可以在初始化redisTemplate时指定，如

    @Bean("redisTemplateDB1")
    public RedisTemplate<String, String> redisTemplateDB1(@Value("${spring.redis.redis-b.host}") String host, @Value("${spring.redis.redis-b.port}") int port
            , @Value("${spring.redis.redis-b.database}") int db, @Value("${spring.redis.redis-b.password}") String password) {

        RedisStandaloneConfiguration redisConfig = new RedisStandaloneConfiguration();
        redisConfig.setHostName(host);
        redisConfig.setPassword(password);
        redisConfig.setPort(port);
        //指定database
        redisConfig.setDatabase(db);
        GenericObjectPoolConfig config=new GenericObjectPoolConfig();
        LettuceClientConfiguration clientConfiguration = LettucePoolingClientConfiguration.builder()
                .poolConfig(config).commandTimeout(Duration.ofMillis(config.getMaxWaitMillis())).build();
        LettuceConnectionFactory lettuceConnectionFactory=new LettuceConnectionFactory(redisConfig, clientConfiguration);
        
        RedisTemplate<String, String> template = new RedisTemplate<>();
        // 配置连接工厂
        template.setConnectionFactory(lettuceConnectionFactory);
        redisConfig tetemplate;
        }

也可以在使用前指定，像这样

            LettuceConnectionFactory jedisConnectionFactory = (LettuceConnectionFactory) redisTemplate
                    .getConnectionFactory();
            //指定db
            jedisConnectionFactory.setDatabase(1);
            redisTemplate.setConnectionFactory(jedisConnectionFactory);