问题

之前在做后端项目遇到一个奇怪的问题，我装配到Spring容器中的一个Bean在另外一个类中无法被注入，该Bean的类型如下：

java复制代码@Component @Data public class FeishuCrawlerConfig{...}

我使用@Component注解将其装配到Spring容器中，然后在另外一个类中将其自动注入，格式如下：

java复制代码@Component public class FeishuTenantTokenEntity {     @Autowired     private FeishuCrawlerConfig feishuConfig;     ... }

正常来讲feishuConfig这个变量会被自动注入完成初始化，但是后面我在使用这个变量时却抛出了NPE。后来经过跟同事的讨论才发现我是在FeishuTenantTokenEntity的构造方法中调用到了feishuConfig的方法，导致了NPE的产生。构造方法如下：

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

产生这个问题的原因是Spring中的Bean在初始化时，会先执行其构造方法，再注入@Autowired注解标注的其他Bean。我们的代码中在构造方法中就使用到了@Autowired注入的feishuConfig对象，这时它还没有初始化完成，自然会抛出NPE。这个问题的解决方法就是使用@PostConstruct注解标注的方法替代FeishuTenantTokenEntity的构造方法，该方法如下：

java复制代码@PostConstruct public void init() throws IOException {     this.tokenValue = refreshTokenValue();     this.accessTime = System.currentTimeMillis(); }

Spring在初始化Bean时，会在注入@Autowired注解标注的Bean后执行@PostConstruct注解标注的方法。我们在init()方法中使用feishuConfig的方法显然是没问题的，并且它还能替代构造方法的作用。Spring中Bean初始化的执行顺序是构造方法>依赖注入（ @Autowired ）> @PostConstruct标注的方法。

@PostConstruct注解

讲到这里，不得不细说一下@PostConstruct注解。

java复制代码/**  * 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 {  }

这是Java官方对@PostConstruct注解的注释文档，可以看到@PostConstruct注解是用于初始化方法上，该方法在依赖注入完成后执行。Java对被@PostConstruct标注的方法做出了如下限制：

• 除拦截器方法外，该方法不能有任何参数

• 被@PostConstruct标注的拦截器方法不能抛出application异常

• 该方法可以被public，protected，package private和private修饰

• 该方法不能为静态的，但是可以被final修饰 简而言之，如果你的类中的构造方法需要使用到依赖注入的变量，你可以用@PostConstruct标注的方法来替代构造方法完成初始化。

Spring如何实现 @PostConstruct注解

我们知道在Spring中，Bean的初始化一般分为实例化，属性赋值，初始化和销毁这四个过程。在实例化阶段的前后，InstantiationAwareBeanPostProcessor接口的postProcessBeforeInstantiation和postProcessAfterInstantiation方法会被调用。在初始化阶段的前后，BeanPostProcessor接口的postProcessBeforeInitialization和postProcessAfterInitialization方法会被调用。开发者也可以继承这些接口拓展功能。如下图所示：

在这四个过程中间，Bean的依赖注入发生在属性赋值这个阶段。Spring会在postProcessBeforeInstantiation方法中也就是依赖注入完成之后调用@PostConstruct注解标注的方法，完成Bean的部分初始化工作。 Spring的具体做法就是在创建Bean时，会将它里面被@PostConstruct注解标注的方法保存到Bean的元数据中，在后面调用postProcessBeforeInstantiation方法时，会利用反射调用Bean的元数据中被 @PostConstruct注解标注的方法，从而完成部分初始化工作。感兴趣的同学可以看看源码。

注意

Java官方已在Java 9中弃用了@PostConstruct注解，并在Java 11中删除了@PostConstruct注解。 实现InitializingBean接口并重写其中的afterPropertiesSet方法也可以实现@PostConstruct注解相同的功能。

java复制代码/**  * 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; }