解析mapper的xml配置檔案
我們來看看mybatis是怎麼讀取mapper的xml配置檔案並解析其中的sql語句。

我們還記得是這樣配置sqlSessionFactory的：

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">   
  <property name="dataSource" ref="dataSource" />  
  <property name="configLocation" value="classpath:configuration.xml"></property>   
  <property name="mapperLocations" value="classpath:com/xxx/mybatis/mapper/*.xml"/>   
  <property name="typeAliasesPackage" value="com.tiantian.mybatis.model" />   
</bean>  

這裡配置了一個mapperLocations屬性，它是一個表示式，sqlSessionFactory會根據這個表示式讀取包com.xxx.mybaits.mapper下面的所有xml格式檔案，那麼具體是怎麼根據這個屬性來讀取配置檔案的呢？

答案就在SqlSessionFactoryBean類中的buildSqlSessionFactory方法中：

if (!isEmpty(this.mapperLocations)) { 
   for (Resource mapperLocation : this.mapperLocations) { 
    if (mapperLocation == null) { 
     continue; 
    } 
 
    try { 
     XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), 
       configuration, mapperLocation.toString(), configuration.getSqlFragments()); 
     xmlMapperBuilder.parse(); 
    } catch (Exception e) { 
     throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e); 
    } finally { 
     ErrorContext.instance().reset(); 
    } 
 
    if (logger.isDebugEnabled()) { 
     logger.debug("Parsed mapper file: '" + mapperLocation + "'"); 
    } 
   } 
  } 
 

mybatis使用XMLMapperBuilder類的例項來解析mapper配置檔案。

public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map<String, XNode> sqlFragments) { 
  this(new XPathParser(reader, true, configuration.getVariables(), new XMLMapperEntityResolver()), 
    configuration, resource, sqlFragments); 
 } 

private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) { 
  super(configuration); 
  this.builderAssistant = new MapperBuilderAssistant(configuration, resource); 
  this.parser = parser; 
  this.sqlFragments = sqlFragments; 
  this.resource = resource; 
 } 

 

接著系統呼叫xmlMapperBuilder的parse方法解析mapper。

public void parse() { 
  //如果configuration物件還沒載入xml配置檔案（避免重複載入，實際上是確認是否解析了mapper節點的屬性及內容， 
  //為解析它的子節點如cache、sql、select、resultMap、parameterMap等做準備）， 
  //則從輸入流中解析mapper節點，然後再將resource的狀態置為已載入 
  if (!configuration.isResourceLoaded(resource)) { 
   configurationElement(parser.evalNode("/mapper")); 
   configuration.addLoadedResource(resource); 
   bindMapperForNamespace(); 
  } 
  //解析在configurationElement函式中處理resultMap時其extends屬性指向的父物件還沒被處理的<resultMap>節點 
  parsePendingResultMaps(); 
  //解析在configurationElement函式中處理cache-ref時其指向的物件不存在的<cache>節點(如果cache-ref先於其指向的cache節點載入就會出現這種情況) 
  parsePendingChacheRefs(); 
  //同上，如果cache沒載入的話處理statement時也會丟擲異常 
  parsePendingStatements(); 
 } 

mybatis解析mapper的xml檔案的過程已經很明顯了，接下來我們看看它是怎麼解析mapper的：

private void configurationElement(XNode context) { 
  try { 
   //獲取mapper節點的namespace屬性 
   String namespace = context.getStringAttribute("namespace"); 
   if (namespace.equals("")) { 
    throw new BuilderException("Mapper's namespace cannot be empty"); 
   } 
   //設定當前namespace 
   builderAssistant.setCurrentNamespace(namespace); 
   //解析mapper的<cache-ref>節點 
   cacheRefElement(context.evalNode("cache-ref")); 
   //解析mapper的<cache>節點 
   cacheElement(context.evalNode("cache")); 
   //解析mapper的<parameterMap>節點 
   parameterMapElement(context.evalNodes("/mapper/parameterMap")); 
   //解析mapper的<resultMap>節點 
   resultMapElements(context.evalNodes("/mapper/resultMap")); 
   //解析mapper的<sql>節點 
   sqlElement(context.evalNodes("/mapper/sql")); 
   //使用XMLStatementBuilder的物件解析mapper的<select>、<insert>、<update>、<delete>節點, 
   //mybaits會使用MappedStatement.Builder類build一個MappedStatement物件, 
   //所以mybaits中一個sql對應一個MappedStatement 
   buildStatementFromContext(context.evalNodes("select|insert|update|delete")); 
  } catch (Exception e) { 
   throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e); 
  } 
 } 

configurationElement函式幾乎解析了mapper節點下所有子節點，至此mybaits解析了mapper中的所有節點，並將其加入到了Configuration物件中提供給sqlSessionFactory物件隨時使用。這裡我們需要補充講一下mybaits是怎麼使用XMLStatementBuilder類的物件的parseStatementNode函數借用MapperBuilderAssistant類物件builderAssistant的addMappedStatement解析MappedStatement並將其關聯到Configuration類物件的：

public void parseStatementNode() { 
  //ID屬性 
  String id = context.getStringAttribute("id"); 
  //databaseId屬性 
  String databaseId = context.getStringAttribute("databaseId"); 
 
  if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { 
   return; 
  } 
  //fetchSize屬性 
  Integer fetchSize = context.getIntAttribute("fetchSize"); 
  //timeout屬性 
  Integer timeout = context.getIntAttribute("timeout"); 
  //parameterMap屬性 
  String parameterMap = context.getStringAttribute("parameterMap"); 
  //parameterType屬性 
  String parameterType = context.getStringAttribute("parameterType"); 
  Class<?> parameterTypeClass = resolveClass(parameterType); 
  //resultMap屬性 
  String resultMap = context.getStringAttribute("resultMap"); 
  //resultType屬性 
  String resultType = context.getStringAttribute("resultType"); 
  //lang屬性 
  String lang = context.getStringAttribute("lang"); 
  LanguageDriver langDriver = getLanguageDriver(lang); 
 
  Class<?> resultTypeClass = resolveClass(resultType); 
  //resultSetType屬性 
  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)); 
  //是否是<select>節點 
  boolean isSelect = sqlCommandType == SqlCommandType.SELECT; 
  //flushCache屬性 
  boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect); 
  //useCache屬性 
  boolean useCache = context.getBooleanAttribute("useCache", isSelect); 
  //resultOrdered屬性 
  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); 
  //resultSets屬性 
  String resultSets = context.getStringAttribute("resultSets"); 
  //keyProperty屬性 
  String keyProperty = context.getStringAttribute("keyProperty"); 
  //keyColumn屬性 
  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 { 
   //useGeneratedKeys屬性 
   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); 
 } 

由以上程式碼可以看出mybaits使用XPath解析mapper的配置檔案後將其中的resultMap、parameterMap、cache、statement等節點使用關聯的builder建立並將得到的物件關聯到configuration物件中，而這個configuration物件可以從sqlSession中獲取的，這就解釋了我們在使用sqlSession對資料庫進行操作時mybaits怎麼獲取到mapper並執行其中的sql語句的問題。