Mybatis|Mybatis #foreach中相同的变量名导致值覆盖的问题解决 Mybatis#foreach中相同的变量名导

背景
问题原因（简略版）
Mybatis流程源码解析（长文警告，按需自取）

一、获取SqlSessionFactory
二、获取SqlSession
三、执行SQL

背景
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 list = new ArrayList<>(); list.add(1); list.add(2); list.add(3); queryCondition.setWidthList(list); System.out.println(mapper.findByCondition(queryCondition)); }}

XML

select * from testand id = #{id,jdbcType=INTEGER}#{width,jdbcType=INTEGER}and width = #{width,jdbcType=INTEGER}

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

org.mybatismybatis3.4.1

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。

Mybatis|Mybatis #foreach中相同的变量名导致值覆盖的问题解决

文章图片

文章图片

问题原因（简略版）

在获取到DefaultSqlSession之后，会获取到Mapper接口的代理类，通过调用代理类的方法来执行查询
真正执行数据库查询之前，需要将可执行的SQL拼接好，此操作在DynamicSqlSource#getBoundSql方法中执行
当解析到foreach标签时，每次循环都会缓存一个item属性值与变量值之间的映射（如：width:1），当foreach标签解析完成后，缓存的参数映射关系中就保留了一个（width:3）
当解析到最后一个if标签时，由于width变量有值，因此if判断为true，正常执行拼接，导致出错
3.4.5版本中，在foreach标签解析完成后，增加了两行代码来解决这个问题。

//foreach标签解析完成后，从bindings中移除itemcontext.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 firstpropertiesElement(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 #631environmentsElement(root.evalNode("environments")); databaseIdProviderElement(root.evalNode("databaseIdProvider")); typeHandlerElement(root.evalNode("typeHandlers")); //这里解析mappermapperElement(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：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|deletebuildStatementFromContext(context.evalNodes("select|insert|update|delete")); } catch (Exception e) {throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e); }}private void buildStatementFromContext(List 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idif (!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 parsingXMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant); includeParser.applyIncludes(context.getNode()); // Parse selectKey after includes and remove them.processSelectKeyNodes(id, parameterTypeClass, langDriver); // Parse the SQL (pre:and 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RawSqlSourceSqlSource 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CachingExecutorfinal 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|deletereturn 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.selectList(command.getName(), param, rowBounds); } else {result = sqlSession.selectList(command.getName(), param); }

来到DefaultSqlSession#selectList方法中

@Overridepublic List 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 List 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SQLBoundSql boundSql = sqlSource.getBoundSql(parameterObject); List 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 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 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elseif (o instanceof Map.Entry) {@SuppressWarnings("unchecked")Map.Entry mapEntry = (Map.Entry) 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 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 #709if (o instanceof Map.Entry) {@SuppressWarnings("unchecked")Map.Entry mapEntry = (Map.Entry) 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itemcontext.getBindings().remove(item); context.getBindings().remove(index); return true; }

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