Mybatis #foreach中相同的变量名导致值覆盖的问题解决
使用Mybatis中执行如下查询：
单元测试
@Testpublic void test1() { String resource = 'mybatis-config.xml'; InputStream inputStream = null; try {inputStream = Resources.getResourceAsStream(resource); } catch (IOException e) {e.printStackTrace(); } SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); try (SqlSession sqlSession = sqlSessionFactory.openSession()) {CommonMapper mapper = sqlSession.getMapper(CommonMapper.class);QueryCondition queryCondition = new QueryCondition();List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(3);queryCondition.setWidthList(list);System.out.println(mapper.findByCondition(queryCondition)); }}
XML
<select parameterType='cn.liupjie.pojo.QueryCondition' resultType='cn.liupjie.pojo.Test'> select * from test <where><if test='id != null'> and id = #{id,jdbcType=INTEGER}</if><if test='widthList != null and widthList.size > 0'> <foreach collection='widthList' open='and width in (' close=')' item='width' separator=','>#{width,jdbcType=INTEGER} </foreach></if><if test='width != null'> and width = #{width,jdbcType=INTEGER}</if> </where></select>
打印的SQL：DEBUG [main] - ==> Preparing: select * from test WHERE width in ( ? , ? , ? ) and width = ? DEBUG [main] - ==> Parameters: 1(Integer), 2(Integer), 3(Integer), 3(Integer)
Mybatis版本
<dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.1</version></dependency>
这是公司的老项目，在迭代的过程中遇到了此问题，以此记录！PS: 此bug在mybatis-3.4.5版本中已经解决。并且Mybatis维护者也建议不要在item/index中使用重复的变量名。
//foreach标签解析完成后，从bindings中移除item context.getBindings().remove(item); context.getBindings().remove(index);Mybatis流程源码解析（长文警告，按需自取）一、获取SqlSessionFactory
入口，跟着build方法走
//获取SqlSessionFactory， 解析完成后，将XML中的内容封装到一个Configuration对象中，//使用此对象构造一个DefaultSqlSessionFactory对象，并返回SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
来到SqlSessionFactoryBuilder#build方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try { //获取XMLConfigBuilder，在XMLConfigBuilder的构造方法中，会创建XPathParser对象 //在创建XPathParser对象时，会将mybatis-config.xml文件转换成Document对象 XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties); //调用XMLConfigBuilder#parse方法开始解析Mybatis的配置文件 return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException('Error building SqlSession.', e); } finally { ErrorContext.instance().reset(); try { inputStream.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } }}
跟着parse方法走，来到XMLConfigBuilder#parseConfiguration方法
private void parseConfiguration(XNode root) { try { Properties settings = settingsAsPropertiess(root.evalNode('settings')); //issue #117 read properties first propertiesElement(root.evalNode('properties')); loadCustomVfs(settings); typeAliasesElement(root.evalNode('typeAliases')); pluginElement(root.evalNode('plugins')); objectFactoryElement(root.evalNode('objectFactory')); objectWrapperFactoryElement(root.evalNode('objectWrapperFactory')); reflectorFactoryElement(root.evalNode('reflectorFactory')); settingsElement(settings); // read it after objectFactory and objectWrapperFactory issue #631 environmentsElement(root.evalNode('environments')); databaseIdProviderElement(root.evalNode('databaseIdProvider')); typeHandlerElement(root.evalNode('typeHandlers')); //这里解析mapper mapperElement(root.evalNode('mappers')); } catch (Exception e) { throw new BuilderException('Error parsing SQL Mapper Configuration. Cause: ' + e, e); }}
来到mapperElement方法
