MyBatis启动（MapperStatement创建）

万事须己运，他得非我贤。这篇文章主要讲述MyBatis启动：MapperStatement创建相关的知识，希望能为你提供帮助。
参考:http://blog.csdn.net/ashan_li/article/details/50351080
MappedStatement说明一个MappedStatement对象对应Mapper配置文件中的一个select/update/insert/delete节点，主要描述的是一条SQL语句。其属性有

//节点中的id属性加要命名空间 private String id; //直接从节点属性中取 private Integer fetchSize; //直接从节点属性中取 private Integer timeout; private StatementType statementType; private ResultSetType resultSetType; //对应一条SQL语句 private SqlSource sqlSource; //每条语句都对就一个缓存，如果有的话。 private Cache cache; //这个已经过时了 private ParameterMap parameterMap; private List< ResultMap> resultMaps; private boolean flushCacheRequired; private boolean useCache; private boolean resultOrdered; //SQL的类型,select/update/insert/detete private SqlCommandType sqlCommandType; private KeyGenerator keyGenerator; private String[] keyProperties; private String[] keyColumns; //是否有内映射 private boolean hasNestedResultMaps; private String databaseId; private Log statementLog; private LanguageDriver lang; private String[] resultSets;

Mapper是接口，用来声明持久层的方法，而Mapper配置对应的XML，决定了方法的执行的内容，决定持久层方法的行为。在MyBatis启动时，会解析这些包含SQL的XML文件，并将其包装成为MapperStatement对象，并将MapperStatement注册到全局的 configuration对象上，接下来就深入的了解代码的实现。

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) { ErrorContext.instance().resource(resource); 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."); } } } } }

从源码中就可以看出，配置Mapper时，可以配置package熟悉，注册包下所有的接口。还可以从资源中比如硬盘上，网络中，去加载XML文件。注册过程是通过注册器MapperRegistry来完成的。注册的容器是一个map，Map< Class< ?> , MapperProxyFactory< ?> > knownMappers = new HashMap< Class< ?> , MapperProxyFactory< ?> > (); 。

key是mapper的接口完整类名，value是mapper的代理工厂。注册完成后，还要做解析XML文件操作。

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 { knownMappers.put(type, new MapperProxyFactory< T> (type)); // It‘s important that the type is added before the parser is run // otherwise the binding may automatically be attempted by the // mapper parser. If the type is already known, it won‘t try. MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }

下面是解析的代码

public void parse() { String resource = type.toString(); if (!configuration.isResourceLoaded(resource)) { loadXmlResource(); configuration.addLoadedResource(resource); assistant.setCurrentNamespace(type.getName()); parseCache(); parseCacheRef(); Method[] methods = type.getMethods(); for (Method method : methods) { try { parseStatement(method); } catch (IncompleteElementException e) { configuration.addIncompleteMethod(new MethodResolver(this, method)); } } } parsePendingMethods(); }

private void loadXmlResource() { // Spring may not know the real resource name so we check a flag // to prevent loading again a resource twice // this flag is set at XMLMapperBuilder#bindMapperForNamespace if (!configuration.isResourceLoaded("namespace:" + type.getName())) { String xmlResource = type.getName().replace(‘.‘, ‘/‘) + ".xml"; InputStream inputStream = null; try { inputStream = Resources.getResourceAsStream(type.getClassLoader(), xmlResource); } catch (IOException e) { // ignore, resource is not required } if (inputStream != null) { XMLMapperBuilder xmlParser = new XMLMapperBuilder(inputStream, assistant.getConfiguration(), xmlResource, configuration.getSqlFragments(), type.getName()); xmlParser.parse(); } } }

MyBatis通过替换mapper完整类名中的“ .” ，替换成为“ /” ，然后加上后缀“ .xml” ，拼成XML资源路径，然后判断是否已加载过XML，没有的话加载XML文件，然后使用xmlMapperBuilder建造者解析XML中的元素。

public void parse() { if (!configuration.isResourceLoaded(resource)) { configurationElement(parser.evalNode("/mapper")); configuration.addLoadedResource(resource); bindMapperForNamespace(); }parsePendingResultMaps(); parsePendingChacheRefs(); parsePendingStatements(); }

resource是创建建造者的构造参数，type.getClass()，就是mapper的类型。判断然后还没有加载mapper，就开始解析XML文件中的mapper节点。

private void configurationElement(XNode context) { try { String namespace = context.getStringAttribute("namespace"); if (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")); buildStatementFromContext(context.evalNodes("select|insert|update|delete")); } catch (Exception e) { throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e); } }

解析时，先设置命名空间。然后解析cache-ref元素，可以使用其他命名空间的的缓存。在configuration对象上有一个 cacheRefMap用来维护引用缓存的关系。并且引用其他命名空间的引用指向助手类的currCache属性上。如果被指向的命名空间还未加载，则抛出异常，并且往configuration对象上添加未处理的缓存引用chcheRef。

private void cacheRefElement(XNode context) { if (context != null) { configuration.addCacheRef(builderAssistant.getCurrentNamespace(), context.getStringAttribute("namespace")); CacheRefResolver cacheRefResolver = new CacheRefResolver(builderAssistant, context.getStringAttribute("namespace")); try { cacheRefResolver.resolveCacheRef(); } catch (IncompleteElementException e) { configuration.addIncompleteCacheRef(cacheRefResolver); } } }

