new BeanHandler＜Student＞(Student.class)；《泛型，反射，一种思想》

素颜马尾好姑娘i 2021-08-28 16:03 246阅读 0赞

## ** ****一、泛型，反射，，一种思想** ##

### 1、new BeanHandler时报错： ###

![format_png][]、

### 2、不知道返回的类型是什么： ###

![format_png 1][]

### 3、应用反射： ###

![format_png 2][]

### 4、找到源代码，找原因，泛型： ###

### ![format_png 3][] ###

### 5、BeanHandler加入泛型，并运用反射，获取返回值： ###

![format_png 4][]

## **二、具体实现** ##

package com.guor.util;
    
    import org.apache.commons.dbutils.QueryRunner;
    import org.apache.commons.dbutils.handlers.BeanHandler;
    
    import java.sql.SQLException;
    
    public class Test {
        public static void testArrayHandler(){
            QueryRunner runner = new QueryRunner(C3P0Demo.getDataSourceWithC3P0());//自动提交事务
            try {
                Student student = runner.query("select * from student",new BeanHandler<Student>(Student.class));
                System.out.println(student.getId()+","+student.getName());
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    
        public static void main(String[] args) {
            testArrayHandler();
        }
    }

查看BeanHandler类：

package org.apache.commons.dbutils.handlers;
    
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import org.apache.commons.dbutils.ResultSetHandler;
    import org.apache.commons.dbutils.RowProcessor;
    
    public class BeanHandler<T> implements ResultSetHandler<T> {
        private final Class<? extends T> type;
        private final RowProcessor convert;
    
        public BeanHandler(Class<? extends T> type) {
            this(type, ArrayHandler.ROW_PROCESSOR);
        }
    
        public BeanHandler(Class<? extends T> type, RowProcessor convert) {
            this.type = type;
            this.convert = convert;
        }
    
        public T handle(ResultSet rs) throws SQLException {
            return rs.next() ? this.convert.toBean(rs, this.type) : null;
        }
    }

BeanHandler：返回结果集中的第一行数据，用对象student接收

反射会通过无参构造来创建对象

![format_png 5][]

[format_png]: /images/20210813/4f1ede60af5f4015818298b337d0d9ac.png
[format_png 1]: /images/20210813/ddee5e246d464e73b65bafb46f11a0f3.png
[format_png 2]: /images/20210813/3fc79785b60d4850b599d05294f31dbc.png
[format_png 3]: /images/20210813/aecfc7b59ef245e3bfa156d00bf24923.png
[format_png 4]: /images/20210813/4ddbb0a14e7b4f21a7eb1890d2a470f3.png
[format_png 5]: /images/20210813/8d18060a23e44d4aadea8080303882f9.png