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

只有一个@Lazy注解的类
一个Singleton类，依赖@Lazy的类
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falseif (!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 {@Autowiredprivate LazyConfig lazyConfig;@Overridepublic 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。