Implementing Dynamic MySQL Master‑Slave Switching in SpringBoot Using AOP and Custom Annotations

The guide demonstrates how to integrate dynamic master‑slave database switching into a SpringBoot project by leveraging AOP and a custom @DataSource annotation, ensuring automatic failover to the master when a slave becomes unavailable.

Why switch data sources? Dynamic switching supports read‑write separation, multi‑tenant architectures, sharding, environment isolation, flexible database management, and fault‑tolerant high availability.

Read‑write separation: writes go to the master, reads to slaves.

Multi‑tenant: different tenants can be routed to different databases.

Sharding: distribute large data sets across multiple databases.

Environment isolation: separate dev, test, and prod databases.

Flexible management: choose data sources at runtime based on business logic.

Failover: automatically switch to a backup when the primary fails.

Implementation steps :

1. pom.xml dependencies

<dependency>
  <groupId>org.projectlombok</groupId>
  <artifactId>lombok</artifactId>
</dependency>

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-aop</artifactId>
</dependency>

<dependency>
  <groupId>com.alibaba</groupId>
  <artifactId>druid-spring-boot-starter</artifactId>
  <version>1.2.20</version>
</dependency>

<dependency>
  <groupId>com.mysql</groupId>
  <artifactId>mysql-connector-j</artifactId>
</dependency>

<dependency>
  <groupId>com.baomidou</groupId>
  <artifactId>mybatis-plus-boot-starter</artifactId>
  <version>3.5.3.1</version>
</dependency>

2. Configuration files

application.yml

server:
  port: 8000
spring:
  profiles:
    active: druid

application-druid.yml

# Data source configuration
spring:
  datasource:
    type: com.alibaba.druid.pool.DruidDataSource
    driverClassName: com.mysql.cj.jdbc.Driver
    druid:
      master:
        url: jdbc:mysql://localhost:3306/study?useUnicode=true&characterEncoding=utf8&zeroDateTimeBehavior=convertToNull&useSSL=true&serverTimezone=GMT%2B8
        username: root
        password: 123456
      slave:
        enabled: true
        url: jdbc:mysql://localhost:3306/t_lyj?useUnicode=true&characterEncoding=utf8&zeroDateTimeBehavior=convertToNull&useSSL=true&serverTimezone=GMT%2B8
        username: root
        password: 123456
      initialSize: 5
      minIdle: 10
      maxActive: 20
      maxWait: 60000
      connectTimeout: 30000
      socketTimeout: 60000
      timeBetweenEvictionRunsMillis: 60000
      minEvictableIdleTimeMillis: 300000
      maxEvictableIdleTimeMillis: 900000

3. DataSourceType enum

public enum DataSourceType {
    MASTER,
    SLAVE,
    SLAVE2 // additional slaves can be added here
}

4. SpringUtils bean helper

@Component
public final class SpringUtils implements BeanFactoryPostProcessor, ApplicationContextAware {
    private static ConfigurableListableBeanFactory beanFactory;
    private static ApplicationContext applicationContext;
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        SpringUtils.beanFactory = beanFactory;
    }
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        SpringUtils.applicationContext = applicationContext;
    }
    @SuppressWarnings("unchecked")
    public static
T getBean(String name) throws BeansException {
        return (T) beanFactory.getBean(name);
    }
    public static
T getBean(Class
clz) throws BeansException {
        return beanFactory.getBean(clz);
    }
    public static boolean containsBean(String name) {
        return beanFactory.containsBean(name);
    }
    public static boolean isSingleton(String name) throws NoSuchBeanDefinitionException {
        return beanFactory.isSingleton(name);
    }
    public static String[] getAliases(String name) throws NoSuchBeanDefinitionException {
        return beanFactory.getAliases(name);
    }
    public static String[] getActiveProfiles() {
        return applicationContext.getEnvironment().getActiveProfiles();
    }
    public static String getActiveProfile() {
        String[] active = getActiveProfiles();
        return active.length > 0 ? active[0] : null;
    }
    public static String getRequiredProperty(String key) {
        return applicationContext.getEnvironment().getRequiredProperty(key);
    }
}

5. Custom @DataSource annotation

@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface DataSource {
    DataSourceType value() default DataSourceType.MASTER;
}

