Spring Architecture Series-8.Implementing Event Publishing and Listening Mechanism

0448

Introduction

Event-driven programming is a powerful paradigm that enables loose coupling between components in an application. In this article, I’ll explore how to implement an event publishing and listening mechanism in a Spring-like framework, based on my miniSpring project’s implementation.

Core Components

The event mechanism implementation consists of several key components: 

src/com/yaruyng/context/
├── ApplicationEvent.java
├── ApplicationListener.java
├── ApplicationEventPublisher.java
├── SimpleApplicationEventPublisher.java
├── ApplicationContextEvent.java
├── ContextRefreshEvent.java
└── ContextRefreshedEvent.java
src/com/yaruyng/context/
├── ApplicationEvent.java
├── ApplicationListener.java
├── ApplicationEventPublisher.java
├── SimpleApplicationEventPublisher.java
├── ApplicationContextEvent.java
├── ContextRefreshEvent.java
└── ContextRefreshedEvent.java
src/com/yaruyng/context/
├── ApplicationEvent.java
├── ApplicationListener.java
├── ApplicationEventPublisher.java
├── SimpleApplicationEventPublisher.java
├── ApplicationContextEvent.java
├── ContextRefreshEvent.java
└── ContextRefreshedEvent.java

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Event Base Class

The ApplicationEvent class serves as the base for all application events: 

<span>public</span> <span>class</span> <span>ApplicationEvent</span> <span>extends</span> <span>EventObject</span> <span>{</span>
    <span>private</span> <span>static</span> <span>final</span> <span>long</span> <span>serialVersionUID</span> <span>=</span> <span>1L</span><span>;</span>
    <span>protected</span> <span>String</span> <span>msg</span> <span>=</span> <span>null</span><span>;</span>
    <span>public</span> <span>ApplicationEvent</span><span>(</span><span>Object</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
        <span>this</span><span>.</span><span>msg</span> <span>=</span> <span>source</span><span>.</span><span>toString</span><span>();</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>ApplicationEvent</span> <span>extends</span> <span>EventObject</span> <span>{</span>
    <span>private</span> <span>static</span> <span>final</span> <span>long</span> <span>serialVersionUID</span> <span>=</span> <span>1L</span><span>;</span>
    <span>protected</span> <span>String</span> <span>msg</span> <span>=</span> <span>null</span><span>;</span>

    <span>public</span> <span>ApplicationEvent</span><span>(</span><span>Object</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
        <span>this</span><span>.</span><span>msg</span> <span>=</span> <span>source</span><span>.</span><span>toString</span><span>();</span>
    <span>}</span>
<span>}</span>
public class ApplicationEvent extends EventObject {
    private static final long serialVersionUID = 1L;
    protected String msg = null;

    public ApplicationEvent(Object source) {
        super(source);
        this.msg = source.toString();
    }
}

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Key features:

  1. Extends EventObject from Java standard library
  2. Serializable support
  3. Source object tracking
  4. Message support

Event Listener Interface

The ApplicationListener interface defines the contract for event listeners: 

<span>public</span> <span>interface</span> <span>ApplicationListener</span><span><</span><span>E</span> <span>extends</span> <span>ApplicationEvent</span><span>></span> <span>extends</span> <span>EventListener</span> <span>{</span>
    <span>void</span> <span>onApplicationEvent</span><span>(</span><span>E</span> <span>event</span><span>);</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>ApplicationListener</span><span><</span><span>E</span> <span>extends</span> <span>ApplicationEvent</span><span>></span> <span>extends</span> <span>EventListener</span> <span>{</span>
    <span>void</span> <span>onApplicationEvent</span><span>(</span><span>E</span> <span>event</span><span>);</span>
<span>}</span>
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    void onApplicationEvent(E event);
}

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Features:

  1. Generic type support
  2. Single responsibility principle
  3. Clear event handling contract

Event Publisher

The SimpleApplicationEventPublisher implements the event publishing mechanism: 

