Spring HTTP Streaming (也就是对 SSE(Server-Sent Events) 得支持

「爱情、让人受尽委屈。」 2022-01-20 00:43 138阅读 0赞

参考: [https://www.jianshu.com/p/ecc6f5168aef][https_www.jianshu.com_p_ecc6f5168aef]

参考: [https://www.logicbig.com/tutorials/spring-framework/spring-web-mvc/http\_streaming.html][https_www.logicbig.com_tutorials_spring-framework_spring-web-mvc_http_streaming.html]

## 简介: ##

控制器可以使用`DeferredResult`或`Callable`对象来异步地计算其返回值，这可以用于实现一些有用的技术，比如 [long polling][]技术，让服务器可以尽可能快地向客户端推送事件。

如果你想在一个HTTP响应中同时推送多个事件，怎么办？这样的技术已经存在，与"Long Polling"相关，叫"HTTP Streaming"。

Spring MVC支持这项技术，你可以通过让方法返回一个`ResponseBodyEmitter`类型对象来实现，该对象可被用于发送多个对象。通常我们所使用的`@ResponseBody`只能返回一个对象，它是通过`HttpMessageConverter`写到响应体中的。

## 说明 ##

通常会被人们称为 **SSE(Server-Sent Events)**,这个技术也是为了解决 后端给前端持续推送得问题,不过 WebSocket 是双通道得(可以后台发前台,也可以前台发后台), 但是 SSE 是单通道得(连接建立后,就只能后台发给前台)

实体:

1. ResponseBodyEmitter

2. SseEmitter

例子:

@RequestMapping("/test")
        public ResponseBodyEmitter test() {
            ResponseBodyEmitter emitter = new ResponseBodyEmitter();
            ExecutorService service = Executors.newSingleThreadExecutor();
    //        ExecutorService service = Executors.newFixedThreadPool(10);
            service.execute(()-> {
                for (int i = 0; i < 100 ; i++) {
                    try {
                        emitter.send(i+"-", MediaType.ALL);
                        Thread.sleep(100L);
                    } catch (IOException | InterruptedException e) {
                        e.printStackTrace();
                        emitter.completeWithError(e);
                        return;
                    }
                }
                emitter.complete();
            });
    
            return emitter;
        }

2. SseEmitter 和对应的HTML5 中的**Server-Sent Events** 特性,互相对应,sse 通道才能实现 服务器端主动向客户端推送数据

参考: [https://www.cnblogs.com/elonlee/p/3914704.html][https_www.cnblogs.com_elonlee_p_3914704.html]

例子:

package com.kzcm.controller;
    
    import com.kzcm.entity.User;
    import org.springframework.stereotype.Controller;
    import org.springframework.ui.ModelMap;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RequestMapping;
    
    @Controller
    @RequestMapping("index")
    public class IndexController {
    
    
        @RequestMapping("testSse")
        public String testSse(ModelMap model) {
            String path = "index/ssedemo";
    //        model.addAttribute("name", name);
            return path;
        }
    }

跳转到 index/ssedemo 目录下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t6Y21pbmc_size_16_color_FFFFFF_t_70][]

如图示,EventSource 访问路径为 /async/testSse

package com.kzcm.controller;
    
    import org.springframework.http.MediaType;
    import org.springframework.stereotype.Controller;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.ResponseBody;
    import org.springframework.web.context.request.async.DeferredResult;
    import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter;
    import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
    
    import java.io.IOException;
    import java.time.LocalTime;
    import java.util.concurrent.Callable;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    
    @Controller
    @RequestMapping("/async")
    public class AsyncController {
    
    
        
    
        @RequestMapping("/testSse")
        public SseEmitter testSse() {
            SseEmitter emitter = new SseEmitter();
            ExecutorService service = Executors.newSingleThreadExecutor();
    //        ExecutorService service = Executors.newFixedThreadPool(10);
            service.execute(()-> {
                for (int i = 0; i < 5 ; i++) {
                    try {
                        emitter.send(LocalTime.now().toString(), MediaType.ALL);
                        Thread.sleep(10);
                    } catch (IOException | InterruptedException e) {
                        e.printStackTrace();
                        emitter.completeWithError(e);
                        return;
                    }
                }
                emitter.complete();
            });
    
            return emitter;
        }
    }

最后结果就像轮询一样,隔一段时间访问一下 event 接口,

[https_www.jianshu.com_p_ecc6f5168aef]: https://www.jianshu.com/p/ecc6f5168aef
[https_www.logicbig.com_tutorials_spring-framework_spring-web-mvc_http_streaming.html]: https://www.logicbig.com/tutorials/spring-framework/spring-web-mvc/http_streaming.html
[long polling]: https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/
[https_www.cnblogs.com_elonlee_p_3914704.html]: https://www.cnblogs.com/elonlee/p/3914704.html
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t6Y21pbmc_size_16_color_FFFFFF_t_70]: /images/20220120/fb7329abd61844b19ccfe161b54e2a6f.png