多数据源H2

实际业务中经常会遇到需要访问多个数据库的情况，有些情况是表和数据库之间的关系是固定的，而有些情况同一张表会存在于多个数据库中。

CASE 1H3

针对第一种情况，通过Spring的配置就能完成，例如

bash
@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(
  entityManagerFactoryRef = "entityManagerFactory",
  basePackages = { "com.foobar.foo.repo" }
)
public class FooDbConfig {
  
  @Primary
  @Bean(name = "dataSource")
  @ConfigurationProperties(prefix = "spring.datasource")
  public DataSource dataSource() {
    return DataSourceBuilder.create().build();
  }
  
  @Primary
  @Bean(name = "entityManagerFactory")
  public LocalContainerEntityManagerFactoryBean 
  entityManagerFactory(
    EntityManagerFactoryBuilder builder,
    @Qualifier("dataSource") DataSource dataSource
  ) {
    return builder
      .dataSource(dataSource)
      .packages("com.foobar.foo.domain")
      .persistenceUnit("foo")
      .build();
  }
    
  @Primary
  @Bean(name = "transactionManager")
  public PlatformTransactionManager transactionManager(
    @Qualifier("entityManagerFactory") EntityManagerFactory 
    entityManagerFactory
  ) {
    return new JpaTransactionManager(entityManagerFactory);
  }
}

bash
@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(
  entityManagerFactoryRef = "barEntityManagerFactory",
  transactionManagerRef = "barTransactionManager",
  basePackages = { "com.foobar.bar.repo" }
)
public class BarDbConfig {
 
  @Bean(name = "barDataSource")
  @ConfigurationProperties(prefix = "bar.datasource")
  public DataSource dataSource() {
    return DataSourceBuilder.create().build();
  }
  
  @Bean(name = "barEntityManagerFactory")
  public LocalContainerEntityManagerFactoryBean 
  barEntityManagerFactory(
    EntityManagerFactoryBuilder builder,
    @Qualifier("barDataSource") DataSource dataSource
  ) {
    return
      builder
        .dataSource(dataSource)
        .packages("com.foobar.bar.domain")
        .persistenceUnit("bar")
        .build();
  }
  @Bean(name = "barTransactionManager")
  public PlatformTransactionManager barTransactionManager(
    @Qualifier("barEntityManagerFactory") EntityManagerFactory
    barEntityManagerFactory
  ) {
    return new JpaTransactionManager(barEntityManagerFactory);
  }
}

CASE 2H3

第二种情况，则通过框架提供的AbstractRoutingDataSource来解决。通常的方案是根据自己的业务需求把要访问的数据库的标识存在TheadLocal中，然后在determineCurrentLookupKey去获取这个值。

bash
public class ContextRoutingDataSource extends AbstractRoutingDataSource {

    @Override
    protected Object determineCurrentLookupKey() {
        return DatabaseContextHolder.getContext();
    }
}

public class DatabaseContextHolder {

    private static final ThreadLocal<String> contextHolder = new ThreadLocal<>();

    public static void setContext(String contextName) {
        Assert.notNull(contextName, "Context cannot be null");
        contextHolder.set(contextName);
    }

    public static String getContext() {
        return contextHolder.get();
    }

    public static void clearContext() {
        contextHolder.remove();
    }
}

事务的处理H2

多个数据源之间的访问，有些情况需要保证在一个事务中进行，这就涉及到分布式事务。Atomikos 是一个维护的较好的内嵌的事务管理器，能够直接集成到Spring应用中。

下面是在 Spring Boot 中配置Datasource的示例，主要在于需要用AtomikosDataSourceBean来包装各个数据库的XA驱动

bash
private DataSource createXaDataSource(DataSourceProperties dataSourceProperties) throws Exception {
        String className = dataSourceProperties.getXa().getDataSourceClassName();
        if (!StringUtils.hasLength(className)) {
            className = DatabaseDriver.fromJdbcUrl(dataSourceProperties.determineUrl())
                .getXaDataSourceClassName();
        }
        Assert.state(StringUtils.hasLength(className),
            "No XA DataSource class name specified");
        XADataSource dataSource = createXaDataSourceInstance(className);
        bindXaProperties(dataSource, dataSourceProperties);
        return wrapDataSource(dataSourceProperties, dataSource);
    }

    private XADataSource createXaDataSourceInstance(String className) {
        try {
            Class<?> dataSourceClass = ClassUtils.forName(className, this.classLoader);
            Object instance = BeanUtils.instantiate(dataSourceClass);
            Assert.isInstanceOf(XADataSource.class, instance);
            return (XADataSource) instance;
        } catch (Exception ex) {
            throw new IllegalStateException(
                "Unable to create XADataSource instance from '" + className + "'");
        }
    }

    private void bindXaProperties(XADataSource target, DataSourceProperties dataSourceProperties) {
        MutablePropertyValues values = new MutablePropertyValues();
        values.add("user", dataSourceProperties.determineUsername());
        values.add("password", dataSourceProperties.determinePassword());
        values.add("url", dataSourceProperties.determineUrl());
        values.addPropertyValues(dataSourceProperties.getXa().getProperties());
        new RelaxedDataBinder(target).withAlias("user", "username").bind(values);
    }

    private DataSource wrapDataSource(DataSourceProperties dataSourceProperties, XADataSource dataSource) throws Exception {
        DataSource wrappedDataSource = this.wrapper.wrapDataSource(dataSource);
        if (wrappedDataSource instanceof AtomikosDataSourceBean) {
            ((AtomikosDataSourceBean) wrappedDataSource).setUniqueResourceName(dataSourceProperties.getName());
            ((AtomikosDataSourceBean) wrappedDataSource).setMaxPoolSize(
                env.getProperty("spring.jta.atomikos.datasource.max-pool-size", Integer.class));
            ((AtomikosDataSourceBean) wrappedDataSource).setMinPoolSize(
                env.getProperty("spring.jta.atomikos.datasource.min-pool-size", Integer.class));
            ((AtomikosDataSourceBean) wrappedDataSource).setTestQuery(
                env.getProperty("spring.jta.atomikos.datasource.test-query"));

        }
        return wrappedDataSource;
    }

Spring框架中一个事务中用到的EntityManger、Connection等都会绑定到TheadLocal中进行复用，这就导致一个问题。为了让两个数据库的操作在一些事务中进行，我们需要用@Transactional注解标示事务，但是在一个事务中由于绑定了ThreadLocal，切换数据源的设置又会无效。

解决的办法是手动清除掉绑定到ThreadLocal的一些对象，实际只要清除EntityManagerFactory即可

bash
TransactionSynchronizationManager.unbindResource(emf);

事务结束时会自动unbind 所有注册的TransactionSynchronization中引用的 EntityManagerFactory。TransactionSynchronization对象是获取EntityManagerFactory后注册的。

由于我们手动清除了一次EntityManagerFactory，再次生成EntityManagerFactory对象时会再registerSynchronization一次，导致synchronizations中两个TransactionSynchronization中引用了同一个TransactionSynchronization，事务结束时会对这个EntityManagerFactory对象unbind两次，导致错误。

所以需要在unbindResource之后把之前的TransactionSynchronization对象清除掉

bash
TransactionSynchronizationManager.clearSynchronization();
TransactionSynchronizationManager.initSynchronization();