由于java类型和jdbc类型的差异，在操作数据库之前要把java类型转换成jdbc类型，拿到数据返回时要把jdbc类型转换成java类型，Mybatis作为持久层的ORM框架必然也做了类型之间的转换，本文将会来看一看Mybatis中是如何实现类型转换的。
@Test
void shouldSelectOneAuthorWithInlineParams() {
try (SqlSession session = sqlMapper.openSession()) {
Author author = session.selectOne(
"org.apache.ibatis.domain.blog.mappers.AuthorMapper.selectAuthorWithInlineParams", new Author(101));
assertEquals(101, author.getId());
}
}
源码中上面的测试类作为测试入口
TypeHandler
在TypeHander中定义了类型转换的基础接口，其他接口都实现或继承此接口做具体功能实现。
/*
* Copyright 2009-2023 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
*
* https://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> {
/**
* java类型转换为jdbc类型
* @param ps
* @param i
* @param parameter
* @param jdbcType
* @throws SQLException
*/
void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
/**
*
* jdbc类型转换为java类型
* Gets the result.
*
* @param rs
* the rs
* @param columnName
* Column name, when configuration <code>useColumnLabel</code> is <code>false</code>
*
* @return the result
*
* @throws SQLException
* the SQL exception
*/
T getResult(ResultSet rs, String columnName) throws SQLException;
T getResult(ResultSet rs, int columnIndex) throws SQLException;
T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}
TypeHandler的使用
TypeHandlerRegistry
TypeHandlerRegistry 保存了jabc类型到java类型之间需要处理器的关系。
public final class TypeHandlerRegistry {
//JdbcType和TypeHandler的对应关系
private final Map<JdbcType, TypeHandler<?>> jdbcTypeHandlerMap = new EnumMap<>(JdbcType.class);
//java类型和jdbc类型以及jdbc类型需要的TypeHandler
private final Map<Type, Map<JdbcType, TypeHandler<?>>> typeHandlerMap = new ConcurrentHashMap<>();
private final TypeHandler<Object> unknownTypeHandler;
//所有的TypeHandler
private final Map<Class<?>, TypeHandler<?>> allTypeHandlersMap = new HashMap<>();
//空类型处理
private static final Map<JdbcType, TypeHandler<?>> NULL_TYPE_HANDLER_MAP = Collections.emptyMap();
//默认枚举类型处理
private Class<? extends TypeHandler> defaultEnumTypeHandler = EnumTypeHandler.class;
在构造方法中注册了一些Mybatis提供的实现类
/**
* The constructor that pass the MyBatis configuration.
*
* @param configuration a MyBatis configuration
* @since 3.5.4
*/
public TypeHandlerRegistry(Configuration configuration) {
this.unknownTypeHandler = new UnknownTypeHandler(configuration);
register(Boolean.class, new BooleanTypeHandler());
register(boolean.class, new BooleanTypeHandler());
register(JdbcType.BOOLEAN, new BooleanTypeHandler());
register(JdbcType.BIT, new BooleanTypeHandler());
register(Byte.class, new ByteTypeHandler());
register(byte.class, new ByteTypeHandler());
register(JdbcType.TINYINT, new ByteTypeHandler());
register(Short.class, new ShortTypeHandler());
register(short.class, new ShortTypeHandler());
register(JdbcType.SMALLINT, new ShortTypeHandler());
register(Integer.class, new IntegerTypeHandler());
register(int.class, new IntegerTypeHandler());
register(JdbcType.INTEGER, new IntegerTypeHandler());
register(Long.class, new LongTypeHandler());
register(long.class, new LongTypeHandler());
register(Float.class, new FloatTypeHandler());
register(float.class, new FloatTypeHandler());
register(JdbcType.FLOAT, new FloatTypeHandler());
register(Double.class, new DoubleTypeHandler());
register(double.class, new DoubleTypeHandler());
register(JdbcType.DOUBLE, new DoubleTypeHandler());
register(Reader.class, new ClobReaderTypeHandler());
register(String.class, new StringTypeHandler());
register(String.class, JdbcType.CHAR, new StringTypeHandler());
register(String.class, JdbcType.CLOB, new ClobTypeHandler());
register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
register(String.class, JdbcType.LONGVARCHAR, new StringTypeHandler());
register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
register(JdbcType.CHAR, new StringTypeHandler());
register(JdbcType.VARCHAR, new StringTypeHandler());
