Spring Boot 多数据源解决方案：dynamic-datasource-spring-boot-starter 的奥秘（下）

在上一篇博客《Spring Boot 多数据源解决方案：dynamic-datasource-spring-boot-starter 的奥秘》介绍了dynamic-datasource-spring-boot-starter的自动配置类和配置属性类之后，本文继续来剖析多数据源是如何切换的，什么时候切换的。

前文中提到dynamic-datasource-spring-boot-starter的自动配置类DynamicDataSourceAutoConfiguration在初始化的时候，会创建一个Advisor bean。

	@Role(2)@Beanpublic Advisor dynamicDatasourceAnnotationAdvisor(DsProcessor dsProcessor) {DynamicDataSourceAnnotationInterceptor interceptor = new DynamicDataSourceAnnotationInterceptor(this.properties.isAllowedPublicOnly(), dsProcessor);DynamicDataSourceAnnotationAdvisor advisor = new DynamicDataSourceAnnotationAdvisor(interceptor);advisor.setOrder(this.properties.getOrder());return advisor;}

该bean负责处理带有@DS注解的类或方法。它发挥的作用和通过@Pointcut+@Before或@Around发挥的作用一致。更多细节可见《探索微服务中的权限控制：一次线上问题排查的思考》

public class DynamicDataSourceAnnotationAdvisor extends AbstractPointcutAdvisor implements BeanFactoryAware {private final Advice advice;private final Pointcut pointcut;public DynamicDataSourceAnnotationAdvisor(@NonNull DynamicDataSourceAnnotationInterceptor dynamicDataSourceAnnotationInterceptor) {// 初始化的时候，传入了dynamicDataSourceAnnotationInterceptor// 说明拦截器发挥了通知的作用。this.advice = dynamicDataSourceAnnotationInterceptor;this.pointcut = buildPointcut();}// 核心方法， 切点获取，类比@Pointcut注解 @Overridepublic Pointcut getPointcut() {return this.pointcut;}// 核心方法，通知获取， 类比@Before, @After, @Around注解@Overridepublic Advice getAdvice() {return this.advice;}@Overridepublic void setBeanFactory(BeanFactory beanFactory) throws BeansException {if (this.advice instanceof BeanFactoryAware) {((BeanFactoryAware) this.advice).setBeanFactory(beanFactory);}}private Pointcut buildPointcut() {Pointcut cpc = new AnnotationMatchingPointcut(DS.class, true);Pointcut mpc = new AnnotationMethodPoint(DS.class);return new ComposablePointcut(cpc).union(mpc);}……
}

DynamicDataSourceAnnotationAdvisor类在初始化的时候，传入了DynamicDataSourceAnnotationInterceptor类型的拦截器作为advice, 说明该拦截器发挥了通知的作用（类似@Before, @Around）。

public class DynamicDataSourceAnnotationInterceptor implements MethodInterceptor {/*** The identification of SPEL.*/private static final String DYNAMIC_PREFIX = "#";private final DataSourceClassResolver dataSourceClassResolver;private final DsProcessor dsProcessor;public DynamicDataSourceAnnotationInterceptor(Boolean allowedPublicOnly, DsProcessor dsProcessor) {dataSourceClassResolver = new DataSourceClassResolver(allowedPublicOnly);this.dsProcessor = dsProcessor;}// 核心方法， dsKey就是@DS注解中的值， 如@DS("cusOB"),那么dsKey = "cusOB"@Overridepublic Object invoke(MethodInvocation invocation) throws Throwable {String dsKey = determineDatasourceKey(invocation);DynamicDataSourceContextHolder.push(dsKey);try {return invocation.proceed();} finally {DynamicDataSourceContextHolder.poll();}}private String determineDatasourceKey(MethodInvocation invocation) {String key = dataSourceClassResolver.findDSKey(invocation.getMethod(), invocation.getThis());return (!key.isEmpty() && key.startsWith(DYNAMIC_PREFIX)) ? dsProcessor.determineDatasource(invocation, key) : key;}
}

在invoke方法执行前，会先把该数据源对应的key加入到线程本地变量的双端队列中，如下。

private static final ThreadLocal<Deque<String>> LOOKUP_KEY_HOLDER = new NamedThreadLocal<Deque<String>>("dynamic-datasource") {@Overrideprotected Deque<String> initialValue() {return new ArrayDeque<>();}};

