MyBatis配置文件(四)--typeHandlers
typeHandlers又叫类型处理器,就像在JDBC中,我们在PreparedStatement中设置预编译sql所需的参数或执行sql后根据结果集ResultSet对象获取得到的数据时,需要将数据库中的类型和java中字段的类型进行转换一样,在MyBatis中使用typeHandler来实现。所以说白了,typeHandlers就是用来完成javaType和jdbcType之间的转换。举个比较简单的例子,我创建一个博客表,表中的创建时间和修改时间用VARCHAR类型,但是在我的POJO对象中,创建时间和修改时间的类型是Date,这样我在向数据库插入数据时,需要将日期类型转化成VARCHAR,而从数据库中查询出的结果中,又需要将VARCHAR类型转换成Date.在MyBatis中,使用typeHandlers配置来实现这个转换过程。和别名一样,MyBatis中的类型处理器也存在系统定义的和自定义两种,MyBatis会根据javaType和jdbcType来决定采用哪个typeHandler来处理这些转换规则,而且系统定义的能满足大部分需求,可以说是非常好用,用户只需要自定义一些特有的转换规则,如枚举类型。下面分别介绍这两种typeHandler。
一、系统定义的typeHandler
同上一节的typeAliases,typeHandler也能通过Configuration对象获取到,如下:
/**
* 获取类型处理器
*/
public static void getTypeHandlers() {
SqlSession sqlSession = getSqlSession();
TypeHandlerRegistry typeHandlerRegistry = sqlSession.getConfiguration().getTypeHandlerRegistry();
Collection<TypeHandler<?>> handlers = typeHandlerRegistry.getTypeHandlers();
System.out.println(handlers.size());
for (TypeHandler<?> typeHandler : handlers) {
System.out.println(typeHandler.getClass().getName());
}
}
执行结果如下:
39
org.apache.ibatis.type.SqlDateTypeHandler
org.apache.ibatis.type.ClobReaderTypeHandler
org.apache.ibatis.type.LocalTimeTypeHandler
org.apache.ibatis.type.YearMonthTypeHandler
org.apache.ibatis.type.NStringTypeHandler
org.apache.ibatis.type.LocalDateTypeHandler
org.apache.ibatis.type.BigIntegerTypeHandler
org.apache.ibatis.type.OffsetDateTimeTypeHandler
org.apache.ibatis.type.ByteObjectArrayTypeHandler
org.apache.ibatis.type.ArrayTypeHandler
org.apache.ibatis.type.BigDecimalTypeHandler
org.apache.ibatis.type.UnknownTypeHandler
org.apache.ibatis.type.OffsetTimeTypeHandler
org.apache.ibatis.type.ByteArrayTypeHandler
org.apache.ibatis.type.DateOnlyTypeHandler
org.apache.ibatis.type.JapaneseDateTypeHandler
org.apache.ibatis.type.TimeOnlyTypeHandler
org.apache.ibatis.type.NClobTypeHandler
org.apache.ibatis.type.BooleanTypeHandler
org.apache.ibatis.type.BlobTypeHandler
org.apache.ibatis.type.YearTypeHandler
org.apache.ibatis.type.BlobByteObjectArrayTypeHandler
org.apache.ibatis.type.MonthTypeHandler
org.apache.ibatis.type.FloatTypeHandler
org.apache.ibatis.type.DateTypeHandler
org.apache.ibatis.type.ClobTypeHandler
org.apache.ibatis.type.BlobInputStreamTypeHandler
org.apache.ibatis.type.ByteTypeHandler
org.apache.ibatis.type.SqlTimestampTypeHandler
org.apache.ibatis.type.ZonedDateTimeTypeHandler
org.apache.ibatis.type.IntegerTypeHandler
org.apache.ibatis.type.LocalDateTimeTypeHandler
org.apache.ibatis.type.CharacterTypeHandler
org.apache.ibatis.type.SqlTimeTypeHandler org.apache.ibatis.type.DoubleTypeHandler
org.apache.ibatis.type.ShortTypeHandler
org.apache.ibatis.type.LongTypeHandler
39 org.apache.ibatis.type.InstantTypeHandler
40 org.apache.ibatis.type.StringTypeHandler
从结果来看,系统总过定义了39个类型处理器。
现在选择其中的StringTypeHandler来进行分析,看看其源代码
1、获取StringTypeHandler的源代码
/**
* Copyright 2009-2015 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.ibatis.type; import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException; /**
* @author Clinton Begin
*/
public class StringTypeHandler extends BaseTypeHandler<String> { @Override
public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)
throws SQLException {
ps.setString(i, parameter);
} @Override
public String getNullableResult(ResultSet rs, String columnName)
throws SQLException {
return rs.getString(columnName);
} @Override
public String getNullableResult(ResultSet rs, int columnIndex)
throws SQLException {
return rs.getString(columnIndex);
} @Override
public String getNullableResult(CallableStatement cs, int columnIndex)
throws SQLException {
return cs.getString(columnIndex);
}
}
从上述代码可以看出它继承了一个叫做BaseTypeHandler<String>的类,这个类的范型是String,即javaType,几个方法如下:
1⃣️setNonNullParameter:这个方法是用来将javaType转换成jdbcTpe
2⃣️getNullableResult:这个方法用来将从结果集根据列名称获取到的数据的jdbcType转换成javaType
3⃣️getNullableResult:这个方法用来将从结果集根据列索引获取到的数据的jdbcType转换成javaType
