SpringBoot：Bean生命周期自定义初始化和销毁

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前言

上篇文章详细讲诉了Bean的生命周期和作用域，在生命周期中提到了如何自定义初始化Bean，可能很多人不知道如何自定义初始化，这里详细补充讲解一下：使用@Bean注解指定初始化和销毁方法、实现InitializingBean接口和DisposableBean接口自定义初始化和销毁、@PostConstruct(初始化逻辑)和@PreDestroy(销毁逻辑)注解、使用BeanPostProcessor接口。

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一、@Bean注解指定初始化和销毁方法

• 创建BeanTest类，自定义初始化方法和销毁方法。
• 在@Bean注解的参数中指定BeanTest自定义的初始化和销毁方法：
• 销毁方法只有在IOC容器关闭的时候才调用。

代码如下：

/**
 * @Version: 1.0.0
 * @Author: Dragon_王
 * @ClassName: dog
 * @Description: TODO描述
 * @Date: 2024/1/21 22:55
 */
public class BeanTest {
    public BeanTest(){
        System.out.println("BeanTest被创建");
    }

    public void init(){
        System.out.println("BeanTest被初始化");
    }

    public void destory(){
        System.out.println("BeanTest被销毁");
    }
}
/**
 * @Version: 1.0.0
 * @Author: Dragon_王
 * @ClassName: MyConfig
 * @Description: TODO描述
 * @Date: 2024/1/21 22:59
 */
@Configuration
@ComponentScan(("com.dragon.restart1"))
public class MyConfig {
    @Bean(initMethod = "init",destroyMethod = "destory")
    public BeanTest beanTest(){
        return new BeanTest();
    }
}
//测试代码
AnnotationConfigApplicationContext ct = new AnnotationConfigApplicationContext(MyConfig.class);
System.out.println("IoC容器创建完成");

• 可以看到调用的是自定义的方法，这里解释一下，测试时，运行完代码块程序就结束了，所哟IoC容器就被关闭，所以调用了IoC销毁方法。同时可以看到初始化方法在对象创建完成后调用。
• 当组件的作用域为单例时在容器启动时即创建对象，而当作用域为原型（PROTOTYPE）时在每次获取对象的时候才创建对象。并且当作用域为原型（PROTOTYPE）时，IOC容器只负责创建Bean但不会管理Bean，所以IOC容器不会调用销毁方法。

二、实现InitializingBean接口和DisposableBean接口

看一下两接口的方法：

public interface InitializingBean {

	/**
	 * Invoked by the containing {@code BeanFactory} after it has set all bean properties
	 * and satisfied {@link BeanFactoryAware}, {@code ApplicationContextAware} etc.
	 * 
This method allows the bean instance to perform validation of its overall
	 * configuration and final initialization when all bean properties have been set.
	 * @throws Exception in the event of misconfiguration (such as failure to set an
	 * essential property) or if initialization fails for any other reason
	 * Bean都装配完成后执行初始化
	 */
	void afterPropertiesSet() throws Exception;
}
====================================================================
public interface DisposableBean {

	/**
	 * Invoked by the containing {@code BeanFactory} on destruction of a bean.
	 * @throws Exception in case of shutdown errors. Exceptions will get logged
	 * but not rethrown to allow other beans to release their resources as well.
	 */
	void destroy() throws Exception;

}

代码如下：

/**
 * @Version: 1.0.0
 * @Author: Dragon_王
 * @ClassName: BeanTest1
 * @Description: TODO描述
 * @Date: 2024/1/21 23:25
 */
public class BeanTest1 implements InitializingBean, DisposableBean {
    @Override
    public void destroy() throws Exception {
        System.out.println("BeanTest1销毁");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("BeanTest1初始化");
    }

    public BeanTest1() {
        System.out.println("BeanTest1被创建");
    }
}
=========================

@Configuration
@ComponentScan(("com.dragon.restart1"))
public class MyConfig {
 @Bean
    public BeanTest1 beanTest1(){
        return new BeanTest1();
    }
}

三、@PostConstruct(初始化逻辑)和@PreDestroy(销毁逻辑)注解

• 被@PostConstruct修饰的方法会在服务器加载Servlet的时候运行，并且只会被服务器调用一次，类似于Serclet的inti()方法。
• 被@PostConstruct修饰的方法会在构造函数之后，init()方法之前运行。
• 被@PreDestroy修饰的方法会在服务器卸载Servlet的时候运行，并且只会被服务器调用一次，类似于Servlet的destroy()方法。被@PreDestroy修饰的方法会在destroy()方法之后运行，在Servlet被彻底卸载之前。

