自定义MyBatis

自定义MyBatis是为了深入了解MyBatis的原理

主要的调用是这样的：

//1.读取配置文件
InputStream in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2.创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3.使用工厂生产SqlSession对象
SqlSession session = factory.openSession();
//4.使用SQLSession创建Dao接口的代理对象
UserDao userDao = session.getMapper(UserDao.class);
//5.使用代理对象执行方法
List<User> users = userDao.findAll();
for (User user : users) {
    System.out.println(user);
}
//6.释放资源
session.close();
in.close();

首先第一步：将配置文件SqlMapConfig.xml转为流文件

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE configuration>

<configuration>
    <!--配置环境-->
    <environments default="mysql">
        <!--配置mysql的环境-->
        <environment id="mysql">
            <!--配置事务类型-->
            <transactionManager type="JDBC"></transactionManager>
            <!--配置数据源（连接池）-->
            <dataSource type="POOLED">
                <!--配置连接数据库的基本信息-->
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/mybatis?useUnicode=true&amp;characterEncoding=utf8"/>
                <property name="username" value="root"/>
                <property name="password" value="1234"/>
            </dataSource>
        </environment>
    </environments>

    <!--指定映射配置文件的位置，映射配置文件指的是每个dao独立的配置文件-->
    <!--<mappers>
        <mapper resource="com/jinke/dao/UserDao.xml"/>
    </mappers>-->
    <!--如果是用注解来配置-->
    <mappers>
        <mapper class="com.jinke.dao.UserDao"/>
    </mappers>
</configuration>

import java.io.InputStream;

/*使用类加载器读取配置文件的类*/
public class Resources {

    public static InputStream getResourceAsStream(String filePath) {
        return Resources.class.getClassLoader().getResourceAsStream(filePath);
    }
}

第二步：解析配置文件

import com.jinke.mybatis.cfg.Configuration;
import com.jinke.mybatis.sqlsession.defaults.DefaultSqlSessionFactory;
import com.jinke.mybatis.utils.XMLConfigBuilder;

import java.io.InputStream;

public class SqlSessionFactoryBuilder {
    public SqlSessionFactory build(InputStream config) {
        Configuration cfg = XMLConfigBuilder.loadConfiguration(config);
        return new DefaultSqlSessionFactory(cfg);
    }
}

主要是通过反射将属性值保存到map中

import com.jinke.mybatis.annotations.Select;
import com.jinke.mybatis.cfg.Configuration;
import com.jinke.mybatis.cfg.Mapper;
import com.jinke.mybatis.io.Resources;
import org.dom4j.Attribute;
import org.dom4j.Document;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class XMLConfigBuilder {
    public static Configuration loadConfiguration(InputStream config) {
        Configuration cfg = new Configuration();
        try {

            SAXReader reader = new SAXReader();

            Document document = reader.read(config);

            Element root = document.getRootElement();
            List<Element> propertyElements = root.selectNodes("//property");

            for (Element propertyElement : propertyElements) {
                String name = propertyElement.attributeValue("name");
                if ("driver".equals(name)) {
                    String driver = propertyElement.attributeValue("value");
                    cfg.setDriver(driver);
                }
                if ("url".equals(name)) {
                    String url = propertyElement.attributeValue("value");
                    cfg.setUrl(url);
                }
                if ("username".equals(name)) {
                    String username = propertyElement.attributeValue("value");
                    cfg.setUsername(username);
                }
                if ("password".equals(name)) {
                    String password = propertyElement.attributeValue("value");
                    cfg.setPassword(password);
                }
            }
            List<Element> mapperElements = root.selectNodes("//mappers/mapper");
            for (Element mapperElement : mapperElements) {
                Attribute attribute = mapperElement.attribute("resource");
                if (attribute != null) {
                    System.out.println("使用的是XML");
                    String mapperPath = attribute.getValue();
                    Map<String, Mapper> mappers = loadMapperConfiguration(mapperPath);
                    cfg.setMappers(mappers);
                } else {
                    System.out.println("使用的是注解");
                    String daoClassPath = mapperElement.attributeValue("class");
                    Map<String, Mapper> mappers = loadMapperAnnotation(daoClassPath);
                    cfg.setMappers(mappers);
                }
            }
            return cfg;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                config.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return cfg;
    }

    private static Map<String, Mapper> loadMapperConfiguration(String mapperPath) throws IOException {
        InputStream in = null;
        Map<String, Mapper> mappers = new HashMap<String, Mapper>();
        try {
            in = Resources.getResourceAsStream(mapperPath);
            SAXReader reader = new SAXReader();
            Document document = reader.read(in);
            Element root = document.getRootElement();
            String namespace = root.attributeValue("namespace");
            List<Element> selectElements = root.selectNodes("//select");
            for (Element selectElement : selectElements) {
                String id = selectElement.attributeValue("id");
                String resultType = selectElement.attributeValue("resultType");
                String queryString = selectElement.getText();
                String key = namespace + "." + id;
                Mapper mapper = new Mapper();
                mapper.setQueryString(queryString);
                mapper.setResultType(resultType);
                mappers.put(key, mapper);
            }
            return mappers;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return mappers;
    }

