轉載自??mybatis源碼閱讀(二)：mybatis初始化上

1.初始化入口

//Mybatis 通過SqlSessionFactory獲取SqlSession, 然后才能通過SqlSession與數據庫進行交互 private static SqlSessionFactory getSessionFactory() {SqlSessionFactory sessionFactory = null;String resource = "configuration.xml";try {sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));} catch (IOException e) {e.printStackTrace();}return sessionFactory; }

那么，我們就先從SqlSessionFactoryBuilder入手， 咱們先看看源碼是怎么實現的

SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {// 讀取配置文件XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);//解析配置得到Configuration對象，創建DefaultSqlSessionFactory對象return build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} finally {//關閉讀取配置文件的輸入流對象ErrorContext.instance().reset();try {inputStream.close();} catch (IOException e) {// Intentionally ignore. Prefer previous error.}} }

XMLConfigBuilder

XMLConfigBuilder是BaseBuilder的眾多子類之一，核心字段如下

//表示是否已經解析過了 private boolean parsed; //用于解析配置文件的對象 private final XPathParser parser; //配置文件中表示<environment>的名稱 默認讀取default屬性 private String environment; // 負責和創建Reflector對象 private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

XMLConfigBuilder.parse()方法是解析mybatis-config.xml配置文件的如。它調用parseConfiguration（）方法實現整個解析過程。具體實現如下：

/*** 解析配置文件的入口* @return*/ public Configuration parse() {if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}parsed = true;parseConfiguration(parser.evalNode("/configuration"));return configuration; }/*** 對配置文件每個節點具體的解析過程* configuration節點為根節點。* 在configuration節點之下，我們可以配置11 個子節點，* 分別為：properties、settings、typeAliases、plugins、objectFactory、objectWrapperFactory、reflectorFactory、* environments、databaseIdProvider、typeHandlers、mappers。* @param root 根節點*/ private void parseConfiguration(XNode root) {try {// 解析properties節點propertiesElement(root.evalNode("properties"));//解析settings節點Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(settings);//設置vfsImpl字段//解析typeAliases節點typeAliasesElement(root.evalNode("typeAliases"));//解析plugins節點pluginElement(root.evalNode("plugins"));//解析objectFactory節點objectFactoryElement(root.evalNode("objectFactory"));//解析objectWrapperFactory節點objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));//解析reflectorFactory節點reflectorFactoryElement(root.evalNode("reflectorFactory"));settingsElement(settings);//解析environments節點environmentsElement(root.evalNode("environments"));//解析databaseIdProvider節點databaseIdProviderElement(root.evalNode("databaseIdProvider"));//解析typeHandlers節點typeHandlerElement(root.evalNode("typeHandlers"));//解析mappers節點mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);} }

propertiesElement()方法會解析配置文件中的properties節點并形成Java.util.Properties對象，之后將改對象設置到XpathParse和Configguration的variables字段中，占位符就是用Properties中的信息替換的，具體實現如下：

/*** 解析properties的具體方法* @param context* @throws Exception*/ private void propertiesElement(XNode context) throws Exception {if (context != null) {// 將子節點的 name 以及value屬性set進properties對象// 這兒可以注意一下順序，xml配置優先， 外部指定properties配置其次Properties defaults = context.getChildrenAsProperties();// 獲取properties節點上 resource屬性的值String resource = context.getStringAttribute("resource");// 獲取properties節點上 url屬性的值, resource和url不能同時配置String url = context.getStringAttribute("url");if (resource != null && url != null) {//url 和resource不能同時配置throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference. Please specify one or the other.");}// 把解析出的properties文件set進Properties對象if (resource != null) {defaults.putAll(Resources.getResourceAsProperties(resource));} else if (url != null) {defaults.putAll(Resources.getUrlAsProperties(url));}// 將configuration對象中已配置的Properties屬性與剛剛解析的融合// configuration這個對象會裝載所解析mybatis配置文件的所有節點元素，以后也會頻頻提到這個對象Properties vars = configuration.getVariables();if (vars != null) {defaults.putAll(vars);}// 把裝有解析配置propertis對象set進解析器， 因為后面可能會用到parser.setVariables(defaults);// set進configuration對象configuration.setVariables(defaults);} }

