PostConstruct注解標(biāo)記類ApplicationContext未加載空指針

更新時(shí)間：2022年11月01日 08:50:45 作者：雨夜之寂

這篇文章主要為大家介紹了@PostConstruct注解標(biāo)記類ApplicationContext未加載空指針示例詳解，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進(jìn)步，早日升職加薪

序

今天Code Review的時(shí)候看到其他項(xiàng)目 static 方法需要使用 bean的實(shí)體方法，是從網(wǎng)上copy的大概是

public class SpringUtils implements ApplicationListener<ApplicationEvent> {
	private static ApplicationContext applicationContext;
    public static ApplicationContext getApplicationContext() {
        return applicationContext;
    }
	public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ContextRefreshedEvent) {
            ContextRefreshedEvent e = (ContextRefreshedEvent)event;
            if (e.getApplicationContext().getParent() == null) {
                applicationContext = e.getApplicationContext();
            }
        }
    }
	public static <T> T getBean(Class<T> clazz){
		return getApplicationContext().getBean(clazz);
	}
}

雖然現(xiàn)在代碼運(yùn)行沒(méi)有毛病，但是我們有公共類SpringUtils 實(shí)現(xiàn)了相同功能，其實(shí)不應(yīng)該重復(fù)在業(yè)務(wù)系統(tǒng)自己寫(xiě)。

但是這個(gè)時(shí)候人家可能會(huì)問(wèn) 我這么寫(xiě)和用公共類的效果不是一樣么？都一樣

區(qū)別

一方面是代碼規(guī)范，公共功能都有現(xiàn)成的，不需要自己開(kāi)發(fā)，節(jié)省錯(cuò)誤的概率和提升效率

開(kāi)發(fā)的時(shí)候有人會(huì)說(shuō) 我哪知道有哪些功能是現(xiàn)在有的，關(guān)于這個(gè) 我會(huì)提供一個(gè)搜索的網(wǎng)頁(yè)，方便進(jìn)行搜索，如果搜索不到就是沒(méi)有，你感覺(jué)是公共功能，可以提交讓別人使用。

你既然給人家推薦用公共類，那你肯定要說(shuō)清楚公共類的好處，才能讓人家信服。你不能說(shuō)效果都一樣，就是用我的吧。。。

講道理

你這種寫(xiě)法是可能出錯(cuò)的

定義一個(gè) Service

@Service
public class TestService{
}

定義一個(gè)初始化方法

@Component
public class TestInit{
    @PostConstruct
    public void init(){
        SpringUtils.getBean(TestService.class);
    }
}

報(bào)錯(cuò)信息

Caused by: java.lang.NullPointerException: null
   at com.example.demo.utils.SpringUtils.getBean(SpringUtils.java:25) ~[classes/:na]
   at com.example.demo.service.TestInit.init(TestInit.java:12) ~[classes/:na]
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_322]
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_322]
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_322]
   at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_322]
   at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(InitDestroyAnnotationBeanPostProcessor.java:363) ~[spring-beans-5.1.5.RELEASE.jar:5.1.5.RELEASE]
   at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeInitMethods(InitDestroyAnnotationBeanPostProcessor.java:307) ~[spring-beans-5.1.5.RELEASE.jar:5.1.5.RELEASE]
   at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization(InitDestroyAnnotationBeanPostProcessor.java:136) ~[spring-beans-5.1.5.RELEASE.jar:5.1.5.RELEASE]
   18 common frames omitted

原因

在spring服務(wù)啟動(dòng)過(guò)程中，spring會(huì)先去注冊(cè)所有的bean，在注冊(cè)過(guò)程中，如果發(fā)現(xiàn)該bean中包涵了被@PostConstruct注釋的函數(shù)，那么就會(huì)先去執(zhí)行這個(gè)函數(shù)，然后再繼續(xù)注冊(cè)其他未注冊(cè)的bean。

但是在springUtils中，無(wú)論是繼承ApplicationListener，還是繼承自ApplicationContextAware，都只有在bean初始化完成后，才會(huì)執(zhí)行注入applicationContext。

