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