Event publishing with ApplicationEventPublisher in Spring Boot - Performance & Optimization

Performance: Event publishing with ApplicationEventPublisher

MEDIUM IMPACT

This affects the responsiveness and throughput of the application by how events are published and handled asynchronously or synchronously.

Publishing events to notify other components without blocking main processing

Spring Boot

@Async
public void handleEvent(CustomEvent event) { /* handle event asynchronously */ }

applicationEventPublisher.publishEvent(new CustomEvent(this, data));

Listeners handle events asynchronously on separate threads, freeing main thread immediately.

📈 Performance GainImproves responsiveness, reduces blocking, better throughput under concurrent load

Publishing events to notify other components without blocking main processing

Spring Boot

applicationEventPublisher.publishEvent(new CustomEvent(this, data)); // synchronous event handling

Events are handled synchronously on the same thread, blocking the main flow until all listeners complete.

📉 Performance CostBlocks main thread, increasing response time and reducing throughput under load

Performance Comparison

Pattern	Thread Blocking	CPU Usage	Response Delay	Verdict
Synchronous Event Publishing	Blocks main thread until listeners finish	Lower CPU concurrency	Increases response time	[X] Bad
Asynchronous Event Publishing with @Async	No blocking of main thread	Higher CPU concurrency due to threads	Reduces response delay	[OK] Good

Rendering Pipeline

Event publishing triggers listener execution either synchronously or asynchronously, affecting thread usage and response timing.

→Event Dispatch

→Thread Scheduling

→Application Logic Execution

⚠️ BottleneckSynchronous listener execution blocks event dispatch thread, delaying subsequent processing.

Optimization Tips

1Avoid synchronous event listeners that block the main thread.

2Use @Async or custom executors to handle events asynchronously.

3Monitor thread pool usage to prevent thread exhaustion.

Performance Quiz - 3 Questions

What is the main performance drawback of synchronous event publishing with ApplicationEventPublisher?

A It causes layout shifts in the UI

B It increases bundle size significantly

C It blocks the main thread until all listeners complete

D It reduces CPU usage drastically

DevTools: Spring Boot Actuator and Java Flight Recorder

How to check: Enable actuator endpoints and use Java Flight Recorder to monitor thread usage and event listener execution times.

What to look for: Look for main thread blocking during event publishing and thread pool utilization for asynchronous listeners.

Practice

1. What is the primary role of ApplicationEventPublisher in a Spring Boot application?

easy

A. To manage HTTP requests and responses

B. To listen and handle events automatically

C. To configure database connections

D. To send or publish events within the application

Solution

1. Step 1: Understand the purpose of ApplicationEventPublisher

   This interface is designed to publish events to listeners within the Spring application context.

2. Step 2: Differentiate from listeners and other components

   Listeners handle events, but ApplicationEventPublisher only sends or publishes them.

3. Final Answer:

   To send or publish events within the application -> Option D

4. Quick Check:

   Event sending = ApplicationEventPublisher [OK]

Hint: Publisher sends events; listeners receive them [OK]

Common Mistakes:

• Confusing publisher with listener
• Thinking it manages HTTP or database
• Assuming it handles event processing

2. Which of the following is the correct way to inject ApplicationEventPublisher in a Spring Boot component?

easy

A. @Autowired private ApplicationEventPublisher publisher;

B. private ApplicationEventPublisher publisher = new ApplicationEventPublisher();

C. @Inject ApplicationEventPublisher publisher = null;

D. ApplicationEventPublisher publisher = ApplicationEventPublisher.getInstance();

Solution

1. Step 1: Recall Spring dependency injection syntax

   Spring uses @Autowired to inject beans automatically into components.

2. Step 2: Check each option for correctness

   @Autowired private ApplicationEventPublisher publisher; uses @Autowired correctly. private ApplicationEventPublisher publisher = new ApplicationEventPublisher(); tries to instantiate interface (not allowed). @Inject ApplicationEventPublisher publisher = null; mixes @Inject with assignment (not typical in Spring). ApplicationEventPublisher publisher = ApplicationEventPublisher.getInstance(); uses a non-existent getInstance method.

