手写mybatis之Mapper XML的解析和注册使用

前言

你是怎么面对功能迭代的？
很多程序员在刚开始做编程或者新加入一家公司时，都没有多少机会可以做一个新项目，大部分时候都是在老项目上不断的迭代更新。在这个过程你可能要学习N个前人留下的各式各样的风格迥异的代码片段，在这些纵横交错的流程中，找到一席之地，把自己的ifelse加进去。
但说回来，其实不能逐步清理一片屎山，让代码在你的手上逐步清晰、整洁、干净，很多时候也是作为码农自身经验的不足，不懂得系统重构、不了解设计原则、不熟悉业务背景、不清楚产品走向等等原因造成的。所以最好的办法是提升自身的能力，每接到一次需求都有一些技术上的改变，既然它是屎山，那就当做打怪升级了，修一点、改一块、补一片，总会在你手上越来越易于维护和扩展的。
设计
上一章节我们使用了 MapperRegistry 对包路径进行扫描注册映射器，并在 DefaultSqlSession 中进行使用。那么在我们可以把这些命名空间、SQL描述、映射信息统一维护到每一个 DAO 对应的 Mapper XML 的文件以后，其实 XML 就是我们的源头了。通过对 XML 文件的解析和处理就可以完成 Mapper 映射器的注册和 SQL 管理。

XML 解析和注册类实现关系

1:SqlSessionFactoryBuilder 作为整个 Mybatis 的入口，提供建造者工厂，包装 XML 解析处理，并返回对应 SqlSessionFactory 处理类。

2:通过解析把 XML 信息注册到 Configuration 配置类中，再通过传递 Configuration 配置类到各个逻辑处理类里，包括 DefaultSqlSession 中，这样就可以在获取映射器和执行SQL的时候，从配置类中拿到对应的内容了。

构建SqlSessionFactory建造者工厂

java 复制代码

package com.lm.mybatis.session;

public class SqlSessionFactoryBuilder {
    public SqlSessionFactory build(Reader reader) {
        XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder(reader);
        return build(xmlConfigBuilder.parse());
    }

    public SqlSessionFactory build(Configuration config) {
        return new DefaultSqlSessionFactory(config);
    }


}

java 复制代码

package com.lm.mybatis.builder.xml;

import com.lm.mybatis.builder.BaseBuilder;
import org.dom4j.Element;

public class XMLConfigBuilder extends BaseBuilder {

    private Element root;

    public XMLConfigBuilder(Reader reader) {
        // 1. 调用父类初始化Configuration
        super(new Configuration());
        // 2. dom4j 处理 xml
        SAXReader saxReader = new SAXReader();
        try {
            Document document = saxReader.read(new InputSource(reader));
            root = document.getRootElement();
        } catch (DocumentException e) {
            e.printStackTrace();
        }
    }

    /**
     * 解析配置；类型别名、插件、对象工厂、对象包装工厂、设置、环境、类型转换、映射器
     *
     * @return Configuration
     */
    public Configuration parse() {
        try {
            // 解析映射器
            mapperElement(root.element("mappers"));
        } catch (Exception e) {
            throw new RuntimeException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
        }
        return configuration;
    }

    private void mapperElement(Element mappers) throws Exception {
        List<Element> mapperList = mappers.elements("mapper");
        for (Element e : mapperList) {
            String resource = e.attributeValue("resource");
            Reader reader = Resources.getResourceAsReader(resource);
            SAXReader saxReader = new SAXReader();
            Document document = saxReader.read(new InputSource(reader));
            Element root = document.getRootElement();
            //命名空间
            String namespace = root.attributeValue("namespace");

            // SELECT
            List<Element> selectNodes = root.elements("select");
            for (Element node : selectNodes) {
                String id = node.attributeValue("id");
                String parameterType = node.attributeValue("parameterType");
                String resultType = node.attributeValue("resultType");
                String sql = node.getText();

                // ? 匹配
                Map<Integer, String> parameter = new HashMap<>();
                Pattern pattern = Pattern.compile("(#\\{(.*?)})");
                Matcher matcher = pattern.matcher(sql);
                for (int i = 1; matcher.find(); i++) {
                    String g1 = matcher.group(1);
                    String g2 = matcher.group(2);
                    parameter.put(i, g2);
                    sql = sql.replace(g1, "?");
                }

                String msId = namespace + "." + id;
                String nodeName = node.getName();
                SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
                MappedStatement mappedStatement = new MappedStatement.Builder(configuration, msId, sqlCommandType, parameterType, resultType, sql, parameter).build();
                // 添加解析 SQL
                configuration.addMappedStatement(mappedStatement);
            }

            // 注册Mapper映射器
            configuration.addMapper(Resources.classForName(namespace));
        }
    }

