Spring Boot 4 Complete Guide · 课时

应用内部事件与监听器

使用在应用上下文中发布和消费的领域事件，使模块解耦。

应用内部事件与监听器 是 CoddyKit 上的免费 Spring Boot 4 Complete Guide 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Spring Boot 4 Complete Guide 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Spring Boot 4 Complete Guide 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Why Decouple Modules with Events?

In a modular monolith, an Order module often needs to trigger work in other modules: send a confirmation email, update inventory, award loyalty points. The naive approach is to have OrderService call EmailService, InventoryService, and LoyaltyService directly.

This creates tight coupling — Order depends on every downstream module.
Adding a new reaction means editing OrderService again.
Modules can no longer evolve or be tested independently.

Spring's internal application events invert this: the Order module simply publishes a domain event, and interested modules listen for it. The publisher never knows who is listening.

The Domain Event Record

An internal event is just a plain object — in Spring Boot 4 (Java 21+) a record is the idiomatic choice. It should be immutable and carry only the data listeners need.

Name it in the past tense (OrderCompleted) — it describes something that already happened.
Keep it inside the owning module's package.
No Spring annotations are required on the event itself.

package com.shop.order;

import java.math.BigDecimal;
import java.util.UUID;

public record OrderCompleted(
        UUID orderId,
        String customerEmail,
        BigDecimal total) {
}

Publishing an Event

The owning module publishes the event through Spring's ApplicationEventPublisher. Inject it via the constructor and call publishEvent() after the business operation succeeds.

The publisher has no compile-time reference to any listener.
By default publishing is synchronous — listeners run on the same thread, inside the same transaction.

package com.shop.order;

import org.springframework.context.ApplicationEventPublisher;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

@Service
public class OrderService {

    private final ApplicationEventPublisher events;
    private final OrderRepository orders;

    public OrderService(ApplicationEventPublisher events, OrderRepository orders) {
        this.events = events;
        this.orders = orders;
    }

    @Transactional
    public void complete(Order order) {
        order.markCompleted();
        orders.save(order);
        events.publishEvent(
            new OrderCompleted(order.getId(), order.getCustomerEmail(), order.getTotal()));
    }
}

Listening with @EventListener

Any Spring bean in another module can react by annotating a method with @EventListener. The single method parameter declares the event type the listener cares about.

The method can have any name — Spring matches on the parameter type.
The listener lives in its own module (com.shop.notification) and never references the Order service.

package com.shop.notification;

import com.shop.order.OrderCompleted;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
public class OrderEmailListener {

    private final MailSender mail;

    public OrderEmailListener(MailSender mail) {
        this.mail = mail;
    }

    @EventListener
    void on(OrderCompleted event) {
        mail.send(event.customerEmail(),
                  "Your order " + event.orderId() + " is complete!");
    }
}

Synchronous by Default — and Its Risks

By default publishEvent() invokes every matching @EventListener synchronously, in order, on the calling thread. Because the listeners run inside the publisher's transaction:

A listener that throws will propagate the exception back to the publisher and roll back the whole transaction.
A slow listener (e.g. sending an email) blocks the business call until it finishes.

This is rarely what you want for side effects like email. The next scenes show how to fix both problems with transaction-bound and asynchronous listeners.

Reacting Only After Commit

You usually want side effects to fire only if the transaction actually commits — you should not send a "your order is complete" email if the order save later rolls back. Use @TransactionalEventListener, which by default binds to the AFTER_COMMIT phase.

The listener runs after the publisher's transaction commits successfully.
If the transaction rolls back, the listener is skipped.
You can choose other phases via phase = BEFORE_COMMIT, AFTER_ROLLBACK, or AFTER_COMPLETION.

package com.shop.notification;

import com.shop.order.OrderCompleted;
import org.springframework.stereotype.Component;
import org.springframework.transaction.event.TransactionPhase;
import org.springframework.transaction.event.TransactionalEventListener;

@Component
public class OrderEmailListener {

    private final MailSender mail;

    public OrderEmailListener(MailSender mail) {
        this.mail = mail;
    }

    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    void on(OrderCompleted event) {
        mail.send(event.customerEmail(),
                  "Your order " + event.orderId() + " is complete!");
    }
}

Making Listeners Asynchronous

To stop a slow listener from blocking the caller, add @Async. The listener then runs on a separate thread from a task executor, so publishEvent() returns immediately.