register(JdbcType.CLOB, new ClobTypeHandler());
register(JdbcType.LONGVARCHAR, new StringTypeHandler());
register(JdbcType.NVARCHAR, new NStringTypeHandler());
register(JdbcType.NCHAR, new NStringTypeHandler());
register(JdbcType.NCLOB, new NClobTypeHandler());
register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
register(JdbcType.ARRAY, new ArrayTypeHandler());
register(BigInteger.class, new BigIntegerTypeHandler());
register(JdbcType.BIGINT, new LongTypeHandler());
register(BigDecimal.class, new BigDecimalTypeHandler());
register(JdbcType.REAL, new BigDecimalTypeHandler());
register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
register(JdbcType.NUMERIC, new BigDecimalTypeHandler());
register(InputStream.class, new BlobInputStreamTypeHandler());
register(Byte[].class, new ByteObjectArrayTypeHandler());
register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
register(byte[].class, new ByteArrayTypeHandler());
register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
register(JdbcType.BLOB, new BlobTypeHandler());
register(Object.class, unknownTypeHandler);
register(Object.class, JdbcType.OTHER, unknownTypeHandler);
register(JdbcType.OTHER, unknownTypeHandler);
register(Date.class, new DateTypeHandler());
register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
register(JdbcType.TIMESTAMP, new DateTypeHandler());
register(JdbcType.DATE, new DateOnlyTypeHandler());
register(JdbcType.TIME, new TimeOnlyTypeHandler());
register(java.sql.Date.class, new SqlDateTypeHandler());
register(java.sql.Time.class, new SqlTimeTypeHandler());
register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());
register(String.class, JdbcType.SQLXML, new SqlxmlTypeHandler());
register(Instant.class, new InstantTypeHandler());
register(LocalDateTime.class, new LocalDateTimeTypeHandler());
register(LocalDate.class, new LocalDateTypeHandler());
register(LocalTime.class, new LocalTimeTypeHandler());
register(OffsetDateTime.class, new OffsetDateTimeTypeHandler());
register(OffsetTime.class, new OffsetTimeTypeHandler());
register(ZonedDateTime.class, new ZonedDateTimeTypeHandler());
register(Month.class, new MonthTypeHandler());
register(Year.class, new YearTypeHandler());
register(YearMonth.class, new YearMonthTypeHandler());
register(JapaneseDate.class, new JapaneseDateTypeHandler());
// issue #273
register(Character.class, new CharacterTypeHandler());
register(char.class, new CharacterTypeHandler());
}
TypeHandlerRegistry的注册
在解析配置文件的时候如果配置了handler将会进行注册
参数设置值类型转换
先来看下查询数据库前，Mybatis是怎么进行参数设置的
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
MetaObject metaObject = null;
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
if (metaObject == null) {
metaObject = configuration.newMetaObject(parameterObject);
}
value = metaObject.getValue(propertyName);
}
//获取参数类型对应的处理器
//我们这里传参是int 拿到的是IntegerTypeHandler
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
测试入参new Author(101)
Author author = session.selectOne(
"org.apache.ibatis.domain.blog.mappers.AuthorMapper.selectAuthorWithInlineParams", new Author(101));
sql查询的传参是int类型 代码26行拿到类型的处理器IntegerTypeHandler
IntegerTypeHandler 类进行参数设置值
获取结果类型转化
拿到数据库返回结果，把结果集映射成java类型是怎么处理的
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject,
String columnPrefix) throws SQLException {
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (!autoMapping.isEmpty()) {
for (UnMappedColumnAutoMapping mapping : autoMapping) {
//拿到结果值
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || configuration.isCallSettersOnNulls() && !mapping.primitive) {
// gcode issue #377, call setter on nulls (value is not 'found')
//最终通过反射设置结果
metaObject.setValue(mapping.property, value);
}
}
}
return foundValues;
}
DefaultResultSetHandler类的applyAutomaticMappings()方法处理结果。代码第8行获取value，15行设置值
结果集第一个是int 类型，所以这里最后调用IntegerTypeHandler获取结果
最后反射设置值
