转载自  mybatis源码阅读(二)：mybatis初始化上

1.初始化入口

//Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
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，我们就先从SqlSessionFactoryBuilder入手， 咱们先看看源码是怎么实现的

SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {// 读取配置文件XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);//解析配置得到Configuration对象，创建DefaultSqlSessionFactory对象return build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} finally {//关闭读取配置文件的输入流对象ErrorContext.instance().reset();try {inputStream.close();} catch (IOException e) {// Intentionally ignore. Prefer previous error.}}
}

XMLConfigBuilder

XMLConfigBuilder是BaseBuilder的众多子类之一，核心字段如下

//表示是否已经解析过了
private boolean parsed;
//用于解析配置文件的对象
private final XPathParser parser;
//配置文件中表示<environment>的名称  默认读取default属性
private String environment;
// 负责和创建Reflector对象
private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

XMLConfigBuilder.parse()方法是解析mybatis-config.xml配置文件的如。它调用parseConfiguration（）方法实现整个解析过程。具体实现如下：

/*** 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;
}/*** 对配置文件每个节点具体的解析过程* configuration节点为根节点。* 在configuration节点之下，我们可以配置11  个子节点，* 分别为：properties、settings、typeAliases、plugins、objectFactory、objectWrapperFactory、reflectorFactory、* environments、databaseIdProvider、typeHandlers、mappers。* @param root 根节点*/
private void parseConfiguration(XNode root) {try {// 解析properties节点propertiesElement(root.evalNode("properties"));//解析settings节点Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(settings);//设置vfsImpl字段//解析typeAliases节点typeAliasesElement(root.evalNode("typeAliases"));//解析plugins节点pluginElement(root.evalNode("plugins"));//解析objectFactory节点objectFactoryElement(root.evalNode("objectFactory"));//解析objectWrapperFactory节点objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));//解析reflectorFactory节点reflectorFactoryElement(root.evalNode("reflectorFactory"));settingsElement(settings);//解析environments节点environmentsElement(root.evalNode("environments"));//解析databaseIdProvider节点databaseIdProviderElement(root.evalNode("databaseIdProvider"));//解析typeHandlers节点typeHandlerElement(root.evalNode("typeHandlers"));//解析mappers节点mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}
}

propertiesElement()方法会解析配置文件中的properties节点并形成Java.util.Properties对象，之后将改对象设置到XpathParse和Configguration的variables字段中，占位符就是用Properties中的信息替换的，具体实现如下：

/*** 解析properties的具体方法* @param context* @throws Exception*/
private void propertiesElement(XNode context) throws Exception {if (context != null) {// 将子节点的 name 以及value属性set进properties对象// 这儿可以注意一下顺序，xml配置优先， 外部指定properties配置其次Properties defaults = context.getChildrenAsProperties();// 获取properties节点上 resource属性的值String resource = context.getStringAttribute("resource");// 获取properties节点上 url属性的值, resource和url不能同时配置String url = context.getStringAttribute("url");if (resource != null && url != null) {//url 和resource不能同时配置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进Properties对象if (resource != null) {defaults.putAll(Resources.getResourceAsProperties(resource));} else if (url != null) {defaults.putAll(Resources.getUrlAsProperties(url));}// 将configuration对象中已配置的Properties属性与刚刚解析的融合// configuration这个对象会装载所解析mybatis配置文件的所有节点元素，以后也会频频提到这个对象Properties vars = configuration.getVariables();if (vars != null) {defaults.putAll(vars);}// 把装有解析配置propertis对象set进解析器， 因为后面可能会用到parser.setVariables(defaults);// set进configuration对象configuration.setVariables(defaults);}
}

settings节点下的配饰是mybatis的全局性配置，修改的是configuration对象的属性，具体说明参考官方文档

/*** settings标签就是设置configuration对象的各种属性，* 具体属性说明可以参考mybatis官方文档* @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元素节点主要配置数据库事物，数据源。可以配置多个environment子节点，假如我们系统的开发环境和正式环境所用的数据库不一样（这是肯定的）， 那么可以设置两个environment, 两个id分别对应开发环境（development）和正式环境（final），那么通过配置environments的default属性就能选择对应的environment了， 例如，我将environments的deault属性的值配置为development, 那么就会选择dev的environment。具体实现如下