解析缓存元素，可以使用type属性配置自定义的缓存，否则使用默认的PERPETUAL。然后用别名注册器注册缓存类。接下来注册缓存的回收算法，缓存大小，过期时间，是否只读等属性。然后由助手类通过反射创建一个具体的Cache对象。然后注册到configuration全局对象上。

private void cacheElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type", "PERPETUAL"); Class< ? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type); String eviction = context.getStringAttribute("eviction", "LRU"); Class< ? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction); Long flushInterval = context.getLongAttribute("flushInterval"); Integer size = context.getIntAttribute("size"); boolean readWrite = !context.getBooleanAttribute("readOnly", false); Properties props = context.getChildrenAsProperties(); builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, props); } }

下一步是解析parameterMap，新版中已经不推荐配置这个属性了，属于老方法。
参数Map映射已经被淘汰，但是结果集映射还很有用。接下来就是解析 resultMap。解析resultMap的元素比较多，解析完成后，还会根据解析到的映射关系创建一个结果处理器对象 resultMapResolver，后面对数据库操作时，用来处理列和属性的类型转换。

private ResultMap resultMapElement(XNode resultMapNode, List< ResultMapping> additionalResultMappings) throws Exception { ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier()); String id = resultMapNode.getStringAttribute("id", resultMapNode.getValueBasedIdentifier()); String type = resultMapNode.getStringAttribute("type", resultMapNode.getStringAttribute("ofType", resultMapNode.getStringAttribute("resultType", resultMapNode.getStringAttribute("javaType")))); String extend = resultMapNode.getStringAttribute("extends"); Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping"); Class< ?> typeClass = resolveClass(type); Discriminator discriminator = null; List< ResultMapping> resultMappings = new ArrayList< ResultMapping> (); resultMappings.addAll(additionalResultMappings); List< XNode> resultChildren = resultMapNode.getChildren(); for (XNode resultChild : resultChildren) { if ("constructor".equals(resultChild.getName())) { processConstructorElement(resultChild, typeClass, resultMappings); } else if ("discriminator".equals(resultChild.getName())) { discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings); } else { ArrayList< ResultFlag> flags = new ArrayList< ResultFlag> (); if ("id".equals(resultChild.getName())) { flags.add(ResultFlag.ID); } resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags)); } } ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping); try { return resultMapResolver.resolve(); } catch (IncompleteElementExceptione) { configuration.addIncompleteResultMap(resultMapResolver); throw e; } }

【MyBatis启动（MapperStatement创建）】解析来继续解析SQL片段，用来复用的SQL。助手类会将SQL片段的ID前面加上当前命名空间和一个点，用来和其他命名空间区别开。然后将SQL片段加载到configuration全局对象的sqlFragments对象上保存。

private void sqlElement(List< XNode> list, String requiredDatabaseId) throws Exception { for (XNode context : list) { String databaseId = context.getStringAttribute("databaseId"); String id = context.getStringAttribute("id"); id = builderAssistant.applyCurrentNamespace(id, false); if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) sqlFragments.put(id, context); } }

最后是重头戏，解析增删改查节点，创建Statement对象。同样是通过建造者模式来创建语句对象，建造者的构造参数包括全局配置信息，当前命名空间助手，XML配置信息和数据库ID。

private void buildStatementFromContext(List< XNode> list, String requiredDatabaseId) { for (XNode context : list) { final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId); try { statementParser.parseStatementNode(); } catch (IncompleteElementException e) { configuration.addIncompleteStatement(statementParser); } } }

首先还是解析XML文件的各个属性，然后处理< include> 和< selectKey> 片段。根据include标签中的refid到全局配置中取对应的SQL片段。根据selectKey的配置信息，创建一个MapperStatement，并且添加到全局配置中，然后移除selectKey节点。

public void parseStatementNode() { String id = context.getStringAttribute("id"); String databaseId = context.getStringAttribute("databaseId"); if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) return; Integer fetchSize = context.getIntAttribute("fetchSize"); Integer timeout = context.getIntAttribute("timeout"); String parameterMap = context.getStringAttribute("parameterMap"); String parameterType = context.getStringAttribute("parameterType"); Class< ?> parameterTypeClass = resolveClass(parameterType); String resultMap = context.getStringAttribute("resultMap"); String resultType = context.getStringAttribute("resultType"); String lang = context.getStringAttribute("lang"); LanguageDriver langDriver = getLanguageDriver(lang); Class< ?> resultTypeClass = resolveClass(resultType); String resultSetType = context.getStringAttribute("resultSetType"); StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString())); ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType); 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) 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); }

接下来的操作，也是根据配置的属性，然后通过建造者创建mappedStatement对象。并添加到configuration全局对象上。