springboot源码解析-管中窥豹系列之aware（六）

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 getBean(Class 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);

public 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 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