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String resource = "mybatis-config.xml";//1.读取resources下面的mybatis-config.xml文件InputStream inputStream = Resources.getResourceAsStream(resource);//2.使用SqlSessionFactoryBuilder创建SqlSessionFactorySqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);//3.通过sqlSessionFactory创建SqlSessionSqlSession sqlSession = sqlSessionFactory.openSession();

public static InputStream getResourceAsStream(String resource) throws IOException { return getResourceAsStream(null, resource);} //loader赋值为nullpublic static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException { InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader); if (in == null) {  throw new IOException("Could not find resource " + resource); }  return in;}//classLoader为nullpublic InputStream getResourceAsStream(String resource, ClassLoader classLoader) { return getResourceAsStream(resource, getClassLoaders(classLoader));} //classLoader类加载InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) { for (ClassLoader cl : classLoader) {  if (null != cl) {   //加载指定路径文件流   InputStream returnValue = cl.getResourceAsStream(resource);   // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource   if (null == returnValue) {    returnValue = cl.getResourceAsStream("/" + resource);   }    if (null != returnValue) {    return returnValue;   }  } }  return null;}

//SqlSessionFactoryBuilder是一个建造者模式SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);public SqlSessionFactory build(InputStream inputStream) { return build(inputStream, null, null);}//XMLConfigBuilder也是建造者模式public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try {  XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);  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.  } }}//接下来进入XMLConfigBuilder构造函数public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) { this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);}//接下来进入this后,初始化Configurationprivate XMLConfigBuilder(XPathParser parser, String environment, Properties props) { super(new Configuration()); ErrorContext.instance().resource("SQL Mapper Configuration"); this.configuration.setVariables(props); this.parsed = false; this.environment = environment; this.parser = parser;}//其中parser.parse()负责解析xml,build(configuration)创建SqlSessionFactoryreturn build(parser.parse());

public Configuration parse() { //判断是否重复解析 if (parsed) {  throw new BuilderException("Each XMLConfigBuilder can only be used once."); }  parsed = true; //读取配置文件一级节点configuration parseConfiguration(parser.evalNode("/configuration")); return configuration;}private void parseConfiguration(XNode root) { try {  //properties 标签,用来配置参数信息，比如最常见的数据库连接信息  propertiesElement(root.evalNode("properties"));  Properties settings = settingsAsProperties(root.evalNode("settings"));  loadCustomVfs(settings);  loadCustomLogImpl(settings);  //实体别名两种方式:1.指定单个实体;2.指定包  typeAliasesElement(root.evalNode("typeAliases"));  //插件  pluginElement(root.evalNode("plugins"));  //用来创建对象(数据库数据映射成java对象时)  objectFactoryElement(root.evalNode("objectFactory"));  objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));  reflectorFactoryElement(root.evalNode("reflectorFactory"));  settingsElement(settings);  // read it after objectFactory and objectWrapperFactory issue #631  //数据库环境  environmentsElement(root.evalNode("environments"));  databaseIdProviderElement(root.evalNode("databaseIdProvider"));  //数据库类型和Java数据类型的转换  typeHandlerElement(root.evalNode("typeHandlers"));  //这个是对数据库增删改查的解析  mapperElement(root.evalNode("mappers")); } catch (Exception e) {  throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); }}

private void mapperElement(XNode parent) throws Exception { if (parent != null) {   for (XNode child : parent.getChildren()) {   //解析<package name=""/>   if ("package".equals(child.getName())) {    String mapperPackage = child.getStringAttribute("name");    //包路径存到mapperRegistry中    configuration.addMappers(mapperPackage);   } else {    //解析<mapper url="" class="" resource=""></mapper>    String resource = child.getStringAttribute("resource");    String url = child.getStringAttribute("url");    String mapperClass = child.getStringAttribute("class");    if (resource != null && url == null && mapperClass == null) {     ErrorContext.instance().resource(resource);     //读取Mapper.xml文件     InputStream inputStream = Resources.getResourceAsStream(resource);     XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream,     configuration, resource, configuration.getSqlFragments());     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.");    }   }  } }}

