使用场景
我们在这样的场景下使用,当ApplicationContext 加载完所有的Bean之后,我们来做一些我们想要的操作。
下面是使用方法:
public class MyRegister implements SmartInitializingSingleton {
Logger logger = LoggerFactory.getLogger(MyRegister.class);
private final DefaultListableBeanFactory beanFactory;
public MyRegister(DefaultListableBeanFactory beanFactory) {
this.beanFactory = beanFactory;
}
@Override
public void afterSingletonsInstantiated() {
Map<String, Tiger> redisCacheProviderMap = beanFactory.getBeansOfType(Tiger.class);
logger.info("tiger pre init");
}
}
接口说明
/**
*<p> Callback interface triggered at the end of the singleton pre-instantiation phase
* during {@link BeanFactory} bootstrap.
*
* <p>This callback variant is somewhat similar to
* {@link org.springframework.context.event.ContextRefreshedEvent} but doesn't
* require an implementation of {@link org.springframework.context.ApplicationListener},
* with no need to filter context references across a context hierarchy etc.
* It also implies a more minimal dependency on just the {@code beans} package
* and is being honored by standalone {@link ListableBeanFactory} implementations,
* not just in an {@link org.springframework.context.ApplicationContext} environment.
*/
public interface SmartInitializingSingleton {}
主要摘取上面一段说明来解答疑惑:
- 这个接口的功能类似ContextRefreshedEvent(由Configua>bleApplicationContext.refresh()触发),ContextRefreshedEvent的具体阶段来看下面具体文档。
Published when the ApplicationContext is initialized or refreshed (for example, by using the refresh() method on the ConfigurableApplicationContext interface). Here, “initialized” means that all beans are loaded, post-processor beans are detected and activated, singletons are pre-instantiated, and the ApplicationContext object is ready for use. As long as the context has not been closed, a refresh can be triggered multiple times, provided that the chosen ApplicationContext actually supports such “hot” refreshes. For example, XmlWebApplicationContext supports hot refreshes, but GenericApplicationContext does not.
对比接收事件的优点:就是不需要实现ApplicationListener,引用也更加简单(不需要获取ApplicationContext),对应性能也会更好一点。
- 文档里说到 pre-instantiation phase 这个比较疑惑,实际上这里的pre-instantiation对应SmartInitializingSingleton的执行过程·
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext, DisposableBean {
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 省略其他步骤.........
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
}
finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory)
{
//省略其他步骤.......
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
所以结论就是SmartInitializingSingleton的方法就是在finishRefresh() 触发ContextRefreshedEvent事件前执行的。所以上面的pre-instantiation phase说的就是beanFactory.preInstantiateSingletons()这方法执行。
实现
beanFactory.preInstantiateSingletons();
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
smartSingleton.afterSingletonsInstantiated();
return null;
}
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
DefaultListableBeanFactory 这个类的实例是怎么构造注入的?构造函数注入。
另外的方法:Bean获取ApplicationContext是需要实现Aware接口的,类似这样:
//spring @EventListner 处理器
public class EventListenerMethodProcessor implements SmartInitializingSingleton, ApplicationContextAware {
private ConfigurableApplicationContext applicationContext;
@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
Assert.isTrue(applicationContext instanceof ConfigurableApplicationContext,
"ApplicationContext does not implement ConfigurableApplicationContext");
this.applicationContext = (ConfigurableApplicationContext) applicationContext;
}