6. DruidConfig for master and slave beans

@Configuration
public class DruidConfig {
    @Bean
    @ConfigurationProperties("spring.datasource.druid.master")
    public DataSource masterDataSource(DruidProperties druidProperties) {
        DruidDataSource ds = DruidDataSourceBuilder.create().build();
        return druidProperties.dataSource(ds);
    }
    @Bean
    @ConfigurationProperties("spring.datasource.druid.slave")
    @ConditionalOnProperty(prefix = "spring.datasource.druid.slave", name = "enabled", havingValue = "true")
    public DataSource slaveDataSource(DruidProperties druidProperties) {
        DruidDataSource ds = DruidDataSourceBuilder.create().build();
        return druidProperties.dataSource(ds);
    }
    @Bean(name = "dynamicDataSource")
    @Primary
    public DynamicDataSource dataSource(DataSource masterDataSource) {
        Map
target = new HashMap<>();
        target.put(DataSourceType.MASTER.name(), masterDataSource);
        setDataSource(target, DataSourceType.SLAVE.name(), "slaveDataSource");
        return new DynamicDataSource(masterDataSource, target);
    }
    private void setDataSource(Map
target, String sourceName, String beanName) {
        try {
            DataSource ds = SpringUtils.getBean(beanName);
            target.put(sourceName, ds);
        } catch (Exception ignored) {}
    }
}

7. DynamicDataSource implementation

public class DynamicDataSource extends AbstractRoutingDataSource {
    public DynamicDataSource(DataSource defaultTarget, Map
targetDataSources) {
        super.setDefaultTargetDataSource(defaultTarget);
        super.setTargetDataSources(targetDataSources);
        super.afterPropertiesSet();
    }
    @Override
    protected Object determineCurrentLookupKey() {
        return DynamicDataSourceContextHolder.getDataSourceType();
    }
}

8. DynamicDataSourceContextHolder

public class DynamicDataSourceContextHolder {
    private static final Logger log = LoggerFactory.getLogger(DynamicDataSourceContextHolder.class);
    private static final ThreadLocal
CONTEXT_HOLDER = new ThreadLocal<>();
    public static void setDataSourceType(String ds) {
        log.info("Switching to {} datasource", ds);
        CONTEXT_HOLDER.set(ds);
    }
    public static String getDataSourceType() {
        return CONTEXT_HOLDER.get() == null ? DataSourceType.MASTER.name() : CONTEXT_HOLDER.get();
    }
    public static void clearDataSourceType() {
        CONTEXT_HOLDER.remove();
    }
}

9. DataSourceAspect for AOP interception

@Aspect
@Order(1)
@Component
public class DataSourceAspect {
    @Pointcut("@annotation(com.LYJ.study.DynamicDataSource.annocation.DataSource) || @within(com.LYJ.study.DynamicDataSource.annocation.DataSource)")
    public void dsPointCut() {}
    @Around("dsPointCut()")
    public Object around(ProceedingJoinPoint joinPoint) throws Throwable {
        DataSource ds = getDataSource(joinPoint);
        if (ds != null) {
            DynamicDataSourceContextHolder.setDataSourceType(ds.value().name());
        }
        try {
            return joinPoint.proceed();
        } finally {
            DynamicDataSourceContextHolder.clearDataSourceType();
        }
    }
    private DataSource getDataSource(ProceedingJoinPoint point) {
        MethodSignature sig = (MethodSignature) point.getSignature();
        DataSource ds = AnnotationUtils.findAnnotation(sig.getMethod(), DataSource.class);
        if (ds != null) return ds;
        return AnnotationUtils.findAnnotation(sig.getDeclaringType(), DataSource.class);
    }
}

10. Usage in service layer

@Service
@RequiredArgsConstructor
@DataSource(DataSourceType.MASTER)
public class UserServiceImpl extends ServiceImpl
implements UserService {
    private final UserMapper userMapper;
    @Override
    @DataSource(DataSourceType.MASTER)
    public List
queryAll() {
        return userMapper.selectList(null);
    }
}

The same annotation can be placed on mapper classes or individual methods to control which datasource is used.

Adding multiple slaves – extend DataSourceType with SLAVE2 , SLAVE3 , etc., add corresponding sections in application-druid.yml , and register beans in DruidConfig using the same setDataSource helper.

Switching to Oracle – add the Oracle driver dependency and a new datasource configuration:

<dependency>
  <groupId>com.oracle</groupId>
  <artifactId>ojdbc6</artifactId>
  <version>11.2.0.3</version>
</dependency>

slave3:
  enabled: true
  url: jdbc:oracle:thin:@127.0.0.1:1521:oracle
  username: root
  password: password

Remember that SQL dialect differences may cause startup errors when mixing MySQL and Oracle.