Spring源码之@Lazy和预实例化

https://www.cnblogs.com/yanze/p/10243348.html

懒加载优缺点

优点：懒加载，对象使用的时候才去创建；启动速度快，节省资源

缺点：不利于提前发现错误；初次请求getBean时慢

三种情况

1. 只有一个@Lazy注解的类
2. 一个Singleton类，依赖@Lazy的类
3. 两个@Lazy的类互相依赖

只有一个@Lazy注解的类分析

@Lazy注解的类在容器初始化时，不执行getBean

singleton 的bean初始化是通过调用AbstractApplicationContext的finishBeanFactoryInitialization方法完成。

当用@Lazy注解时，执行到DefaultListableBeanFactory的preInstantiateSingletons方法时，不满足条件，故在容器初始化时，不会进行预实例化。不会调用后面getBean方法。

if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
    ......
}

调用链：

SpringApplication#run() --> SpringApplication#refreshContext() --> SpringApplication#refresh() --> ServletWebServerApplicationContext#refresh() --> AbstractApplicationContext#refresh() --> AbstractApplicationContext#finishBeanFactoryInitialization() --> DefaultListableBeanFactory#preInstantiateSingletons()

preInstantiateSingletons源码：

@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("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.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		//!bd.isLazyInit()为false
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					FactoryBean<?> factory = (FactoryBean<?>) bean;
					boolean isEagerInit;
					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 {
				getBean(beanName);
			}
		}
	}

	// Trigger post-initialization callback for all applicable beans...
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
					.tag("beanName", beanName);
			SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					smartSingleton.afterSingletonsInstantiated();
					return null;
				}, getAccessControlContext());
			}
			else {
				smartSingleton.afterSingletonsInstantiated();
			}
			smartInitialize.end();
		}
	}
}

第一次对@Lazy修饰的类调用getBean方法

一种写法：

public String getLazyBean() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(LazyConfig.class);
    Object lazyConfig = context.getBean("lazyConfig");
    return lazyConfig.toString();
}

@Component
@Lazy
public class LazyConfig {
    ......
}

AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(LazyConfig.class);会调用AnnotationConfigApplicationContext初始化方法，进行refresh()

public AnnotationConfigApplicationContext(String... basePackages) {
	this();
	scan(basePackages);
	refresh();
}

而@Lazy注解的类真正初始化则在context.getBean("lazyConfig");过程，调用到AbstractApplicationContext类getBean方法

@Override
public Object getBean(String name) throws BeansException {
	assertBeanFactoryActive();
	return getBeanFactory().getBean(name);
}

然后调用到AbstractBeanFactory#doGetBean，后面和预实例化中过程一样，最后调用到AbstractAutowireCapableBeanFactory#doCreateBean();

	protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (instanceWrapper == null) {
			// 实例化对象
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();

        ......

		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
		    //属性注入
			populateBean(beanName, mbd, instanceWrapper);
			//初始化对象
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}

        ......

		return exposedObject;
	}

调用链：

AbstractApplicationContext#getBean() --> AbstractBeanFactory#getBean() --> AbstractBeanFactory#doGetBean() --> AbstractAutowireCapableBeanFactory#createBean()

--> AbstractAutowireCapableBeanFactory#doCreateBean() --> AbstractAutowireCapableBeanFactory#createBeanInstance()、populateBean()、initializeBean()

一个Singleton类，依赖@Lazy的类

一个例子

@Service
public class LazyServiceImpl implements LazyService {

    @Autowired
    private LazyConfig lazyConfig;

    @Override
    public void lazyDependent() {
        ......
    }
}

@Component
@Lazy
public class LazyConfig {
    ......
}

分析

在容器初始化时，preInstantiateSingletons会对上面的LazyServiceImpl进行getBean的处理，执行到AbstractAutowireCapableBeanFactory类populateBean方法进行属性注入时，通过如下调用链对上面的LazyConfig类进行getBean处理

AbstractAutowireCapableBeanFactory#populateBean() --> AutowiredAnnotationBeanPostProcessor#postProcessProperties() --> InjectionMetadata#inject() --> AutowiredAnnotationBeanPostProcessor#inject() --> DefaultListableBeanFactory#resolveDependency() --> DefaultListableBeanFactory#doResolveDependency() --> DependencyDescriptor#resolveCandidate() --> AbstractBeanFactory#getBean() --> AbstractBeanFactory#doGetBean()

从而一个Singleton类，依赖@Lazy的类，这个被依赖的@Lazy注释的类，也会被初始化

两个@Lazy的类互相依赖

容器初始化时都不调用getBean进行初始化，在其中一个getBean时，后面和singleton的循环依赖一样处理，详见前文。