/*** 解析enviroments元素节点的方法* @param context* @throws Exception*/
private void environmentsElement(XNode context) throws Exception {if (context != null) {if (environment == null) {//获取 <environments default="development"> 中的default值environment = context.getStringAttribute("default");}// 循环environments的子节点for (XNode child : context.getChildren()) {// 获取 <environment id="development"> z中的idString id = child.getStringAttribute("id");if (isSpecifiedEnvironment(id)) {//根据由environments的default属性去选择对应的enviroment// 事物 mybatis有两种：JDBC 和 MANAGED, 配置为JDBC则直接使用JDBC的事务，配置为MANAGED则是将事务托管给容器// <transactionManager type="JDBC"/>TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));//enviroment节点下面就是dataSource节点了，解析dataSource节点DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));DataSource dataSource = dsFactory.getDataSource();Environment.Builder environmentBuilder = new Environment.Builder(id).transactionFactory(txFactory).dataSource(dataSource);// 将dataSource设置进configuration对象configuration.setEnvironment(environmentBuilder.build());}}}
}

typeAliases节点主要用来设置别名，其实这是挺好用的一个功能， 通过配置别名，我们不用再指定完整的包名

/*** 解析typeAliases 节点* <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()) {//如果子节点是package, 那么就获取package节点的name属性， mybatis会扫描指定的packageif ("package".equals(child.getName())) {String typeAliasPackage = child.getStringAttribute("name");//TypeAliasRegistry 负责管理别名， 这儿就是通过TypeAliasRegistry 进行别名注册configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);} else {//如果子节点是typeAlias节点，那么就获取alias属性和type的属性值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);}}}}
}

具体的别名注册类

public class TypeAliasRegistry {// 别名通过一个HashMap来实现， key为别名， value就是别名对应的类型（class对象）private final Map<String, Class<?>> TYPE_ALIASES = new HashMap<String, Class<?>>();/*** mybatis默认为我们注册的别名*/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);}/*** 处理别名， 直接从保存有别名的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);}}/*** 配置文件中配置为package的时候，扫描包下的Javabean ，然后自动注册别名* 默认会使用 Bean 的首字母小写的非限定类名来作为它的别名* 也可在javabean 加上注解@Alias 来自定义别名， 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中注册别名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);}}/*** 获取保存别名的HashMap, Configuration对象持有对TypeAliasRegistry的引用，* 因此，如果需要，我们可以通过Configuration对象获取*/public Map<String, Class<?>> getTypeAliases() {return Collections.unmodifiableMap(TYPE_ALIASES);}}

typeHandlers节点的解析和typeAlianses节点的解析类似

/*** 解析typeHandlers节点* 无论是 MyBatis 在预处理语句（PreparedStatement）中设置一个参数时，* 还是从结果集中取出一个值时，都会用类型处理器将获取的值以合适的方式转换成 Java 类型。* Mybatis默认为我们实现了许多TypeHandler, 当我们没有配置指定TypeHandler时，* Mybatis会根据参数或者返回结果的不同，默认为我们选择合适的TypeHandler处理。* @param parent* @throws Exception*/
private void typeHandlerElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {//子节点为package时，获取其name属性的值，然后自动扫描package下的自定义typeHandlerif ("package".equals(child.getName())) {String typeHandlerPackage = child.getStringAttribute("name");typeHandlerRegistry.register(typeHandlerPackage);} else {//子节点为typeHandler时， 可以指定javaType属性， 也可以指定jdbcType, 也可两者都指定//javaType 是指定java类型//jdbcType 是指定jdbc类型（数据库类型： 如varchar）String javaTypeName = child.getStringAttribute("javaType");String jdbcTypeName = child.getStringAttribute("jdbcType");//handler就是我们配置的typeHandlerString handlerTypeName = child.getStringAttribute("handler");Class<?> javaTypeClass = resolveClass(javaTypeName);//JdbcType是一个枚举类型，resolveJdbcType方法是在获取枚举类型的值JdbcType jdbcType = resolveJdbcType(jdbcTypeName);Class<?> typeHandlerClass = resolveClass(handlerTypeName);//注册typeHandler, typeHandler通过TypeHandlerRegistry这个类管理if (javaTypeClass != null) {if (jdbcType == null) {typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);} else {typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);}} else {typeHandlerRegistry.register(typeHandlerClass);}}}}
}

插件是mybatis提供的扩展机制，用户可以通过添加自定义插件在SQL语句执行的过程中某一环节进行拦截，mybatis中的自定义插件只需实现Interceptor接口，并通过注解指定拦截的方法签名，这个后面具体介绍。

/*** 解析plugins标签* mybatis中的plugin其实就是个interceptor，* 它可以拦截Executor 、ParameterHandler 、ResultSetHandler 、StatementHandler 的部分方法，处理我们自己的逻辑。* @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();// 我们在定义一个interceptor的时候，需要去实现InterceptorInterceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();interceptorInstance.setProperties(properties);// 向configuration对象中注册拦截器configuration.addInterceptor(interceptorInstance);}}
}

mybatis初始化时，出了加载mybatis-config.xml的全局配置文件，还会加载全部的映射配置文件，即mappers节点配置的mapper.

/*** 解析mapper文件，mapper可以理解为dao的实现* @param parent* @throws Exception*/
private void mapperElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {//如果mappers节点的子节点是package, 那么就扫描package下的文件, 注入进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 三选一if (resource != null && url == null && mapperClass == null) {ErrorContext.instance().resource(resource);InputStream inputStream = Resources.getResourceAsStream(resource);//mapper映射文件都是通过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初始化过程中对mybatis-config.xml配置文件的解析过程到这吧，下一个就叫啥mapper配置文件的解析过程。