Java学习第36天 - 响应式编程、虚拟线程与现代化技术栈

一、学习目标

• 掌握 Reactor 的核心概念与常用操作符（Mono、Flux、背压、map/flatMap/zip/then 等）。
• 掌握 Spring WebFlux 的基本使用：响应式 Controller、R2DBC、WebClient。
• 理解 虚拟线程（Virtual Threads，Java 21+） 与 结构化并发（Structured Concurrency） 的基本用法。
• 理解并实践 可观测性：指标（Metrics）、链路追踪（Tracing）、结构化日志（Logging）。
• 能够在现有 Spring MVC 项目中，逐步引入 WebFlux、虚拟线程和可观测性，完成现代化改造雏形。

---

二、响应式编程基础（Reactor）

2.1 核心概念：Publisher / Subscriber / 背压

1. 响应式流标准（Reactive Streams）包含四个核心接口：

   • Publisher：数据发布者
   • Subscriber：数据订阅者
   • Subscription：订阅关系
   • Processor：既是 Publisher 又是 Subscriber

2. Reactor 中常用类型：

   • Mono<T>：0 或 1 个元素
   • Flux<T>：0 到 N 个元素

3. 基本示例：

import reactor.core.publisher.Mono;
import reactor.core.publisher.Flux;

import java.util.Optional;

public class ReactorBasics {

    // Mono：0 或 1 个元素
    public static Mono<String> monoExample() {
        return Mono.just("Hello")
                .map(String::toUpperCase)
                .doOnNext(s -> System.out.println("Mono: " + s))
                .doOnSuccess(s -> System.out.println("Mono 完成"));
    }

    // Flux：0~N 个元素
    public static Flux<Integer> fluxExample() {
        return Flux.range(1, 5)
                .map(i -> i * 2)
                .doOnNext(i -> System.out.println("Flux: " + i))
                .doOnComplete(() -> System.out.println("Flux 完成"));
    }

    // 从 Optional 创建 Mono
    public static Mono<String> fromOptional(Optional<String> opt) {
        return Mono.justOrEmpty(opt);
    }

    // 背压示例
    public static void backpressureExample() {
        Flux.range(1, 100)
                .log()
                .limitRate(10) // 每次最多请求 10 个
                .subscribe(
                        i -> System.out.println("onNext: " + i),
                        err -> err.printStackTrace(),
                        () -> System.out.println("完成")
                );
    }

    // 错误处理：onErrorResume / onErrorReturn
    public static Mono<String> errorHandlingExample() {
        return Mono.<String>error(new RuntimeException("模拟错误"))
                .onErrorResume(e -> {
                    System.out.println("发生错误: " + e.getMessage());
                    return Mono.just("降级值");
                })
                .onErrorReturn("默认值");
    }
}

2.2 常用操作符：map、flatMap、zip、then、buffer

import reactor.core.publisher.Mono;
import reactor.core.publisher.Flux;

import java.time.Duration;
import java.util.List;

public class ReactorOperators {

    // map：同步转换
    public static Flux<String> mapExample() {
        return Flux.just("a", "b", "c")
                .map(String::toUpperCase);
    }

    // flatMap：异步转换
    public static Flux<String> flatMapExample() {
        return Flux.just(1, 2, 3)
                .flatMap(i ->
                        Mono.just("item-" + i)
                                .delayElement(Duration.ofMillis(100))
                );
    }

    // zip：组合多个 Publisher
    public static Mono<String> zipExample() {
        Mono<String> first = Mono.just("Hello");
        Mono<String> second = Mono.just("Reactor");
        return Mono.zip(first, second)
                .map(tuple -> tuple.getT1() + " " + tuple.getT2());
    }

    // then：只关心前面完成信号，不关心值
    public static Mono<Void> thenExample() {
        return Mono.just("step1")
                .doOnNext(System.out::println)
                .then(Mono.just("step2").doOnNext(System.out::println))
                .then();
    }

    // merge / concat：合并流
    public static Flux<Integer> mergeExample() {
        Flux<Integer> even = Flux.just(2, 4, 6);
        Flux<Integer> odd = Flux.just(1, 3, 5);
        return Flux.merge(even, odd);
    }

    public static Flux<Integer> concatExample() {
        Flux<Integer> first = Flux.just(1, 2, 3);
        Flux<Integer> second = Flux.just(4, 5, 6);
        return Flux.concat(first, second);
    }

    // buffer：缓冲
    public static Flux<List<Integer>> bufferExample() {
        return Flux.range(1, 10).buffer(3); // [1,2,3], [4,5,6], [7,8,9], [10]
    }
}