settings節點下的配飾是mybatis的全局性配置，修改的是configuration對象的屬性，具體說明參考官方文檔

/*** settings標簽就是設置configuration對象的各種屬性，* 具體屬性說明可以參考mybatis官方文檔* @param props* @throws Exception*/ private void settingsElement(Properties props) throws Exception {configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));@SuppressWarnings("unchecked")Class<? extends TypeHandler> typeHandler = (Class<? extends TypeHandler>)resolveClass(props.getProperty("defaultEnumTypeHandler"));configuration.setDefaultEnumTypeHandler(typeHandler);configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));configuration.setLogPrefix(props.getProperty("logPrefix"));@SuppressWarnings("unchecked")Class<? extends Log> logImpl = (Class<? extends Log>)resolveClass(props.getProperty("logImpl"));configuration.setLogImpl(logImpl);configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory"))); }

environments元素節點主要配置數據庫事物，數據源。可以配置多個environment子節點，假如我們系統的開發環境和正式環境所用的數據庫不一樣（這是肯定的）， 那么可以設置兩個environment, 兩個id分別對應開發環境（development）和正式環境（final），那么通過配置environments的default屬性就能選擇對應的environment了， 例如，我將environments的deault屬性的值配置為development, 那么就會選擇dev的environment。具體實現如下

/*** 解析enviroments元素節點的方法* @param context* @throws Exception*/ private void environmentsElement(XNode context) throws Exception {if (context != null) {if (environment == null) {//獲取 <environments default="development"> 中的default值environment = context.getStringAttribute("default");}// 循環environments的子節點for (XNode child : context.getChildren()) {// 獲取 <environment id="development"> z中的idString id = child.getStringAttribute("id");if (isSpecifiedEnvironment(id)) {//根據由environments的default屬性去選擇對應的enviroment// 事物 mybatis有兩種：JDBC 和 MANAGED, 配置為JDBC則直接使用JDBC的事務，配置為MANAGED則是將事務托管給容器// <transactionManager type="JDBC"/>TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));//enviroment節點下面就是dataSource節點了，解析dataSource節點DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));DataSource dataSource = dsFactory.getDataSource();Environment.Builder environmentBuilder = new Environment.Builder(id).transactionFactory(txFactory).dataSource(dataSource);// 將dataSource設置進configuration對象configuration.setEnvironment(environmentBuilder.build());}}} }

typeAliases節點主要用來設置別名，其實這是挺好用的一個功能， 通過配置別名，我們不用再指定完整的包名

/*** 解析typeAliases 節點* <typeAliases>* <!--<package name="com.lpf.entity"></package>-->* <typeAlias alias="UserEntity" type="com.lpf.entity.User"/>* </typeAliases>* @param parent*/ private void typeAliasesElement(XNode parent) {if (parent != null) {for (XNode child : parent.getChildren()) {//如果子節點是package, 那么就獲取package節點的name屬性， mybatis會掃描指定的packageif ("package".equals(child.getName())) {String typeAliasPackage = child.getStringAttribute("name");//TypeAliasRegistry 負責管理別名， 這兒就是通過TypeAliasRegistry 進行別名注冊configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);} else {//如果子節點是typeAlias節點，那么就獲取alias屬性和type的屬性值String alias = child.getStringAttribute("alias");String type = child.getStringAttribute("type");try {Class<?> clazz = Resources.classForName(type);if (alias == null) {typeAliasRegistry.registerAlias(clazz);} else {typeAliasRegistry.registerAlias(alias, clazz);}} catch (ClassNotFoundException e) {throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);}}}} }

