mybatis核心组件关系图
mybatis主要组件
SqlSession：是mybatis提供的面向开发者的框架底层API，开发者通过获取一个是sqlSession实例来进行具体的数据库增删改查操作
Executor：mybatis的执行器，用于执行sql，生成结果并进行结果缓存等操作，类似于controller
StatementHandler：对JDBC的Statement进行封装
ParameterHandler：参数转换，将用户传递的参数转换成JDBC需要的参数
ResultSetHandler：将JDBC返回的结果集转换成java对象，并返回给调用者
TypeHandler：java数据类型和JDBC数据类型之间的映射
MappedStatement：一个MappedStatement维护一条<select|update|delete|insert>节点的封装
SqlSource：根据用户传递的参数动态生成sql语句，信息封装搭配BoundSql中，BoundSql表示动态生成的sql语句及参数信息
Configuration：持有mybatis所有的配置信息
mybatis主干流程（以query为例）
1: SqlSession
/** * Retrieve a single row mapped from the statement key * @param <T> the returned object type * @param statement * @return Mapped object */ <T> T selectOne(String statement);
2:DefaultSessionSql
@Override
public <T> T selectOne(String statement) { return this.<T>selectOne(statement, null);
} @Override
public <T> T selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter); if (list.size() == 1) { return list.get(0);
} else if (list.size() > 1) { throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else { return null;
}
} @Override
public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT);
} @Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try {
MappedStatement ms = configuration.getMappedStatement(statement); return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}3:BaseExecutor
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql); return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
} @SuppressWarnings("unchecked") @Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId()); if (closed) { throw new ExecutorException("Executor was closed.");
} if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list; try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
} if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
} // issue #601
deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { // issue #482
clearLocalCache();
}
} return list;
} private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER); try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
} return list;
}4:SimpleExecutor
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null; try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog()); return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}5:SimpleStatementHandler
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql); return resultSetHandler.<E>handleResultSets(statement);
}6:DefaultResultSetHandler
//
// HANDLE RESULT SETS
//
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId()); final List<Object> multipleResults = new ArrayList<Object>(); int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt);
List<ResultMap> resultMaps = mappedStatement.getResultMaps(); int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount); while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
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);
} 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);
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());
}
} public 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 void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { final DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
Object rowValue = previousRowValue; while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) { final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null); final CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);
Object partialObject = nestedResultObjects.get(rowKey); // issue #577 && #542
if (mappedStatement.isResultOrdered()) { if (partialObject == null && rowValue != null) {
nestedResultObjects.clear();
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
} else {
rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject); if (partialObject == null) {
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
} if (rowValue != null && mappedStatement.isResultOrdered() && shouldProcessMoreRows(resultContext, rowBounds)) {
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
previousRowValue = null;
} else if (rowValue != null) {
previousRowValue = rowValue;
}
} private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, CacheKey combinedKey, String columnPrefix, Object partialObject) throws SQLException { final String resultMapId = resultMap.getId();
Object rowValue = partialObject; if (rowValue != null) { final MetaObject metaObject = configuration.newMetaObject(rowValue);
putAncestor(rowValue, resultMapId, columnPrefix);
applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
ancestorObjects.remove(resultMapId);
} else { final ResultLoaderMap lazyLoader = new ResultLoaderMap();
rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix); if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) { final MetaObject metaObject = configuration.newMetaObject(rowValue); boolean foundValues = this.useConstructorMappings; if (shouldApplyAutomaticMappings(resultMap, true)) {
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
putAncestor(rowValue, resultMapId, columnPrefix);
foundValues = applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
ancestorObjects.remove(resultMapId);
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = (foundValues || configuration.isReturnInstanceForEmptyRow()) ? rowValue : null;
} if (combinedKey != CacheKey.NULL_CACHE_KEY) {
nestedResultObjects.put(combinedKey, rowValue);
}
} return rowValue;
