BasicDAO+druid 运行异常

Question

您好，goods商品表是mysql数据库中的一张商品信息表，GoodsDAO继承BasicDAO，末段代码为运行时报错内容，其中druid配置信息等如下，求解，非常3Q

```java
#key=value
driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/lhc_db03?rewriteBatchedStatements=true
username=root
password=lhc
#initial connection Size
initialSize=10
#min idle connecton size
minIdle=5
#max active connection size
maxActive=50
#max wait time (5000 mil seconds)
maxWait=5000

```java
package dao_.dao;

import dao_.utils.JDBCUtilsByDruid;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import org.apache.commons.dbutils.handlers.ScalarHandler;

import java.sql.Connection;
import java.sql.SQLException;
import java.util.List;

public class BasicDAO<T> {
    private QueryRunner qr = new QueryRunner();
    public int update(String sql,Object...parameters){
        Connection connection = null;
        try {

            connection = JDBCUtilsByDruid.getConnection();
            int update = qr.update(connection,sql,parameters);
            return update;
        } catch (SQLException e) {
           throw new RuntimeException(e);
        } finally {
            JDBCUtilsByDruid.close(null,null,connection);
        }
    }

    public List<T> queryMulti(String sql,Class<T> clazz,Object...parameters){
        Connection connection = null;
        try {
            connection = JDBCUtilsByDruid.getConnection();
            return qr.query(connection,sql,new BeanListHandler<T>(clazz),parameters);
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            JDBCUtilsByDruid.close(null,null,connection);
        }
    }

    public T querySingle(String sql,Class<T> clazz,Object...parameters){
        Connection connection = null;
        try {
            connection=JDBCUtilsByDruid.getConnection();
            return qr.query(connection,sql,new BeanHandler<T>(clazz),parameters);
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            JDBCUtilsByDruid.close(null,null,connection);
        }
    }

    public Object queryScalar(String sql,Object...parameters){
        Connection connection = null;
        try {
            connection = JDBCUtilsByDruid.getConnection();
            return qr.query(connection,sql,new ScalarHandler(),parameters);
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            JDBCUtilsByDruid.close(null,null,connection);
        }

    }
}

package dao_.dao;
public class GoodsDAO extends BasicDAO{


}

```java
package dao_.domain;

public class Goods {
    private int id;
    private String goods_name;
    private double price;

    public Goods(int id, String goods_name, double price) {
        this.id = id;
        this.goods_name = goods_name;
        this.price = price;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getGoods_name() {
        return goods_name;
    }

    public void setGoods_name(String goods_name) {
        this.goods_name = goods_name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Goods{" +
                "id=" + id +
                ", goods_name='" + goods_name + '\'' +
                ", price=" + price +
                '}';
    }
}

package dao_.domain;

public class Goods {
    private int id;
    private String goods_name;
    private double price;

    public Goods(int id, String goods_name, double price) {
        this.id = id;
        this.goods_name = goods_name;
        this.price = price;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getGoods_name() {
        return goods_name;
    }

    public void setGoods_name(String goods_name) {
        this.goods_name = goods_name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Goods{" +
                "id=" + id +
                ", goods_name='" + goods_name + '\'' +
                ", price=" + price +
                '}';
    }
}

```java
一月 31, 2024 9:50:27 上午 com.alibaba.druid.pool.DruidDataSource info
信息: {dataSource-1} inited
Exception in thread "main" java.lang.RuntimeException: java.sql.SQLException: Cannot create dao_.domain.Goods: dao_.domain.Goods Query: select * from goods where price >= ? Parameters: [2000]
    at dao_.dao.BasicDAO.queryMulti(BasicDAO.java:35)
    at dao_.test.TestGoodsDAO.main(TestGoodsDAO.java:11)
Caused by: java.sql.SQLException: Cannot create dao_.domain.Goods: dao_.domain.Goods Query: select * from goods where price >= ? Parameters: [2000]
    at org.apache.commons.dbutils.QueryRunner.rethrow(QueryRunner.java:542)
    at org.apache.commons.dbutils.QueryRunner.query(QueryRunner.java:399)
    at dao_.dao.BasicDAO.queryMulti(BasicDAO.java:33)
    ... 1 more

Process finished with exit code 1

Answer 1

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问题分析： 根据报错信息，异常发生在BasicDAO的queryMulti()方法中，具体是在qr.query()方法执行时出错。报错信息为：java.sql.SQLException: Cannot create dao_.domain.Goods: dao_.domain.Goods Query: select * from goods where price >= ? Parameters: [2000]。

解决方案： 根据异常信息，无法创建Goods对象。这可能是由于Goods类缺少无参构造函数。您可以添加一个无参构造函数来解决此问题，如下所示：

public Goods() {}

请注意，当使用BeanHandler或BeanListHandler处理器时，需要保证被处理的类具有无参构造函数。

尝试添加无参构造函数后，重新运行您的代码，看是否仍然报错。