ORM框架的设计理念

完成一个ORM框架---目标: 能够理解反射泛型注解应用。
要求:JDBC+反射+泛型+注解+maven。

1 创建一个工程Maven.

2 引用依赖。

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.am</groupId>
    <artifactId>orm</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.28</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.2.4</version>
        </dependency>
    </dependencies>

</project>

3 创建一个连接数据库的工具类

db.properties

package com.am.util;
import com.alibaba.druid.pool.DruidDataSource;
import com.alibaba.druid.pool.DruidDataSourceFactory;
import javax.sql.DataSource;
import java.io.InputStream;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Properties;

/**
 * @program: orm
 * @description:
 * @author: 阿木
 * @create: 2022-02-12 08:49
 **/
public class ConnectionUtil {
    private static DataSource dataSource;//声明一个连接池对象
    static{ //静态代码块。随着类的加载而被加载到JVM内存 而且只会被加载一次。
        try {
            InputStream inputStream = ConnectionUtil.class.getClassLoader()
                     .getResourceAsStream("db.properties");

            Properties properties = new Properties(); //读取属性文件内容。
            properties.load(inputStream);

            dataSource = DruidDataSourceFactory.createDataSource(properties);//通过Druid的工厂类对象创建数据源对象
        }catch (Exception e){
            e.printStackTrace();
        }
    }
    //从连接池中获取连接对象。
    public static Connection getConnection() throws Exception{
        Connection connection = dataSource.getConnection();
        return connection;
    }

    //关闭连接对象。
    public static void close(ResultSet rs, PreparedStatement ps,Connection connection){
        try {
            if(rs!=null){
                 rs.close();
            }
            if(ps!=null){
                 ps.close();
            }
            if(connection!=null){
                 connection.close();
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
    }

}

4 创建一个公共的父类。

4.1 添加功能

    //添加操作。
    public int insert(T t) {
        try {
            //sql="insert into 表名 values('值1','值2'....)";
            StringBuffer sql = new StringBuffer("insert into ");
            //获取实体类的反射类对象
            Class<?> aClass = t.getClass();
            //获取类上指定的注解类对象。
            Table table = aClass.getAnnotation(Table.class);
            //如何获取表名。
            String tableName = table.value();

            sql.append(tableName + " values ");

            //获取t中相应的值。new Product(11,"已发送",25.5,300)
            Field[] fields = aClass.getDeclaredFields();
            List<Object> values = new ArrayList<Object>();
            for (Field f : fields) {
                f.setAccessible(true);
                values.add("'" + f.get(t) + "'");
            }
            String replace = values.toString().replace("[", "(").replace("]", ")");

            sql.append(replace);
            System.out.println(sql);

            //执行sql语句。
            connection = ConnectionUtil.getConnection();
            ps = connection.prepareStatement(sql.toString());
            int row = ps.executeUpdate();
            return row;
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            ConnectionUtil.close(rs,ps,connection);
        }
        return 0;
    }

4.2 修改

    //修改。
    public int update(T t) {
        try {
            //sql: update 表名 set 列名='值',列名=值.... where 主键=值；
            StringBuffer sql=new StringBuffer("update ");
            //获取反射类
            Class<?> aClass = t.getClass();
            //获取表名
            String tableName = aClass.getAnnotation(Table.class).value();
            sql.append(tableName+" set ");
            //获取所有的属性对象。
            Field[] fields = aClass.getDeclaredFields();
            String set="";
            String where=" where ";
            for (Field f:fields){
                f.setAccessible(true);
                TableId tableId = f.getAnnotation(TableId.class);
                if(tableId==null){ //不是主键
                    set+=f.getName()+"='"+f.get(t)+"',";
                }else{
                    if(tableId.value().equals("")){
                       where+=f.getName()+"='"+f.get(t)+"'";
                    }else{
                       where+=tableId.value()+"='"+f.get(t)+"'";
                    }
                }
            }
            sql.append(set.substring(0,set.lastIndexOf(","))).append(where);

            //执行sql
            connection=ConnectionUtil.getConnection();
            ps=connection.prepareStatement(sql.toString());
            int row = ps.executeUpdate();
            return row;
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            ConnectionUtil.close(rs,ps,connection);
        }

        return 0;

    }

4.3 删除

    //删除 sql delete from 表名 where 主键=id
    public int delete(Object id) {
        try {
            StringBuffer sql = new StringBuffer("delete from ");
            String tableName = aClass.getAnnotation(Table.class).value();
            sql.append(tableName + " where ");

            //获取所有的属性对象
            Field[] fields = aClass.getDeclaredFields();
            for (Field field : fields) {
                TableId tableId = field.getAnnotation(TableId.class);
                if (tableId != null) {
                    String value = tableId.value();
                    if (value.equals("")) {
                        sql.append(field.getName() + "='" + id + "'");
                    } else {
                        sql.append(tableId.value() + "='" + id + "'");
                    }
                }
            }

            //执行sql
            connection = ConnectionUtil.getConnection();
            ps = connection.prepareStatement(sql.toString());
            int row = ps.executeUpdate();
            return row;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.close(rs, ps, connection);
        }
        return 0;
    }

4.4 查询所有

  //查询所有 select * from 表名 where 主键=值
    public List<T> findAll()  {
        List<T> list=new ArrayList<T>();
        try {
            StringBuffer sql = new StringBuffer("select * from ");
            String tableName = aClass.getAnnotation(Table.class).value();
            sql.append(tableName);

            connection=ConnectionUtil.getConnection();
            ps=connection.prepareStatement(sql.toString());
            rs=ps.executeQuery();
            while(rs.next()){
                T o = (T)aClass.newInstance();
                Field[] fields = aClass.getDeclaredFields();
                for (Field f:fields){
                     f.setAccessible(true);
                    TableId tableId = f.getAnnotation(TableId.class);
                    if(tableId==null){
                        f.set(o,rs.getObject(f.getName()));
                    }else{
                        String value = tableId.value();
                        if(value.equals("")){
                             f.set(o,rs.getObject(f.getName()));
                        }else{
                            f.set(o,rs.getObject(value));
                        }
                    }
                }
                list.add(o);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.close(rs,ps,connection);
        }

        return list;

    }