Mybatis整体流程及关键组件
整体流程
mybatis主流程如下图所示,
- SqlSesison:面向用户的对外API
- Configuration:描述Mybatis的主配置信息
- Executor:SQL执行器,MyBatis中对数据库所有的增删改查操作都是由Executor组件完成的
- StatementHandler:StatementHandler封装了对JDBC Statement对象的操作,比如为Statement对象设置参数,调用Statement接口提供的方法与数据库交互
- ParameterHandler:MyBatis框架使用的Statement类型为CallableStatement和PreparedStatement时,ParameterHandler用于为Statement对象参数占位符设置值
- ResultSetHandler:ResultSetHandler封装了对JDBC中的ResultSet对象操作,当执行SQL类型为SELECT语句时,ResultSetHandler用于将查询结果转换成Java对象
- TypeHandler:TypeHandler是MyBatis中的类型处理器,用于处理Java类型与JDBC类型之间的映射。
核心组件
Executor
在Mybatis中对数据库的操作都是通过Executor进行操作,Executor核心类图如下所示:
- SimpleExecutor是基础实现,能够进行基础的增删改查操作。
- ResueExecutor对JDBC中的Statement对象做了缓存,当执行相同的SQL语句时,直接从缓存中取出Statement对象进行复用,避免了频繁创建和销毁Statement对象,从而提升系统性能
- BatchExecutor则会对调用同一个Mapper执行的update、insert和delete操作,调用Statement对象的批量操作功能
- 当开启二级缓存时,CachingExecutor对SimpleExecutor、ResueExecutor、BatchExecutor进行装饰,为查询操作增加二级缓存功能
StatementHandler
以org.apache.ibatis.executor.SimpleExecutor#doUpdate
方法为例,通过构建StatementHandler对象调用StatementHandler的update方法进行实际的处理逻辑,下面看下StatementHandler的相关实现。
@Override
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.update(stmt);
} finally {
closeStatement(stmt);
}
}
StatementHandler的实现类图如下所示:
- SimpleStatementHandler继承至BaseStatementHandler,封装了对JDBC Statement对象的操作
- PreparedStatementHandler封装了对JDBC PreparedStatement对象的操作
- CallableStatementHandler则封装了对JDBC CallableStatement对象的操作
- RoutingStatementHandler会根据Mapper配置中的statementType属性(取值为STATEMENT、PREPARED或CALLABLE)创建对应的StatementHandler实现
ParameterHandler
Mybatis是如何实现占位符的替换?org.apache.ibatis.executor.statement.PreparedStatementHandler#parameterize
实现参数的替换,具体逻辑如下:
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
setParameters方法如下:
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
//获取所有参数的映射信息
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
//根据参数属性名称获取参数值
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 {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
//获取参数对应的TypeHandler
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
//调用TypeHandler的setParameter方法为占位符设置值
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
核心逻辑:获取对应的TypeHandler然后调用setParameter方法设置参数值。
TypeHandler
TypeHandler类图如下:
public interface TypeHandler<T> {
// PreparedStatement对象参数的占位符设置值
void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
/**
* 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
*/
//从ResultSet获取值
T getResult(ResultSet rs, String columnName) throws SQLException;
//从ResultSet获取值
T getResult(ResultSet rs, int columnIndex) throws SQLException;
//从存储过程结果获取值
T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}
MyBatis通过TypeHandlerRegistry建立JDBC类型、Java类型与TypeHandler之间的映射关系,
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());
}
ResultSetHandler
ResultSetHandler用于在StatementHandler对象执行完查询操作或存储过程后,对结果集或存储过程的执行结果进行处理。ResultSetHandler接口定义如下:
public interface ResultSetHandler {
<E> List<E> handleResultSets(Statement stmt) throws SQLException;
<E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
void handleOutputParameters(CallableStatement cs) throws SQLException;
}
handleResultSets方法默认实现如下图所示:
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
// 从Statement对象中获取ResultSet对象,然后将ResultSet包装为ResultSetWrapper对象
ResultSetWrapper rsw = getFirstResultSet(stmt);
// 获取解析Mapper接口及Mapper SQL配置生成的ResultMap信息,一条语句一般对应一个ResultMap
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
// 调用handleResultSet()方法对ResultSetWrapper对象进行处理,将生成的实体对象存放在multipleResults列表中
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}
总结
至此,对于Mybatis的执行流程已经有一个大致的了解。SqlSesison提供了对用户暴露的API,使用了外观模式,Executor才是真正的执行器,Executor调用了StatementHandler去执行相关操作,使用了ParameterHandler、TypeHandler、ResultSetHandler进行参数占位符的替换、结果的转换操作。