解決

可以直接拿著用

@Component
public class SpringUtils implements BeanFactoryPostProcessor, ApplicationContextAware {
    private static ConfigurableListableBeanFactory beanFactory;
    private static ApplicationContext applicationContext;
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        SpringUtils.beanFactory = beanFactory;
    }
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) {
        SpringUtils.applicationContext = applicationContext;
    }
    /**
     * 獲取{@link ApplicationContext}
     *
     * @return {@link ApplicationContext}
     */
    public ApplicationContext getApplicationContext() {
        return applicationContext;
    }
    public ListableBeanFactory getBeanFactory() {
        return null == beanFactory ? applicationContext : beanFactory;
    }
    public ConfigurableListableBeanFactory getConfigurableBeanFactory() throws UtilException {
        final ConfigurableListableBeanFactory factory;
        if (null != beanFactory) {
            factory = beanFactory;
        } else if (applicationContext instanceof ConfigurableApplicationContext) {
            factory = ((ConfigurableApplicationContext) applicationContext).getBeanFactory();
        } else {
            throw new UtilException("No ConfigurableListableBeanFactory from context!");
        }
        return factory;
    }
    @SuppressWarnings("unchecked")
    public <T> T getBean(String name) {
        return (T) getBeanFactory().getBean(name);
    }
    /**
     * 通過(guò)class獲取Bean
     *
     * @param <T>   Bean類型
     * @param clazz Bean類
     * @return Bean對(duì)象
     */
    public <T> T getBean(Class<T> clazz) {
        return getBeanFactory().getBean(clazz);
    }
    /**
     * 通過(guò)name,以及Clazz返回指定的Bean
     *
     * @param <T>   bean類型
     * @param name  Bean名稱
     * @param clazz bean類型
     * @return Bean對(duì)象
     */
    public <T> T getBean(String name, Class<T> clazz) {
        return getBeanFactory().getBean(name, clazz);
    }
    /**
     * 從spring容器中獲取相關(guān)降級(jí)的bean
     *
     * @param fallbackClass 降級(jí)的Class類對(duì)象
     * @param paramValues   參數(shù)值
     * @return 相關(guān)降級(jí)的bean
     */
    public Object getBean(Class<?> fallbackClass, Object[] paramValues) {
        return getBeanFactory().getBean(fallbackClass, paramValues);
    }
    /**
     * 通過(guò)類型參考返回帶泛型參數(shù)的Bean
     *
     * @param reference 類型參考，用于持有轉(zhuǎn)換后的泛型類型
     * @param <T>       Bean類型
     * @return 帶泛型參數(shù)的Bean
     * @since 5.4.0
     */
    @SuppressWarnings("unchecked")
    public <T> T getBean(TypeReference<T> reference) {
        final ParameterizedType parameterizedType = (ParameterizedType) reference.getType();
        final Class<T> rawType = (Class<T>) parameterizedType.getRawType();
        final Class<?>[] genericTypes = Arrays.stream(parameterizedType.getActualTypeArguments()).map(type -> (Class<?>) type).toArray(Class[]::new);
        final String[] beanNames = getBeanFactory().getBeanNamesForType(ResolvableType.forClassWithGenerics(rawType, genericTypes));
        return getBean(beanNames[0], rawType);
    }
    /**
     * 獲取指定類型對(duì)應(yīng)的所有Bean，包括子類
     *
     * @param <T>  Bean類型
     * @param type 類、接口，null表示獲取所有bean
     * @return 類型對(duì)應(yīng)的bean，key是bean注冊(cè)的name，value是Bean
     * @since 5.3.3
     */
    public <T> Map<String, T> getBeansOfType(Class<T> type) {
        return getBeanFactory().getBeansOfType(type);
    }
    /**
     * 獲取指定類型對(duì)應(yīng)的Bean名稱，包括子類
     *
     * @param type 類、接口，null表示獲取所有bean名稱
     * @return bean名稱
     * @since 5.3.3
     */
    public String[] getBeanNamesForType(Class<?> type) {
        return getBeanFactory().getBeanNamesForType(type);
    }
    /**
     * 獲取配置文件配置項(xiàng)的值
     *
     * @param key 配置項(xiàng)key
     * @return 屬性值
     * @since 5.3.3
     */
    public String getProperty(String key) {
        if (null == applicationContext) {
            return null;
        }
        return applicationContext.getEnvironment().getProperty(key);
    }
    /**
     * 獲取應(yīng)用程序名稱
     *
     * @return 應(yīng)用程序名稱
     * @since 5.7.12
     */
    public String getApplicationName() {
        return getProperty("spring.application.name");
    }
    /**
     * 獲取當(dāng)前的環(huán)境配置，無(wú)配置返回null
     *
     * @return 當(dāng)前的環(huán)境配置
     * @since 5.3.3
     */
    public static String[] getActiveProfiles() {
        if (null == applicationContext) {
            return null;
        }
        return applicationContext.getEnvironment().getActiveProfiles();
    }
    /**
     * 獲取當(dāng)前的環(huán)境配置，當(dāng)有多個(gè)環(huán)境配置時(shí)，只獲取第一個(gè)
     *
     * @return 當(dāng)前的環(huán)境配置
     * @since 5.3.3
     */
    public String getActiveProfile() {
        final String[] activeProfiles = getActiveProfiles();
        return ArrayUtil.isNotEmpty(activeProfiles) ? activeProfiles[0] : null;
    }
    /**
     * 動(dòng)態(tài)向Spring注冊(cè)Bean
     * <p>
     * 由{@link org.springframework.beans.factory.BeanFactory} 實(shí)現(xiàn)，通過(guò)工具開(kāi)放API
     * <p>
     * 更新: shadow 2021-07-29 17:20:44 增加自動(dòng)注入，修復(fù)注冊(cè)bean無(wú)法反向注入的問(wèn)題
     *
     * @param <T>      Bean類型
     * @param beanName 名稱
     * @param bean     bean
     * @author shadow
     * @since 5.4.2
     */
    public <T> void registerBean(String beanName, T bean) {
        final ConfigurableListableBeanFactory factory = getConfigurableBeanFactory();
        factory.autowireBean(bean);
        factory.registerSingleton(beanName, bean);
    }
    /**
     * 注銷(xiāo)bean
     * <p>
     * 將Spring中的bean注銷(xiāo)，請(qǐng)謹(jǐn)慎使用
     *
     * @param beanName bean名稱
     * @author shadow
     * @since 5.7.7
     */
    public void unregisterBean(String beanName) {
        final ConfigurableListableBeanFactory factory = getConfigurableBeanFactory();
        if (factory instanceof DefaultSingletonBeanRegistry) {
            DefaultSingletonBeanRegistry registry = (DefaultSingletonBeanRegistry) factory;
            registry.destroySingleton(beanName);
        } else {
            throw new UtilException("Can not unregister bean, the factory is not a DefaultSingletonBeanRegistry!");
        }
    }
    /**
     * 發(fā)布事件
     *
     * @param event the event to publish
     * @since 5.7.12
     */
    public void publishEvent(ApplicationEvent event) {
        if (null != applicationContext) {
            applicationContext.publishEvent(event);
        }
    }
}