//本次mappers配置：<mapper resource='xml/CommomMapper.xml'/>private void mapperElement(XNode parent) throws Exception { if (parent != null) { for (XNode child : parent.getChildren()) { if ('package'.equals(child.getName())) {String mapperPackage = child.getStringAttribute('name');configuration.addMappers(mapperPackage); } else {String resource = child.getStringAttribute('resource');String url = child.getStringAttribute('url');String mapperClass = child.getStringAttribute('class');if (resource != null && url == null && mapperClass == null) { //因此走这里，读取xml文件，并开始解析 ErrorContext.instance().resource(resource); InputStream inputStream = Resources.getResourceAsStream(resource); //这里同上文创建XMLConfigBuilder对象一样，在内部构造时，也将xml文件转换为了一个Document对象 XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); //解析 mapperParser.parse();} else if (resource == null && url != null && mapperClass == null) { ErrorContext.instance().resource(url); InputStream inputStream = Resources.getUrlAsStream(url); XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments()); mapperParser.parse();} else if (resource == null && url == null && mapperClass != null) { Class<?> mapperInterface = Resources.classForName(mapperClass); configuration.addMapper(mapperInterface);} else { throw new BuilderException('A mapper element may only specify a url, resource or class, but not more than one.');} } } }}
XMLMapperBuilder类，负责解析SQL语句所在XML中的内容
//parse方法public void parse() { if (!configuration.isResourceLoaded(resource)) { //解析mapper标签 configurationElement(parser.evalNode('/mapper')); configuration.addLoadedResource(resource); bindMapperForNamespace(); } parsePendingResultMaps(); parsePendingChacheRefs(); parsePendingStatements();}//configurationElement方法private void configurationElement(XNode context) { try { String namespace = context.getStringAttribute('namespace'); if (namespace == null || namespace.equals('')) { throw new BuilderException('Mapper’s namespace cannot be empty'); } builderAssistant.setCurrentNamespace(namespace); cacheRefElement(context.evalNode('cache-ref')); cacheElement(context.evalNode('cache')); parameterMapElement(context.evalNodes('/mapper/parameterMap')); resultMapElements(context.evalNodes('/mapper/resultMap')); sqlElement(context.evalNodes('/mapper/sql')); //解析各种类型的SQL语句：select|insert|update|delete buildStatementFromContext(context.evalNodes('select|insert|update|delete')); } catch (Exception e) { throw new BuilderException('Error parsing Mapper XML. Cause: ' + e, e); }}private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { for (XNode context : list) { //创建XMLStatementBuilder对象 final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId); try { //解析 statementParser.parseStatementNode(); } catch (IncompleteElementException e) { configuration.addIncompleteStatement(statementParser); } }}
XMLStatementBuilder负责解析单个select|insert|update|delete节点
public void parseStatementNode() { String id = context.getStringAttribute('id'); String databaseId = context.getStringAttribute('databaseId'); //判断databaseId是否匹配，将namespace+’.’+id拼接，判断是否已经存在此id if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { return; } Integer fetchSize = context.getIntAttribute('fetchSize'); Integer timeout = context.getIntAttribute('timeout'); String parameterMap = context.getStringAttribute('parameterMap'); //获取参数类型 String parameterType = context.getStringAttribute('parameterType'); //获取参数类型的class对象 Class<?> parameterTypeClass = resolveClass(parameterType); String resultMap = context.getStringAttribute('resultMap'); String resultType = context.getStringAttribute('resultType'); String lang = context.getStringAttribute('lang'); LanguageDriver langDriver = getLanguageDriver(lang); //获取resultType的class对象 Class<?> resultTypeClass = resolveClass(resultType); String resultSetType = context.getStringAttribute('resultSetType'); StatementType statementType = StatementType.valueOf(context.getStringAttribute('statementType', StatementType.PREPARED.toString())); ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType); //获取select|insert|update|delete类型 String nodeName = context.getNode().getNodeName(); SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH)); boolean isSelect = sqlCommandType == SqlCommandType.SELECT; boolean flushCache = context.getBooleanAttribute('flushCache', !isSelect); boolean useCache = context.getBooleanAttribute('useCache', isSelect); boolean resultOrdered = context.getBooleanAttribute('resultOrdered', false); // Include