public void parse() { if (!configuration.isResourceLoaded(resource)) {  //解析所有的子标签  configurationElement(parser.evalNode("/mapper"));  configuration.addLoadedResource(resource);  //把namespace（接口类型）和工厂类绑定起来  bindMapperForNamespace(); } parsePendingResultMaps(); parsePendingCacheRefs(); parsePendingStatements();} //这里面解析的是Mapper.xml的标签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);  //对其他命名空间缓存配置的引用  cacheRefElement(context.evalNode("cache-ref"));  //对给定命名空间的缓存配置  cacheElement(context.evalNode("cache"));  parameterMapElement(context.evalNodes("/mapper/parameterMap"));  //是最复杂也是最强大的元素，用来描述如何从数据库结果集中来加载对象  resultMapElements(context.evalNodes("/mapper/resultMap"));  //可被其他语句引用的可重用语句块  sqlElement(context.evalNodes("/mapper/sql"));  //获得MappedStatement对象(增删改查标签)  buildStatementFromContext(context.evalNodes("select|insert|update|delete")); } catch (Exception e) {  throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e); }}//获得MappedStatement对象(增删改查标签)private void buildStatementFromContext(List<XNode> list) { if (configuration.getDatabaseId() != null) {  buildStatementFromContext(list, configuration.getDatabaseId()); }  buildStatementFromContext(list, null);}//获得MappedStatement对象(增删改查标签)private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { //循环增删改查标签 for (XNode context : list) {  final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);  try {   //解析insert/update/select/del中的标签   statementParser.parseStatementNode();  } catch (IncompleteElementException e) {   configuration.addIncompleteStatement(statementParser);  } }}public void parseStatementNode() { //在命名空间中唯一的标识符,可以被用来引用这条语句 String id = context.getStringAttribute("id"); //数据库厂商标识 String databaseId = context.getStringAttribute("databaseId"); if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {  return; }  String nodeName = context.getNode().getNodeName(); SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH)); boolean isSelect = sqlCommandType == SqlCommandType.SELECT; //flushCache和useCache都和二级缓存有关 //将其设置为true后,只要语句被调用,都会导致本地缓存和二级缓存被清空,默认值:false boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect); //将其设置为 true 后，将会导致本条语句的结果被二级缓存缓存起来，默认值：对 select 元素为 true 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()); //会传入这条语句的参数类的完全限定名或别名 String parameterType = context.getStringAttribute("parameterType"); Class<?> parameterTypeClass = resolveClass(parameterType); String lang = context.getStringAttribute("lang"); LanguageDriver langDriver = getLanguageDriver(lang); // Parse selectKey after includes and remove them. processSelectKeyNodes(id, parameterTypeClass, langDriver); // Parse the SQL (pre: <selectKey> and <include> were parsed and removed) 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)) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE; }  SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass); StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString())); Integer fetchSize = context.getIntAttribute("fetchSize"); Integer timeout = context.getIntAttribute("timeout"); String parameterMap = context.getStringAttribute("parameterMap"); //从这条语句中返回的期望类型的类的完全限定名或别名 String resultType = context.getStringAttribute("resultType"); Class<?> resultTypeClass = resolveClass(resultType); //外部resultMap的命名引用 String resultMap = context.getStringAttribute("resultMap"); String resultSetType = context.getStringAttribute("resultSetType"); ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType); String keyProperty = context.getStringAttribute("keyProperty"); String keyColumn = context.getStringAttribute("keyColumn"); String resultSets = context.getStringAttribute("resultSets"); builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType, fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass, resultSetTypeEnum, flushCache, useCache, resultOrdered, keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);}public MappedStatement addMappedStatement( String id, SqlSource sqlSource, StatementType statementType, SqlCommandType sqlCommandType, Integer fetchSize, Integer timeout, String parameterMap, Class<?> parameterType, String resultMap, Class<?> resultType, ResultSetType resultSetType, boolean flushCache, boolean useCache, boolean resultOrdered, KeyGenerator keyGenerator, String keyProperty, String keyColumn, String databaseId, LanguageDriver lang, String resultSets) { if (unresolvedCacheRef) {  throw new IncompleteElementException("Cache-ref not yet resolved"); }   id = applyCurrentNamespace(id, false);  boolean isSelect = sqlCommandType == SqlCommandType.SELECT;  MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration,  id, sqlSource, sqlCommandType)  .resource(resource)  .fetchSize(fetchSize)  .timeout(timeout)  .statementType(statementType)  .keyGenerator(keyGenerator)  .keyProperty(keyProperty)  .keyColumn(keyColumn)  .databaseId(databaseId)  .lang(lang)  .resultOrdered(resultOrdered)  .resultSets(resultSets)  .resultMaps(getStatementResultMaps(resultMap, resultType, id))  .resultSetType(resultSetType)  .flushCacheRequired(valueOrDefault(flushCache, !isSelect))  .useCache(valueOrDefault(useCache, isSelect))  .cache(currentCache);  ParameterMap statementParameterMap = getStatementParameterMap(parameterMap,  parameterType, id);  if (statementParameterMap != null) {   statementBuilder.parameterMap(statementParameterMap);  }   MappedStatement statement = statementBuilder.build();  //持有在configuration中  configuration.addMappedStatement(statement);  return statement;}public void addMappedStatement(MappedStatement ms){//ms.getId = mapper.UserMapper.getUserById//ms = MappedStatement等于每一个增删改查的标签的里的数据 mappedStatements.put(ms.getId(), ms);}//最终存放到mappedStatements中,mappedStatements存放的是一个个的增删改查protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection").conflictMessageProducer((savedValue, targetValue) ->". please check " + savedValue.getResource() + " and " + targetValue.getResource());

