spring boot 写一个count
时间: 2023-10-30 09:11:15 浏览: 35
假设你要统计一个列表中的元素数量,可以在Spring Boot中使用以下代码:
```java
import java.util.List;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class CountController {
private List<String> myList = List.of("apple", "banana", "orange");
@GetMapping("/count")
public int count() {
return myList.size();
}
}
```
在上面的代码中,我们首先定义了一个字符串列表`myList`,然后在`count()`方法中返回该列表的大小即可。
当你访问`/count`接口时,将会返回列表中元素的数量。
相关问题
spring boot实现count groupby
在Spring Boot中,可以使用Spring Data JPA和JPQL(Java Persistence Query Language)来实现count group by操作。
首先,需要定义一个Entity类,包含要分组和计数的属性。假设我们有一个实体类叫做Order,包含属性id、customerId和price:
```java
@Entity
@Table(name = "orders")
public class Order {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column(name = "customer_id")
private Long customerId;
private Double price;
// getters and setters
}
```
然后,使用Spring Data JPA定义一个Repository接口,继承JpaRepository,并声明一个查询方法,使用JPQL实现count group by操作:
```java
@Repository
public interface OrderRepository extends JpaRepository<Order, Long> {
@Query("SELECT o.customerId, COUNT(o.id) FROM Order o GROUP BY o.customerId")
List<Object[]> countGroupByCustomerId();
}
```
在这个方法中,我们使用JPQL查询语句"SELECT o.customerId, COUNT(o.id) FROM Order o GROUP BY o.customerId"来实现count group by操作。这个查询语句会返回一个Object数组的列表,每个数组包含两个元素,第一个元素是customerId,第二个元素是该customerId下的订单数量。
最后,可以在Service层调用这个方法并处理返回的结果:
```java
@Service
public class OrderService {
@Autowired
private OrderRepository orderRepository;
public Map<Long, Long> countGroupByCustomerId() {
List<Object[]> list = orderRepository.countGroupByCustomerId();
Map<Long, Long> map = new HashMap<>();
for (Object[] obj : list) {
Long customerId = (Long) obj[0];
Long count = (Long) obj[1];
map.put(customerId, count);
}
return map;
}
}
```
在Service层中,我们调用OrderRepository的countGroupByCustomerId方法获取结果列表,然后将结果转换为一个Map<Long, Long>,其中key是customerId,value是该customerId下的订单数量。
这样,就实现了在Spring Boot中使用Spring Data JPA和JPQL实现count group by操作。
写一个spring boot的rabitmq限流demo
以下是一个使用Spring Boot和RabbitMQ的限流示例:
1. 首先,需要在pom.xml文件中添加以下依赖项:
```
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-amqp</artifactId>
</dependency>
```
2. 创建一个RabbitMQ配置类,用于定义连接工厂和RabbitMQ模板:
```
@Configuration
public class RabbitMQConfig {
@Value("${spring.rabbitmq.host}")
private String host;
@Value("${spring.rabbitmq.port}")
private int port;
@Value("${spring.rabbitmq.username}")
private String username;
@Value("${spring.rabbitmq.password}")
private String password;
@Value("${spring.rabbitmq.virtual-host}")
private String virtualHost;
@Bean
public ConnectionFactory connectionFactory() {
CachingConnectionFactory connectionFactory = new CachingConnectionFactory();
connectionFactory.setHost(host);
connectionFactory.setPort(port);
connectionFactory.setUsername(username);
connectionFactory.setPassword(password);
connectionFactory.setVirtualHost(virtualHost);
return connectionFactory;
}
@Bean
public RabbitTemplate rabbitTemplate(ConnectionFactory connectionFactory) {
RabbitTemplate rabbitTemplate = new RabbitTemplate(connectionFactory);
return rabbitTemplate;
}
}
```
3. 创建一个消息接收者,并使用@RabbitListener注释指定要监听的队列:
```
@Component
public class MessageReceiver {
@RabbitListener(queues = "test.queue")
public void receiveMessage(String message) {
System.out.println("Received message: " + message);
}
}
```
4. 创建一个消息发布者,并使用RabbitTemplate发送消息:
```
@Component
public class MessageSender {
@Autowired
private RabbitTemplate rabbitTemplate;
public void sendMessage(String message) {
rabbitTemplate.convertAndSend("test.exchange", "test.routingkey", message);
}
}
```
5. 创建一个限流过滤器,用于限制消息的发送速率:
```
@Component
public class RateLimitFilter implements Filter {
private static final int MAX_MESSAGES_PER_SECOND = 10;
private static final int INTERVAL = 1000;
private final AtomicLong lastTime = new AtomicLong(System.currentTimeMillis());
private final AtomicInteger count = new AtomicInteger(0);
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
long currentTime = System.currentTimeMillis();
long timeSinceLast = currentTime - lastTime.get();
if (timeSinceLast > INTERVAL) {
int currentCount = count.getAndSet(0);
lastTime.set(currentTime);
return true;
}
else {
int currentCount = count.incrementAndGet();
if (currentCount > MAX_MESSAGES_PER_SECOND) {
response.setStatus(HttpStatus.TOO_MANY_REQUESTS.value());
return false;
}
else {
return true;
}
}
}
}
```
6. 将限流过滤器添加到应用程序中:
```
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Autowired
private RateLimitFilter rateLimitFilter;
@Override
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(rateLimitFilter);
}
}
```
7. 在应用程序中使用消息发布者发送消息:
```
@Autowired
private MessageSender messageSender;
@GetMapping("/send")
public void sendMessage() {
messageSender.sendMessage("Hello, RabbitMQ!");
}
```
现在,应用程序将限制每秒发送的消息数量,以避免过度负载RabbitMQ服务器。
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