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Sharding JDBC自动配置的原理

与所有starter一样，shardingsphere-jdbc-core-spring-boot-starter也是通过SPI自动配置的原理实现分库分表配置加载，spring.factories文件中的自动配置类shardingsphere-jdbc-core-spring-boot-starter功不可没，他主要是自动创建了模式bean、事务类型bean和数据源bean，配置加载的过程可以归纳如下：

其中，创建数据源bean时会根据不同的模式创建不同的bean，本地模式直接从配置文件中加载，配置中心模式就从配置中心加载。ShardingSphereAutoConfiguration的实现如下：

@Configuration
@ComponentScan("org.apache.shardingsphere.spring.boot.converter")
@EnableConfigurationProperties(SpringBootPropertiesConfiguration.class)
@AutoConfigureBefore(DataSourceAutoConfiguration.class)
@RequiredArgsConstructor
public class ShardingSphereAutoConfiguration implements EnvironmentAware {private String databaseName;private final SpringBootPropertiesConfiguration props;private final Map<String, DataSource> dataSourceMap = new LinkedHashMap<>();/*** 模式配置** @return mode configuration*/@Beanpublic ModeConfiguration modeConfiguration() {return null == props.getMode() ? null : new YamlModeConfigurationSwapper().swapToObject(props.getMode());}/*** 单机模式：从本地配置中加载DataSource** @param rules rules configuration* @param modeConfig mode configuration* @return data source bean* @throws SQLException SQL exception*/@Bean@Conditional(LocalRulesCondition.class)@Autowired(required = false)public DataSource shardingSphereDataSource(final ObjectProvider<List<RuleConfiguration>> rules, final ObjectProvider<ModeConfiguration> modeConfig) throws SQLException {Collection<RuleConfiguration> ruleConfigs = Optional.ofNullable(rules.getIfAvailable()).orElseGet(Collections::emptyList);return ShardingSphereDataSourceFactory.createDataSource(databaseName, modeConfig.getIfAvailable(), dataSourceMap, ruleConfigs, props.getProps());}/*** 集群模式：从配置中心中加载DataSource** @param modeConfig mode configuration* @return data source bean* @throws SQLException SQL exception*/@Bean@ConditionalOnMissingBean(DataSource.class)public DataSource dataSource(final ModeConfiguration modeConfig) throws SQLException {return !dataSourceMap.isEmpty() ? ShardingSphereDataSourceFactory.createDataSource(databaseName, modeConfig, dataSourceMap, Collections.emptyList(), props.getProps()): ShardingSphereDataSourceFactory.createDataSource(databaseName, modeConfig);}/*** 事务类型扫描bean** @return transaction type scanner*/@Beanpublic TransactionTypeScanner transactionTypeScanner() {return new TransactionTypeScanner();}@Overridepublic final void setEnvironment(final Environment environment) {dataSourceMap.putAll(DataSourceMapSetter.getDataSourceMap(environment));databaseName = DatabaseNameSetter.getDatabaseName(environment);}
}

下面以单机模式出发点，理一下加载的过程。

@Bean
@Conditional(LocalRulesCondition.class)
@Autowired(required = false)
public DataSource shardingSphereDataSource(final ObjectProvider<List<RuleConfiguration>> rules, final ObjectProvider<ModeConfiguration> modeConfig) throws SQLException {Collection<RuleConfiguration> ruleConfigs = Optional.ofNullable(rules.getIfAvailable()).orElseGet(Collections::emptyList);// 通过ShardingSphereDataSourceFactory工厂创建数据源return ShardingSphereDataSourceFactory.createDataSource(databaseName, modeConfig.getIfAvailable(), dataSourceMap, ruleConfigs, props.getProps());
}

通过对源码的跟踪，可以发现，ShardingSphereDataSourceFactory.createDataSource创建数据源经历了如下的过程

分片规则加载原理

分片规则、审计规则、key生成规则都是通过SPI的方式加载，自动配置类ShardingSphereAutoConfiguration中创建ShardingSphereDataSource的时候，会加载配置的分片规则，创建核心配置类ShardingRule，在ShardingRule的创建中会通过SPI的方式加载分片规则。加载的过程如下：

SPI核心实现类ShardingSphereServiceLoader中会将SPI接口进行Map缓存管理，需要时直接获取。如果Map中不存在，就通过反射的方式新建服务实例，具体实现源码如下：

public final class ShardingSphereServiceLoader {// 缓存service实例，// 缓存的Key,如：//  org.apache.shardingsphere.sharding.spi.ShardingAlgorithm//  org.apache.shardingsphere.sharding.spi.KeyGenerateAlgorithm//  org.apache.shardingsphere.transaction.spi.ShardingSphereTransactionManagerprivate static final Map<Class<?>, Collection<Object>> SERVICES = new ConcurrentHashMap<>();/*** 注册服务实例** @param 服务接口*/public static void register(final Class<?> serviceInterface) {if (!SERVICES.containsKey(serviceInterface)) {SERVICES.put(serviceInterface, load(serviceInterface));}}private static <T> Collection<Object> load(final Class<T> serviceInterface) {Collection<Object> result = new LinkedList<>();for (T each : ServiceLoader.load(serviceInterface)) {result.add(each);}return result;}/*** 获取服务实例* * @param 服务接口* @param <T> 服务类型* @return 服务实例*/public static <T> Collection<T> getServiceInstances(final Class<T> serviceInterface) {return null == serviceInterface.getAnnotation(SingletonSPI.class) ? createNewServiceInstances(serviceInterface) : getSingletonServiceInstances(serviceInterface);}@SneakyThrows(ReflectiveOperationException.class)@SuppressWarnings("unchecked")private static <T> Collection<T> createNewServiceInstances(final Class<T> serviceInterface) {if (!SERVICES.containsKey(serviceInterface)) {return Collections.emptyList();}Collection<Object> services = SERVICES.get(serviceInterface);if (services.isEmpty()) {return Collections.emptyList();}Collection<T> result = new LinkedList<>();for (Object each : services) {// 通过反射新建实例result.add((T) each.getClass().getDeclaredConstructor().newInstance());}return result;}@SuppressWarnings("unchecked")private static <T> Collection<T> getSingletonServiceInstances(final Class<T> serviceInterface) {return (Collection<T>) SERVICES.getOrDefault(serviceInterface, Collections.emptyList());}
}