<span>public</span> <span>class</span> <span>SimpleApplicationEventPublisher</span> <span>implements</span> <span>ApplicationEventPublisher</span> <span>{</span>
    <span>List</span><span><</span><span>ApplicationListener</span><span>></span> <span>listeners</span> <span>=</span> <span>new</span> <span>ArrayList</span><span><>();</span>
    <span>@Override</span>
    <span>public</span> <span>void</span> <span>publishEvent</span><span>(</span><span>ApplicationEvent</span> <span>event</span><span>)</span> <span>{</span>
        <span>for</span> <span>(</span><span>ApplicationListener</span> <span>listener</span> <span>:</span> <span>listeners</span><span>)</span> <span>{</span>
            <span>listener</span><span>.</span><span>onApplicationEvent</span><span>(</span><span>event</span><span>);</span>
        <span>}</span>
    <span>}</span>
    <span>@Override</span>
    <span>public</span> <span>void</span> <span>addApplicationListener</span><span>(</span><span>ApplicationListener</span> <span>listener</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>listeners</span><span>.</span><span>add</span><span>(</span><span>listener</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>SimpleApplicationEventPublisher</span> <span>implements</span> <span>ApplicationEventPublisher</span> <span>{</span>
    <span>List</span><span><</span><span>ApplicationListener</span><span>></span> <span>listeners</span> <span>=</span> <span>new</span> <span>ArrayList</span><span><>();</span>

    <span>@Override</span>
    <span>public</span> <span>void</span> <span>publishEvent</span><span>(</span><span>ApplicationEvent</span> <span>event</span><span>)</span> <span>{</span>
        <span>for</span> <span>(</span><span>ApplicationListener</span> <span>listener</span> <span>:</span> <span>listeners</span><span>)</span> <span>{</span>
            <span>listener</span><span>.</span><span>onApplicationEvent</span><span>(</span><span>event</span><span>);</span>
        <span>}</span>
    <span>}</span>

    <span>@Override</span>
    <span>public</span> <span>void</span> <span>addApplicationListener</span><span>(</span><span>ApplicationListener</span> <span>listener</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>listeners</span><span>.</span><span>add</span><span>(</span><span>listener</span><span>);</span>
    <span>}</span>
<span>}</span>
public class SimpleApplicationEventPublisher implements ApplicationEventPublisher {
    List<ApplicationListener> listeners = new ArrayList<>();

    @Override
    public void publishEvent(ApplicationEvent event) {
        for (ApplicationListener listener : listeners) {
            listener.onApplicationEvent(event);
        }
    }

    @Override
    public void addApplicationListener(ApplicationListener listener) {
        this.listeners.add(listener);
    }
}

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Key aspects:

  1. Listener management
  2. Event broadcasting
  3. Synchronous event processing

Context Events

1. Context Refresh Event 

<span>public</span> <span>class</span> <span>ContextRefreshEvent</span> <span>extends</span> <span>ApplicationContextEvent</span> <span>{</span>
    <span>public</span> <span>ContextRefreshEvent</span><span>(</span><span>ApplicationContext</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>ContextRefreshEvent</span> <span>extends</span> <span>ApplicationContextEvent</span> <span>{</span>
    <span>public</span> <span>ContextRefreshEvent</span><span>(</span><span>ApplicationContext</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
    <span>}</span>
<span>}</span>
public class ContextRefreshEvent extends ApplicationContextEvent {
    public ContextRefreshEvent(ApplicationContext source) {
        super(source);
    }
}

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2. Context Refreshed Event 

<span>public</span> <span>class</span> <span>ContextRefreshedEvent</span> <span>extends</span> <span>ApplicationContextEvent</span> <span>{</span>
    <span>public</span> <span>ContextRefreshedEvent</span><span>(</span><span>ApplicationContext</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>ContextRefreshedEvent</span> <span>extends</span> <span>ApplicationContextEvent</span> <span>{</span>
    <span>public</span> <span>ContextRefreshedEvent</span><span>(</span><span>ApplicationContext</span> <span>source</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
    <span>}</span>
<span>}</span>
public class ContextRefreshedEvent extends ApplicationContextEvent {
    public ContextRefreshedEvent(ApplicationContext source) {
        super(source);
    }
}

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Integration with Application Context

The AbstractApplicationContext integrates event support: 