private void bindMapperForNamespace() { //当前Mapper的命名空间 String namespace = builderAssistant.getCurrentNamespace(); if (namespace != null) {  Class<?> boundType = null;  try {   //interface mapper.UserMapper这种   boundType = Resources.classForName(namespace);  } catch (ClassNotFoundException e) {  }   if (boundType != null) {   if (!configuration.hasMapper(boundType)) {    configuration.addLoadedResource("namespace:" + namespace);    configuration.addMapper(boundType);   }  } }}public <T> void addMapper(Class<T> type) { mapperRegistry.addMapper(type);} public <T> void addMapper(Class<T> type) { if (type.isInterface()) {  if (hasMapper(type)) {   throw new BindingException("Type " + type + " is already known to the MapperRegistry.");  }   boolean loadCompleted = false;  try {   //接口类型(key)->工厂类   knownMappers.put(type, new MapperProxyFactory<>(type));   MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);   parser.parse();   loadCompleted = true;  } finally {   if (!loadCompleted) {    knownMappers.remove(type);   }  } }}

public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config);}

SqlSession sqlSession = sqlSessionFactory.openSession();public SqlSession openSession() { //configuration中有默认赋值protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}<environments default="development"> <environment id="development">  <transactionManager type="JDBC"/>  <dataSource type="POOLED">   <property name="driver" value="${driver}"/>   <property name="url" value="${url}"/>   <property name="username" value="${username}"/>   <property name="password" value="${password}"/>  </dataSource> </environment></environments>

//ExecutorType是SIMPLE,一共有三种SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor)private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try {  //xml中的development节点  final Environment environment = configuration.getEnvironment();  //type配置的是Jbdc所以生成的是JbdcTransactionFactory工厂类  final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);  //Jdbc生成JbdcTransactionFactory生成JbdcTransaction  tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);  //创建CachingExecutor执行器  final Executor executor = configuration.newExecutor(tx, execType);  //创建DefaultSqlSession属性包括 Configuration、Executor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(); }}