在invoke方法执行后，会先把该数据源对应的key从线程本地变量的双端队列中移除。

看到这里，我们知道，dynamic-datasource-spring-boot-starter会通过DynamicDataSourceAnnotationInterceptor 拦截器获取到@DS注解标识的方法或者类的代理。而@DS注解标注的就是数据源的Key。有了该key， Mybatis就可以获取到该key对应的Druid数据库连接池了。具体的策略，dynamic-datasource-spring-boot-starter提供了轮询和随机两种，这里就不再赘述了。

数据源的切换逻辑在DynamicRoutingDataSource类中。它继承了AbstractRoutingDataSource类，该类实现了getConnection()方法。

public abstract class AbstractRoutingDataSource extends AbstractDataSource {protected abstract DataSource determineDataSource();@Overridepublic Connection getConnection() throws SQLException {String xid = TransactionContext.getXID();if (StringUtils.isEmpty(xid)) {// 核心方法determineDataSourcereturn determineDataSource().getConnection();} else {String ds = DynamicDataSourceContextHolder.peek();ConnectionProxy connection = ConnectionFactory.getConnection(ds);return connection == null ? getConnectionProxy(ds, determineDataSource().getConnection()) : connection;}}@Overridepublic Connection getConnection(String username, String password) throws SQLException {String xid = TransactionContext.getXID();if (StringUtils.isEmpty(xid)) {return determineDataSource().getConnection(username, password);} else {String ds = DynamicDataSourceContextHolder.peek();ConnectionProxy connection = ConnectionFactory.getConnection(ds);return connection == null ? getConnectionProxy(ds, determineDataSource().getConnection(username, password)): connection;}}
}

DynamicDataSourceAnnotationInterceptor的invoke方法负责放数据源的key，
determineDataSource方法负责从线程本地变量的双端队列中取出数据源的key。

public DataSource determineDataSource() {String dsKey = DynamicDataSourceContextHolder.peek();return getDataSource(dsKey);}

getDataSource方法用来获取具体的数据库连接。

    private final Map<String, DataSource> dataSourceMap = new ConcurrentHashMap<>();public DataSource getDataSource(String ds) {if (StringUtils.isEmpty(ds)) {return determinePrimaryDataSource();} else if (!groupDataSources.isEmpty() && groupDataSources.containsKey(ds)) {log.debug("dynamic-datasource switch to the datasource named [{}]", ds);return groupDataSources.get(ds).determineDataSource();} else if (dataSourceMap.containsKey(ds)) {log.debug("dynamic-datasource switch to the datasource named [{}]", ds);// map中存了所有的数据库连接信息return dataSourceMap.get(ds);}if (strict) {throw new CannotFindDataSourceException("dynamic-datasource could not find a datasource named" + ds);}return determinePrimaryDataSource();}

那么问题来了，dataSourceMap的数据库相关信息是什么时候存进去的呢？

大家还记得上一篇博客中，我们提到dynamic-datasource-spring-boot-starter的自动配置类DynamicDataSourceAutoConfiguration在初始化的时候还创建了一个DynamicDataSourceProvider类型的bean嘛？没错，dataSourceMap是它从DynamicDataSourceProperties配置类中获取，然后存储进去的，key是数据源的唯一标识，如value是数据库的相关信息，包括url, username, password等。

	@Bean@ConditionalOnMissingBean // 确保Spring容器中不存在 DynamicDataSourceProvider 类型的 Bean 时才创建该 Beanpublic DynamicDataSourceProvider dynamicDataSourceProvider() {Map<String, DataSourceProperty> datasourceMap = this.properties.getDatasource();return new YmlDynamicDataSourceProvider(datasourceMap);}public class YmlDynamicDataSourceProvider extends AbstractDataSourceProvider {/*** 所有数据源*/private final Map<String, DataSourceProperty> dataSourcePropertiesMap;@Overridepublic Map<String, DataSource> loadDataSources() {// 核心方法return createDataSourceMap(dataSourcePropertiesMap);}
}

protected Map<String, DataSource> createDataSourceMap(Map<String, DataSourceProperty> dataSourcePropertiesMap) {// dataSourcePropertiesMap 就是从DynamicDataSourceProperties配置类中拿到的所有数据库信息Map<String, DataSource> dataSourceMap = new HashMap<>(dataSourcePropertiesMap.size() * 2);for (Map.Entry<String, DataSourceProperty> item : dataSourcePropertiesMap.entrySet()) {DataSourceProperty dataSourceProperty = item.getValue();String poolName = dataSourceProperty.getPoolName();if (poolName == null || "".equals(poolName)) {poolName = item.getKey();}dataSourceProperty.setPoolName(poolName);dataSourceMap.put(poolName, defaultDataSourceCreator.createDataSource(dataSourceProperty));}return dataSourceMap;}