Fragments before parsing XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant); includeParser.applyIncludes(context.getNode()); // Parse selectKey after includes and remove them. processSelectKeyNodes(id, parameterTypeClass, langDriver); // Parse the SQL (pre: <selectKey> and <include> were parsed and removed) //获取SqlSource对象，langDriver为默认的XMLLanguageDriver，在new Configuration时设置 //若sql中包含元素节点或$，则返回DynamicSqlSource，否则返回RawSqlSource SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass); String resultSets = context.getStringAttribute('resultSets'); String keyProperty = context.getStringAttribute('keyProperty'); String keyColumn = context.getStringAttribute('keyColumn'); KeyGenerator keyGenerator; String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX; keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true); if (configuration.hasKeyGenerator(keyStatementId)) { keyGenerator = configuration.getKeyGenerator(keyStatementId); } else { keyGenerator = context.getBooleanAttribute('useGeneratedKeys',configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))? new Jdbc3KeyGenerator() : new NoKeyGenerator(); } builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType, fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass, resultSetTypeEnum, flushCache, useCache, resultOrdered, keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);}二、获取SqlSession
由上文可知，此处的SqlSessionFactory使用的是DefaultSqlSessionFactory
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); //创建执行器，默认是SimpleExecutor //如果在配置文件中开启了缓存（默认开启），则是CachingExecutor final Executor executor = configuration.newExecutor(tx, execType); //返回DefaultSqlSession对象 return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException('Error opening session. Cause: ' + e, e); } finally { ErrorContext.instance().reset(); }}
这里获取到了一个DefaultSqlSession对象
三、执行SQL获取CommonMapper的对象，这里CommonMapper是一个接口，因此是一个代理对象，代理类是MapperProxy
org.apache.ibatis.binding.MapperProxy@72cde7cc
执行Query方法，来到MapperProxy的invoke方法
@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (Object.class.equals(method.getDeclaringClass())) { try { return method.invoke(this, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } //缓存 final MapperMethod mapperMethod = cachedMapperMethod(method); //执行操作：select|insert|update|delete return mapperMethod.execute(sqlSession, args);}
执行操作时，根据SELECT操作，以及返回值类型（反射方法获取）确定executeForMany方法
caseSELECT: 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;
来到executeForMany方法中，就可以看到执行查询的操作，由于这里没有进行分页查询，因此走else
if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<E>selectList(command.getName(), param, rowBounds);} else { result = sqlSession.<E>selectList(command.getName(), param);}
来到DefaultSqlSession#selectList方法中
@Overridepublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { //根据key（namespace+'.'+id）来获取MappedStatement对象 //MappedStatement对象中封装了解析好的SQL信息 MappedStatement ms = configuration.getMappedStatement(statement); //通过CachingExecutor#query执行查询 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(); }}
CachingExecutor#query
@Overridepublic <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { //解析SQL为可执行的SQL BoundSql boundSql = ms.getBoundSql(parameter); //获取缓存的key CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql); //执行查询 return query(ms, parameter, rowBounds, resultHandler, key, boundSql);}
MappedStatement#getBoundSql
public BoundSql getBoundSql(Object parameterObject) { //解析SQL BoundSql boundSql = sqlSource.getBoundSql(parameterObject); List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); if (parameterMappings == null || parameterMappings.isEmpty()) { boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject); } //检查是否有嵌套的ResultMap // check for nested result maps in parameter mappings (issue #30) for (ParameterMapping pm : boundSql.getParameterMappings()) { String rmId = pm.getResultMapId(); if (rmId != null) { ResultMap rm = configuration.getResultMap(rmId); if (rm != null) {hasNestedResultMaps |= rm.hasNestedResultMaps(); } } } return boundSql;}
