springboot提供了两种异步处理方案，一是传统同步web处理加异步service层，二是异步web处理框架webFlux。

一、传统同步web处理加异步service层

使用流程：在springboot application启动类上添加@EnableAsync注解。在service层类或方法上添加@Async注解，同时在异步方法所在的service层类上添加@Component或@service 等注解，该方法必须返回AsyncResult或CompletableFuture、ListenableFuture、Future之一，然后controller层通过@Autowired自动注入service类并使用异步类。

controller和service层代码如下：

@RestController
public class UserCtroller {@AutowiredUserService userService;@RequestMapping( "/n/{name}")@ResponseBodypublic User getUserByName(@PathVariable String name){AsyncResult> user = userService.getUserByName( name );try{return user.get().isEmpty() ? null: user.get().get() ;} catch (ExecutionException e) {return null;}}@RequestMapping( "/l")@ResponseBodypublic List getAll(){return userService.getAll();}}

@Service
public class UserServiceImpl implements UserService {List users = Arrays.asList(new User( "a", 20 , "aaaa"),new User( "b", 20 , "aaaa"),new User( "c", 20 , "aaaa"));@Override@Asyncpublic AsyncResult> getUserByName(String name){Optional user = users.stream().filter( (u) ->  Objects.equals( u.getName(), name ) ).findFirst();return new AsyncResult<>( user );}@Override@Asyncpublic List getAll(){return  users;}
}

在service的 getUserByName方法内置断点，执行监控调用堆栈，结果如下。可以看出spring采用AOP的方式将@Async的注解的方法封装为异步执行。

getUserByName:26, UserServiceImpl (com.alun.serviceasync)
invoke0:-1, NativeMethodAccessorImpl (jdk.internal.reflect)
invoke:77, NativeMethodAccessorImpl (jdk.internal.reflect)
invoke:43, DelegatingMethodAccessorImpl (jdk.internal.reflect)
invoke:568, Method (java.lang.reflect)
invokeJoinpointUsingReflection:344, AopUtils (org.springframework.aop.support)
invokeJoinpoint:198, ReflectiveMethodInvocation (org.springframework.aop.framework)
proceed:163, ReflectiveMethodInvocation (org.springframework.aop.framework)
lambda$invoke$0:115, AsyncExecutionInterceptor (org.springframework.aop.interceptor)
call:-1, AsyncExecutionInterceptor$$Lambda$778/0x0000000800fb3710 (org.springframework.aop.interceptor)
run$$$capture:264, FutureTask (java.util.concurrent)
run:-1, FutureTask (java.util.concurrent)- 异步堆栈跟踪
:132, FutureTask (java.util.concurrent)
:44, ListenableFutureTask (org.springframework.util.concurrent)
submitListenable:412, ThreadPoolTaskExecutor (org.springframework.scheduling.concurrent)
doSubmit:286, AsyncExecutionAspectSupport (org.springframework.aop.interceptor)
invoke:129, AsyncExecutionInterceptor (org.springframework.aop.interceptor)
proceed:186, ReflectiveMethodInvocation (org.springframework.aop.framework)
invoke:215, JdkDynamicAopProxy (org.springframework.aop.framework)
getUserByName:-1, $Proxy50 (jdk.proxy2)
getUserByName:27, UserCtroller (com.alun.serviceasync)

使用数据库事务时一定要注意：在Async方法上标注@Transactional，事务无效。 在Async方法调用的方法上标注@Transactional 有效。

Springboot中使用@Async实现异步处理及注意事项  这篇文章总结的不错，值得大家参考。

二、异步web处理框架webFlux

spring5提供了webFlux异步处理框架，提高系统吞吐量和伸缩性，以 Reactor为核心，基于Servlet3.1规范实现，即从servlet线程就以异步方式处理请求。springboot提供了TomcatReactiveWebServerFactory、NettyReactiveWebServerFactory、UndertowReactiveWebServerFactory、JettyReactiveWebServerFactory四种ReactiveWebServer实现。默认使用容器是 Netty，Netty 是高性能的 NIO 框架。

controller层的方法需要返回Mono或Flux，service代码不需要改动。如：

@RestController
public class UserCtroller {@AutowiredUserService userService;@RequestMapping( "/n/{name}")public Mono getUserByName( @PathVariable String name){return userService.getUserByName( name );}@RequestMapping( "/l")public Flux getAll(){return userService.getAll();}
}

SpringWebflux 提供两种编程模式：注解编程模型和函数式编程模型。

函数式编程模型参见Spring Webflux

两种方式的gradle文件的配置如下，如需切换server提供方式，可以如下修改：

web mvc方式配置：

    // webimplementation ('org.springframework.boot:spring-boot-starter-web'){exclude  group:'org.springframework.boot', module:'spring-boot-starter-tomcat'}implementation 'org.springframework.boot:spring-boot-starter-undertow'

webFlux方式的配置：

// webfluximplementation ('org.springframework.boot:spring-boot-starter-webflux'){exclude group:'org.springframework.boot', module:'spring-boot-starter-netty'}implementation 'org.springframework.boot:spring-boot-starter-undertow'