參考:http://blog.csdn.net/ashan_li/article/details/50351080

MappedStatement說明

一個MappedStatement對象對應Mapper配置文件中的一個select/update/insert/delete節點，主要描述的是一條SQL語句。其屬性有

//節點中的id屬性加要命名空間  
private String id;  
//直接從節點屬性中取  
private Integer fetchSize;  
//直接從節點屬性中取  
private Integer timeout;  
private StatementType statementType;  
private ResultSetType resultSetType;  
//對應一條SQL語句  
private SqlSource sqlSource;  //每條語句都對就一個緩存，如果有的話。  
private Cache cache;  
//這個已經過時了  
private ParameterMap parameterMap;  
private List<ResultMap> resultMaps;  
private boolean flushCacheRequired;  
private boolean useCache;  
private boolean resultOrdered;  
//SQL的類型,select/update/insert/detete  
private SqlCommandType sqlCommandType;  
private KeyGenerator keyGenerator;  
private String[] keyProperties;  
private String[] keyColumns;  //是否有內映射  
private boolean hasNestedResultMaps;  
private String databaseId;  
private Log statementLog;  
private LanguageDriver lang;  
private String[] resultSets; 

　　

Mapper是接口，用來聲明持久層的方法，而Mapper配置對應的XML，決定了方法的執行的內容，決定持久層方法的行為。在MyBatis啟 動時，會解析這些包含SQL的XML文件，并將其包裝成為MapperStatement對象，并將MapperStatement注冊到全局的 configuration對象上，接下來就深入的了解代碼的實現。

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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.");  }  }  }  }  }  

　　

從 源碼中就可以看出，配置Mapper時，可以配置package熟悉，注冊包下所有的接口。還可以從資源中比如硬盤上，網絡中，去加載XML文件。注冊過 程是通過注冊器MapperRegistry來完成的。注冊的容器是一個map，Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();。

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key是mapper的接口完整類名，value是mapper的代理工廠。注冊完成后，還要做解析XML文件操作。

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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);  }  }  }  
}

　　

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下面 是解析的代碼

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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();  }  }  
}  

　　

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MyBatis通過替換mapper完整類名中的“.”，替換成為“/”，然后加上后綴“.xml”，拼成XML資源路徑，然后判斷是否已加載過XML，沒有的話加載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是創建建造者的構造參數，type.getClass()，就是mapper的類型。判斷然后還沒有加載mapper，就開始解析XML文件中的mapper節點。

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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);  }  }  

　　

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解 析時，先設置命名空間。然后解析cache-ref元素，可以使用其他命名空間的的緩存。在configuration對象上有一個 cacheRefMap用來維護引用緩存的關系。并且引用其他命名空間的引用指向助手類的currCache屬性上。如果被指向的命名空間還未加載，則拋 出異常，并且往configuration對象上添加未處理的緩存引用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);  }  }  
}  

　　

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解析緩存元素，可以使用type屬性配置自定義的緩存，否則使用默認 的PERPETUAL。然后用別名注冊器注冊緩存類。接下來注冊緩存的回收算法，緩存大小，過期時間，是否只讀等屬性。然后由助手類通過反射創建一個具體 的Cache對象。然后注冊到configuration全局對象上。

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);  }  
}  

　　

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下一步是解析parameterMap，新版中已經不推薦配置這個屬性了，屬于老方法。

參數Map映射已經被淘汰，但是結果集映射還很有用。接下來就是解析 resultMap。解析resultMap的元素比較多，解析完成后，還會根據解析到的映射關系創建一個結果處理器對象 resultMapResolver，后面對數據庫操作時，用來處理列和屬性的類型轉換。

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 (IncompleteElementException  e) {  configuration.addIncompleteResultMap(resultMapResolver);  throw e;  }  
}  

　　

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解析來繼續解析SQL片段，用來復用的SQL。助手類會將SQL片段的ID前面加上當前命名空間和一個點，用來和其他命名空間區別開。然后將SQL片段加載到configuration全局對象的sqlFragments對象上保存。

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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);  }  
}  

　　

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最后是重頭戲，解析增刪改查節點，創建Statement對象。同樣是通過建造者模式來創建語句對象，建造者的構造參數包括全局配置信息，當前命名空間助手，XML配置信息和數據庫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);  }  }  
}  

　　

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首先還是解析XML文件的各個屬性，然后處理<include>和<selectKey>片段。根據include標簽中的refid到全局配置中取對應的SQL片段。根據selectKey的配置信息，創建一個MapperStatement，并且添加到全局配置中，然后移除selectKey節點。

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);  
}  

　　

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接下來的操作，也是根據配置的屬性，然后通過建造者創建mappedStatement對象。并添加到configuration全局對象上。

轉載于:https://www.cnblogs.com/qlqwjy/p/7866982.html

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