<span>public</span> <span>abstract</span> <span>class</span> <span>AbstractApplicationContext</span> <span>implements</span> <span>ApplicationContext</span> <span>{</span>
    <span>private</span> <span>ApplicationEventPublisher</span> <span>applicationEventPublisher</span><span>;</span>
    <span>public</span> <span>abstract</span> <span>void</span> <span>registerListeners</span><span>();</span>
    <span>public</span> <span>abstract</span> <span>void</span> <span>initApplicationEventPublisher</span><span>();</span>
    <span>public</span> <span>void</span> <span>refresh</span><span>()</span> <span>throws</span> <span>BeansException</span><span>,</span> <span>IllegalStateException</span> <span>{</span>
        <span>// Initialize event publisher</span>
        <span>initApplicationEventPublisher</span><span>();</span>
        <span>// Register listeners</span>
        <span>registerListeners</span><span>();</span>
        <span>// Other initialization steps...</span>
        <span>// Publish refresh event</span>
        <span>finishRefresh</span><span>();</span>
    <span>}</span>
    <span>public</span> <span>void</span> <span>finishRefresh</span><span>()</span> <span>{</span>
        <span>publishEvent</span><span>(</span><span>new</span> <span>ContextRefreshedEvent</span><span>(</span><span>this</span><span>));</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>abstract</span> <span>class</span> <span>AbstractApplicationContext</span> <span>implements</span> <span>ApplicationContext</span> <span>{</span>
    <span>private</span> <span>ApplicationEventPublisher</span> <span>applicationEventPublisher</span><span>;</span>

    <span>public</span> <span>abstract</span> <span>void</span> <span>registerListeners</span><span>();</span>
    <span>public</span> <span>abstract</span> <span>void</span> <span>initApplicationEventPublisher</span><span>();</span>

    <span>public</span> <span>void</span> <span>refresh</span><span>()</span> <span>throws</span> <span>BeansException</span><span>,</span> <span>IllegalStateException</span> <span>{</span>
        <span>// Initialize event publisher</span>
        <span>initApplicationEventPublisher</span><span>();</span>

        <span>// Register listeners</span>
        <span>registerListeners</span><span>();</span>

        <span>// Other initialization steps...</span>

        <span>// Publish refresh event</span>
        <span>finishRefresh</span><span>();</span>
    <span>}</span>

    <span>public</span> <span>void</span> <span>finishRefresh</span><span>()</span> <span>{</span>
        <span>publishEvent</span><span>(</span><span>new</span> <span>ContextRefreshedEvent</span><span>(</span><span>this</span><span>));</span>
    <span>}</span>
<span>}</span>
public abstract class AbstractApplicationContext implements ApplicationContext {
    private ApplicationEventPublisher applicationEventPublisher;

    public abstract void registerListeners();
    public abstract void initApplicationEventPublisher();

    public void refresh() throws BeansException, IllegalStateException {
        // Initialize event publisher
        initApplicationEventPublisher();

        // Register listeners
        registerListeners();

        // Other initialization steps...

        // Publish refresh event
        finishRefresh();
    }

    public void finishRefresh() {
        publishEvent(new ContextRefreshedEvent(this));
    }
}

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Event Processing Flow

1. Event Register 

<span>public</span> <span>void</span> <span>registerListeners</span><span>()</span> <span>{</span>
    <span>String</span><span>[]</span> <span>beanDefinitionNames</span> <span>=</span> <span>this</span><span>.</span><span>getBeanFactory</span><span>().</span><span>getBeanDefinitionNames</span><span>();</span>
    <span>for</span> <span>(</span><span>String</span> <span>bdName</span> <span>:</span> <span>beanDefinitionNames</span><span>)</span> <span>{</span>
        <span>Object</span> <span>bean</span> <span>=</span> <span>getBean</span><span>(</span><span>bdName</span><span>);</span>
        <span>if</span><span>(</span><span>bean</span> <span>instanceof</span> <span>ApplicationListener</span><span>)</span> <span>{</span>
            <span>this</span><span>.</span><span>getApplicationEventPublisher</span><span>()</span>
                <span>.</span><span>addApplicationListener</span><span>((</span><span>ApplicationListener</span><span><?>)</span> <span>bean</span><span>);</span>
        <span>}</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>void</span> <span>registerListeners</span><span>()</span> <span>{</span>
    <span>String</span><span>[]</span> <span>beanDefinitionNames</span> <span>=</span> <span>this</span><span>.</span><span>getBeanFactory</span><span>().</span><span>getBeanDefinitionNames</span><span>();</span>
    <span>for</span> <span>(</span><span>String</span> <span>bdName</span> <span>:</span> <span>beanDefinitionNames</span><span>)</span> <span>{</span>
        <span>Object</span> <span>bean</span> <span>=</span> <span>getBean</span><span>(</span><span>bdName</span><span>);</span>
        <span>if</span><span>(</span><span>bean</span> <span>instanceof</span> <span>ApplicationListener</span><span>)</span> <span>{</span>
            <span>this</span><span>.</span><span>getApplicationEventPublisher</span><span>()</span>
                <span>.</span><span>addApplicationListener</span><span>((</span><span>ApplicationListener</span><span><?>)</span> <span>bean</span><span>);</span>
        <span>}</span>
    <span>}</span>
<span>}</span>
public void registerListeners() {
    String[] beanDefinitionNames = this.getBeanFactory().getBeanDefinitionNames();
    for (String bdName : beanDefinitionNames) {
        Object bean = getBean(bdName);
        if(bean instanceof ApplicationListener) {
            this.getApplicationEventPublisher()
                .addApplicationListener((ApplicationListener<?>) bean);
        }
    }
}