代码如下：

/**
 * @Version: 1.0.0
 * @Author: Dragon_王
 * @ClassName: BeanTest2
 * @Description: TODO描述
 * @Date: 2024/1/21 23:32
 */
public class BeanTest2 {
    public BeanTest2(){
        System.out.println("BeanTest2被创建");
    }

    @PostConstruct
    public void init(){
        System.out.println("BeanTest2被初始化");
    }

    @PreDestroy
    public void destory(){
        System.out.println("BeanTest2被销毁");
    }
}
========================
//
@Configuration
@ComponentScan(("com.dragon.restart1"))
public class MyConfig {
 @Bean
    public BeanTest2 beanTest2(){
        return new BeanTest2();
    }
}

四、BeanPostProcessor接口

BeanPostProcessor又叫Bean的后置处理器，是Spring框架中IOC容器提供的一个扩展接口，在Bean初始化的前后进行一些处理工作。

BeanPostProcessor的源码如下：

public interface BeanPostProcessor {

	/**
	 * Apply this BeanPostProcessor to the given new bean instance before any bean
	 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
	 * or a custom init-method). The bean will already be populated with property values.
	 * The returned bean instance may be a wrapper around the original.
	 * 
The default implementation returns the given {@code bean} as-is.
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return the bean instance to use, either the original or a wrapped one;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 */
	@Nullable
	  //bean初始化方法调用前被调用
	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

	/**
	 * Apply this BeanPostProcessor to the given new bean instance after any bean
	 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
	 * or a custom init-method). The bean will already be populated with property values.
	 * The returned bean instance may be a wrapper around the original.
	 * 
In case of a FactoryBean, this callback will be invoked for both the FactoryBean
	 * instance and the objects created by the FactoryBean (as of Spring 2.0). The
	 * post-processor can decide whether to apply to either the FactoryBean or created
	 * objects or both through corresponding {@code bean instanceof FactoryBean} checks.
	 * 
This callback will also be invoked after a short-circuiting triggered by a
	 * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
	 * in contrast to all other BeanPostProcessor callbacks.
	 * 
The default implementation returns the given {@code bean} as-is.
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return the bean instance to use, either the original or a wrapped one;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 * @see org.springframework.beans.factory.FactoryBean
	 */
	@Nullable
	//bean初始化方法调用后被调用
	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

代码如下：

/**
 * @Version: 1.0.0
 * @Author: Dragon_王
 * @ClassName: MyBeanPostProcess
 * @Description: TODO描述
 * @Date: 2024/1/21 23:40
 */
@Component
public class MyBeanPostProcess implements BeanPostProcessor {
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("postProcessBeforeInitialization..."+beanName+"=>"+bean);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("postProcessAfterInitialization..."+beanName+"=>"+bean);
        return bean;
    }
}
============================
@Configuration
@ComponentScan(("com.dragon.restart1"))
public class MyConfig {
    @Bean
    public BeanTest1 beanTest1(){
        return new BeanTest1();
    }

    @Bean
    public BeanTest2 beanTest2(){
        return new BeanTest2();
    }
}

运行结果如下：

BeanTest1被创建
postProcessBeforeInitialization...beanTest1=>com.dragon.restart1.BeanTest1@111d5c97
BeanTest1初始化
postProcessAfterInitialization...beanTest1=>com.dragon.restart1.BeanTest1@111d5c97
BeanTest2被创建
postProcessBeforeInitialization...beanTest2=>com.dragon.restart1.BeanTest2@29c17c3d
BeanTest2被初始化
postProcessAfterInitialization...beanTest2=>com.dragon.restart1.BeanTest2@29c17c3d
IoC容器创建完成
BeanTest2被销毁
BeanTest1销毁

通过上述运行结果可以发现使用BeanPostProcessor的运行顺序为：
IOC容器实例化Bean—->调用BeanPostProcessor的postProcessBeforeInitialization方法—->调用bean实例的初始化方法—->调用BeanPostProcessor的postProcessAfterInitialization方法。

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总结

以上就是Bean生命周期自定义初始化和销毁的讲解。