3. Final Answer:

   @Autowired private ApplicationEventPublisher publisher; -> Option A

4. Quick Check:

   @Autowired injects beans properly [OK]

Hint: Use @Autowired for Spring bean injection [OK]

Common Mistakes:

• Trying to instantiate ApplicationEventPublisher directly
• Mixing injection annotations incorrectly
• Using static getInstance() which doesn't exist

3. Given the following code snippet, what will be printed when publishEvent() is called?

public class MyEvent extends ApplicationEvent {
    public MyEvent(Object source) { super(source); }
}

@Component
public class MyPublisher {
    @Autowired
    private ApplicationEventPublisher publisher;

    public void publishEvent() {
        publisher.publishEvent(new MyEvent(this));
    }
}

@Component
public class MyListener {
    @EventListener
    public void handleMyEvent(MyEvent event) {
        System.out.println("Event received");
    }
}

medium

A. No output

B. Compilation error

C. Event received

D. Runtime exception

Solution

1. Step 1: Understand event publishing and listening

   The publisher sends a MyEvent instance. The listener listens for MyEvent and prints "Event received".

2. Step 2: Confirm no errors in code

   Code uses correct annotations and event class extending ApplicationEvent, so no errors or exceptions.

3. Final Answer:

   Event received -> Option C

4. Quick Check:

   Listener prints message on event [OK]

Hint: Listener prints message when event is published [OK]

Common Mistakes:

• Forgetting @EventListener annotation
• Not extending ApplicationEvent in event class
• Expecting output without publishing event

4. Identify the error in the following Spring Boot event publishing code:

@Component
public class EventSender {
    private ApplicationEventPublisher publisher;

    public void send() {
        publisher.publishEvent(new CustomEvent(this));
    }
}

public class CustomEvent extends ApplicationEvent {
    public CustomEvent(Object source) { super(source); }
}

medium

A. send method should be static

B. ApplicationEventPublisher is not injected, so publisher is null

C. publishEvent method does not exist

D. CustomEvent does not extend ApplicationEvent

Solution

1. Step 1: Check ApplicationEventPublisher usage

   The publisher field is declared but never injected or assigned, so it remains null.

2. Step 2: Consequence of null publisher

   Calling publishEvent on null causes NullPointerException at runtime.

3. Final Answer:

   ApplicationEventPublisher is not injected, so publisher is null -> Option B

4. Quick Check:

   Missing injection causes null publisher [OK]

Hint: Always inject ApplicationEventPublisher before use [OK]

Common Mistakes:

• Forgetting @Autowired on publisher
• Assuming interface can be instantiated
• Thinking method must be static

5. You want to publish a custom event UserCreatedEvent and ensure multiple listeners react differently. Which approach correctly implements this in Spring Boot?

hard

A. Create UserCreatedEvent extending ApplicationEvent, inject ApplicationEventPublisher, publish event, and annotate multiple methods with @EventListener for UserCreatedEvent

B. Create UserCreatedEvent as a simple POJO, call listeners manually without publishing events

C. Use ApplicationEventPublisher to publish String messages instead of event objects

D. Create UserCreatedEvent but do not extend ApplicationEvent; rely on @EventListener to catch any object

Solution

1. Step 1: Define event class properly

   Custom events should extend ApplicationEvent for Spring to recognize them.

2. Step 2: Publish event and create multiple listeners

   Inject ApplicationEventPublisher to publish event. Use @EventListener on multiple methods to react differently.

3. Step 3: Evaluate other options

   Create UserCreatedEvent as a simple POJO, call listeners manually without publishing events skips event system, losing decoupling. Use ApplicationEventPublisher to publish String messages instead of event objects misuses publisher with Strings. Create UserCreatedEvent but do not extend ApplicationEvent; rely on @EventListener to catch any object breaks event contract, listeners may not work.

4. Final Answer:

   Create UserCreatedEvent extending ApplicationEvent, inject ApplicationEventPublisher, publish event, and annotate multiple methods with @EventListener for UserCreatedEvent -> Option A

5. Quick Check:

   Proper event class + publisher + listeners = correct [OK]

Hint: Extend ApplicationEvent and use @EventListener for multiple reactions [OK]

Common Mistakes:

• Not extending ApplicationEvent for custom events
• Calling listeners manually instead of publishing
• Publishing wrong object types like String