Enable async support once with @EnableAsync on a configuration class.
Important: an @Async listener runs in its own thread with no transaction and no access to the publisher's context. Combine it with @TransactionalEventListener(AFTER_COMMIT) when you want "after commit, on a background thread."
Async listeners that throw are logged but do not affect the publisher — handle failures yourself.

package com.shop.notification;

import com.shop.order.OrderCompleted;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
import org.springframework.transaction.event.TransactionalEventListener;

@Component
public class OrderEmailListener {

    private final MailSender mail;

    public OrderEmailListener(MailSender mail) {
        this.mail = mail;
    }

    @Async
    @TransactionalEventListener
    void on(OrderCompleted event) {
        mail.send(event.customerEmail(),
                  "Your order " + event.orderId() + " is complete!");
    }
}

Spring Modulith Event Externalization

Plain Spring events are in-memory only: if the application crashes between commit and the listener running, the event is lost. Spring Modulith adds durability with its Event Publication Registry.

Add the spring-modulith-events-jpa (or JDBC/MongoDB) starter.
Each @TransactionalEventListener invocation is recorded in an event_publication table within the same transaction.
Once the listener completes, its publication is marked as completed. Incomplete publications can be republished on restart, guaranteeing at-least-once delivery.

Configuring the Publication Registry

With the events starter on the classpath, Modulith auto-creates the registry schema and wires interception. A little YAML controls recovery behaviour.

republish-outstanding-events-on-restart resubmits any publications that never completed before the last shutdown.
Modulith also exposes the registry so you can monitor or manually resubmit stuck events.

spring:
  modulith:
    events:
      jdbc:
        schema-initialization:
          enabled: true
      republish-outstanding-events-on-restart: true
      completion-mode: delete

Filtering Events by Condition

Sometimes a listener should only react to some events of a type. The condition attribute takes a SpEL expression evaluated against the event; the listener runs only when it returns true.

Reference the event payload with #root.event or simply the property via # root.
This keeps filtering declarative instead of an if at the top of the method.

package com.shop.loyalty;

import com.shop.order.OrderCompleted;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
public class HighValueOrderListener {

    // Only fires for orders over 100
    @EventListener(condition = "#event.total().compareTo(new java.math.BigDecimal('100')) > 0")
    void onBigOrder(OrderCompleted event) {
        // award bonus loyalty points
    }
}

A Standalone Pub/Sub Mental Model

Strip away Spring and the pattern is just a registry of listeners keyed by event type. The snippet below is a plain Java program that mimics the synchronous publish/subscribe behaviour — useful to internalize what ApplicationEventPublisher does under the hood.

import java.util.*;
import java.util.function.Consumer;

public class MiniEventBus {
    private final Map<Class<?>, List<Consumer<Object>>> listeners = new HashMap<>();

    <T> void subscribe(Class<T> type, Consumer<T> listener) {
        listeners.computeIfAbsent(type, k -> new ArrayList<>())
                 .add(e -> listener.accept(type.cast(e)));
    }

    void publish(Object event) {
        listeners.getOrDefault(event.getClass(), List.of())
                 .forEach(l -> l.accept(event));
    }

    record OrderCompleted(String id, double total) {}

    public static void main(String[] args) {
        MiniEventBus bus = new MiniEventBus();
        bus.subscribe(OrderCompleted.class,
                e -> System.out.println("Email: order " + e.id() + " done"));
        bus.subscribe(OrderCompleted.class,
                e -> System.out.println("Loyalty: +" + (int) e.total() + " points"));
        bus.publish(new OrderCompleted("A-42", 150.0));
    }
}

Quick Check: Choosing the Right Listener

You publish an OrderCompleted event inside a @Transactional order-save method. The email listener must run only if the order is actually persisted, and it must not delay or break the order-save call. Which listener configuration is correct?

Recap: Internal Events for Module Decoupling

You learned how to decouple modules with in-application domain events:

Model events as immutable past-tense records owned by the publishing module.
Publish via ApplicationEventPublisher.publishEvent() — the publisher never references listeners.
React with @EventListener (synchronous, in-transaction) or @TransactionalEventListener (runs in a chosen transaction phase, default AFTER_COMMIT).
Add @Async + @EnableAsync to run side effects off the caller's thread.
Filter declaratively with the condition SpEL attribute.
Use Spring Modulith's Event Publication Registry to persist publications and republish incomplete ones on restart for reliable, at-least-once delivery.

The result: modules that communicate through events stay independently testable, replaceable, and free of direct dependencies.

常见问题解答

「应用内部事件与监听器」课时是免费的吗？

是的 — 「应用内部事件与监听器」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Spring Boot 4 Complete Guide 课程的其余内容，请升级到 CoddyKit PRO。 Spring Boot 4 Complete Guide 课程共包含 4 节课。

「应用内部事件与监听器」这节课中我会学到什么？

使用在应用上下文中发布和消费的领域事件，使模块解耦。你通过在浏览器中直接运行的动手代码来练习 Spring Boot 4 Complete Guide，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Spring Boot 4 Complete Guide 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Spring Boot 4 Complete Guide 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「应用内部事件与监听器」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Spring Boot 4 Complete Guide 课中编写并运行代码吗？

能。每节 Spring Boot 4 Complete Guide 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 Spring Boot 4 Complete Guide