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MyBatis的 类型处理器 在设置预处理语句中的参数或从结果集中取出一个值时， 都会用类型处理器将获取到的值以合适的方式转换成 Java 类型。
本篇文章基于mybatis-3.5.7分支进行分析，梳理MyBatis类型处理器在源码层面的实现，包含类型处理解析、请求集处理、结果集处理三个方向。
类型处理器解析
private static class ParameterMappingTokenHandler extends BaseBuilder implements TokenHandler {
private List<ParameterMapping> parameterMappings = new ArrayList<>();
private Class<?> parameterType;
private MetaObject metaParameters;
private ParameterMapping buildParameterMapping(String content) {
Map<String, String> propertiesMap = parseParameterMapping(content);
// 省略相关代码
ParameterMapping.Builder builder = new ParameterMapping.Builder(configuration, property, propertyType);
Class<?> javaType = propertyType;
String typeHandlerAlias = null;
for (Map.Entry<String, String> entry : propertiesMap.entrySet()) {
String name = entry.getKey();
String value = entry.getValue();
if ("javaType".equals(name)) {
javaType = resolveClass(value);
builder.javaType(javaType);
} else if ("jdbcType".equals(name)) {
builder.jdbcType(resolveJdbcType(value));
} else if ("mode".equals(name)) {
builder.mode(resolveParameterMode(value));
} else if ("numericScale".equals(name)) {
builder.numericScale(Integer.valueOf(value));
} else if ("resultMap".equals(name)) {
builder.resultMapId(value);
// 解析对应的typeHandler参数
} else if ("typeHandler".equals(name)) {
typeHandlerAlias = value;
} else if ("jdbcTypeName".equals(name)) {
builder.jdbcTypeName(value);
} else if ("property".equals(name)) {
// Do Nothing
} else if ("expression".equals(name)) {
// 省略相关代码
} else {
// 省略相关代码
}
}
// 设置对应的typeHandlerAlias参数
if (typeHandlerAlias != null) {
builder.typeHandler(resolveTypeHandler(javaType, typeHandlerAlias));
}
// 构建对应的ParameterMapping对象
return builder.build();
}
}
- 在参数解析的方法ParameterMappingTokenHandler中通过resolveTypeHandler实现类型处理器的解析。
- ParameterMappingTokenHandler负责生成ParameterMapping对象，里面包含typeHandler对象。
请求集处理
public class DefaultParameterHandler implements ParameterHandler {
private final TypeHandlerRegistry typeHandlerRegistry;
private final MappedStatement mappedStatement;
private final Object parameterObject;
private final BoundSql boundSql;
private final Configuration configuration;
public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
this.mappedStatement = mappedStatement;
this.configuration = mappedStatement.getConfiguration();
this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
this.parameterObject = parameterObject;
this.boundSql = boundSql;
}
@Override
public Object getParameterObject() {
return parameterObject;
}
@Override
public void setParameters(PreparedStatement ps) {
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
// 遍历所有的参数映射
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 {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
// 获取参数类型处理器TypeHandler对象
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
// 通过类型处理器进行参数解析
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
}
}
}
}
}
}
- DefaultParameterHandler#setParameters方法会遍历parameterMappings并调用typeHandler.setParameter的处理参数并进行设置。
结果集处理
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;
}
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;
}
public abstract class BaseTypeHandler<T> extends TypeReference<T> implements TypeHandler<T> {
protected Configuration configuration;
}
- 结果集处理的接口ResultSetHandler包含上述的方法，针对结果处理的方法内会执行类型处理器TypeHandler。
- 类型处理器TypeHandler开放的接口如上述定义，自定义类型处理器需要实现上述功能。
- 考虑实现TypeHandler的接口的复杂度较高，可以通过继承BaseTypeHandler来简化接口操作。
public class DefaultResultSetHandler implements ResultSetHandler {
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap,
List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
// 处理每行的结果值
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}
private void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap,
ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) {
ensureNoRowBounds();
checkResultHandler();
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}
}
private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject,
ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
if (propertyMapping.getNestedQueryId() != null) {
return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
} else if (propertyMapping.getResultSet() != null) {
addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
return DEFERED;
} else {
final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
// 通过typeHandler进行处理
return typeHandler.getResult(rs, column);
}
}
}
- 结果集的处理核心在DefaultResultSetHandler当中，在内部逻辑中会调用到typeHandler。
- 结果集处理的例子之一是通过typeHandler.getResult来处理结果。