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);public <T> T getMapper(Class<T> type) { return configuration.getMapper(type, this);}

public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession);}

public <T> T getMapper(Class<T> type, SqlSession sqlSession) { final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) {  throw new BindingException("Type " + type + " is not known to the MapperRegistry."); }  try {  return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) {  throw new BindingException("Error getting mapper instance. Cause: " + e, e); }}public T newInstance(SqlSession sqlSession) { final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy);}

protected T newInstance(MapperProxy<T> mapperProxy) { //mapperInterface是interface mapper.UserMapper  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

User user = userMapper.getUserById(1);

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try {  //判断是否需要去执行SQL还是直接执行方法  if (Object.class.equals(method.getDeclaringClass())) {   return method.invoke(this, args);   //这里判断的是接口中的默认方法Default等  } else if (isDefaultMethod(method)) {   return invokeDefaultMethod(proxy, method, args);  } } catch (Throwable t) {  throw ExceptionUtil.unwrapThrowable(t); }     //获取缓存,保存了方法签名和接口方法的关系 final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args);}

public Object execute(SqlSession sqlSession, Object[] args) { Object result; //根据命令类型走不行的操作command.getType()是select switch (command.getType()) {  case INSERT: {   Object param = method.convertArgsToSqlCommandParam(args);   result = rowCountResult(sqlSession.insert(command.getName(), param));   break;  }   case UPDATE: {   Object param = method.convertArgsToSqlCommandParam(args);   result = rowCountResult(sqlSession.update(command.getName(), param));   break;  }   case DELETE: {   Object param = method.convertArgsToSqlCommandParam(args);   result = rowCountResult(sqlSession.delete(command.getName(), param));   break;  }   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 {    //将参数转换为SQL的参数    Object param = method.convertArgsToSqlCommandParam(args);    result = sqlSession.selectOne(command.getName(), param);    if (method.returnsOptional()    && (result == null ||    !method.getReturnType().equals(result.getClass()))) {     result = Optional.ofNullable(result);    }   }   break;  case FLUSH:   result = sqlSession.flushStatements();   break;  default:   throw new BindingException("Unknown execution method for: " + command.getName()); }  if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {  throw new BindingException("Mapper method '" + command.getName() + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ")."); }  return result;}

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.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; }}public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try {  //从Configuration里的mappedStatements里根据key(id的全路径)获取MappedStatement 对象  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(); }}

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { //创建缓存Key BoundSql boundSql = ms.getBoundSql(parameterObject); //key = -575461213:-771016147:mapper.UserMapper.getUserById:0:2147483647:select * from test_user where id = ?:1:development CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) {  flushCacheIfRequired(ms);  if (ms.isUseCache() && resultHandler == null) {   ensureNoOutParams(ms, boundSql);   @SuppressWarnings("unchecked")   List<E> list = (List<E>) tcm.getObject(cache, key);   if (list == null) {    list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);    tcm.putObject(cache, key, list); // issue #578 and #116   }    return list;  } } return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}

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."); }  //queryStack 用于记录查询栈，防止递归查询重复处理缓存 //flushCache=true 的时候，会先清理本地缓存（一级缓存） if (queryStack == 0 && ms.isFlushCacheRequired()) {  //清空本地缓存  clearLocalCache(); }  List<E> list; try {  queryStack++;  list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;  if (list != null) {   handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);  } else {   //如果没有缓存,会从数据库查询：queryFromDatabase()   list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);  } } finally {  queryStack--; }  if (queryStack == 0) {  for (DeferredLoad deferredLoad : deferredLoads) {  deferredLoad.load();  }   // issue #601  deferredLoads.clear();  //如果 LocalCacheScope == STATEMENT,会清理本地缓存  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; //先在缓存用占位符占位 localCache.putObject(key, EXECUTION_PLACEHOLDER); try {  //执行Executor 的 doQuery(),默认是SimpleExecutor  list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally {  //执行查询后,移除占位符  localCache.removeObject(key); }  //从新放入数据 localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) {  localOutputParameterCache.putObject(key, parameter); }  return list;}

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