Spring5.0源码学习系列之浅谈懒加载机制原理
前言介绍
附录：Spring源码学习专栏
在上一章的学习中，我们对Bean的创建有了一个粗略的了解，接着本文挑一个比较重要的知识点Bean的懒加载进行学习
1、什么是懒加载？
懒加载（Lazy-initialized beans）：懒加载模式是bean在第一次调用时候被实例，而不是spring容器启动时候，默认是不开启的，（ A lazy-initialized bean tells the IoC container to create a bean instance when it is first requested, rather than at startup.），通过改配置lazy-init="true"
2、实验环境准备
实验环境：
- SpringFramework版本
- Springframework5.0.x
- 开发环境
- JAR管理：gradle 4.9/ Maven3.+
- 开发IDE：IntelliJ IDEA 2018.2.5
- JDK：jdk1.8.0_31
- Git Server：Git fro window 2.8.3
- Git Client：SmartGit18.1.5（可选）
3、lazy-init使用
import org.springframework.beans.factory.InitializingBean;
/**
* <pre>
* SpringBean
* </pre>
*
* <pre>
* @author mazq
* 修改记录
* 修改后版本: 修改人： 修改日期: 2020/11/05 10:50 修改内容:
* </pre>
*/
public class SpringBean implements InitializingBean {
public SpringBean(){
System.out.println("SpringBean构造函数");
}
@Override
public void afterPropertiesSet() throws Exception {
System.out.println("SpringBean afterPropertiesSet");
}
}
xml配置方式，在applicationContext.xml加载配置
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="springBean" lazy-init="true" class="com.example.bean.SpringBean" ></bean>
</beans>
注解方式，使用@Lazy即可
import com.example.bean.SpringBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.example.bean.A;
import org.springframework.context.annotation.Lazy;
/**
* <pre>
* AppConfiguration
* </pre>
*
* <pre>
* @author mazq
* 修改记录
* 修改后版本: 修改人： 修改日期: 2020/11/05 10:26 修改内容:
* </pre>
*/
@Configuration
public class AppConfiguration {
@Bean
@Lazy // 开启懒加载
// @Lazy(value = false) 默认
public SpringBean springBean() {
return new SpringBean();
}
}
ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
// lazy-init开启的bean，context.getBean调用时候才会被实例
SpringBean springBean = context.getBean(SpringBean.class);
System.out.println(springBean);
4、lazy加载机制原理
为什么设置为lazy-init之后，Spring IoC容器启动时候bean不会被实例？可以基于上一章内容找到答案
{@link org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization}
/**
* Finish the initialization of this context's bean factory,
* initializing all remaining singleton beans.
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
// 初始化ConversionService，这个bean用于将前端传过来的参数和后端的 controller 方法上的参数进行绑定
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
// 先初始化LoadTimeWeaverAware 类型的Bean
// AspectJ 的内容，IoC的源码学习，先跳过
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
// 冻结配置，不让bean 定义解析、加载、注册
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
// 实例所有非懒加载的单例Bean
beanFactory.preInstantiateSingletons();
}
找到关键代码beanFactory.preInstantiateSingletons();
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
// 获取beanName列表，this.beanDefinitionNames 保存了所有的 beanNames
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
// 触发所有非懒加载的单例bean初始化操作（lazy-init=false）
for (String beanName : beanNames) {
// 合并rootBean中的配置， <bean id="a" class="a" parent="p" />
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非抽象（abstract = false）、非懒加载（lazy-init=false）的单例Bean（scope=singleton）
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 处理FactoryBean，注意对比BeanFactory和FactoryBean
if (isFactoryBean(beanName)) {
// factoryBean调用在beanName加载前缀符号‘&’
// 为什么要加‘&’，应该是做下标记，不过在bean创建过程要进行转换，详情请看下文
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
// FactoryBean是SmartFactoryBean 的基类
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
// 普通的Bean，调这个方法进行实例，往下跟
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
// SmartInitializingSingleton 的基类在这里回调
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
代码里有isLazyInit()的校验，所以设置lazy-init=true的bean都不会随着ioc容器启动而被实例加载
学习归纳
综上所述：
- 对于被修饰为lazy-init=false（默认情况）的bean Spring 容器初始化阶段会被依赖注入，并且实例到单例池里
- 对于懒加载的bean，context.getBean的时候会从缓存里获取，因为容器初始化阶段 Bean 已经初始化完成并缓存了起来，但是还没被实例，第一次调用时候就会被实例