springboot 長輪詢實(shí)現(xiàn)

@SpringBootApplication@EnableAsyncpublic class DemoApplication { public static void main(String[] args) { SpringApplication.run(DemoApplication.class, args); }}

@RequestMapping('/async')@RestControllerpublic class AsyncRequestDemo { @Autowired private AsyncRequestService asyncRequestService; @GetMapping('/value') public String getValue() { String msg = null; Future<String> result = null; try{ result = asyncRequestService.getValue(); msg = result.get(10, TimeUnit.SECONDS); }catch (Exception e){ e.printStackTrace(); }finally { if (result != null){ result.cancel(true); } } return msg; } @PostMapping('/value') public void postValue(String msg) { asyncRequestService.postValue(msg); }}

@Servicepublic class AsyncRequestService { private String msg = null; @Async public Future<String> getValue() throws InterruptedException { while (true){ synchronized (this){ if (msg != null){ String resultMsg = msg; msg = null; return new AsyncResult(resultMsg); } } Thread.sleep(100); } } public synchronized void postValue(String msg) { this.msg = msg; }}備注

@EnableAsync 開啟異步

@Sync 標(biāo)記異步方法

Future 用于接收異步返回值

result.get(10, TimeUnit.SECONDS); 阻塞，超時(shí)獲取結(jié)果

Future.cancel() 中斷線程

補(bǔ)充：通過spring提供的DeferredResult實(shí)現(xiàn)長輪詢服務(wù)端推送消息

DeferredResult字面意思就是推遲結(jié)果，是在servlet3.0以后引入了異步請求之后，spring封裝了一下提供了相應(yīng)的支持，也是一個(gè)很老的特性了。DeferredResult可以允許容器線程快速釋放以便可以接受更多的請求提升吞吐量，讓真正的業(yè)務(wù)邏輯在其他的工作線程中去完成。

最近再看apollo配置中心的實(shí)現(xiàn)原理，apollo的發(fā)布配置推送變更消息就是用DeferredResult實(shí)現(xiàn)的，apollo客戶端會像服務(wù)端發(fā)送長輪訓(xùn)http請求，超時(shí)時(shí)間60秒，當(dāng)超時(shí)后返回客戶端一個(gè)304 httpstatus,表明配置沒有變更，客戶端繼續(xù)這個(gè)步驟重復(fù)發(fā)起請求，當(dāng)有發(fā)布配置的時(shí)候，服務(wù)端會調(diào)用DeferredResult.setResult返回200狀態(tài)碼，然后輪訓(xùn)請求會立即返回（不會超時(shí)），客戶端收到響應(yīng)結(jié)果后，會發(fā)起請求獲取變更后的配置信息。

springboot啟動類：

@SpringBootApplicationpublic class DemoApplication implements WebMvcConfigurer { public static void main(String[] args) { SpringApplication.run(DemoApplication.class, args); } @Bean public ThreadPoolTaskExecutor mvcTaskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(10); executor.setQueueCapacity(100); executor.setMaxPoolSize(25); return executor; } //配置異步支持，設(shè)置了一個(gè)用來異步執(zhí)行業(yè)務(wù)邏輯的工作線程池，設(shè)置了默認(rèn)的超時(shí)時(shí)間是60秒 @Override public void configureAsyncSupport(AsyncSupportConfigurer configurer) { configurer.setTaskExecutor(mvcTaskExecutor()); configurer.setDefaultTimeout(60000L); }}

