这两天用到了Spring boot + Mybatis 做项目,使用了resultType="HashMap" 接收Mybatis查询返回的数据。以列名作为key,值作为value。结果发现部分列没有返回相对应的k-v对,导致了程序发生了错误。
然后开始debug大法,这里开始调用invoke方法,使用MapperMethod 的execute方法。
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } else if (isDefaultMethod(method)) { return invokeDefaultMethod(proxy, method, args); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }
MapperMethod 类 通过判断sql的执行类型和返回值类型来选择执行方法,这里是select语句,并且返回值类型是list,就会选择executeForMany方法。
public Object execute(SqlSession sqlSession, Object[] args) { Object result; switch (command.getType()) { .... 省略部分代码 case SELECT: if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); } else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args); } else { Object param = method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(command.getName(), param); } break; case FLUSH: result = sqlSession.flushStatements(); break; default: throw new BindingException("Unknown execution method for: " + command.getName()); } .... 省略部分代码 return result; }
然后执行selectList 方法,
private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<E>selectList(command.getName(), param, rowBounds); } else { result = sqlSession.<E>selectList(command.getName(), param); } // issue #510 Collections & arrays support if (!method.getReturnType().isAssignableFrom(result.getClass())) { if (method.getReturnType().isArray()) { return convertToArray(result); } else { return convertToDeclaredCollection(sqlSession.getConfiguration(), result); } } return result; }
再通过method 的invoke方法反射调用
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { SqlSession sqlSession = getSqlSession( SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator); try { Object result = method.invoke(sqlSession, args); 。。。。 return result; } 。。。 省略 } }
然后执行select方法
@Override public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); }
其实也就是调用了selectList方法,再执行Executor 的query方法。
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(); } }
然后到了,主要看query方法
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); }
然后是
@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()); 。。。 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--; } 。。。 return list; }
来看看 queryFromDatabase方法,这里会加入缓存,然后执行query方法。
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; }
这里主要先获取了Mybatis的配置类,把拦截器放入到Handler类中,再生成具体的Statement,比如PreparedStatement等,然后执行Handler的query方法
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); } }
再反射调用query方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { Set<Method> methods = signatureMap.get(method.getDeclaringClass()); if (methods != null && methods.contains(method)) { return interceptor.intercept(new Invocation(target, method, args)); } return method.invoke(target, args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } }
就是这里
@Override public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { return delegate.<E>query(statement, resultHandler); }
进去,这里的PreparedStatement开始执行execute方法,执行查询数据库,获取到具体的数据,然后对查询到的结果集进行处理。
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { PreparedStatement ps = (PreparedStatement) statement; ps.execute(); return resultSetHandler.<E> handleResultSets(ps); }
看看handleResultSets方法,注释也说清楚了,处理返回的结果集。我们主要看看为什么没有返回数据是null的k-v对。
// // 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); }
DefaultResultSetHandler 类的handleResultSet方法,
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()); } }
然后看看handleRowValues方法
// // HANDLE ROWS FOR SIMPLE RESULTMAP // 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); } }
然后是handleRowValuesForSimpleResultMap
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>(); skipRows(rsw.getResultSet(), rowBounds); while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) { ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null); Object rowValue = getRowValue(rsw, discriminatedResultMap); storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet()); } }
接着是获取每一行的值
// // GET VALUE FROM ROW FOR SIMPLE RESULT MAP // private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException { final ResultLoaderMap lazyLoader = new ResultLoaderMap(); Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null); if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) { final MetaObject metaObject = configuration.newMetaObject(rowValue); boolean foundValues = this.useConstructorMappings; if (shouldApplyAutomaticMappings(resultMap, false)) { foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues; } foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues; foundValues = lazyLoader.size() > 0 || foundValues; rowValue = (foundValues || configuration.isReturnInstanceForEmptyRow()) ? rowValue : null; } return rowValue; }
再看看applyAutomaticMappings 方法,final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
就是获取到每一行具体那一列的值,
value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive
会判断value是否为null,当某一行某一列的值是null的时候,会callSettersOnNulls这个配置的值,是否把这一行这一列以列名为key,null为值返回。
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) { 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') metaObject.setValue(mapping.property, value); } } } return foundValues; }
到了这里,也就知道了mybatis 中callSettersOnNulls 参数的真正作用是查询的某一行某一列为null,是否返回。