JetLinks 响应式编程

发表于 2023/05/25

作者 deathwhispers

12 分钟阅读

JetLinks 响应式编程

响应式

JetLinks使用Project Reactor作为响应式编程框架,从网络层(webflux,vert.x)到持久层(r2dbc,elastic)全部封装为非阻塞,响应式调用.

响应式可以理解为观察者模式,通过订阅和发布数据流中的数据对数据进行处理. Project Reactor提供了强大的API,简化多线程和异步编程开发,降低了对数据各种处理方式的复杂度,如果你已经大量使用了java8 stream api,使用reactor将很容易上手.

注意

响应式与传统编程最大的区别是:

响应式中的方法调用是在构造一个流以及处理流中数据的逻辑,当流中产生了数据(发布,订阅),才会执行构造好的逻辑.

传统编程则是直接执行逻辑获取结果.

优点

非阻塞,大大简化多线程异步编程. 集成netty等框架可实现更高的网络并发处理能力. API丰富,实现很多复杂的功能只需要几行代码,例如:

前端展示实时数据处理进度.
请求撤销,可获取到连接断开事件.
定时( interval ),延迟( delay ),超时( timout ),以及细粒度的流量控制( limitRate ).
分组( groupBy ),聚合( collect , reduce )操作等

缺点

调试不易,异常栈难跟踪,对开发人员有更高的要求.

此问题可以通过优化代码结构来解决,比如: 避免在响应式操作符中直接业务逻辑, 正确的做法是将业务逻辑抽离为独立的函数(方法),然后使用响应式来进行组合.

注意

响应式只是一个编程模型,并不能直接提高系统的并发处理能力. 通常与netty(reactor-netty)等框架配合,从上(网络)到下(持久化)全套实现非阻塞,响应式才有意义.

选择合适的操作符

系统中大量使用到了reactor,其核心类只有2个Flux(0-n个数据的流),Mono(0-1个数据的流). 摒弃传统编程的思想,熟悉Flux,Mono操作符(API),就可以很好的使用响应式编程了.

常用操作符:

map
转换流中的元素: flux.map(UserEntity::getId)
mapNotNull
转换流中的元素,并忽略null值.( reactor 3.4 提供)
flatMap
转换流中的元素为新的流: flux.flatMap(this::findById)
flatMapMany
转换Mono中的元素为Flux(1个转多个): mono.flatMapMany(this::findChildren)
concat
将多个流连接在一起组成一个流(按顺序订阅) : Flux.concat(header,body)
merge
将多个流合并在一起,同时订阅流: Flux.merge(save(info),saveDetail(detail))
zip
压缩多个流中的元素: Mono.zip(getData(id),getDetail(id),UserInfo::of)
then
上游流完成后执行其他的操作.
doOnNext
流中产生数据时执行.
doOnError
发送错误时执行.
doOnCancel
流被取消时执行.如: http未响应前,客户端断开了连接.
onErrorContinue
流发生错误时,继续处理数据而不是终止整个流.
defaultIfEmpty
当流为空时,使用默认值.
switchIfEmpty
当流为空时,切换为另外一个流.
as
将流作为参数,转为另外一个结果: flux.as(this::save)

完整文档请查看官方文档

代码格式化

使用reactor时,应该注意代码尽量以.换行并做好相应到缩进.例如:

```plain text //错误 return paramMono.map(param->param.getString(“id”)).flatMap(this::findById);

//建议 return paramMono .map(param->param.getString(“id”)) .flatMap(this::findById);

## lamdba

避免在一个lambda中编写大量的逻辑代码,推荐参考领域模型,将具体当逻辑放到对应到实体或者领域对象中.例如:

```plain text
//错误
return devicePropertyMono
        .map(prop->{
            Map<String,Object> map = new HashMap<>();
            map.put("property",prop.getProperty());
            ....
            return map;
        })
        .flatMap(this::doSomeThing)

//建议
//在DeviceProperty中编写toMap方法实现上面lambda中到逻辑.
return devicePropertyMono
        .map(DeviceProperty::toMap)
        .flatMap(this::doSomeThing)

null处理

数据流中到元素不允许为null,因此在进行数据转换到时候要注意null处理.例如:

```plain text //存在缺陷 return this.findById(id) .map(UserEntity::getDescription); //getDescription可能返回null,为null时会抛出空指针,

在reactor 3.4后可以使用以下方式来处理可能存在null的map操作

```plain text
return this.findById(id)
           .mapNotNull(UserEntity::getDescription);