由上文，此次语句由于SQL中包含元素节点，因此是DynamicSqlSource。由此来到DynamicSqlSource#getBoundSql。rootSqlNode.apply(context);这段代码便是在执行SQL解析。
@Overridepublic BoundSql getBoundSql(Object parameterObject) { DynamicContext context = new DynamicContext(configuration, parameterObject); //执行SQL解析 rootSqlNode.apply(context); SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration); Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass(); SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings()); BoundSql boundSql = sqlSource.getBoundSql(parameterObject); for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) { boundSql.setAdditionalParameter(entry.getKey(), entry.getValue()); } return boundSql;}
打上断点，跟着解析流程，来到解析foreach标签的代码，ForEachSqlNode#apply
@Overridepublic boolean apply(DynamicContext context) { Map<String, Object> bindings = context.getBindings(); final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings); if (!iterable.iterator().hasNext()) { return true; } boolean first = true; //解析open属性 applyOpen(context); int i = 0; for (Object o : iterable) { DynamicContext oldContext = context; if (first) { context = new PrefixedContext(context, ''); } else if (separator != null) { context = new PrefixedContext(context, separator); } else {context = new PrefixedContext(context, ''); } int uniqueNumber = context.getUniqueNumber(); // Issue #709 //集合中的元素是Integer，走else if (o instanceof Map.Entry) { @SuppressWarnings('unchecked') Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o; applyIndex(context, mapEntry.getKey(), uniqueNumber); applyItem(context, mapEntry.getValue(), uniqueNumber); } else { //使用index属性 applyIndex(context, i, uniqueNumber); //使用item属性 applyItem(context, o, uniqueNumber); } //当foreach中使用#号时，会将变量替换为占位符（类似__frch_width_0）（StaticTextSqlNode） //当使用$符号时，会将值直接拼接到SQL中（TextSqlNode） contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber)); if (first) { first = !((PrefixedContext) context).isPrefixApplied(); } context = oldContext; i++; } applyClose(context); return true;}private void applyItem(DynamicContext context, Object o, int i) { if (item != null) {//在参数映射中绑定item属性值与集合值的关系//第一次：（width:1）//第二次：（width:2）//第三次：（width:3）context.bind(item, o);//在参数映射中绑定处理后的item属性值与集合值的关系//第一次：（__frch_width_0:1）//第二次：（__frch_width_1:2）//第三次：（__frch_width_2:3）context.bind(itemizeItem(item, i), o); } }
到这里，结果就清晰了，在解析foreach标签时，每次循环都会将item属性值与参数集合中的值进行绑定，到最后就会保留（width:3）的映射关系，而在解析完foreach标签后，会解析最后一个if标签，此时在判断if标签是否成立时，答案是true，因此最终拼接出来一个错误的SQL。
在3.4.5版本中，代码中增加了context.getBindings().remove(item);在foreach标签解析完成后移除bindings中的参数映射。以下是源码：
@Overridepublic boolean apply(DynamicContext context) { Map<String, Object> bindings = context.getBindings(); final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings); if (!iterable.iterator().hasNext()) { return true; } boolean first = true; applyOpen(context); int i = 0; for (Object o : iterable) { DynamicContext oldContext = context; if (first || separator == null) { context = new PrefixedContext(context, ''); } else { context = new PrefixedContext(context, separator); } int uniqueNumber = context.getUniqueNumber(); // Issue #709 if (o instanceof Map.Entry) { @SuppressWarnings('unchecked') Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o; applyIndex(context, mapEntry.getKey(), uniqueNumber); applyItem(context, mapEntry.getValue(), uniqueNumber); } else { applyIndex(context, i, uniqueNumber); applyItem(context, o, uniqueNumber); } contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber)); if (first) { first = !((PrefixedContext) context).isPrefixApplied(); } context = oldContext; i++; } applyClose(context); //foreach标签解析完成后，从bindings中移除item context.getBindings().remove(item); context.getBindings().remove(index); return true;}
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