問題

之前在做后端項(xiàng)目遇到一個(gè)奇怪的問題，我裝配到Spring容器中的一個(gè)Bean在另外一個(gè)類中無法被注入，該Bean的類型如下：

java復(fù)制代碼@Component @Data public class FeishuCrawlerConfig{...}

我使用@Component注解將其裝配到Spring容器中，然后在另外一個(gè)類中將其自動(dòng)注入，格式如下：

java復(fù)制代碼@Component public class FeishuTenantTokenEntity { ?? ?@Autowired ?? ?private FeishuCrawlerConfig feishuConfig; ?? ?... }

正常來講feishuConfig這個(gè)變量會(huì)被自動(dòng)注入完成初始化，但是后面我在使用這個(gè)變量時(shí)卻拋出了NPE。后來經(jīng)過跟同事的討論才發(fā)現(xiàn)我是在FeishuTenantTokenEntity的構(gòu)造方法中調(diào)用到了feishuConfig的方法，導(dǎo)致了NPE的產(chǎn)生。構(gòu)造方法如下：

java復(fù)制代碼public FeishuTenantTokenEntity() IOException { ?? ?// refreshTokenValue()方法中使用到feishuConfig的方法 ?? ?this.tokenValue = refreshTokenValue(); ?? ?this.accessTime = System.currentTimeMillis(); }

產(chǎn)生這個(gè)問題的原因是Spring中的Bean在初始化時(shí)，會(huì)先執(zhí)行其構(gòu)造方法，再注入@Autowired注解標(biāo)注的其他Bean。我們的代碼中在構(gòu)造方法中就使用到了@Autowired注入的feishuConfig對(duì)象，這時(shí)它還沒有初始化完成，自然會(huì)拋出NPE。這個(gè)問題的解決方法就是使用@PostConstruct注解標(biāo)注的方法替代FeishuTenantTokenEntity的構(gòu)造方法，該方法如下：

java復(fù)制代碼@PostConstruct public void init() throws IOException { ?? ?this.tokenValue = refreshTokenValue(); ?? ?this.accessTime = System.currentTimeMillis(); }

Spring在初始化Bean時(shí)，會(huì)在注入@Autowired注解標(biāo)注的Bean后執(zhí)行@PostConstruct注解標(biāo)注的方法。我們?cè)?code>init()方法中使用feishuConfig的方法顯然是沒問題的，并且它還能替代構(gòu)造方法的作用。Spring中Bean初始化的執(zhí)行順序是構(gòu)造方法>依賴注入（ @Autowired ）> @PostConstruct標(biāo)注的方法。

@PostConstruct注解

講到這里，不得不細(xì)說一下@PostConstruct注解。

java復(fù)制代碼/** ?* The PostConstruct annotation is used on a method that needs to be executed ?* after dependency injection is done to perform any initialization. This ?* method MUST be invoked before the class is put into service. This ?* annotation MUST be supported on all classes that support dependenc ?* injection. The method annotated with PostConstruct MUST be invoked even ?* if the class does not request any resources to be injected. Only one ?* method can be annotated with this annotation. The method on which the ?* PostConstruct annotation is applied MUST fulfill all of the following ?* criteria: ?* <p> ?* <ul> ?* <li>The method MUST NOT have any parameters except in the case of ?* interceptors in which case it takes an InvocationContext object as ?* defined by the Interceptors specification.</li> ?* <li>The method defined on an interceptor class MUST HAVE one of the ?* following signatures: ?* <p> ?* void &#060;METHOD&#062;(InvocationContext) ?* <p> ?* Object &#060;METHOD&#062;(InvocationContext) throws Exception ?* <p> ?* <i>Note: A PostConstruct interceptor method must not throw application ?* exceptions, but it may be declared to throw checked exceptions including ?* the java.lang.Exception if the same interceptor method interposes on ?* business or timeout methods in addition to lifecycle events. If a ?* PostConstruct interceptor method returns a value, it is ignored by ?* the container.</i> ?* </li> ?* <li>The method defined on a non-interceptor class MUST HAVE the ?* following signature: ?* <p> ?* void &#060;METHOD&#062;() ?* </li> ?* <li>The method on which PostConstruct is applied MAY be public, protected, ?* package private or private.</li> ?* <li>The method MUST NOT be static except for the application client.</li> ?* <li>The method MAY be final.</li> ?* <li>If the method throws an unchecked exception the class MUST NOT be put into ?* service except in the case of EJBs where the EJB can handle exceptions and ?* even recover from them.</li></ul> ?* @since Common Annotations 1.0 ?* @see javax.annotation.PreDestroy ?* @see javax.annotation.Resource ?*/ @Documented @Retention (RUNTIME) @Target(METHOD) public @interface PostConstruct { ?}

