java反射与注解结合使用(根据传入对象输出查询sql)

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

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
    String value();
}

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

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
    String value();
}

/**
 * 数据库 user表，对应表字段为 userid/username/age/addredd
 */
@Table("user")
public class User {
    @Column("userid")
    private String userid;
    @Column("username")
    private String username;
    @Column("age")
    private int age;
    @Column("sl")
    private double sl;
    @Column("address")
    private String address;
    // 添加一个不是数据库的字段: 用于测试
    private String noField;

    public double getSl() {
        return sl;
    }

    public void setSl(double sl) {
        this.sl = sl;
    }

    public String getNoField() {
        return noField;
    }

    public void setNoField(String noField) {
        this.noField = noField;
    }

    public String getUserid() {
        return userid;
    }

    public void setUserid(String userid) {
        this.userid = userid;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Objects;

/**
 * 注解
 */
public class AnnotationUtil {
    /**
     * 根据传入的对象 来生成query sql
     *
     * @param object
     * @return
     */
    public static String getSqlByAnnotation(Object object) throws Exception {
        // 使用StringBuilder 线程安全
        StringBuilder sb = new StringBuilder();
        // 检查参数是否为空
        Objects.requireNonNull(object, "该对象不能为空!");
        // 1.通过反射获取该对象的类类型
        Class c = object.getClass();
        // 2.获取该对象的 类注解： 标明表名
        boolean table_flag = c.isAnnotationPresent(Table.class);
        if (!table_flag) {
            throw new NullPointerException();
        }
        Table table = (Table) c.getDeclaredAnnotation(Table.class);
        sb.append("SELECT * FROM ").append(table.value()).append(" WHERE 1 = 1");
        // 3.获取该对象的 字段注解：显示查询字段
        Field[] fields = c.getDeclaredFields();
        boolean field_flag;
        Column column;
        String field_name;
        Method menthod;
        Object val;
        for (Field field : fields) {
            // 首先得拿到字段名称：通过字段上的注解标明该指定是数据库表对应的字段
            field_flag = field.isAnnotationPresent(Column.class);
            if (!field_flag) {
                continue;
            }
            column = field.getDeclaredAnnotation(Column.class);
            // 其次得拿到字段值 : 通过反射调用类的getXxxx方法
            field_name = field.getName();
            menthod = c.getDeclaredMethod("get" + Character.toUpperCase(field_name.charAt(0)) + field_name.substring(1));
            val = menthod.invoke(object);
            // 判断该字段是否为空，不为空，才进行append
            // 这里要注意：有些属性不赋值时候，java会自动生成默认值，如Double默认为0.0
            // 还有其他类型，小伙伴们可以自行测试
            if (!Objects.isNull(val)) {
                if((val instanceof Double) && (double) val == 0.0){
                    continue;
                }
                sb.append(" AND ").append(column.value()).append(" = ");
                sb.append(val);
            }
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        User user = new User();
        user.setUserid("0001");
        user.setAge(23);
        // 用于测试没有带Column注解的字段
        user.setNoField("field");
        try {
            System.out.println(AnnotationUtil.getSqlByAnnotation(user));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}