1、问题描述
某些spring 托管的bean，被标注Autowired注解的属性，没有自动注入对象。比如下面这个类。
/**
* Created by zhengwei on 2017/6/21.
*/
@Component
public classGetProductByProductIdRequestHandlerextendsPcRequestHandler {
@Autowired
privateProductQueryManagerproductQueryManager;
@Override
protectedGetProductByProductIdResponsedoHandle(GetProductByProductIdRequest request) {
GetProductByProductIdResponse response =newGetProductByProductIdResponse();
ProductDO product;
try{
product =this.productQueryManager.getProductBaseInfoById(request.getProductId());
}catch(Exception e) {
logger.error("getProductBaseInfoById error, productId="+ request.getProductId(),e);
response.setSuccess(false);
response.setResultCode(BizErrorCode.COMMON_ERROR.getErrorCode());
response.setResultMsg(BizErrorCode.COMMON_ERROR.getErrorMsg());
returnresponse;
}
response.setProduct(product);
response.setSuccess(true);
returnresponse;
}
@Override
publicClasssupportRequest() {
returnGetProductByProductIdRequest.class;
}
}
运行的过程中，上图GetProductByProductIdRequestHandler中的属性productQueryManager为null，导致空指针异常。
2、问题产生原因
项目启用了配置中心，通过PropertyPlaceholderConfigurer实现，因此它是个BeanFactoryPostProcessor，而此bean在初始化spring时，会去配置中心获取配置项到本地内存，其中该bean初始化过程中，有这么一段代码，如下图：
private void initListener(PropertiesConfiguration config) {
Map listeners = applicationContext.getBeansOfType(MessageConfCenterListener.class);
if (listeners == null) {
listeners = new HashMap();
}
// add default contextPropertiesListener
listeners.put("contextPropertiesListener", new MessageConfCenterListener() {
@Override
public void notifyEvent(ConfigurationEvent event) {
switch (event.getType()) {
case ADD:
case UPDATE:
contextProperties.put(event.getPropertyName(), event.getPropertyValue());
break;
case CLEAR:
contextProperties.remove(event.getPropertyName());
}
}
});
config.addConfigurationListener(new MessageConfigurationListener(listeners.values()));
}
可以看到其中第2行代码是，
Map listeners = applicationContext.getBeansOfType(MessageConfCenterListener.class);
也就是从applicationContext获取MessageConfCenterListener，getBeansOfType此方法是根据name遍历整个bean factory里的bean，如果是FactoryBean的话，会先创建此bean，问题就出在这里了。这里需要看看AbstractApplicationContext里的refresh方法，如下图：
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
1、prepareBeanFactory(beanFactory)方法，表示准备动作，初始化一些默认的配置。
2、invokeBeanFactoryPostProcessors(beanFactory)，执行BeanFactoryPostProcessor，而我们的配置中心是通过PropertyPlaceholderConfigurer来实现的，而PropertyPlaceholderConfigurer是一个BeanFactoryPostProcessor，因此这个时候会被执行。
3、registerBeanPostProcessors(beanFactory)，注册BeanPostProcessor，这个BeanPostProcessor干啥的呢，spring的Autowired注解就是通过AutowiredAnnotationBeanPostProcessor来实现的。
因此当执行BeanFactoryPostProcessor的时候，spring还没有往容器里注册AutowiredAnnotationBeanPostProcessor，问题就这么发生了。
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