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2. Event Publishing 

<span>public</span> <span>void</span> <span>publishEvent</span><span>(</span><span>ApplicationEvent</span> <span>event</span><span>)</span> <span>{</span>
    <span>this</span><span>.</span><span>getApplicationEventPublisher</span><span>().</span><span>publishEvent</span><span>(</span><span>event</span><span>);</span>
<span>}</span>
<span>public</span> <span>void</span> <span>publishEvent</span><span>(</span><span>ApplicationEvent</span> <span>event</span><span>)</span> <span>{</span>
    <span>this</span><span>.</span><span>getApplicationEventPublisher</span><span>().</span><span>publishEvent</span><span>(</span><span>event</span><span>);</span>
<span>}</span>
public void publishEvent(ApplicationEvent event) {
    this.getApplicationEventPublisher().publishEvent(event);
}

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Usage Example

1. Creating Custom Events 

<span>public</span> <span>class</span> <span>UserRegisteredEvent</span> <span>extends</span> <span>ApplicationEvent</span> <span>{</span>
    <span>private</span> <span>final</span> <span>User</span> <span>user</span><span>;</span>
    <span>public</span> <span>UserRegisteredEvent</span><span>(</span><span>Object</span> <span>source</span><span>,</span> <span>User</span> <span>user</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
        <span>this</span><span>.</span><span>user</span> <span>=</span> <span>user</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>User</span> <span>getUser</span><span>()</span> <span>{</span>
        <span>return</span> <span>user</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>UserRegisteredEvent</span> <span>extends</span> <span>ApplicationEvent</span> <span>{</span>
    <span>private</span> <span>final</span> <span>User</span> <span>user</span><span>;</span>

    <span>public</span> <span>UserRegisteredEvent</span><span>(</span><span>Object</span> <span>source</span><span>,</span> <span>User</span> <span>user</span><span>)</span> <span>{</span>
        <span>super</span><span>(</span><span>source</span><span>);</span>
        <span>this</span><span>.</span><span>user</span> <span>=</span> <span>user</span><span>;</span>
    <span>}</span>

    <span>public</span> <span>User</span> <span>getUser</span><span>()</span> <span>{</span>
        <span>return</span> <span>user</span><span>;</span>
    <span>}</span>
<span>}</span>
public class UserRegisteredEvent extends ApplicationEvent {
    private final User user;

    public UserRegisteredEvent(Object source, User user) {
        super(source);
        this.user = user;
    }

    public User getUser() {
        return user;
    }
}

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2. Implementing Event Listeners 

<span>@Component</span>
<span>public</span> <span>class</span> <span>EmailNotificationListener</span> <span>implements</span> <span>ApplicationListener</span><span><</span><span>UserRegisteredEvent</span><span>></span> <span>{</span>
    <span>@Override</span>
    <span>public</span> <span>void</span> <span>onApplicationEvent</span><span>(</span><span>UserRegisteredEvent</span> <span>event</span><span>)</span> <span>{</span>
        <span>User</span> <span>user</span> <span>=</span> <span>event</span><span>.</span><span>getUser</span><span>();</span>
        <span>// Send welcome email</span>
        <span>sendWelcomeEmail</span><span>(</span><span>user</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>@Component</span>
<span>public</span> <span>class</span> <span>EmailNotificationListener</span> <span>implements</span> <span>ApplicationListener</span><span><</span><span>UserRegisteredEvent</span><span>></span> <span>{</span>
    <span>@Override</span>
    <span>public</span> <span>void</span> <span>onApplicationEvent</span><span>(</span><span>UserRegisteredEvent</span> <span>event</span><span>)</span> <span>{</span>
        <span>User</span> <span>user</span> <span>=</span> <span>event</span><span>.</span><span>getUser</span><span>();</span>
        <span>// Send welcome email</span>
        <span>sendWelcomeEmail</span><span>(</span><span>user</span><span>);</span>
    <span>}</span>
<span>}</span>
@Component
public class EmailNotificationListener implements ApplicationListener<UserRegisteredEvent> {
    @Override
    public void onApplicationEvent(UserRegisteredEvent event) {
        User user = event.getUser();
        // Send welcome email
        sendWelcomeEmail(user);
    }
}