4⃣️getNullableResult:这个方法用在存储过程中
其实主要就是完成不同类型之间的转换,下面来看一下它所继承的BaseTypeHandler<String>类
2、BaseTypeHandler<String>类源码
/**
* Copyright 2009-2015 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.ibatis.type; import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException; import org.apache.ibatis.executor.result.ResultMapException;
import org.apache.ibatis.session.Configuration; /**
* @author Clinton Begin
* @author Simone Tripodi
*/
public abstract class BaseTypeHandler<T> extends TypeReference<T> implements TypeHandler<T> { protected Configuration configuration; public void setConfiguration(Configuration c) {
this.configuration = c;
} @Override
public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
if (parameter == null) {
if (jdbcType == null) {
throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
}
try {
ps.setNull(i, jdbcType.TYPE_CODE);
} catch (SQLException e) {
throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
"Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. " +
"Cause: " + e, e);
}
} else {
try {
setNonNullParameter(ps, i, parameter, jdbcType);
} catch (Exception e) {
throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
"Try setting a different JdbcType for this parameter or a different configuration property. " +
"Cause: " + e, e);
}
}
} @Override
public T getResult(ResultSet rs, String columnName) throws SQLException {
T result;
try {
result = getNullableResult(rs, columnName);
} catch (Exception e) {
throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set. Cause: " + e, e);
}
if (rs.wasNull()) {
return null;
} else {
return result;
}
} @Override
public T getResult(ResultSet rs, int columnIndex) throws SQLException {
T result;
try {
result = getNullableResult(rs, columnIndex);
} catch (Exception e) {
throw new ResultMapException("Error attempting to get column #" + columnIndex+ " from result set. Cause: " + e, e);
}
if (rs.wasNull()) {
return null;
} else {
return result;
}
} @Override
public T getResult(CallableStatement cs, int columnIndex) throws SQLException {
T result;
try {
result = getNullableResult(cs, columnIndex);
} catch (Exception e) {
throw new ResultMapException("Error attempting to get column #" + columnIndex+ " from callable statement. Cause: " + e, e);
}
if (cs.wasNull()) {
return null;
} else {
return result;
}
} public abstract void setNonNullParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException; public abstract T getNullableResult(ResultSet rs, String columnName) throws SQLException; public abstract T getNullableResult(ResultSet rs, int columnIndex) throws SQLException; public abstract T getNullableResult(CallableStatement cs, int columnIndex) throws SQLException; }
此类是一个抽象类,重写了四个接口之外,还有四个抽象方法,也就是在StringTypeHandler中实现的接口,那么重写的四个接口是什么呢?我们去看一下它所实现的TypeHandler<T>接口
3、接口TypeHandler<T>源代码
/**
* Copyright 2009-2015 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.ibatis.type; import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException; /**
* @author Clinton Begin
*/
public interface TypeHandler<T> { void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException; T getResult(ResultSet rs, String columnName) throws SQLException; T getResult(ResultSet rs, int columnIndex) throws SQLException; T getResult(CallableStatement cs, int columnIndex) throws SQLException; }
这里也定义了四个接口,一个set方法用来将javaType转为jdbcType,三个get方法用来将jdbcType转为javaType,而在BaseTypeHandler中实现的就是这四个方法。
所以当我们自定义自己的typeHandler时有两种方法:
第一种:继承BaseTypeHandler类
第二种:实现TypeHandler接口
二、自定义typeHandler
我使用实现TypeHandler接口的方式创建一个MyDateHandler,用来完成javaType中的Date类型与jdbcType中的varchar类型之间的转化。
创建MyDateHandler
package com.daily.handler; import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.text.SimpleDateFormat;
import java.util.Date; import org.apache.ibatis.type.DateTypeHandler;
import org.apache.ibatis.type.JdbcType;
