indexOf = value.indexOf("大区");
                if (indexOf != -1 && value.length() == 4) {
                    if (areaName.equals(value.substring(0,2))) {
                        field.setAccessible(true);
                        field.set(areaStatistic,(Integer) field.get(areaStatistic) + 1);
                        for (Field idField : fields) {
                            ColumnProperty idProperty = idField.getAnnotation(ColumnProperty.class);
                            String idValue = idProperty.value();
                            if (idValue.equals(areaName+"id")) {
                                idField.setAccessible(true);
                                idField.set(areaStatistic,areaId);
                                break;
                            }
                        }
                        break;
                    }
                }
            }
        }
    }

    /**
     * 根据注解累加字段值
     */
    @Test
    public void accumulate() throws IllegalAccessException {
        AreaStatistic statistic = new AreaStatistic();
        statistic.setEastChina(2);
        statistic.setNorthChina(3);
        statistic.setSouthChina(1);
        int sum = 0;
        Class staticClass = statistic.getClass();
        Field[] fields = staticClass.getDeclaredFields();
        for (Field field : fields) {
            ColumnProperty property = field.getAnnotation(ColumnProperty.class);
            String value = property.value();
            if (value.indexOf("大区") != -1) {
                field.setAccessible(true);
                sum += field.get(statistic) == null ? 0 : (Integer) field.get(statistic);
            }
        }
        System.out.println(sum);
    }

}

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