@Bean
public PlatformTransactionManager aTransactionManager() {
    return new DataSourceTransactionManager(aDataSource());
}

@Bean
public PlatformTransactionManager bTransactionManager() {
    return new DataSourceTransactionManager(bDataSource());
}

org.springframework.transaction.interceptor.TransactionInterceptor#invoke
?? ?org.springframework.transaction.interceptor.TransactionAspectSupport#invokeWithinTransaction
?? ?
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
//根據(jù)事務(wù)屬性獲取獲取事務(wù)管理器，這里一般是 DataSourceTransactionManager，我們也可以自己去指定事務(wù)管理器
final PlatformTransactionManager tm = determineTransactionManager(txAttr);?? ?

org.springframework.transaction.annotation.SpringTransactionAnnotationParser#parseTransactionAnnotation(org.springframework.core.annotation.AnnotationAttributes)

/**
 * 這個方法，是來獲取事務(wù)管理器的，如果我們有自己指定，就可以在使用@Transactional注解的時候，通過value，指定我們要使用的事務(wù)管理器
 * Determine the specific transaction manager to use for the given transaction.
 */
@Nullable
protected PlatformTransactionManager determineTransactionManager(@Nullable TransactionAttribute txAttr) {
	// Do not attempt to lookup tx manager if no tx attributes are set
	if (txAttr == null || this.beanFactory == null) {
		return getTransactionManager();
	}

	// 1.由于在解析事務(wù)注解的時候，將value，放到了qualifier屬性上，所以要這樣獲取，我們只需要知道，這里的qualifier就是事務(wù)注解中的value對應(yīng)的值
	String qualifier = txAttr.getQualifier();
	// 2.如果在@Transaction的value指定事務(wù)管理器，會執(zhí)行這里，其實(shí)就是從spring容器中獲取指定的事務(wù)管理器
	if (StringUtils.hasText(qualifier)) {
		return determineQualifiedTransactionManager(this.beanFactory, qualifier);
	}
	// 3.
	else if (StringUtils.hasText(this.transactionManagerBeanName)) {
		return determineQualifiedTransactionManager(this.beanFactory, this.transactionManagerBeanName);
	}
	else {
		// 4.先判斷是否有指定事務(wù)管理器，如果沒有，就獲取默認(rèn)的
		PlatformTransactionManager defaultTransactionManager = getTransactionManager();
		if (defaultTransactionManager == null) {
			// 獲取默認(rèn)的事務(wù)管理器
			defaultTransactionManager = this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);
			if (defaultTransactionManager == null) {
				//由于我們沒有指定，會從spring容器中按照類型獲取，所以我們只需要往spring容器中注入一個事務(wù)管理器即可，springboot應(yīng)用會默認(rèn)注入一個事務(wù)管理器
				defaultTransactionManager = this.beanFactory.getBean(PlatformTransactionManager.class);
				this.transactionManagerCache.putIfAbsent(
						DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);
			}
		}
		return defaultTransactionManager;
	}
}

@Configuration
@ConditionalOnClass({ JdbcTemplate.class, PlatformTransactionManager.class })
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceTransactionManagerAutoConfiguration {

	@Configuration
	@ConditionalOnSingleCandidate(DataSource.class)
	static class DataSourceTransactionManagerConfiguration {

		private final DataSource dataSource;

		private final TransactionManagerCustomizers transactionManagerCustomizers;

		DataSourceTransactionManagerConfiguration(DataSource dataSource,
				ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
			this.dataSource = dataSource;
			this.transactionManagerCustomizers = transactionManagerCustomizers
					.getIfAvailable();
		}

		@Bean
		@ConditionalOnMissingBean(PlatformTransactionManager.class)
		public DataSourceTransactionManager transactionManager(
				DataSourceProperties properties) {
			// 根據(jù)DataSource，初始化一個DataSourceTransactionManager對象，然后return；所以，這里就會把初始化的事務(wù)管理器放到spring容器中
			DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(
					this.dataSource);
			if (this.transactionManagerCustomizers != null) {
				this.transactionManagerCustomizers.customize(transactionManager);
			}
			return transactionManager;
		}
	}
}