    private static Map<String, Mapper> loadMapperAnnotation(String daoClassPath) throws Exception {
        Map<String, Mapper> mappers = new HashMap<String, Mapper>();
        Class daoClass = Class.forName(daoClassPath);
        Method[] methods = daoClass.getMethods();
        for (Method method : methods) {
            boolean isAnnotated = method.isAnnotationPresent(Select.class);
            if (isAnnotated) {
                Mapper mapper = new Mapper();
                Select selectAnno = method.getAnnotation(Select.class);
                String queryString = selectAnno.value();
                mapper.setQueryString(queryString);
                Type type = method.getGenericReturnType();
                if (type instanceof ParameterizedType) {
                    ParameterizedType ptype = (ParameterizedType) type;
                    Type[] types = ptype.getActualTypeArguments();
                    Class domainClass = (Class) types[0];
                    String resultType = domainClass.getName();
                    mapper.setResultType(resultType);
                }
                String methodName = method.getName();
                String className = method.getDeclaringClass().getName();
                String key = className + "." + methodName;
                mappers.put(key, mapper);
            }
        }
        return mappers;
    }
}

第三步：DefaultSqlSessionFactory工厂生产出DefaultSqlSession对象

import com.jinke.mybatis.cfg.Configuration;
import com.jinke.mybatis.sqlsession.SqlSession;
import com.jinke.mybatis.sqlsession.SqlSessionFactory;

public class DefaultSqlSessionFactory implements SqlSessionFactory {

    private Configuration cfg;

    public DefaultSqlSessionFactory(Configuration cfg) {
        this.cfg = cfg;
    }

    public SqlSession openSession() {
        return new DefaultSqlSession(cfg);
    }
}

第四步：DefaultSqlSession执行动态代理

import com.jinke.mybatis.cfg.Configuration;
import com.jinke.mybatis.sqlsession.SqlSession;
import com.jinke.mybatis.sqlsession.proxy.MapperProxy;
import com.jinke.mybatis.utils.DataSourceUtil;

import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.sql.SQLException;

public class DefaultSqlSession implements SqlSession {

    private Configuration cfg;
    private Connection connection;

    public DefaultSqlSession(Configuration cfg) {
        this.cfg = cfg;
        this.connection = DataSourceUtil.getConnection(cfg);
    }

    public <T> T getMapper(Class<T> daoInterfaceClass) {
        return (T) Proxy.newProxyInstance(daoInterfaceClass.getClassLoader(), new Class[]{daoInterfaceClass}, new MapperProxy(cfg.getMappers(), connection));
    }

    public void close() {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

执行sql语句

import com.jinke.mybatis.cfg.Mapper;
import com.jinke.mybatis.utils.Executor;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.sql.Connection;
import java.util.Map;

public class MapperProxy implements InvocationHandler {

    private Map<String, Mapper> mappers;
    private Connection connection;

    public MapperProxy(Map<String, Mapper> mappers, Connection connection) {
        this.mappers = mappers;
        this.connection = connection;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        String methodName = method.getName();
        String className = method.getDeclaringClass().getName();
        String key = className + "." + methodName;
        Mapper mapper = mappers.get(key);
        if (mapper == null) {
            throw new IllegalArgumentException("传入的参数有误");
        }
        return new Executor().selectList(mapper, connection);
    }
}

import com.jinke.mybatis.cfg.Mapper;

import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

public class Executor {
    public <E> List<E> selectList(Mapper mapper, Connection conn) {
        PreparedStatement pstm = null;
        ResultSet rs = null;
        try {
            String queryString = mapper.getQueryString();
            String resultType = mapper.getResultType();
            Class domainClass = Class.forName(resultType);
            pstm = conn.prepareStatement(queryString);
            rs = pstm.executeQuery();
            List<E> list = new ArrayList<E>();
            while (rs.next()) {
                E obj = (E) domainClass.newInstance();
                ResultSetMetaData rsmd = rs.getMetaData();
                int columnCount = rsmd.getColumnCount();
                for (int i = 1; i < columnCount; i++) {
                    String columnName = rsmd.getColumnName(i);
                    Object columnValue = rs.getObject(columnName);
                    PropertyDescriptor pd = new PropertyDescriptor(columnName, domainClass);
                    Method writeMethod = pd.getWriteMethod();
                    writeMethod.invoke(obj, columnValue);
                }
                list.add(obj);
            }
            return list;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            release(pstm, rs);
        }
        return null;
    }

    private void release(PreparedStatement pstm, ResultSet rs) {
        if (rs != null) {
            try {
                rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        if (pstm != null) {
            try {
                pstm.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

最后放一张文件结构图

代码地址 https://github.com/king1039/MyBatis

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