}

java 复制代码

package com.lm.mybatis.io;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;

public class Resources {

    public static Reader getResourceAsReader(String resource) throws IOException {
        return new InputStreamReader(getResourceAsStream(resource));
    }

    private static InputStream getResourceAsStream(String resource) throws IOException {
        ClassLoader[] classLoaders = getClassLoaders();
        for (ClassLoader classLoader : classLoaders) {
            InputStream inputStream = classLoader.getResourceAsStream(resource);
            if (null != inputStream) {
                return inputStream;
            }
        }
        throw new IOException("Could not find resource " + resource);
    }

    private static ClassLoader[] getClassLoaders() {
        return new ClassLoader[]{
                ClassLoader.getSystemClassLoader(),
                Thread.currentThread().getContextClassLoader()};
    }

    public static Class<?> classForName(String className) throws ClassNotFoundException {
        return Class.forName(className);
    }

}

java 复制代码

package com.lm.mybatis.mapping;

import com.lm.mybatis.session.Configuration;

import java.util.Map;

public class MappedStatement {
    private Configuration configuration;
    private String id;
    private SqlCommandType sqlCommandType;

    private String parameterType;
    private String resultType;
    private String sql;
    private Map<Integer, String> parameter;

    MappedStatement() {
        // constructor disabled
    }

    /**
     * 建造者
     */
    public static class Builder {

        private MappedStatement mappedStatement = new MappedStatement();

        public Builder(Configuration configuration, String id, SqlCommandType sqlCommandType, String parameterType, String resultType, String sql, Map<Integer, String> parameter) {
            mappedStatement.configuration = configuration;
            mappedStatement.id = id;
            mappedStatement.sqlCommandType = sqlCommandType;
            mappedStatement.parameterType = parameterType;
            mappedStatement.resultType = resultType;
            mappedStatement.sql = sql;
            mappedStatement.parameter = parameter;
        }

        public MappedStatement build() {
            assert mappedStatement.configuration != null;
            assert mappedStatement.id != null;
            return mappedStatement;
        }

    }

    public Configuration getConfiguration() {
        return configuration;
    }

    public void setConfiguration(Configuration configuration) {
        this.configuration = configuration;
    }

    public String getId() {
        return id;
    }

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

    public SqlCommandType getSqlCommandType() {
        return sqlCommandType;
    }

    public void setSqlCommandType(SqlCommandType sqlCommandType) {
        this.sqlCommandType = sqlCommandType;
    }

    public String getParameterType() {
        return parameterType;
    }

    public void setParameterType(String parameterType) {
        this.parameterType = parameterType;
    }

    public String getResultType() {
        return resultType;
    }

    public void setResultType(String resultType) {
        this.resultType = resultType;
    }

    public String getSql() {
        return sql;
    }

    public void setSql(String sql) {
        this.sql = sql;
    }

    public Map<Integer, String> getParameter() {
        return parameter;
    }

    public void setParameter(Map<Integer, String> parameter) {
        this.parameter = parameter;
    }

}

java 复制代码

package com.lm.mybatis.mapping;

public enum SqlCommandType {
    /**
     * 未知
     */
    UNKNOWN,
    /**
     * 插入
     */
    INSERT,
    /**
     * 更新
     */
    UPDATE,
    /**
     * 删除
     */
    DELETE,
    /**
     * 查找
     */
    SELECT;

}

java 复制代码

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.lm.test.dao.IUserDao">

    <select id="queryUserInfoById" parameterType="java.lang.Long" resultType="com.lm.test.pojo.User">
        SELECT id, userId, userHead, createTime
        FROM user
        where id = #{id}
    </select>

</mapper>

java 复制代码

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>

    <mappers>
        <mapper resource="mapper/User_Mapper.xml"/>
    </mappers>

</configuration>

java 复制代码

package com.lm.test;

import com.lm.mybatis.binding.MapperProxyFactory;
import com.lm.mybatis.binding.MapperRegistry;
import com.lm.mybatis.io.Resources;
import com.lm.mybatis.session.SqlSession;
import com.lm.mybatis.session.SqlSessionFactory;
import com.lm.mybatis.session.SqlSessionFactoryBuilder;
import com.lm.mybatis.session.defaults.DefaultSqlSessionFactory;
import com.lm.test.dao.IUserDao;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

public class TestApi {
    private Logger logger = LoggerFactory.getLogger(TestApi.class);

    @Test
    public void test_SqlSessionFactory() throws IOException {
        // 1. 从SqlSessionFactory中获取SqlSession
        Reader reader = Resources.getResourceAsReader("mybatis-config-datasource.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        // 2. 获取映射器对象
        IUserDao userDao = sqlSession.getMapper(IUserDao.class);

        // 3. 测试验证
        String res = userDao.queryUserInfoById("10001");
        logger.info("测试结果：{}", res);
    }

}

java 复制代码

package com.lm.test.pojo;

import java.util.Date;

public class User {
    private Long id;
    private String userId;          // 用户ID
    private String userHead;        // 头像
    private Date createTime;        // 创建时间
    private Date updateTime;        // 更新时间

    public Long getId() {
        return id;
    }

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

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public String getUserHead() {
        return userHead;
    }

    public void setUserHead(String userHead) {
        this.userHead = userHead;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Date getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Date updateTime) {
        this.updateTime = updateTime;
    }

}

好了到这里就结束了手写mybatis之Mapper XML的解析和注册使用的学习，大家一定要跟着动手操作起来。需要的源码的可si我获取；