} private boolean applyNestedResultMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String parentPrefix, CacheKey parentRowKey, boolean newObject) { boolean foundValues = false; for (ResultMapping resultMapping : resultMap.getPropertyResultMappings()) { final String nestedResultMapId = resultMapping.getNestedResultMapId(); if (nestedResultMapId != null && resultMapping.getResultSet() == null) { try { final String columnPrefix = getColumnPrefix(parentPrefix, resultMapping); final ResultMap nestedResultMap = getNestedResultMap(rsw.getResultSet(), nestedResultMapId, columnPrefix); if (resultMapping.getColumnPrefix() == null) { // try to fill circular reference only when columnPrefix
// is not specified for the nested result map (issue #215)
Object ancestorObject = ancestorObjects.get(nestedResultMapId); if (ancestorObject != null) { if (newObject) {
linkObjects(metaObject, resultMapping, ancestorObject); // issue #385
} continue;
}
}
final CacheKey rowKey = createRowKey(nestedResultMap, rsw, columnPrefix); final CacheKey combinedKey = combineKeys(rowKey, parentRowKey);
Object rowValue = nestedResultObjects.get(combinedKey); boolean knownValue = (rowValue != null);
instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject); // mandatory
if (anyNotNullColumnHasValue(resultMapping, columnPrefix, rsw)) {
rowValue = getRowValue(rsw, nestedResultMap, combinedKey, columnPrefix, rowValue); if (rowValue != null && !knownValue) {
linkObjects(metaObject, resultMapping, rowValue);
foundValues = true;
}
}
} catch (SQLException e) { throw new ExecutorException("Error getting nested result map values for '" + resultMapping.getProperty() + "'. Cause: " + e, e);
}
}
} return foundValues;
} private ResultMap getNestedResultMap(ResultSet rs, String nestedResultMapId, String columnPrefix) throws SQLException {
ResultMap nestedResultMap = configuration.getResultMap(nestedResultMapId); return resolveDiscriminatedResultMap(rs, nestedResultMap, columnPrefix);
} private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, CacheKey combinedKey, String columnPrefix, Object partialObject) throws SQLException { final String resultMapId = resultMap.getId();
Object rowValue = partialObject; if (rowValue != null) { final MetaObject metaObject = configuration.newMetaObject(rowValue);
putAncestor(rowValue, resultMapId, columnPrefix);
applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
ancestorObjects.remove(resultMapId);
} else { final ResultLoaderMap lazyLoader = new ResultLoaderMap();
rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix); if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) { final MetaObject metaObject = configuration.newMetaObject(rowValue); boolean foundValues = this.useConstructorMappings; if (shouldApplyAutomaticMappings(resultMap, true)) {
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
putAncestor(rowValue, resultMapId, columnPrefix);
foundValues = applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
ancestorObjects.remove(resultMapId);
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = (foundValues || configuration.isReturnInstanceForEmptyRow()) ? rowValue : null;
} if (combinedKey != CacheKey.NULL_CACHE_KEY) {
nestedResultObjects.put(combinedKey, rowValue);
}
} return rowValue;
} private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Object rowValue) { final Object collectionProperty = instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject); if (collectionProperty != null) { final MetaObject targetMetaObject = configuration.newMetaObject(collectionProperty);
targetMetaObject.add(rowValue);
} else {
metaObject.setValue(resultMapping.getProperty(), rowValue);
}
}其实流程到达JDBC获取到结果集这里都不算复杂，最为复杂的地方（对于query）就是ResultSetHandler这一部分，当然也分两种情况，如果没有关联查询，那就不难理解了，直接使用ResultType的相关规则生成结果对象即可，具体就是通过resultType先create一个新的结果对象，然后根据字段映射规则将DB中查询出来的结果集映射到java对象中来即可。麻烦的是对于关联查询，就不能简单的这么处理了，可能还存在一对多的情况，先看下面两个方法：
private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Object rowValue) { final Object collectionProperty = instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject); if (collectionProperty != null) { final MetaObject targetMetaObject = configuration.newMetaObject(collectionProperty);
targetMetaObject.add(rowValue);
} else {
metaObject.setValue(resultMapping.getProperty(), rowValue);
}
} private Object instantiateCollectionPropertyIfAppropriate(ResultMapping resultMapping, MetaObject metaObject) { final String propertyName = resultMapping.getProperty();
Object propertyValue = metaObject.getValue(propertyName); if (propertyValue == null) {
Class<?> type = resultMapping.getJavaType(); if (type == null) {
type = metaObject.getSetterType(propertyName);
} try { if (objectFactory.isCollection(type)) {
propertyValue = objectFactory.create(type);
metaObject.setValue(propertyName, propertyValue); return propertyValue;
}
} catch (Exception e) { throw new ExecutorException("Error instantiating collection property for result '" + resultMapping.getProperty() + "'. Cause: " + e, e);
}
} else if (objectFactory.isCollection(propertyValue.getClass())) { return propertyValue;
} return null;
}instantiateCollectionPropertyIfAppropriate方法会在需要的时候创建一个集合（处理一对多的情况），在上面的流程中提到过linkObjects方法，该方法会将一个查询好的对象（数据库中的一行查询结果）关联到结果对象中去，如果关联查询到的是一个集合，则添加该行数据到集合中去，否则设置一个结果属性。
最后再说一件事情，getNestedResultMap 方法会根据resultMap id取到正确的resultMap，然后就可以获取到resultClass，createResultObject会生成一个新的用于接收查询结果集的映射对象，getRowValue会生根据设置的映射规则填充这个生成的对象，然后会通过createRowKey方法创建一个cacheKey，put到nestedResultObjects中去，然后接着就可以通过linkObjects将这个对象设置到映射结果对象中去了。
作者：一字马胡
链接：https://www.jianshu.com/p/119d6cf6b8fb