import com.google.common.collect.HashMultimap;import com.google.common.collect.Multimap;import com.google.common.collect.Multimaps;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.web.bind.annotation.PathVariable;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RequestMethod;import org.springframework.web.bind.annotation.RestController;import org.springframework.web.context.request.async.DeferredResult; import java.util.Collection; @RestControllerpublic class ApolloController { private final Logger logger = LoggerFactory.getLogger(this.getClass()); //guava中的Multimap，多值map,對map的增強(qiáng)，一個(gè)key可以保持多個(gè)value private Multimap<String, DeferredResult<String>> watchRequests = Multimaps.synchronizedSetMultimap(HashMultimap.create()); //模擬長輪詢 @RequestMapping(value = '/watch/{namespace}', method = RequestMethod.GET, produces = 'text/html') public DeferredResult<String> watch(@PathVariable('namespace') String namespace) { logger.info('Request received'); DeferredResult<String> deferredResult = new DeferredResult<>(); //當(dāng)deferredResult完成時(shí)（不論是超時(shí)還是異常還是正常完成），移除watchRequests中相應(yīng)的watch key deferredResult.onCompletion(new Runnable() { @Override public void run() { System.out.println('remove key:' + namespace); watchRequests.remove(namespace, deferredResult); } }); watchRequests.put(namespace, deferredResult); logger.info('Servlet thread released'); return deferredResult; } //模擬發(fā)布namespace配置 @RequestMapping(value = '/publish/{namespace}', method = RequestMethod.GET, produces = 'text/html') public Object publishConfig(@PathVariable('namespace') String namespace) { if (watchRequests.containsKey(namespace)) { Collection<DeferredResult<String>> deferredResults = watchRequests.get(namespace); Long time = System.currentTimeMillis(); //通知所有watch這個(gè)namespace變更的長輪訓(xùn)配置變更結(jié)果 for (DeferredResult<String> deferredResult : deferredResults) { deferredResult.setResult(namespace + ' changed:' + time); } } return 'success'; }}

當(dāng)請求超時(shí)的時(shí)候會產(chǎn)生AsyncRequestTimeoutException,我們定義一個(gè)全局異常捕獲類:

import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.http.HttpStatus;import org.springframework.web.bind.annotation.ControllerAdvice;import org.springframework.web.bind.annotation.ExceptionHandler;import org.springframework.web.bind.annotation.ResponseBody;import org.springframework.web.bind.annotation.ResponseStatus;import org.springframework.web.context.request.async.AsyncRequestTimeoutException; import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse; @ControllerAdviceclass GlobalControllerExceptionHandler { protected static final Logger logger = LoggerFactory.getLogger(GlobalControllerExceptionHandler.class); @ResponseStatus(HttpStatus.NOT_MODIFIED)//返回304狀態(tài)碼 @ResponseBody @ExceptionHandler(AsyncRequestTimeoutException.class) //捕獲特定異常 public void handleAsyncRequestTimeoutException(AsyncRequestTimeoutException e, HttpServletRequest request) { System.out.println('handleAsyncRequestTimeoutException'); }}

然后我們通過postman工具發(fā)送請求http://localhost:8080/watch/mynamespace，請求會掛起，60秒后，DeferredResult超時(shí)，客戶端正常收到了304狀態(tài)碼，表明在這個(gè)期間配置沒有變更過。

然后我們在模擬配置變更的情況，再次發(fā)起請求http://localhost:8080/watch/mynamespace，等待個(gè)10秒鐘（不要超過60秒），然后調(diào)用http://localhost:8080/publish/mynamespace,發(fā)布配置變更。這時(shí)postman會立刻收到response響應(yīng)結(jié)果:

mynamespace changed:1538880050147

表明在輪訓(xùn)期間有配置變更過。

這里我們用了一個(gè)MultiMap來存放所有輪訓(xùn)的請求，Key對應(yīng)的是namespace,value對應(yīng)的是所有watch這個(gè)namespace變更的異步請求DeferredResult，需要注意的是：在DeferredResult完成的時(shí)候記得移除MultiMap中相應(yīng)的key,避免內(nèi)存溢出請求。

采用這種長輪詢的好處是，相比一直循環(huán)請求服務(wù)器，實(shí)例一多的話會對服務(wù)器產(chǎn)生很大的壓力，http長輪詢的方式會在服務(wù)器變更的時(shí)候主動推送給客戶端，其他時(shí)間客戶端是掛起請求的，這樣同時(shí)滿足了性能和實(shí)時(shí)性。

以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持好吧啦網(wǎng)。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教。