I don't configure initialize() method when configuring a custom thread pool, why does the program still run normally?

Deeply understand initialize() method of custom thread pools

When building a custom thread pool, you may notice initialize() method. Many developers can also run normally without explicitly calling this method, which raises questions: What is the function of initialize() method?

Question: I omitted initialize() method when configuring a custom thread pool, and the program is still running normally. Why is this?

Answer: The key lies in the automatic call of the Spring framework.

Let's first look at an example that does not use Spring:

 public class SomeTest {
    public static void main(String[] args) {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.submit(() -> System.out.println("!"));
    }
}

Running this code will throw an exception: "ThreadPoolTaskExecutor not initialized". This is because ThreadPoolTaskExecutor is not initialized correctly.

Now, let's use Spring Boot to manage thread pools:

 @SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DemoApplication.class, args);
        ThreadPoolTaskExecutor myExecutor = context.getBean("myExecutor", ThreadPoolTaskExecutor.class);
        myExecutor.submit(() -> System.out.println("Hello!"));
    }

    @Bean
    public ThreadPoolTaskExecutor myExecutor() {
        return new ThreadPoolTaskExecutor();
    }
}

This code works normally. This is because the Spring framework automatically calls initialize() method.

How does Spring do this? The answer lies in the ExecutorConfigurationSupport class:

 @Override
public void afterPropertiesSet() {
    initialize();
}

afterPropertiesSet() method is part of the InitializingBean interface. During the life cycle of Spring Bean, after the bean is initialized, Spring will automatically call afterPropertiesSet() , and this method internally calls initialize() method to complete the initialization of the thread pool.

Therefore, when you use thread pools in Spring containers, Spring will automatically complete the initialization process even if you do not explicitly call initialize() , so the program can run normally. This does not mean that initialize() method is optional. It is still responsible for the key initialization steps of the thread pool. It is just that Spring helps you complete this call.

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