Java通過反射注解賦值的方法詳解

更新時間：2022年07月22日 09:35:16 作者：小碼code

這篇文章主要為大家詳細(xì)介紹了Java語言如何通過反射實現(xiàn)注解賦值，文中的示例代碼講解詳細(xì)，具有一定的借鑒價值，需要的可以參考一下

問題描述

查詢公司列表，分別是公司id、區(qū)域id、區(qū)域名稱:

公司id	區(qū)域id	區(qū)域名稱
1	1	華南
2	2	華北
3	2	華北
4	3	華東
5	3	華東

創(chuàng)建公司類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;

    /**
     * 區(qū)域id
     */
    private Integer areaId;

    /**
     * 區(qū)域名稱
     */
    private String areaName;
     
    // 省略get/set方法 

}

最終解決

要求匯總各個區(qū)域公司數(shù)量,得到如下匯總：

區(qū)域id	區(qū)域名稱	公司總數(shù)
1	華南	1
2	華北	2
3	華東	2

最終區(qū)域?qū)嶓wAreaStatistic:

public class AreaStatistic {

    @ColumnProperty("華東大區(qū)")
    private Integer eastChina = 0;

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

    @ColumnProperty("華南大區(qū)")
    private Integer southChina = 0;

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

    @ColumnProperty("華北大區(qū)")
    private Integer northChina = 0;

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

    @Override
    public String toString() {
        return "AreaStatistic{\n" +
                "華東Id=" + eastChinaId +
                ",華東=" + eastChina +
                ", \n華南Id=" + southChinaId +
                ", 華南=" + southChina +
                ", \n華北Id=" + northChinaId +
                ", 華北=" + 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 ("華南".equals(areaName)) {
        areaStatistic.setSouthChina(areaStatistic.getSouthChina()+1);
        areaStatistic.setSouthChinaId(company.getAreaId());
    } else if ("華北".equals(areaName)) {
        areaStatistic.setNorthChina(areaStatistic.getNorthChina()+1);
        areaStatistic.setNorthChinaId(company.getAreaId());
    } else if ("華東".equals(areaName)) {
        areaStatistic.setEastChina(areaStatistic.getEastChina()+1);
        areaStatistic.setEastChinaId(company.getAreaId());
    }
}

輸出：

華東Id=3,華東=2,
華南Id=1, 華南=1,
華北Id=2, 華北=2

這種做法的缺點(diǎn)：

要寫大量的條件判斷語句，非常的繁瑣。
增加和減少統(tǒng)計區(qū)域，都要修改代碼。

針對上面的缺點(diǎn)，使用反射獲取注解，通過注解獲取屬性賦值。

通過反射注解賦值屬性

解題思路

1.遍歷公司列表，獲取到區(qū)域id和區(qū)域名稱。

2.創(chuàng)建自定義注解@ColumnProperty:

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

    String value() default "";

}

3.通過反射獲取屬性，然后遍歷字段屬性獲取注解。

在AreaStatistic字段屬性上添加注解：

@ColumnProperty("華東大區(qū)")
private Integer eastChina = 0;

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

@ColumnProperty("華南大區(qū)")
private Integer southChina = 0;

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

@ColumnProperty("華北大區(qū)")
private Integer northChina = 0;

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

4.通過反射獲取屬性，然后遍歷字段屬性獲取注解。

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

5.匹配區(qū)域名稱和字段屬性，比如遍歷公司區(qū)域是華東，就遍歷到華東大區(qū)注解對應(yīng)的字段，并賦值或者獲取字段值。

if (value != null) {
    int indexOf = value.indexOf("大區(qū)");
    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.區(qū)域id賦值也是相同的解題思路。

根據(jù)上面的思路，有如下代碼匯總：

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

private void setAreaProperty(AreaStatistic areaStatistic,String areaName,Integer areaId) throws IllegalAccessException {
    // 反射獲取注解 
    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("大區(qū)");
            if (indexOf != -1 && value.length() == 4) {
                // 匹配到注解屬性并賦值 
                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;
                }
            }
        }
    }
}

輸出：

華東Id=3,華東=2,
華南Id=1, 華南=1,
華北Id=2, 華北=2

匯總某些字段的和

上面算出各個區(qū)域的匯總之后，還要算出全部區(qū)域的總和，這里還是使用到注解，把屬性字段包含大區(qū)都累加起來:

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("大區(qū)") != -1) {
        field.setAccessible(true);
        sum += field.get(statistic) == null ? 0 : (Integer) field.get(statistic);
    }
}
System.out.println(sum);

輸出結(jié)果：

6

總結(jié)

1.自定義注解，通過反射獲取注解

2.通過匹配注解值，獲取或者復(fù)制對應(yīng)的字段屬性。

賦值主要代碼為:

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

源碼

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: 通過注解設(shè)值
 */
public class SetValueByAnnotation {

    /**
     * 通過屬性賦值
     */
    @Test
    public void test() throws IllegalAccessException {
        // 添加數(shù)據(jù)
        Company company5 = new Company(1,1,"華南");
        Company company1 = new Company(2,2,"華北");
        Company company2 = new Company(3,2,"華北");
        Company company3 = new Company(4,3,"華東");
        Company company4 = new Company(5,3,"華東");
        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 ("華南".equals(areaName)) {
                areaStatistic.setSouthChina(areaStatistic.getSouthChina()+1);
                areaStatistic.setSouthChinaId(company.getAreaId());
            } else if ("華北".equals(areaName)) {
                areaStatistic.setNorthChina(areaStatistic.getNorthChina()+1);
                areaStatistic.setNorthChinaId(company.getAreaId());
            } else if ("華東".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("大區(qū)");
                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;
                    }
                }
            }
        }
    }

    /**
     * 根據(jù)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("大區(qū)") != -1) {
                field.setAccessible(true);
                sum += field.get(statistic) == null ? 0 : (Integer) field.get(statistic);
            }
        }
        System.out.println(sum);
    }

}

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