Dynamic Data Source Switching in Spring Boot Using ThreadLocal & AbstractRoutingDataSource

1 Introduction

When a business requirement needs to read from different databases and write to the current one, data source switching is required. The author first tried the dynamic-datasource-spring-boot-starter but it failed, so a custom solution using ThreadLocal and AbstractRoutingDataSource was implemented.

2 Code Implementation

2.1 Implement ThreadLocal

DataSourceContextHolder

provides get, set and remove methods to store the current thread's data source name in a ThreadLocal variable.

/**
 * @author: jiangjs
 * @description:
 * @date: 2023/7/27 11:21
 */
public class DataSourceContextHolder {
    private static final ThreadLocal<String> DATASOURCE_HOLDER = new ThreadLocal<>();

    /**
     * Set data source name
     * @param dataSourceName data source name
     */
    public static void setDataSource(String dataSourceName){
        DATASOURCE_HOLDER.set(dataSourceName);
    }

    /**
     * Get current thread's data source name
     * @return data source name
     */
    public static String getDataSource(){
        return DATASOURCE_HOLDER.get();
    }

    /**
     * Remove current data source
     */
    public static void removeDataSource(){
        DATASOURCE_HOLDER.remove();
    }
}

2.2 Implement AbstractRoutingDataSource

DynamicDataSource

extends AbstractRoutingDataSource and overrides determineCurrentLookupKey to return the data source name from DataSourceContextHolder.

/**
 * @author: jiangjs
 * @description: Implement dynamic data source, route to different data sources via AbstractRoutingDataSource
 * @date: 2023/7/27 11:18
 */
public class DynamicDataSource extends AbstractRoutingDataSource {

    public DynamicDataSource(DataSource defaultDataSource, Map<Object, Object> targetDataSources){
        super.setDefaultTargetDataSource(defaultDataSource);
        super.setTargetDataSources(targetDataSources);
    }

    @Override
    protected Object determineCurrentLookupKey() {
        return DataSourceContextHolder.getDataSource();
    }
}

2.3 Configure Databases

The application.yml defines master and slave data sources using Druid.

# Set data sources
spring:
  datasource:
    type: com.alibaba.druid.pool.DruidDataSource
    druid:
      master:
        url: jdbc:mysql://xxxxxx:3306/test1?characterEncoding=utf-8&allowMultiQueries=true&zeroDateTimeBehavior=convertToNull&useSSL=false
        username: root
        password: 123456
        driver-class-name: com.mysql.cj.jdbc.Driver
      slave:
        url: jdbc:mysql://xxxxx:3306/test2?characterEncoding=utf-8&allowMultiQueries=true&zeroDateTimeBehavior=convertToNull&useSSL=false
        username: root
        password: 123456
        driver-class-name: com.mysql.cj.jdbc.Driver
      initial-size: 15
      min-idle: 15
      max-active: 200
      max-wait: 60000
      time-between-eviction-runs-millis: 60000
      min-evictable-idle-time-millis: 300000
      validation-query: ""
      test-while-idle: true
      test-on-borrow: false
      test-on-return: false
      pool-prepared-statements: false
      connection-properties: false

2.4 Test

SQL scripts create a test_user table with a single column user_name in both master and slave databases, then insert distinct rows.

create table test_user(
  user_name varchar(255) not null comment '用户名'
);

insert into test_user (user_name) value ('master');

insert into test_user (user_name) value ('slave');

A controller method sets the data source, queries the table, and removes the data source.

@GetMapping("/getData.do/{datasourceName}")
public String getMasterData(@PathVariable("datasourceName") String datasourceName){
    DataSourceContextHolder.setDataSource(datasourceName);
    TestUser testUser = testUserMapper.selectOne(null);
    DataSourceContextHolder.removeDataSource();
    return testUser.getUserName();
}

Results show different values are returned when master or slave is passed.

2.5 Optimization and Extension

2.5.1 Annotation‑Based Data Source Switching

Define a @DS annotation and an AOP aspect that sets the data source before method execution and clears it afterwards.

