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mybatis源码分析——TypeHandler的使用与原理

一：TypeHandler的定义

　　mybatis是orm(对象关系模型)框架，需要实现pojo与数据库jdbcType的转换，当设置参数时，会调用到DefaultParameterHandler的setParameter方法，根据参数寻找不同的TypeHandler，将参数设置到PreparedStatement语句中。当返回结果时，会调用DefaultResultSetHandler的getPropertyMappingValue方法，根据propertyMapping找到TypeHandler，

调用TypeHandler的getResult方法，从ResultSet中读取数据，然后封装到pojo上

二：自定义TypeHandler的使用

1：定义一个币种的枚举

public enum Currency {

    cny("CNY","人民币"),
    usd("USD","美元");

    private String code;
    private String desc;

    Currency(String code,String desc){
        this.code = code;
        this.desc = desc;
    }

    private static Map<String,Currency> map = new ConcurrentHashMap<>();

    static{
        for(Currency currency:values()){
            map.put(currency.code,currency);
        }
    }

    public static Currency getCurrencyByCode(String code){
        return map.get(code);
    }


    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getDesc() {
        return desc;
    }

    public void setDesc(String desc) {
        this.desc = desc;
    }
}

定义一个关于币种转换的自定义TypeHandler，设置参数时，将枚举的币种值转换为字符串，设置到statement中，返回

结果时，将字符串的值转换为枚举值

public class CurrencyTypeHandler extends BaseTypeHandler<Currency> {

    @Override public void setNonNullParameter(PreparedStatement ps, int i, Currency parameter, JdbcType jdbcType)
        throws SQLException {
        ps.setString(i,parameter.getCode());
    }

    @Override public Currency getNullableResult(ResultSet rs, String columnName) throws SQLException {
        String currency = rs.getString(columnName);
        return Currency.getCurrencyByCode(currency);
    }

    @Override public Currency getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        return null;
    }

    @Override public Currency getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        return null;
    }
}

mybatis-config.xml中的配置：

    <typeHandlers>
        <typeHandler javaType="com.example.mybatis.model.Currency" jdbcType="VARCHAR"
                     handler="com.example.mybatis.model.CurrencyTypeHandler"/>
    </typeHandlers>

mapper.xml中resultMap元素直接配置result子元素：

<mapper namespace="com.example.mybatis.mapper.UserMapper">

	<resultMap id="baseResultMap" type="com.example.mybatis.model.User">
		<result column="id" property="id" jdbcType="INTEGER"></result>
		<result column="name" property="name" jdbcType="VARCHAR"></result>
		<result column="age" property="age" jdbcType="VARCHAR"></result>
		<result column="currency" property="currency" jdbcType="VARCHAR"
		typeHandler="com.example.mybatis.model.CurrencyTypeHandler"></result>
	</resultMap>

	<resultMap id="test" type="com.example.mybatis.model.User"/>

	<insert id="insert" parameterType="com.example.mybatis.model.User">
		insert into user (name,age) value(#{name},#{age})
	</insert>

	<select id="listUsers" resultMap="baseResultMap" parameterType="com.example.mybatis.model.User">
		select * from user where name = #{name} and currency = #{currency,javaType=com.example.mybatis.model.Currency,
		jdbcType=VARCHAR,typeHandler=com.example.mybatis.model.CurrencyTypeHandler}
	</select>

</mapper>

测试代码：

    public static void main(String[] args) throws IOException {
        // 将mybatis-config的配置文件读入内存，生成字符流对象
        Reader reader = Resources.getResourceAsReader("mybatis-config.xml");

        // 解析全局配置文件mybatis-config.xml
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = builder.build(reader);

        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        //       PageHelper.startPage(1,2);

        // 测试一级缓存：
        List<User> list = userMapper.listUsers("hello105", Currency.USD);
        System.out.println("第一次查询结果：" + list.size());
        System.out.println("第一次查询结果：" + list);
    }

运行结果：字符串已经转换为枚举值：