非阻塞与阻塞

默认情况下,reactor的调度器由数据的生产者(Publisher)决定,在WebFlux中则是netty的工作线程, 为了防止工作线程被阻塞导致服务崩溃,在一个请求的流中,禁止执行存在阻塞(如执行JDBC)可能的操作的,如果无法避免阻塞操作,应该指定调度器如:

```plain text paramMono .publishOn(Schedulers.elastic()) //指定调度器去执行下面的操作 .map(param-> jdbcService.select(param))

## 上下文

在响应式中,大部分情况是禁止使用ThreadLocal的(可能造成内存泄漏).因此基于ThreadLocal的功能都无法使用,reactor中引入了上下文,在一个流中,可共享此上下文 ,通过上下文进行变量共享以例如:事务,权限等功能.例如:

```plain text
//从上下文中获取
@GetMapping
public Mono<UserInfo> getCurrentUser(){
    return Mono.subscriberContext()
            .map(ctx->userService.findById(ctx.getOrEmpty("userId").orElseThrow(IllegalArgumentException::new));
}

//定义过滤器设置数据到上下文中
class MyFilter implements WebFilter{
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain){
        return chain.filter(exchange)
            .subscriberContext(Context.of("userId",...))
    }
}

注意

在开发中应该将多个流组合为一个流,而不是分别处理.例如:

```plain text //错误 return flux.doOnNext(data->this.save(data).subscribe());

//正确 return flux.flatMap(this::save);

//错误,没有将流组合在一起 request.flatMap(this::save); Mono result = this.notifySaveSuccess(); return result;

//正确 return request .flatMap(this::save) .then(this.notifySaveSuccess());

## FAQ: 我写的操作看上去是正确的,但是没有执行.

有3种可能: 上游流为空,多个流未组合在一起,在不支持响应式的地方使用了响应式

上游流为空:

例:

```plain text
public Mono<Response> handleRequest(Request request){

 return this
      .findOldData(request)
      .flatMap(old -> {
            //这里为什么不执行?
            return ....
      })

}

说明

当findOldData返回的流为空时,下游的flatMap等需要操作流中元素的操作符是不会执行的. 可以通过switchIfEmpty操作符来处理空流的情况. 例如:

```plain text return this .findOldData(request) //处理没获取到数据的情况 .switchIfEmpty(Mono.error(()->new NotFoundException(“error.data_not_found”))) .flatMap(old -> { return …. })

如果flatMap和switchIfEmpty中的逻辑都没执行,那可能是下面一种情况.

多个流未组合在一起

例:

```plain text
public Result handleRequest(Request request){

     //处理请求
     service.handleRequest(request);

     return ok;

}

注意

只要方法返回值是 Mono 或者 Flux ,都不能单独行动.
只要方法中调用了任何响应式操作.那这个方法也应该是响应式.(返回Mono或者Flux)

因此正确的写法是:

```plain text public Mono handleRequest(Request request){ return service //处理请求 .handleRequest(request) //记录日志 .then(saveLog(request)) //返回结果 .thenReturn(ok); }

在不支持响应式的地方使用响应式

```plain text
public Mono<Result> handleRequest(Request request){

return service
         //处理请求
         .handleRequest(request)
         //记录日志 此为错误的用法
         .doOnNext(response-> saveLog(request,response) )
         //返回结果
         .thenReturn(ok);
}

说明

从doOnNext方法的语义以及参数Consumer可知,此方法是不支持响应式的（Consumer只有参数没有返回值）,因此不能在此方法中使用响应式操作.

正确的写法:

```plain text return service //处理请求 .handleRequest(request) //记录日志此为错误的用法 .flatMap(response-> saveLog(request,response) ) //返回结果 .thenReturn(ok);

## FAQ: 我想获取流中的元素怎么办

不要试图从流中获取数据出来,而是先思考需要对流中元素做什么, 需要对流中的数据进行操作时,都应该使用操作符来处理,根据Flux或者Mono提供的操作符API进行组合操作.比如:

传统:

```plain text
public List<Book> getAllBooks(){
    List<BookEntity> bookEntities = repository.findAll();

    List<Book> books = new ArrayList(bookEntities.size());

    for(BookEntity entity : bookEntities){
        Book book = entity.copyTo(new Book());
        books.add(book);
    }

    return books;
}

响应式:

```plain text public Flux getAllBooks(){ return repository .findAll() .map(entity-> entity.copyTo(new Book())) }

## FAQ: 我需要在非响应式方法中使用响应式怎么办

如果需要阻塞获取结果,可以使用flux.block(timeout).

如果需要异步获取结果,可以使用flux.subscribe(data->{ },error->{ })

如:

```plain text
public void handleRequest(Request request){

  //logService.saveLog(request).block()

  logService
    .saveLog(request)
    .subscribe(
        result->{
            log.debug("保存成功 {}",request)
        },
        error->{
            log.warn("保存失败 {}",request,error);
        }
    )
}

相关资料

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IoT, JetLinks

JetLinks

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