/**
 * @author: jiangjs
 * @description:
 * @date: 2023/7/27 14:39
 */
@Target({ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface DS {
    String value() default "master";
}

@Aspect
@Component
@Slf4j
public class DSAspect {

    @Pointcut("@annotation(com.jiashn.dynamic_datasource.dynamic.aop.DS)")
    public void dynamicDataSource(){}

    @Around("dynamicDataSource()")
    public Object datasourceAround(ProceedingJoinPoint point) throws Throwable {
        MethodSignature signature = (MethodSignature)point.getSignature();
        Method method = signature.getMethod();
        DS ds = method.getAnnotation(DS.class);
        if (Objects.nonNull(ds)){
            DataSourceContextHolder.setDataSource(ds.value());
        }
        try {
            return point.proceed();
        } finally {
            DataSourceContextHolder.removeDataSource();
        }
    }
}

Controller examples:

@GetMapping("/getMasterData.do")
public String getMasterData(){
    TestUser testUser = testUserMapper.selectOne(null);
    return testUser.getUserName();
}

@GetMapping("/getSlaveData.do")
@DS("slave")
public String getSlaveData(){
    TestUser testUser = testUserMapper.selectOne(null);
    return testUser.getUserName();
}

2.5.2 Dynamic Data Source Addition

DataSourceEntity

holds connection parameters and a key used in the dynamic map.

/**
 * @author: jiangjs
 * @description: Data source entity
 * @date: 2023/7/27 15:55
 */
@Data
@Accessors(chain = true)
public class DataSourceEntity {
    private String url;
    private String userName;
    private String passWord;
    private String driverClassName;
    private String key;
}

The extended DynamicDataSource maintains a targetDataSourceMap and provides createDataSource to validate, instantiate, and add new Druid data sources at runtime.

@Slf4j
public class DynamicDataSource extends AbstractRoutingDataSource {

    private final Map<Object,Object> targetDataSourceMap;

    public DynamicDataSource(DataSource defaultDataSource, Map<Object, Object> targetDataSources){
        super.setDefaultTargetDataSource(defaultDataSource);
        super.setTargetDataSources(targetDataSources);
        this.targetDataSourceMap = targetDataSources;
    }

    @Override
    protected Object determineCurrentLookupKey() {
        return DataSourceContextHolder.getDataSource();
    }

    /**
     * Add data sources dynamically
     */
    public void createDataSource(List<DataSourceEntity> dataSources){
        try {
            if (CollectionUtils.isNotEmpty(dataSources)){
                for (DataSourceEntity ds : dataSources) {
                    Class.forName(ds.getDriverClassName());
                    DriverManager.getConnection(ds.getUrl(), ds.getUserName(), ds.getPassWord());
                    DruidDataSource dataSource = new DruidDataSource();
                    BeanUtils.copyProperties(ds, dataSource);
                    dataSource.setTestOnBorrow(true);
                    dataSource.setTestWhileIdle(true);
                    dataSource.setValidationQuery("select 1 ");
                    dataSource.init();
                    this.targetDataSourceMap.put(ds.getKey(), dataSource);
                }
                super.setTargetDataSources(this.targetDataSourceMap);
                super.afterPropertiesSet();
                return Boolean.TRUE;
            }
        } catch (ClassNotFoundException | SQLException e) {
            log.error("---程序报错---:{}", e.getMessage());
        }
        return Boolean.FALSE;
    }

    public boolean existsDataSource(String key){
        return Objects.nonNull(this.targetDataSourceMap.get(key));
    }
}

A CommandLineRunner loads rows from a test_db_info table, converts them to DataSourceEntity, and calls dynamicDataSource.createDataSource at application startup.

@Component
public class LoadDataSourceRunner implements CommandLineRunner {
    @Resource
    private DynamicDataSource dynamicDataSource;
    @Resource
    private TestDbInfoMapper testDbInfoMapper;

    @Override
    public void run(String... args) throws Exception {
        List<TestDbInfo> testDbInfos = testDbInfoMapper.selectList(null);
        if (CollectionUtils.isNotEmpty(testDbInfos)) {
            List<DataSourceEntity> ds = new ArrayList<>();
            for (TestDbInfo testDbInfo : testDbInfos) {
                DataSourceEntity sourceEntity = new DataSourceEntity();
                BeanUtils.copyProperties(testDbInfo, sourceEntity);
                sourceEntity.setKey(testDbInfo.getName());
                ds.add(sourceEntity);
            }
            dynamicDataSource.createDataSource(ds);
        }
    }
}

After the application starts, the newly added data sources are available for queries just like the static ones.

Note: Exclude DataSourceAutoConfiguration when starting the Spring Boot application to avoid circular dependency errors. <code>@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)</code>

Overall, the article demonstrates a complete workflow for custom dynamic data source routing, annotation‑driven switching, and runtime addition of new data sources in a Spring Boot environment.