2.3 调度器（Scheduler）与线程模型

import reactor.core.publisher.Flux;
import reactor.core.scheduler.Schedulers;

public class ReactorSchedulers {

    // subscribeOn：影响数据源执行线程
    public static Flux<Integer> subscribeOnBoundedElastic() {
        return Flux.range(1, 5)
                .subscribeOn(Schedulers.boundedElastic())
                .map(i -> {
                    System.out.println("subscribeOn 线程: " + Thread.currentThread().getName());
                    return i;
                });
    }

    // publishOn：影响后续操作符执行线程
    public static Flux<Integer> publishOnParallel() {
        return Flux.range(1, 5)
                .publishOn(Schedulers.parallel())
                .map(i -> {
                    System.out.println("publishOn 线程: " + Thread.currentThread().getName());
                    return i;
                });
    }

    // 组合使用
    public static Flux<Integer> mixedSchedulers() {
        return Flux.range(1, 5)
                .subscribeOn(Schedulers.boundedElastic())
                .map(i -> i * 2)
                .publishOn(Schedulers.parallel())
                .map(i -> i + 1);
    }
}

---

三、Spring WebFlux 与响应式 Web

3.1 依赖与启动类

（以 Maven 为例）

<!-- 引入 WebFlux 与 R2DBC -->
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-r2dbc</artifactId>
    </dependency>
</dependencies>

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class WebFluxApplication {

    public static void main(String[] args) {
        SpringApplication.run(WebFluxApplication.class, args);
    }
}

3.2 响应式 Controller（使用 Mono / Flux）

import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.server.ResponseStatusException;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Flux;

import javax.validation.Valid;
import java.time.Duration;

@RestController
@RequestMapping("/api/reactive")
public class ReactiveUserController {

    private final ReactiveUserService userService;

    public ReactiveUserController(ReactiveUserService userService) {
        this.userService = userService;
    }

    @GetMapping("/users/{id}")
    public Mono<UserDTO> getById(@PathVariable Long id) {
        return userService.findById(id)
                .switchIfEmpty(Mono.error(
                        new ResponseStatusException(HttpStatus.NOT_FOUND, "用户不存在")
                ));
    }

    @GetMapping("/users")
    public Flux<UserDTO> list(
            @RequestParam(defaultValue = "0") int page,
            @RequestParam(defaultValue = "10") int size) {
        return userService.findAll(page, size);
    }

    @PostMapping("/users")
    public Mono<UserDTO> create(@RequestBody @Valid UserCreateRequest request) {
        return userService.create(request);
    }

    // SSE：流式推送用户数据
    @GetMapping(value = "/users/stream", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<UserDTO> stream() {
        return userService.findAllStream()
                .delayElements(Duration.ofSeconds(1)); // 模拟每秒推送一条
    }
}

3.3 响应式 Service 与 R2DBC Repository

import org.springframework.data.domain.PageRequest;
import org.springframework.stereotype.Service;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Flux;

@Service
public class ReactiveUserService {

    private final ReactiveUserRepository repository;

    public ReactiveUserService(ReactiveUserRepository repository) {
        this.repository = repository;
    }

    public Mono<UserDTO> findById(Long id) {
        return repository.findById(id).map(this::toDTO);
    }

    public Flux<UserDTO> findAll(int page, int size) {
        return repository
                .findAllBy(PageRequest.of(page, size))
                .map(this::toDTO);
    }

    public Mono<UserDTO> create(UserCreateRequest request) {
        User entity = new User();
        entity.setUsername(request.getUsername());
        entity.setEmail(request.getEmail());
        // 其他字段...

        return repository.save(entity).map(this::toDTO);
    }

    public Flux<UserDTO> findAllStream() {
        return repository.findAll().map(this::toDTO);
    }

    private UserDTO toDTO(User user) {
        UserDTO dto = new UserDTO();
        dto.setId(user.getId());
        dto.setUsername(user.getUsername());
        dto.setEmail(user.getEmail());
        return dto;
    }
}

import org.springframework.data.domain.Pageable;
import org.springframework.data.repository.reactive.ReactiveCrudRepository;
import reactor.core.publisher.Flux;

public interface ReactiveUserRepository extends ReactiveCrudRepository<User, Long> {

    Flux<User> findAllBy(Pageable pageable);
}

3.4 WebClient：替代 RestTemplate 的响应式 HTTP 客户端

import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.HttpStatusCode;
import org.springframework.stereotype.Service;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Flux;

