【问题定位】通过看Mybatis源码解决系统问题

开发需求好好的，运维同事突然发现了一个问题，某个任务的详情页面加载不出来。看日志，系统在进行查询操作的时候抛出空指针异常。感觉是Mybatis内部异常，所以就跟踪源码看下Mybatis运行到哪一步报错的。

DefaultSqlSession#selectOne(java.lang.String, java.lang.Object)，空指针异常的地方是从DefaultSqlSession报出来的，由于查询结果返回null，导致调用size方法报错。

  @Overridepublic <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.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;}}

BaseExecutor#query()，BaseExecutor的query方法是Mybatis执行查询sql的必经之路，这边有一级缓存和二级缓存的知识点。

  @Overridepublic <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 #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;}

SimpleExecutor#doQuery，这个方法有两个重要的方法调用，prepareStatement()主要是给预编译的sql设置参数，handler.query(stmt, resultHandler);是获取数据，并进行对象封装。

  @Overridepublic <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.query(stmt, resultHandler);} finally {closeStatement(stmt);}}

DefaultResultSetHandler#handleResultSets，将查询出来的数据集合挨个解析。

  @Overridepublic List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());final List<Object> multipleResults = new ArrayList<>();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#applyPropertyMappings，该方法是将挨个数据库中的列数据映射到属性中。当系统遍历到日期字段，end的时候，该方法报错，跳出。

  private boolean applyPropertyMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, ResultLoaderMap lazyLoader, String columnPrefix)throws SQLException {final List<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix);boolean foundValues = false;final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();for (ResultMapping propertyMapping : propertyMappings) {String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);if (propertyMapping.getNestedResultMapId() != null) {// the user added a column attribute to a nested result map, ignore itcolumn = null;}if (propertyMapping.isCompositeResult()|| (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH)))|| propertyMapping.getResultSet() != null) {Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);// issue #541 make property optionalfinal String property = propertyMapping.getProperty();if (property == null) {continue;} else if (value == DEFERRED) {foundValues = true;continue;}if (value != null) {foundValues = true;}if (value != null || (configuration.isCallSettersOnNulls() && !metaObject.getSetterType(property).isPrimitive())) {// gcode issue #377, call setter on nulls (value is not 'found')metaObject.setValue(property, value);}}}return foundValues;}