這是Java官方對(duì)@PostConstruct注解的注釋文檔，可以看到@PostConstruct注解是用于初始化方法上，該方法在依賴注入完成后執(zhí)行。Java對(duì)被@PostConstruct標(biāo)注的方法做出了如下限制：

• 除攔截器方法外，該方法不能有任何參數(shù)

• 被@PostConstruct標(biāo)注的攔截器方法不能拋出application異常

• 該方法可以被public，protected，package private和private修飾

• 該方法不能為靜態(tài)的，但是可以被final修飾 簡(jiǎn)而言之，如果你的類中的構(gòu)造方法需要使用到依賴注入的變量，你可以用@PostConstruct標(biāo)注的方法來替代構(gòu)造方法完成初始化。

Spring如何實(shí)現(xiàn) @PostConstruct注解

我們知道在Spring中，Bean的初始化一般分為實(shí)例化，屬性賦值，初始化和銷毀這四個(gè)過程。在實(shí)例化階段的前后，InstantiationAwareBeanPostProcessor接口的postProcessBeforeInstantiation和postProcessAfterInstantiation方法會(huì)被調(diào)用。在初始化階段的前后，BeanPostProcessor接口的postProcessBeforeInitialization和postProcessAfterInitialization方法會(huì)被調(diào)用。開發(fā)者也可以繼承這些接口拓展功能。如下圖所示：

在這四個(gè)過程中間，Bean的依賴注入發(fā)生在屬性賦值這個(gè)階段。Spring會(huì)在postProcessBeforeInstantiation方法中也就是依賴注入完成之后調(diào)用@PostConstruct注解標(biāo)注的方法，完成Bean的部分初始化工作。 Spring的具體做法就是在創(chuàng)建Bean時(shí)，會(huì)將它里面被@PostConstruct注解標(biāo)注的方法保存到Bean的元數(shù)據(jù)中，在后面調(diào)用postProcessBeforeInstantiation方法時(shí)，會(huì)利用反射調(diào)用Bean的元數(shù)據(jù)中被 @PostConstruct注解標(biāo)注的方法，從而完成部分初始化工作。感興趣的同學(xué)可以看看源碼。

注意

Java官方已在Java 9中棄用了@PostConstruct注解，并在Java 11中刪除了@PostConstruct注解。 實(shí)現(xiàn)InitializingBean接口并重寫其中的afterPropertiesSet方法也可以實(shí)現(xiàn)@PostConstruct注解相同的功能。

java復(fù)制代碼/** ?* Interface to be implemented by beans that need to react once all their properties ?* have been set by a {@link BeanFactory}: e.g. to perform custom initialization, ?* or merely to check that all mandatory properties have been set. ?* ?* <p>An alternative to implementing {@code InitializingBean} is specifying a custom ?* init method, for example in an XML bean definition. For a list of all bean ?* lifecycle methods, see the {@link BeanFactory BeanFactory javadocs}. ?* ?* @author Rod Johnson ?* @author Juergen Hoeller ?* @see DisposableBean ?* @see org.springframework.beans.factory.config.BeanDefinition#getPropertyValues() ?* @see org.springframework.beans.factory.support.AbstractBeanDefinition#getInitMethodName() ?*/ public interface InitializingBean { ?? /** ?? ?* Invoked by the containing {@code BeanFactory} after it has set all bean properties ?? ?* and satisfied {@link BeanFactoryAware}, {@code ApplicationContextAware} etc. ?? ?* <p>This method allows the bean instance to perform validation of its overall ?? ?* configuration and final initialization when all bean properties have been set. ?? ?* @throws Exception in the event of misconfiguration (such as failure to set an ?? ?* essential property) or if initialization fails for any other reason ?? ?*/ ?? void afterPropertiesSet() throws Exception; }