Interviewer: young man, let's talk about the life cycle of Java object creation and spring bean

Bright future 2021-01-24 15:17:03
interviewer young man let talk


Understanding objects and Bean The relationship between

java It's an object-oriented language , In short , Everything is the object .Bean Nature is also the object , It's just that it's hosted for Bean What the factory manages .

java How objects are created

When writing code , We usually use the following statement to create an object :

 A a=new A();

So in the process of creating objects , What happened . In fact, the above simple sentence , A lot of things happen in the program .
First , An object needs to be stored in memory . So there's a process of allocating memory . After allocating memory ,jvm You start creating objects , And assign it to a Variable . And then initialize it A Some properties in , And implement A Construction method of . During the initialization , Will execute first static Code block , Then execute the construction method . besides , If there is a parent class , Will take precedence over the execution of the parent class . It's roughly as follows ( Figure 1 ) Shown .

How to verify the process of object initialization ? Verify with the following code . This code is very simple , Initialization of static variables , There's a way to structure , Have inherited .

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class InitTest {
private static final Logger logger = LoggerFactory.getLogger(InitTest.class);
// 1. Static variable initialization
static String staticWord = "hello";
// 2. Static code block
static {
logger.info("staticWord = "+staticWord);
}
public InitTest(){
logger.info("father construct method invoke...");
}
public static void main(String[] args) {
new Son();
}
static class Son extends InitTest{
static {
logger.info("son staticWord init in static");
}
public Son(){
logger.info("son construct method invoke...");
}
}
}

Run the printed log as follows , By analyzing logs , We can figure out , Static code blocks precede construction methods , The order in which the parent class precedes the child class .

00:55:18.869 [main] INFO com.fc.study.InitTest - staticWord = hello
00:55:18.877 [main] INFO com.fc.study.InitTest - son staticWord init in static
00:55:18.877 [main] INFO com.fc.study.InitTest - father construct method invoke...
00:55:18.877 [main] INFO com.fc.study.InitTest - son construct method invoke...

Spring Bean Life cycle of

well , With the initialization order of objects , We can continue to analyze bean The life cycle of . We can first recall how we usually define a bean Of .

@Component
public class TestBean{
}
@Bean
public Object myObject(){
}

The above two are commonly used : The first is through Component Annotation annotation class ; The second way is to do it methodically @Bean Annotations . We all know , Annotated methods or classes , You'll be killed spring scanning , And finally generate a bean. This article does not discuss in detail bean The process of scanning , Only analyze bean Some interfaces in the initialization process .
that ,Spring establish Bean It can be divided into two steps , The first step is by Springboot Scan and get BeanDefinition; Second parts , Is initialization Bean.spring stay bean The initialization process of provides us with many interfaces , We can use them in bean Do something in the process of generating . All of these interfaces are callbacks , The following is the introduction of some interfaces and the timing of callback .

Interface explain The callback time
BeanNameAware If your bean Implements the interface of setName Method , Then we can get bean name It happened in bean Early in the life cycle , Prior to the construction method
ApplicationContextAware If one bean Implements the interface of setApplicationContext Method , Then we can get ApplicationContext Called at the beginning of the life cycle , stay BeanNameAware And construction methods
InitializingBean The method of this interface is afterPropertiesSet stay bean The factory is set up bean After all the attributes of , Will call back this method . The callback time is after the construction method
BeanPostProcessor This interface has postProcessBeforeInitialization、postProcessAfterInitialization Two methods , They correspond to each other Bean Two callbacks in the life cycle These two methods are also after the construction method , But they are InitializingBean Before and after

If you add the above interface to , be bean The life cycle is roughly as follows ( Figure 2 ):

Again , Let's use the code to verify the callback order . For testing Bean The code is as follows , This test bean No other parent class is inherited , For verification only springboot The interface of is bean The call timing of the life cycle :

package com.fc.study.beanLife;
import com.fc.study.InitTest;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;
@Component
public class TestBean implements BeanNameAware, InitializingBean, ApplicationContextAware {
private static final Logger logger = LoggerFactory.getLogger(InitTest.class);
private String beanName;
static String staticWord;
static {
logger.info("father staticWord init in static");
staticWord="hi";
}
public TestBean(){
logger.info("testBean construct method invoke...");
}
public void setBeanName(String name) {
logger.info("setBeanName");
this.beanName = name;
}
public void afterPropertiesSet() throws Exception {
logger.info("afterProperties Set");
}
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
logger.info("applictionContextAware");
}
}

meanwhile , I defined a BeanPostProcessor as follows :

package com.fc.study.beanLife;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;
@Component
@Order(Ordered.HIGHEST_PRECEDENCE)
public class DefaultBeanPostProcessor implements BeanPostProcessor, ApplicationContextAware {
private static Logger logger = LoggerFactory.getLogger(DefaultBeanPostProcessor.class);
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if(beanName.equals("testBean")) {
logger.info(beanName + " postProcessBeforeInitialization perform ");
}
return bean;
}
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if(beanName.equals("testBean")) {
logger.info(beanName + " postProcessAfterInitialization perform ");
}
return bean;
}
private ApplicationContext applicationContext;
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
}

Next is the startup class :

package com.fc.study.beanLife;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
@ComponentScan("com.fc.study")
public class SimpleSpringBoot {
private static final Logger logger = LoggerFactory.getLogger(SimpleSpringBoot.class);
public static void main(String[] args) {
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(SimpleSpringBoot.class);
logger.info("before get Bean");
context.getBean(TestBean.class);
logger.info("after get bean");
}
}

Run startup class , Print out the log as follows :

2021-01-23 02:18:09,764 INFO InitTest:29 - father staticWord init in static
2021-01-23 02:18:09,768 INFO InitTest:34 - testBean construct method invoke...
2021-01-23 02:18:09,768 INFO InitTest:38 - setBeanName
2021-01-23 02:18:09,768 INFO InitTest:48 - applictionContextAware
2021-01-23 02:18:09,768 INFO DefaultBeanPostProcessor:27 - testBean postProcessBeforeInitialization perform
2021-01-23 02:18:09,768 INFO InitTest:44 - afterProperties Set
2021-01-23 02:18:09,768 INFO DefaultBeanPostProcessor:34 - testBean postProcessAfterInitialization perform
2021-01-23 02:18:11,449 INFO SimpleSpringBoot:24 - before get Bean
2021-01-23 02:18:11,449 INFO SimpleSpringBoot:26 - after get bean

Look at this log , Confirm the conclusion of Figure 2 about the call time of each interface .

summary

Object initialization , It's about creating objects , And the process of initializing its properties . The first is to load the class file , Second, the memory required by the object . Then the static code block is called , Finally, the construction method .
Spring Bean The initialization , In addition to the steps of creating objects , There are also some life cycle interfaces in it . First, after the class is loaded , You'll get BeanDefinition, And then initialize it with this definition , Instead of generating objects directly from loaded class objects . Between static code blocks and construction methods ,Spring There are several Aware Interface , As shown in the table BeanNameAware and ApplicationContextAware. At the end of the constructor call , also springboot to bean set After all the attributes are added , Would call Initializing Interface and BeanPostProcessor.
above , That's what I understand spring bean Life cycle , It is spring In the process of helping us initialize objects and manage objects, we did some extra things .

版权声明
本文为[Bright future]所创,转载请带上原文链接,感谢
https://javamana.com/2021/01/20210124151557757J.html

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