@Service
public class ReactiveOrderClient {

    private final WebClient webClient;

    public ReactiveOrderClient(WebClient.Builder builder) {
        this.webClient = builder
                .baseUrl("http://order-service")
                .defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
                .build();
    }

    public Mono<OrderDTO> getOrderById(Long id) {
        return webClient.get()
                .uri("/api/orders/{id}", id)
                .retrieve()
                .onStatus(HttpStatusCode::is4xxClientError,
                        resp -> Mono.error(new RuntimeException("订单不存在")))
                .bodyToMono(OrderDTO.class);
    }

    public Flux<OrderDTO> getOrdersByUserId(Long userId) {
        return webClient.get()
                .uri(uriBuilder -> uriBuilder
                        .path("/api/orders")
                        .queryParam("userId", userId)
                        .build())
                .retrieve()
                .bodyToFlux(OrderDTO.class);
    }

    public Mono<OrderDTO> createOrder(OrderCreateRequest request) {
        return webClient.post()
                .uri("/api/orders")
                .bodyValue(request)
                .retrieve()
                .bodyToMono(OrderDTO.class);
    }
}

---

四、虚拟线程（Java 21+）与结构化并发

4.1 虚拟线程的创建与使用

import java.util.concurrent.Executors;

public class VirtualThreadDemo {

    // 方式一：虚拟线程池
    public static void runWithVirtualThreads() throws Exception {
        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            executor.submit(() ->
                    System.out.println("Task 1, virtual=" + Thread.currentThread().isVirtual()));
            executor.submit(() ->
                    System.out.println("Task 2, virtual=" + Thread.currentThread().isVirtual()));
        }
    }

    // 方式二：直接创建虚拟线程
    public static void threadOfVirtual() {
        Thread.ofVirtual()
                .name("vt-", 0)
                .start(() ->
                        System.out.println("虚拟线程名称: " + Thread.currentThread().getName()));
    }
}

4.2 结构化并发（StructuredTaskScope）

import java.util.concurrent.StructuredTaskScope;
import java.util.concurrent.StructuredTaskScope.ShutdownOnFailure;

public class StructuredConcurrencyDemo {

    public static class Result {
        String user;
        String order;
        String product;
    }

    public static Result fetchUserOrderProduct(long userId, long orderId, long productId) throws Exception {
        try (var scope = new ShutdownOnFailure()) {
            var userTask = scope.fork(() -> fetchUser(userId));
            var orderTask = scope.fork(() -> fetchOrder(orderId));
            var productTask = scope.fork(() -> fetchProduct(productId));

            scope.join();          // 等待所有子任务完成
            scope.throwIfFailed(); // 任一失败则抛异常

            Result r = new Result();
            r.user = userTask.get();
            r.order = orderTask.get();
            r.product = productTask.get();
            return r;
        }
    }

    private static String fetchUser(long id) throws InterruptedException {
        Thread.sleep(100);
        return "User-" + id;
    }

    private static String fetchOrder(long id) throws InterruptedException {
        Thread.sleep(150);
        return "Order-" + id;
    }

    private static String fetchProduct(long id) throws InterruptedException {
        Thread.sleep(80);
        return "Product-" + id;
    }
}

4.3 Spring Boot 中启用虚拟线程

application.yml：

spring:
  threads:
    virtual:
      enabled: true

自定义异步任务使用虚拟线程：

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

@Configuration
public class VirtualThreadConfig {

    @Bean
    public Executor virtualThreadExecutor() {
        return Executors.newVirtualThreadPerTaskExecutor();
    }
}

@Service
class AsyncService {

    @Async("virtualThreadExecutor")
    public CompletableFuture<String> asyncTask() {
        return CompletableFuture.completedFuture(
                "run on: " + Thread.currentThread().getName() +
                        ", virtual=" + Thread.currentThread().isVirtual()
        );
    }
}

---

五、可观测性：指标、链路追踪与结构化日志

5.1 Micrometer + Prometheus 指标

import io.micrometer.core.instrument.Counter;
import io.micrometer.core.instrument.Timer;
import io.micrometer.core.instrument.MeterRegistry;
import org.springframework.stereotype.Service;

import java.time.Duration;

@Service
public class OrderMetricsService {

    private final Counter orderCreatedCounter;
    private final Timer orderCreateTimer;

    public OrderMetricsService(MeterRegistry registry) {
        this.orderCreatedCounter = registry.counter("order.created", "env", "dev");
        this.orderCreateTimer = registry.timer("order.create.duration");
    }

    public void recordOrderCreated() {
        orderCreatedCounter.increment();
    }

    public void recordOrderCreateDuration(Runnable runnable) {
        orderCreateTimer.record(runnable);
    }

    public void recordOrderCreateDuration(Duration duration) {
        orderCreateTimer.record(duration);
    }
}

application.yml 配置：

management:
  endpoints:
    web:
      exposure:
        include: health,info,prometheus
  endpoint:
    prometheus:
      enabled: true
  metrics:
    tags:
      application: ${spring.application.name}

