# http-long-polling

**Repository Path**: ssp198/http-long-polling

## Basic Information

- **Project Name**: http-long-polling
- **Description**: 基于HTTP的长轮询实现
- **Primary Language**: Java
- **License**: Not specified
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 3
- **Forks**: 0
- **Created**: 2022-08-09
- **Last Updated**: 2024-02-01

## Categories & Tags

**Categories**: Uncategorized

**Tags**: SpringBoot, Java, Http, long-polling

## README

### A demo of http-long-polling ——基于HTTP的长轮询

#### 长轮询简介

* *HTTP的通信基本特点：半双工通信、“一请求一响应”。*
* *简单轮询：简单轮询方式是由客户端控制轮询调用间隔。在使用HTTP协议且有一定的实时性要求的业务场景下，一般会通过减小资源调用方对接口的轮询时间间隔来确保“一定的”实时性，但过小的轮询间隔会增大客户端服务端双方CPU和网络等宽等资源消耗，为了权衡资源消耗和实时性，轮询间隔一般不会太短，大多是秒级，从而避免不了有恒定的延时。*
* *长轮询：长轮询机制可缓解简单轮询带有的资源浪费问题，又可以保证更高的实时性。实现思路就是交由服务端控制http-rpc轮询周期来保证实时性，客户端调大``socket-timeout``读超时时长，服务端在超时前返回响应，这个超时时长可以设置的长一点，如30秒。*

#### 长轮询特点
~~~
基于HTTP协议的长轮询在实时性上比上websocket稍有不足，比下HTTP简单轮询有余。
~~~

#### 实现：基于AsyncContext——servlet异步请求

* servlet对异步请求做出了规范
```java
package javax.servlet;

public interface ServletRequest {

    public AsyncContext startAsync();
    
    public AsyncContext startAsync(ServletRequest servletRequest, ServletResponse servletResponse);

}
```
* 直接在SpringMVC中使用
```java
    HttpServletRequest request; 
    HttpServletResponse response;

    AsyncContext asyncContext = request.startAsync((request, response); //开启异步请求上下文
```
#### 快速使用
*详见`io.longpolling.test.controller.ResourceController`用例*

* 对客户端请求开启异步任务.
 ```java
public class ResourceController {

    private static final String RESOURCE_SAY_HELLO = "SayHello";

    @GetMapping("/waitSayHello")
    public void waitSayHello(HttpServletRequest request, HttpServletResponse response, String who, Integer timeout) {
    
        //请求基础信息
        RequestParam curRequestParam = RequestParam.newInstance(RESOURCE_SAY_HELLO, who);
        //响应超时设置及返回结果
        TimeoutResult timeoutResult = TimeoutResult.newInstance(timeout, "Resp Timeout, but hello world for you!", HttpStatus.REQUEST_TIMEOUT);
        
        //开启异步响应
        AsyncResponseTask.buildAndStart(request, response, curRequestParam, timeoutResult);
    }
}
```

* 处理异步请求-响应任务
```java
public class ResourceController {

    private static final String RESOURCE_SAY_HELLO = "SayHello";

    @PutMapping("/sayHello")
    public String sayHelloForWe() {
        final Collection<AsyncResponseTask> sayHelloRespTaskWaitList = AsyncResponseTask.getAsyncTask(RESOURCE_SAY_HELLO);
        final int total = sayHelloRespTaskWaitList.size();

        sayHelloRespTaskWaitList.forEach(asyncResp -> {
            RequestParam requestParam = asyncResp.getRequestParam();
            String who = (String) requestParam.getParam();

            String result = String.format("Hello %s!", who);
            asyncResp.returnJsonResult(result, HttpStatus.OK); //完成异步响应

            log.debug("--->say hello for {} success!", who);
        });

        return String.format("say hello for %s people!", total);
    }
}
```