具體的別名注冊類

public class TypeAliasRegistry {// 別名通過一個HashMap來實現， key為別名， value就是別名對應的類型（class對象）private final Map<String, Class<?>> TYPE_ALIASES = new HashMap<String, Class<?>>();/*** mybatis默認為我們注冊的別名*/public TypeAliasRegistry() {registerAlias("string", String.class);registerAlias("byte", Byte.class);registerAlias("long", Long.class);registerAlias("short", Short.class);registerAlias("int", Integer.class);registerAlias("integer", Integer.class);registerAlias("double", Double.class);registerAlias("float", Float.class);registerAlias("boolean", Boolean.class);registerAlias("byte[]", Byte[].class);registerAlias("long[]", Long[].class);registerAlias("short[]", Short[].class);registerAlias("int[]", Integer[].class);registerAlias("integer[]", Integer[].class);registerAlias("double[]", Double[].class);registerAlias("float[]", Float[].class);registerAlias("boolean[]", Boolean[].class);registerAlias("_byte", byte.class);registerAlias("_long", long.class);registerAlias("_short", short.class);registerAlias("_int", int.class);registerAlias("_integer", int.class);registerAlias("_double", double.class);registerAlias("_float", float.class);registerAlias("_boolean", boolean.class);registerAlias("_byte[]", byte[].class);registerAlias("_long[]", long[].class);registerAlias("_short[]", short[].class);registerAlias("_int[]", int[].class);registerAlias("_integer[]", int[].class);registerAlias("_double[]", double[].class);registerAlias("_float[]", float[].class);registerAlias("_boolean[]", boolean[].class);registerAlias("date", Date.class);registerAlias("decimal", BigDecimal.class);registerAlias("bigdecimal", BigDecimal.class);registerAlias("biginteger", BigInteger.class);registerAlias("object", Object.class);registerAlias("date[]", Date[].class);registerAlias("decimal[]", BigDecimal[].class);registerAlias("bigdecimal[]", BigDecimal[].class);registerAlias("biginteger[]", BigInteger[].class);registerAlias("object[]", Object[].class);registerAlias("map", Map.class);registerAlias("hashmap", HashMap.class);registerAlias("list", List.class);registerAlias("arraylist", ArrayList.class);registerAlias("collection", Collection.class);registerAlias("iterator", Iterator.class);registerAlias("ResultSet", ResultSet.class);}/*** 處理別名， 直接從保存有別名的hashMap中取出即可*/@SuppressWarnings("unchecked")// throws class cast exception as well if types cannot be assignedpublic <T> Class<T> resolveAlias(String string) {try {if (string == null) {return null;}// issue #748String key = string.toLowerCase(Locale.ENGLISH);Class<T> value;if (TYPE_ALIASES.containsKey(key)) {value = (Class<T>) TYPE_ALIASES.get(key);} else {value = (Class<T>) Resources.classForName(string);}return value;} catch (ClassNotFoundException e) {throw new TypeException("Could not resolve type alias '" + string + "'. Cause: " + e, e);}}/*** 配置文件中配置為package的時候，掃描包下的Javabean ，然后自動注冊別名* 默認會使用 Bean 的首字母小寫的非限定類名來作為它的別名* 也可在javabean 加上注解@Alias 來自定義別名， 例如： @Alias(user)*/public void registerAliases(String packageName){registerAliases(packageName, Object.class);}public void registerAliases(String packageName, Class<?> superType){ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();resolverUtil.find(new ResolverUtil.IsA(superType), packageName);Set<Class<? extends Class<?>>> typeSet = resolverUtil.getClasses();for(Class<?> type : typeSet){// Ignore inner classes and interfaces (including package-info.java)// Skip also inner classes. See issue #6if (!type.isAnonymousClass() && !type.isInterface() && !type.isMemberClass()) {registerAlias(type);}}}public void registerAlias(Class<?> type) {String alias = type.getSimpleName();Alias aliasAnnotation = type.getAnnotation(Alias.class);if (aliasAnnotation != null) {alias = aliasAnnotation.value();} registerAlias(alias, type);}//向hashMap中注冊別名public void registerAlias(String alias, Class<?> value) {if (alias == null) {throw new TypeException("The parameter alias cannot be null");}// issue #748String key = alias.toLowerCase(Locale.ENGLISH);if (TYPE_ALIASES.containsKey(key) && TYPE_ALIASES.get(key) != null && !TYPE_ALIASES.get(key).equals(value)) {throw new TypeException("The alias '" + alias + "' is already mapped to the value '" + TYPE_ALIASES.get(key).getName() + "'.");}TYPE_ALIASES.put(key, value);}public void registerAlias(String alias, String value) {try {registerAlias(alias, Resources.classForName(value));} catch (ClassNotFoundException e) {throw new TypeException("Error registering type alias "+alias+" for "+value+". Cause: " + e, e);}}/*** 獲取保存別名的HashMap, Configuration對象持有對TypeAliasRegistry的引用，* 因此，如果需要，我們可以通過Configuration對象獲取*/public Map<String, Class<?>> getTypeAliases() {return Collections.unmodifiableMap(TYPE_ALIASES);}}