5.2 链路追踪：Micrometer Tracing + Zipkin

依赖示例：

<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-tracing-bridge-brave</artifactId>
</dependency>
<dependency>
    <groupId>io.zipkin.reporter2</groupId>
    <artifactId>zipkin-reporter-brave</artifactId>
</dependency>

配置示例：

management:
  tracing:
    sampling:
      probability: 1.0
  zipkin:
    tracing:
      endpoint: http://localhost:9411/api/v2/spans

在业务代码中打 Span：

import io.micrometer.tracing.Tracer;
import org.springframework.stereotype.Service;

@Service
public class OrderService {

    private final Tracer tracer;

    public OrderService(Tracer tracer) {
        this.tracer = tracer;
    }

    public OrderDTO createOrder(CreateOrderRequest request) {
        var span = tracer.nextSpan().name("createOrder").start();
        try (Tracer.SpanInScope ws = tracer.withSpan(span)) {
            span.tag("order.userId", String.valueOf(request.getUserId()));
            // 订单创建逻辑...
            return new OrderDTO();
        } finally {
            span.end();
        }
    }
}

5.3 结构化日志（JSON + MDC）

logback-spring.xml 简化示例：

<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>

    <appender name="JSON" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
            <includeMdcKeyName>traceId</includeMdcKeyName>
        </encoder>
    </appender>

    <root level="INFO">
        <appender-ref ref="JSON"/>
    </root>
</configuration>

在拦截器中写入 traceId：

import org.slf4j.MDC;
import org.springframework.web.servlet.HandlerInterceptor;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.UUID;

public class MdcInterceptor implements HandlerInterceptor {

    private static final String TRACE_ID = "traceId";

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        String traceId = request.getHeader("X-Trace-Id");
        if (traceId == null || traceId.isEmpty()) {
            traceId = UUID.randomUUID().toString();
        }
        MDC.put(TRACE_ID, traceId);
        response.setHeader("X-Trace-Id", traceId);
        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response,
                                Object handler, Exception ex) {
        MDC.remove(TRACE_ID);
    }
}

---

六、综合实战：MVC + WebFlux + 虚拟线程混合

6.1 在传统 MVC 项目中引入 WebClient

import org.springframework.stereotype.Service;
import reactor.core.publisher.Mono;

@Service
public class OrderFacadeService {

    private final OrderService orderService;           // 传统阻塞 service
    private final ReactiveOrderClient reactiveClient;  // 响应式 HTTP 客户端

    public OrderFacadeService(OrderService orderService,
                              ReactiveOrderClient reactiveClient) {
        this.orderService = orderService;
        this.reactiveClient = reactiveClient;
    }

    // 阻塞式接口内部使用 WebClient（建议放在虚拟线程中）
    public OrderAggregateDTO getOrderWithDetails(Long orderId) {
        OrderDTO order = orderService.getById(orderId);
        if (order == null) {
            return null;
        }

        Long userId = order.getUserId();
        Long productId = order.getProductId();

        Mono<UserDTO> userMono = reactiveClient.getUserById(userId);
        Mono<ProductDTO> productMono = reactiveClient.getProductById(productId);

        UserDTO user = userMono.block();       // 在虚拟线程中阻塞开销更小
        ProductDTO product = productMono.block();

        OrderAggregateDTO agg = new OrderAggregateDTO();
        agg.setOrder(order);
        agg.setUser(user);
        agg.setProduct(product);
        return agg;
    }
}

6.2 健康检查与就绪探针

import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.stereotype.Component;
import javax.sql.DataSource;
import org.springframework.data.redis.connection.RedisConnectionFactory;

@Component
public class CustomHealthIndicator implements HealthIndicator {

    private final DataSource dataSource;
    private final RedisConnectionFactory redisConnectionFactory;

    public CustomHealthIndicator(DataSource dataSource,
                                 RedisConnectionFactory redisConnectionFactory) {
        this.dataSource = dataSource;
        this.redisConnectionFactory = redisConnectionFactory;
    }

    @Override
    public Health health() {
        try {
            dataSource.getConnection().close();
        } catch (Exception e) {
            return Health.down().withException(e).build();
        }
        try {
            redisConnectionFactory.getConnection().ping();
        } catch (Exception e) {
            return Health.down().withException(e).build();
        }
        return Health.up().build();
    }
}

application.yml：

management:
  endpoint:
    health:
      probes:
        enabled: true
  health:
    livenessState:
      enabled: true
    readinessState:
      enabled: true