一、前言

• Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
• 我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。

二、ApplicationContextAware

• 假设我们想使用某个bean, 如果是在@Component类下面，直接用@Autowired引用就行了
• 假设我们想在某个静态方法里面用，就不能用上面的方法了
• 你可能想用new Bean()的方式，new一个，但是这个bean里面的@Autowired引用用不了
• 如果有一个静态的全局ApplicationContext就好了，用spring的能力获取bean: ApplicationContext.getBean(clazz)
• ApplicationContextAware就是这个用处

public interface ApplicationContextAware extends Aware {

void setApplicationContext(ApplicationContext applicationContext) throws BeansException;

}

public interface Aware {

}

我们写一个实现类：

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;

@Component
public class SpringContextUtil implements ApplicationContextAware {

private static ApplicationContext applicationContext;

@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
SpringContextUtil.applicationContext = applicationContext;
}

private static ApplicationContext getApplicationContext() {
return applicationContext;
}

public static <T> T getBean(Class<T> clazz){
return getApplicationContext().getBean(clazz);
}

}

• 通过setApplicationContext，把applicationContext赋值到本地静态变量
• 通过ApplicationContext的getBean就可以在静态方法中使用任何bean的能力了

三、源码分析

我们进入SpringApplication的run方法：

public ConfigurableApplicationContext run(String... args) {

...

try {

...

refreshContext(context);

...
}
catch (Throwable ex) {

...

}

...

return context;
}

我们进入refreshContext(context)内部：

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);
}

destroyBeans();

cancelRefresh(ex);

throw ex;
}

finally {
resetCommonCaches();
}
}
}

这个refresh是spring的核心方法，以后会多次用到，内容太多，我们这次只关注一个方法：

• prepareBeanFactory(beanFactory);

publi
15a8
c void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
...

// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);

...
}
}

我们先看prepareBeanFactory(beanFactory)：

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {

...

// Configure the bean factory with context callbacks.
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));

...

}

我们看一下这个addBeanPostProcessor方法

private final List<BeanPostProcessor> beanPostProcessors = new CopyOnWriteArrayList<>();

@Override
public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
// Remove from old position, if any
this.beanPostProcessors.remove(beanPostProcessor);
// Track whether it is instantiation/destruction aware
if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
this.hasInstantiationAwareBeanPostProcessors = true;
}
if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
this.hasDestructionAwareBeanPostProcessors = true;
}
// Add to end of list
this.beanPostProcessors.add(beanPostProcessor);
}

• 先remove,再add
• beanPostProcessors是一个线程安全的list: CopyOnWriteArrayList
• 我们往下看看new ApplicationContextAwareProcessor(this)，注意：this是ApplicationContext

class ApplicationContextAwareProcessor implements BeanPostProcessor {

private final ConfigurableApplicationContext applicationContext;

private final StringValueResolver embeddedValueResolver;

/**
* Create a new ApplicationContextAwareProcessor for the given context.
*/
public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
this.applicationContext = applicationContext;
this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
}

@Override
@Nullable
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
return bean;
}

AccessControlContext acc = null;

if (System.getSecurityManager() != null) {
acc = this.applicationContext.getBeanFactory().getAccessControlContext();
}

if (acc != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareInterfaces(bean);
return null;
}, acc);
}
else {
invokeAwareInterfaces(bean);
}

return bean;
}

private void invokeAwareInterfaces(Object bean) {
if (bean instanceof EnvironmentAware) {
((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
}
if (bean instanceof EmbeddedValueResolverAware) {
((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
}
if (bean instanceof ResourceLoaderAware) {
((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
}
if (bean instanceof ApplicationEventPublisherAware) {
((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
}
if (bean instanceof MessageSourceAware) {
((MessageSourceAware) bean).setMessageSource(this.applicationContext);
}
if (bean instanceof ApplicationContextAware) {
((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
}
}

}

• 构造方法，把applicationContext设到本地变量上
• 实现接口的方法：postProcessBeforeInitialization，回调的时候会用，主要是校验权限
• 最下面的invokeAwareInterfaces是个私有的核心回调方法，根据不同类型，有不同回调

我们看到除了ApplicationContextAware，还有其它的aware, 总共6个

• EnvironmentAware：环境变量
• EmbeddedValueResolverAware：值解析器
• ResourceLoaderAware：资源加载器
• ApplicationEventPublisherAware：事件发布器
• MessageSourceAware：信息处理器
• ApplicationContextAware：spring容器

比如我们想用全局的环境变量，就有EnvironmentAware，想用spring的事件就用ApplicationEventPublisherAware，等等

• 来源找到了，ApplicationContextAwareProcessor什么时候执行的呢？

• 这个比较麻烦，我们后面单开一节再详细的去看。

• 目录：
  springboot源码解析-管中窥豹系列

欢迎关注微信公众号：丰极，更多技术学习分享。