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3. Publishing Event 

<span>@Service</span>
<span>public</span> <span>class</span> <span>UserService</span> <span>{</span>
    <span>@Autowired</span>
    <span>private</span> <span>ApplicationEventPublisher</span> <span>eventPublisher</span><span>;</span>
    <span>public</span> <span>void</span> <span>registerUser</span><span>(</span><span>User</span> <span>user</span><span>)</span> <span>{</span>
        <span>// Save user</span>
        <span>userRepository</span><span>.</span><span>save</span><span>(</span><span>user</span><span>);</span>
        <span>// Publish event</span>
        <span>eventPublisher</span><span>.</span><span>publishEvent</span><span>(</span><span>new</span> <span>UserRegisteredEvent</span><span>(</span><span>this</span><span>,</span> <span>user</span><span>));</span>
    <span>}</span>
<span>}</span>
<span>@Service</span>
<span>public</span> <span>class</span> <span>UserService</span> <span>{</span>
    <span>@Autowired</span>
    <span>private</span> <span>ApplicationEventPublisher</span> <span>eventPublisher</span><span>;</span>

    <span>public</span> <span>void</span> <span>registerUser</span><span>(</span><span>User</span> <span>user</span><span>)</span> <span>{</span>
        <span>// Save user</span>
        <span>userRepository</span><span>.</span><span>save</span><span>(</span><span>user</span><span>);</span>

        <span>// Publish event</span>
        <span>eventPublisher</span><span>.</span><span>publishEvent</span><span>(</span><span>new</span> <span>UserRegisteredEvent</span><span>(</span><span>this</span><span>,</span> <span>user</span><span>));</span>
    <span>}</span>
<span>}</span>
@Service
public class UserService {
    @Autowired
    private ApplicationEventPublisher eventPublisher;

    public void registerUser(User user) {
        // Save user
        userRepository.save(user);

        // Publish event
        eventPublisher.publishEvent(new UserRegisteredEvent(this, user));
    }
}

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Key Features

1.Event Types

  • Context events
  • Custom events
  • Event hierarchy ### 2. Listener Management
  • Dynamic registration
  • Type-safe handling
  • Multiple listeners ### 3. Event Publishing
  • Synchronous processing
  • Error handling
  • Event ordering

Best Practice

  1. Event Design
    • Clear event hierarchy
    • Immutable event data
    • Meaningful event names
  2. Listener Implementation
    • Single responsibility
    • Error handling
    • Performance consideration
  3. Event Publishing
    • Appropriate timing
    • Error propagation
    • Transaction boundaries

Common Challenges and Solutions

  1. Event Ordering
    • Listener priority
    • Synchronous processing
    • Event queuing
  2. Error Handling
    • Exception propagation
    • Listener isolation
    • Recovery mechanisms
  3. Performance
    • Asynchronous processing
    • Event filtering
    • Listener optimization

Conclusion

Implementing an event mechanism provides:

  • Loose coupling between components
  • Asynchronous communication
  • Extensible architecture
  • Decoupled business logic Key takeaways:
  • Understanding event-driven architecture
  • Event listener patterns
  • Event publishing mechanisms
  • Integration with IoC container

This implementation demonstrates how to create a robust event system while maintaining simplicity and flexibility.

原文链接：Spring Architecture Series-8.Implementing Event Publishing and Listening Mechanism

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杀人蜂的头像-拾光赋

杀人蜂5天前0

我很好奇这个网站能提供什么
Vito的头像-拾光赋

Vito6个月前2

你就是我心中的那首忐忑，总是让我惊心动魄
Jason.Meng的头像-拾光赋钻石会员

Jason.Meng徽章-人气大使-拾光赋6个月前2

总是会在人生的不同阶段反复爱上大佬的文章~
每日一言
Hard-working is actually a cool thing.
努力学习其实是一件很酷的事