扩展jdk动态代理：手搭mybaits（Mapper映射）

mybaits的资源定位，解析，注册实际上都和springIOC容器的初始化过程大同小易。最关键的一点就是Mapper文件的映射原理。和Hibernate不同的是mybaits只从数据库映射到pojo,是单向的。

mybaits基本原理

仿照mybaits实现最简单的mapper映射

模拟已经解析完成的xml

package eproxy;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class MapperXML {
    static final String nameSapce="eproxy.UserMapper";
    static Map<String,String> map=new ConcurrentHashMap<String,String>();
    static{
        map.put("findById", "SELECT USERNAME,SEX,ADDRESS FROM USER WHERE ID=%d");
        
        
    }
    

}

第一步：创建SqlSession

package eproxy;

import java.lang.reflect.Proxy;

public class MySqlSession {
    private final Executor executor=new SimpleExecutor();
    
    public <T> T  selectOne(String statement, Object parameter){
        return executor.query(statement, parameter);
    }
    
    @SuppressWarnings("unchecked")
    public <T> T getMapper(Class<T> type){
        
        return(T)Proxy.newProxyInstance(MySqlSession.class.getClassLoader(), 
                new Class[]{type}, new MapperProxy<T>(type,this));
        
    }

}

第二步：创建MyProxyMapper代理类

package eproxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class MapperProxy<T> implements InvocationHandler {
    private final Class<T>  mapperinterface;
    private final MySqlSession sqlSession;
    
    public MapperProxy(Class<T> mapperinterface, MySqlSession sqlSession) {
        this.mapperinterface= mapperinterface;
        this.sqlSession=sqlSession;
        
    }
      //核心代码
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if(method.getDeclaringClass().getName().equals(MapperXML.nameSapce)){
            String sql = MapperXML.map.get(method.getName());
            System.out.println(String.format("sql[%s],paramters[%s]", sql,args[0]));
            //存在硬编码
            return sqlSession.selectOne(sql, args[0]);
        }else{
            throw new Exception("name space is not exists");
        }
    }

}

第三步：创建Executor

package eproxy;


import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;




public class SimpleExecutor implements Executor {

    @Override
    public <T> T query(String statement, Object parameter) {
        User user=null;
        try {
            Connection con = getConection();
            PreparedStatement ps = con
                    .prepareStatement(String.format(statement, Integer.parseInt(parameter.toString())));
            ResultSet rs = ps.executeQuery();
            // 硬编码
            user= new User();
            while (rs.next()) {
                user.setUsername(rs.getString(1));
                user.setSex(rs.getString(2));
                user.setAddress(rs.getString(3));
            }
            

        } catch (SQLException e) {
            e.printStackTrace();

        }
        return (T) user;

    }

    private Connection getConection() throws SQLException {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            
            e.printStackTrace();
        }
        Connection connection = DriverManager.
                getConnection("jdbc:mysql://localhost:3306/mybaits?characterEncoding=UTF8","root","admin");
        
        return connection;
    }

}