简介

这篇文章主要介绍了Spring中InitializingBean的使用详细解析，InitializingBean是Spring提供的拓展性接口，提供了属性初始化后的处理方法,它只有一个afterPropertiesSet方法，凡是继承该接口的类，在bean的属性初始化后都会执行该方法。

源码分析

我们大概看一下初始化涉及的源码部分，主要为这两个方法，
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean 和#invokeInitMethods

/**
	 * @return the initialized bean instance (potentially wrapped)
	 * @see BeanNameAware
	 * @see BeanClassLoaderAware
	 * @see BeanFactoryAware
	 * @see #applyBeanPostProcessorsBeforeInitialization
	 * @see #invokeInitMethods
	 * @see #applyBeanPostProcessorsAfterInitialization
	 */
	protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
		if (System.getSecurityManager() != null) {
			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
				invokeAwareMethods(beanName, bean);
				return null;
			}, getAccessControlContext());
		}
		else {
			invokeAwareMethods(beanName, bean);
		}

		Object wrappedBean = bean;
		if (mbd == null || !mbd.isSynthetic()) {
			//执行BeanPostProcessor接口实现类的postProcessBeforeInitialization方法
			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
		}

		try {
			//如果bean实现了InitializingBean或者自定义了initMethod，
       		//会在这里执行InitializingBean#afterPropertiesSet和initMethod方法
			invokeInitMethods(beanName, wrappedBean, mbd);
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					(mbd != null ? mbd.getResourceDescription() : null),
					beanName, "Invocation of init method failed", ex);
		}
		if (mbd == null || !mbd.isSynthetic()) {
			//执行BeanPostProcessor接口实现类的postProcessAfterInitialization方法
			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
		}

		return wrappedBean;
	}

	protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
			throws Throwable {
		//判断bean是否实现了InitializingBean
		boolean isInitializingBean = (bean instanceof InitializingBean);
		if (isInitializingBean && (mbd == null || !mbd.hasAnyExternallyManagedInitMethod("afterPropertiesSet"))) {
			if (logger.isTraceEnabled()) {
				logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
			}
			if (System.getSecurityManager() != null) {
				try {
					AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
						((InitializingBean) bean).afterPropertiesSet();
						return null;
					}, getAccessControlContext());
				}
				catch (PrivilegedActionException pae) {
					throw pae.getException();
				}
			}
			else {
				//如果实现了InitializingBean，真接调用afterPropertiesSet()，
				((InitializingBean) bean).afterPropertiesSet();
			}
		}

		if (mbd != null && bean.getClass() != NullBean.class) {
			//获取bean自定义的init-method方法
			String initMethodName = mbd.getInitMethodName();
			if (StringUtils.hasLength(initMethodName) &&
					!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
					!mbd.hasAnyExternallyManagedInitMethod(initMethodName)) {
				//如果自定义了init-method方法，在这里开始执行；
				invokeCustomInitMethod(beanName, bean, mbd);
			}
		}
	}

看完源码部分，我们可以很清楚地知道InitializingBean#afterPropertiesSet方法和init-method是什么时候用什么方式来调用的（ 通过反射调用init-method），且InitializingBean#afterPropertiesSet方法的执行时机要稍早于init-method。另外，如果调用afterPropertiesSet方法时出错，则不调用init-method指定的方法

应用场景

这个扩展点是比较有用的一个扩展点，可以用于

• 修改默认设置的属性
• 添加补充额外的属性值
• 或者针对关键属性做校验
• spring MVC内容有一个经典的实现场景，RequestMappingHandlerMapping完成URL与controller的映射工作

public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {
	@Override
	public void afterPropertiesSet() {
		initHandlerMethods();
	}	

	protected void initHandlerMethods() {
		//循环所有的bean
		for (String beanName : getCandidateBeanNames()) {
			//如果bean名字不是以scopedTarget.开头
			if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
				processCandidateBean(beanName);
			}
		}
		//日志输出
		handlerMethodsInitialized(getHandlerMethods());
	}
	
	protected void processCandidateBean(String beanName) {
		Class<?> beanType = null;
		try {
			beanType = obtainApplicationContext().getType(beanName);
		}
		catch (Throwable ex) {
			if (logger.isTraceEnabled()) {
				logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
			}
		}
		//他们是否是Handler(类上标注@Controller注解或者@RequestMapping注解)
		if (beanType != null && isHandler(beanType)) {
			//获取这个类上所有标注@RequestMapping的方法信息，以RequestMappingInfo形式把他们储存到MappingRegistry中
			detectHandlerMethods(beanName);
		}
	}

代码示例

完成一个rsa对象的初始化，方便该类方法调用使用

@Slf4j
@Configuration
public class ExtendInitializingBean implements InitializingBean {

    private RSA rsa;

    @Override
    public void afterPropertiesSet() {
        String priv = "MIICdQIBADANBgkqhkiG9w0BAQEFAASCAl8wggJbAgEAAoGBAJzDhvCJBIIuZzNu\n" +
                "+dHWY5inSAN6d/yqWOTl/5rWofT2+m59qjKoFuJ9kkuYUCkIpoEtLhu5PfOAdR9R\n" +
                "bsVnfXThgwOQFHx1/v4URXwOBrBHRYhPy8fApJSfdBWgKMs9GZ5WbDWVkPm7xhvJ\n" +
                "t716Wtx/leCjvq87jWVg8oo83uKtAgMBAAECgYAW77bNInG3QxHUy9kf03XpY8Rc\n" +
                "xJeiBMyShSJjuyFMMFy45jMM9rIOe8qiu+DdTXI14UXnRtsNHHpGgEmBoYrl4IZ9\n" +
                "Wgbxn0fkrcHaw4F8GoAapyqAgNkyU3kAf8of6G+ZeMulcZtbb7NXVkwTaRHOkzBg\n" +
                "2a44HCSQ2n/fHPxfwwJBANFhUKWMOV3SqEN7RjsxWNAk8P25xHzIWq6KCyhOx5Vm\n" +
                "KLIttnYUiA3UA3q5RJK8Hqh0a2JGcg8O9HawOuMWAAsCQQC/qxNG08pX8dm5mh5B\n" +
                "9d/Wc09U9V7Xni9gqqS5adjxChm97t5h8k3wRxTqDWHCJyZTkezy7pmZvaFWqwLO\n" +
                "xkMnAkBLEbIBeNIEkoejkiovBNVL40LIJXeOKaNhmrq/W+dw6CzFHWCi21+gXZhV\n" +
                "wuEUHwB68yqhFJLGVV1OxIaIbpXbAkBBY0WiRiKjuhiDHzbvXchNSu1nmIYQnSCg\n" +
                "o9aqgpfGM6HwkAtB8v3v2PAjoQkWyXBj1ka14fx43J6sCI9ep3jFAkBDi3wnlIzc\n" +
                "LpJxumw84P8f3pVJ1xKx9tchAh1W+hhcaq+/S+u3rIJcXfXWLojUp7yMyKjzy2lU\n" +
                "PNC8Qke0R/qC";
        String pub = "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCcw4bwiQSCLmczbvnR1mOYp0gD\n" +
                "enf8qljk5f+a1qH09vpufaoyqBbifZJLmFApCKaBLS4buT3zgHUfUW7FZ3104YMD\n" +
                "kBR8df7+FEV8DgawR0WIT8vHwKSUn3QVoCjLPRmeVmw1lZD5u8Ybybe9elrcf5Xg\n" +
                "o76vO41lYPKKPN7irQIDAQAB";
        rsa = new RSA(priv, pub);
        log.info("afterPropertiesSet--Extend--run");
    }

}

运行示例