Mybatis3源码分析（一）：从sqlSession说起

WindySky 2018-03-01

展开全文

分析MyBatis3的源码首先得从sqlSessionFactory开始，先来看一段spring配置文件中Mybaits的sqlSessionFactory的配置。

<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<property name="driverClassName" value="${jdbc.driver}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</bean>
<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/tiantian/mybatis/mapper/*.xml"/>
<property name="typeAliasesPackage" value="com.tiantian.mybatis.model" />
</bean>
<bean id="blogMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
<property name="mapperInterface"
value="com.tiantian.mybatis.mapper.BlogMapper" />
<property name="sqlSessionFactory" ref="sqlSessionFactory" />
</bean>

spring会调用sqlSessionFactory的无参构造函数创建实例，并将dataSource和映射文件路径注入进去初始化sqlSessionFactory。这里的SqlSessionFactoryBean的实现依赖于mybatis社区自行开发的支持包，我们来看看它是怎么读取xml配置文件创建sessionFactory的:

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protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
//代表配置的对象
Configuration configuration;
//xml配置创建者
XMLConfigBuilder xmlConfigBuilder = null;
//spring中注入的配置文件路径，如果不存在使用默认配置
if (this.configLocation != null) {
xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
configuration = xmlConfigBuilder.getConfiguration();
} else {
if (logger.isDebugEnabled()) {
logger.debug("Property 'configLocation' not specified, using default MyBatis Configuration");
}
configuration = new Configuration();
configuration.setVariables(this.configurationProperties);
}
//如果配置了objectFactory使用此配置类
if (this.objectFactory != null) {
configuration.setObjectFactory(this.objectFactory);
}
//同上
if (this.objectWrapperFactory != null) {
configuration.setObjectWrapperFactory(this.objectWrapperFactory);
}
//如果配置了typeAliases（实体类别名标签）的基包则不用在configuration文件中配置
if (hasLength(this.typeAliasesPackage)) {
String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeAliasPackageArray) {
configuration.getTypeAliasRegistry().registerAliases(packageToScan,
typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
if (logger.isDebugEnabled()) {
logger.debug("Scanned package: '" + packageToScan + "' for aliases");
}
}
}
//如果是直接配置的实体类别名则在配置对象中注册
if (!isEmpty(this.typeAliases)) {
for (Class<?> typeAlias : this.typeAliases) {
configuration.getTypeAliasRegistry().registerAlias(typeAlias);
if (logger.isDebugEnabled()) {
logger.debug("Registered type alias: '" + typeAlias + "'");
}
}
}
//是否配置mybaits的一些插件
if (!isEmpty(this.plugins)) {
for (Interceptor plugin : this.plugins) {
configuration.addInterceptor(plugin);
if (logger.isDebugEnabled()) {
logger.debug("Registered plugin: '" + plugin + "'");
}
}
}
//是否配置typeHandler的基包
if (hasLength(this.typeHandlersPackage)) {
String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeHandlersPackageArray) {
configuration.getTypeHandlerRegistry().register(packageToScan);
if (logger.isDebugEnabled()) {
logger.debug("Scanned package: '" + packageToScan + "' for type handlers");
}
}
}
//是否直接配置了typeHandler
if (!isEmpty(this.typeHandlers)) {
for (TypeHandler<?> typeHandler : this.typeHandlers) {
configuration.getTypeHandlerRegistry().register(typeHandler);
if (logger.isDebugEnabled()) {
logger.debug("Registered type handler: '" + typeHandler + "'");
}
}
}
//如果xmlConfigBuilder存在则开始分析创建Configuration对象
if (xmlConfigBuilder != null) {
try {
xmlConfigBuilder.parse();
if (logger.isDebugEnabled()) {
logger.debug("Parsed configuration file: '" + this.configLocation + "'");
}
} catch (Exception ex) {
throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
} finally {
ErrorContext.instance().reset();
}
}
//事务工厂是否配置
if (this.transactionFactory == null) {
this.transactionFactory = new SpringManagedTransactionFactory();
}
//设置Configuration的环境对象
Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);
configuration.setEnvironment(environment);
if (this.databaseIdProvider != null) {
try {
configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
} catch (SQLException e) {
throw new NestedIOException("Failed getting a databaseId", e);
}
}
//是否配置mapperLocations，如果存在解析创建这些mapper，mapper是mybaits管理各种sql的容器
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 + "'");
}
}
} else {
if (logger.isDebugEnabled()) {
logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
}
}
//创建sqlSessionFactoryBean
return this.sqlSessionFactoryBuilder.build(configuration);
}

从mybatis创建sqlSessionFactoryBean的过程可以看出，mybaits默认使用spring的事务管理功能或者由第三方实现，它自身并没有提供事务管理能力。其次，它拥有跟其它框架差不多的解析xml过程，都是将xml解析成一个Configuration对象随时取用。到这里我们就得到了一个sqlSessionFactoryBean对象（实际是mybatis本身的DefaultSqlSessionFactory对象），我们可以在DAO中注入这个对象并利用openSession的方法获取SqlSession对象从而进行各种数据库操作。