import org.apache.ibatis.type.TypeHandler;
import org.postgresql.jdbc2.optional.SimpleDataSource; /**
* 自定义一个TypeHandler用来将javatype的日期类型和jdbctype的VARCHAR进行转换
*/
public class MyDateHandler implements TypeHandler<Date> {
@Override
// 设置sql中指定索引的参数,即将javaType转化为jdbcType
public void setParameter(PreparedStatement ps, int i, Date parameter, JdbcType jdbcType) throws SQLException {
//设置数据存储到数据库中的格式
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
ps.setString(i, sdf.format(parameter));
} @Override
// 根据列名称从结果集获取值,并将jdbcType转换成javaType
public Date getResult(ResultSet rs, String columnName) throws SQLException {
String columnValue = rs.getString(columnName);
if (null != columnValue) {
return new Date(Long.valueOf(columnValue));
}
return null;
} @Override
// 根据列名称从结果集获取值,并将jdbcType转换成javaType
public Date getResult(ResultSet rs, int columnIndex) throws SQLException {
String columnValue = rs.getString(columnIndex);
if (null != columnValue) {
return new Date(Long.valueOf(columnValue));
}
return null;
} @Override
public Date getResult(CallableStatement cs, int columnIndex) throws SQLException {
String columnValue = cs.getString(columnIndex);
if (null != columnValue) {
return new Date(Long.valueOf(columnValue));
}
return null;
} }
下面用一个例子来说明这个handler的使用。
1⃣️创建一个blog表,表中的时间用varchar类型
2⃣️创建POJO对象,将create_time和modify_time的类型定义为Date
package com.daily.pojo; import java.util.Date; public class Blog {
private Integer blogId; private String blogTitle; private String blogContent; private Date createTime; private Date modifyTime; public Integer getBlogId() {
return blogId;
} public void setBlogId(Integer blogId) {
this.blogId = blogId;
} public String getBlogTitle() {
return blogTitle;
} public void setBlogTitle(String blogTitle) {
this.blogTitle = blogTitle == null ? null : blogTitle.trim();
} public String getBlogContent() {
return blogContent;
} public void setBlogContent(String blogContent) {
this.blogContent = blogContent == null ? null : blogContent.trim();
} public Date getCreateTime() {
return createTime;
} public void setCreateTime(Date createTime) {
this.createTime = createTime == null ? null : createTime;
} public Date getModifyTime() {
return modifyTime;
} public void setModifyTime(Date modifyTime) {
this.modifyTime = modifyTime == null ? null : modifyTime;
}
}
3⃣️在mybatis-config.xml文件中注册刚才自定义的MyDateHandler
<!--类型处理器 -->
<typeHandlers>
<!-- 注册自定义handler,说明它作用的jdbcType和javaType -->
<typeHandler jdbcType="VARCHAR" javaType="date" handler="com.daily.handler.MyDateHandler" />
</typeHandlers>
4⃣️在BlogMapper.xml文件中使用
<?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.daily.dao.BlogMapper">
<resultMap id="BaseResultMap" type="com.daily.pojo.Blog">
<id column="blog_id" jdbcType="INTEGER" property="blogId" />
<result column="blog_title" jdbcType="VARCHAR" property="blogTitle" />
<result column="blog_content" jdbcType="VARCHAR" property="blogContent" />
<!-- 不使用MyDateHandler-->
<result column="create_time" jdbcType="VARCHAR" property="createTime" />
<!-- 使用MyDateHandler-->
<result column="modify_time" typeHandler="com.daily.handler.MyDateHandler" property="modifyTime" />
</resultMap> <insert id="insert" parameterType="com.daily.pojo.Blog">
insert into blog (blog_id, blog_title, blog_content,
create_time, modify_time)
values (#{blogId,jdbcType=INTEGER}, #{blogTitle,jdbcType=VARCHAR},
#{blogContent,jdbcType=VARCHAR},
#{createTime,jdbcType=VARCHAR}, #{modifyTime,typeHandler=com.daily.handler.MyDateHandler})
</insert>
</mapper>
5⃣️查看结果
第一种:create_time不设置typeHandler,modify_time设置typeHandler
从上面的结果来看,虽然create_time没有设置typeHandler,但是结果跟使用了typeHandler的modify_time是一样的,那么我把两个都去掉呢?
第二种:两个都不设置
可以看到结果也是一样,下面我去掉注册的MyDateHandler呢?
第三种:去掉MyDateHandler
从上述结果可以看到,当去掉自定义的处理器时,MyBatis会根据结果自动选择合适的handler进行转换。
日常开发中,一般不需要定义,使用默认的就可以,除非是像枚举这种特殊类型就需要自己实现。
以上就是MyBatis配置文件中typeHandlers的配置内容。
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