typeHandlers節點的解析和typeAlianses節點的解析類似

/*** 解析typeHandlers節點* 無論是 MyBatis 在預處理語句（PreparedStatement）中設置一個參數時，* 還是從結果集中取出一個值時，都會用類型處理器將獲取的值以合適的方式轉換成 Java 類型。* Mybatis默認為我們實現了許多TypeHandler, 當我們沒有配置指定TypeHandler時，* Mybatis會根據參數或者返回結果的不同，默認為我們選擇合適的TypeHandler處理。* @param parent* @throws Exception*/ private void typeHandlerElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {//子節點為package時，獲取其name屬性的值，然后自動掃描package下的自定義typeHandlerif ("package".equals(child.getName())) {String typeHandlerPackage = child.getStringAttribute("name");typeHandlerRegistry.register(typeHandlerPackage);} else {//子節點為typeHandler時， 可以指定javaType屬性， 也可以指定jdbcType, 也可兩者都指定//javaType 是指定java類型//jdbcType 是指定jdbc類型（數據庫類型： 如varchar）String javaTypeName = child.getStringAttribute("javaType");String jdbcTypeName = child.getStringAttribute("jdbcType");//handler就是我們配置的typeHandlerString handlerTypeName = child.getStringAttribute("handler");Class<?> javaTypeClass = resolveClass(javaTypeName);//JdbcType是一個枚舉類型，resolveJdbcType方法是在獲取枚舉類型的值JdbcType jdbcType = resolveJdbcType(jdbcTypeName);Class<?> typeHandlerClass = resolveClass(handlerTypeName);//注冊typeHandler, typeHandler通過TypeHandlerRegistry這個類管理if (javaTypeClass != null) {if (jdbcType == null) {typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);} else {typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);}} else {typeHandlerRegistry.register(typeHandlerClass);}}}} }

插件是mybatis提供的擴展機制，用戶可以通過添加自定義插件在SQL語句執行的過程中某一環節進行攔截，mybatis中的自定義插件只需實現Interceptor接口，并通過注解指定攔截的方法簽名，這個后面具體介紹。

/*** 解析plugins標簽* mybatis中的plugin其實就是個interceptor，* 它可以攔截Executor 、ParameterHandler 、ResultSetHandler 、StatementHandler 的部分方法，處理我們自己的邏輯。* @param parent* @throws Exception*/ private void pluginElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {String interceptor = child.getStringAttribute("interceptor");Properties properties = child.getChildrenAsProperties();// 我們在定義一個interceptor的時候，需要去實現InterceptorInterceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();interceptorInstance.setProperties(properties);// 向configuration對象中注冊攔截器configuration.addInterceptor(interceptorInstance);}} }

mybatis初始化時，出了加載mybatis-config.xml的全局配置文件，還會加載全部的映射配置文件，即mappers節點配置的mapper.

/*** 解析mapper文件，mapper可以理解為dao的實現* @param parent* @throws Exception*/ private void mapperElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {//如果mappers節點的子節點是package, 那么就掃描package下的文件, 注入進configurationif ("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");//resource, url, class 三選一if (resource != null && url == null && mapperClass == null) {ErrorContext.instance().resource(resource);InputStream inputStream = Resources.getResourceAsStream(resource);//mapper映射文件都是通過XMLMapperBuilder解析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);//同上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.");}}}} }

mybatis初始化過程中對mybatis-config.xml配置文件的解析過程到這吧，下一個就叫啥mapper配置文件的解析過程。

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