BeanFactoryPostProcessor 為什么能解決這個(gè)問(wèn)題？

@FunctionalInterface
public interface BeanFactoryPostProcessor {
    void postProcessBeanFactory(ConfigurableListableBeanFactory var1) throws BeansException;
}

從注釋可以看出來(lái)：

BeanFactoryPostProcessor接口允許修改上下文中Bean的定義（definitions），可以調(diào)整Bean的屬性
上下文可以自動(dòng)檢測(cè)BeanFactoryPostProcessor，并且在Bean實(shí)例化之前調(diào)用

源碼分析

BeanFactoryPostProcessor是在Bean被實(shí)例化之前對(duì)Bean的定義信息進(jìn)行修改，那么Spring是如何實(shí)現(xiàn)對(duì)自定義BeanFactoryPostProcessor的調(diào)用的，下面通過(guò)源碼來(lái)看一下，首先還是從refresh()方法入手，在refresh()方法中會(huì)調(diào)用invokeBeanFactoryPostProcessors(beanFactory);

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        //主要是這一行
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
		// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
		// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
		if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}
public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
		/**因代碼太長(zhǎng)，省略了***/
		//這里從beanFacoty中通過(guò)BeanFactoryPostProcessor類型來(lái)獲取Bean名稱，就可以拿到我們自定義的BeanFactoryPostProcessor
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
		List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
		List<String> orderedPostProcessorNames = new ArrayList<>();
		List<String> nonOrderedPostProcessorNames = new ArrayList<>();
		for (String ppName : postProcessorNames) {
			if (processedBeans.contains(ppName)) {
				// skip - already processed in first phase above
			}
			//這里是優(yōu)先級(jí)的處理，如果我們有多個(gè)自定義的BeanFactoryPostProcessor,可以通過(guò)優(yōu)先級(jí)來(lái)定義執(zhí)行順序
			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
			}
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}
		// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
		//這里先處理實(shí)現(xiàn)了PriorityOrdered接口的BeanFactoryPostProcessor，也就是定義了優(yōu)先級(jí)的先處理
		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
		// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
		//再處理實(shí)現(xiàn)了Ordered接口的BeanFactoryPostProcessor
		List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : orderedPostProcessorNames) {
			orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		sortPostProcessors(orderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
		// Finally, invoke all other BeanFactoryPostProcessors.
		List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : nonOrderedPostProcessorNames) {
			nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		//這里才到了處理普通的自定義BeanFactoryPostProcessors
		invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
		// Clear cached merged bean definitions since the post-processors might have
		// modified the original metadata, e.g. replacing placeholders in values...
		beanFactory.clearMetadataCache();
	}
private static void invokeBeanFactoryPostProcessors(
			Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
		for (BeanFactoryPostProcessor postProcessor : postProcessors) {
			postProcessor.postProcessBeanFactory(beanFactory);
		}
	}

invokeBeanFactoryPostProcessors()方法的邏輯很簡(jiǎn)單，就是去遍歷容器中的BeanFactoryPostProcessor，然后調(diào)用postProcessBeanFactory()方法，這個(gè)方法就是我們自定義BeanFactoryPostProcessor時(shí)需要去實(shí)現(xiàn)的方法，至此整個(gè)流程就已經(jīng)很清晰了

以上就是PostConstruct注解標(biāo)記類ApplicationContext未加載空指針的詳細(xì)內(nèi)容，更多關(guān)于PostConstruct ApplicationContext的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

您可能感興趣的文章: