ApplicationContextAware原理

1、紧接上文BeanPostProcessor处理器的应用，ApplicationContextAware也可以说是spring框架中对BeanPostProcessor的一个应用，还包括一些其他的Aware,如BeanFactoryAware、ResourceLoaderAware、ServletContextAware等等。
2 、先来一段代码看看

@Component
    public class BeansUtils implements ApplicationContextAware {

        private static ApplicationContext context;

        public static <T> T getBean(Class<T> bean) {
            return context.getBean(bean);
        }
        public static <T> T getBean(String var1, @Nullable Class<T> var2){
            return context.getBean(var1, var2);
        }

        public static ApplicationContext getContext() {
            return context;
        }

        @Override
        public void setApplicationContext(ApplicationContext context) throws BeansException {
            BeansUtils.context = context;
        }
    }

2.1 这样的代码相信用过spring框架的人大部分都知道；通过这样的代码，就可以得到spring容器的一些基础功能,可以通过getBean方法获取容器中得对象等等。那么它的原理是怎样实现的呢？毋庸置虑是肯定setApplicationContext把ApplicationContext设置进来的；下面就看看具体的流程。
3、查看源码
3.1 首先还是定位到入口类ClassPathXmlApplicationContext->AbstractApplicationContext refresh方法 ->prepareBeanFactory方法

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // Tell the internal bean factory to use the context's class loader etc.
         // 设置 BeanFactory 的类加载器，我们知道 BeanFactory 需要加载类，也就需要类加载器，
           // 这里设置为当前 ApplicationContext 的类加载器
        beanFactory.setBeanClassLoader(getClassLoader());
         // 设置 BeanExpressionResolver
        beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver());
        beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

        // Configure the bean factory with context callbacks.
         // 添加一个 BeanPostProcessor，这个 processor 比较简单，
           // 实现了 Aware 接口的几个特殊的 beans 在初始化的时候，这个 processor 负责回调
        beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
         // 下面几行的意思就是，如果某个 bean 依赖于以下几个接口的实现类，在自动装配的时候忽略它们，
           // Spring 会通过其他方式来处理这些依赖。
        beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
        beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
        beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
        beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

        // BeanFactory interface not registered as resolvable type in a plain factory.
        // MessageSource registered (and found for autowiring) as a bean.

        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);

        // Detect a LoadTimeWeaver and prepare for weaving, if found.

        if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            // Set a temporary ClassLoader for type matching.
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }

          /**
            * 从下面几行代码我们可以知道，Spring 往往很 "智能" 就是因为它会帮我们默认注册一些有用的 bean，
            * 我们也可以选择覆盖
            */
        // Register default environment beans.
         // 如果没有定义 "environment" 这个 bean，那么 Spring 会 "手动" 注册一个
        if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
        }

         // 如果没有定义 "systemProperties" 这个 bean，那么 Spring 会 "手动" 注册一个
        if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
        }

         // 如果没有定义 "systemEnvironment" 这个 bean，那么 Spring 会 "手动" 注册一个
        if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
        }
    }

3.2 此次要关注的重点就是beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this)); ，spring源码里面将ApplicationContextAwareProcessor加入到BeanPostProcessor处理器里面了，并且传的是一个ApplicationContext类型参数进去。
3.3 再来看看ApplicationContextAwareProcessor里面做了什么事儿。

/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.support;

import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.Aware;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationEventPublisherAware;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.EmbeddedValueResolverAware;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.MessageSourceAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.util.StringValueResolver;

/**
 * {@link org.springframework.beans.factory.config.BeanPostProcessor}
 * implementation that passes the ApplicationContext to beans that
 * implement the {@link EnvironmentAware}, {@link EmbeddedValueResolverAware},
 * {@link ResourceLoaderAware}, {@link ApplicationEventPublisherAware},
 * {@link MessageSourceAware} and/or {@link ApplicationContextAware} interfaces.
 *
 * <p>Implemented interfaces are satisfied in order of their mention above.
 *
 * <p>Application contexts will automatically register this with their
 * underlying bean factory. Applications do not use this directly.
 *
 * @author Juergen Hoeller
 * @author Costin Leau
 * @author Chris Beams
 * @since 10.10.2003
 * @see org.springframework.context.EnvironmentAware
 * @see org.springframework.context.EmbeddedValueResolverAware
 * @see org.springframework.context.ResourceLoaderAware
 * @see org.springframework.context.ApplicationEventPublisherAware
 * @see org.springframework.context.MessageSourceAware
 * @see org.springframework.context.ApplicationContextAware
 * @see org.springframework.context.support.AbstractApplicationContext#refresh()
 */
class ApplicationContextAwareProcessor implements BeanPostProcessor {

    private final ConfigurableApplicationContext applicationContext;


    /**
     * Create a new ApplicationContextAwareProcessor for the given context.
     */
    //beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));调用此构造方法把ApplicationContext传过来
    public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }


    //实例化之前进行的处理
    public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
        AccessControlContext acc = null;

        if (System.getSecurityManager() != null &&
                (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                        bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                        bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
            acc = this.applicationContext.getBeanFactory().getAccessControlContext();
        }

        if (acc != null) {
            AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    //给Aware的实现类set值进去
                    invokeAwareInterfaces(bean);
                    return null;
                }
            }, acc);
        }
        else {
            //给Aware的实现类set值进去
            invokeAwareInterfaces(bean);
        }

        return bean;
    }

    private void invokeAwareInterfaces(Object bean) {
        if (bean instanceof Aware) {
            if (bean instanceof EnvironmentAware) {
                ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
            }
            if (bean instanceof EmbeddedValueResolverAware) {
                ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(
                        new EmbeddedValueResolver(this.applicationContext.getBeanFactory()));
            }
            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);
            }
            //判读是否是属于ApplicationContextAware接口的类
            if (bean instanceof ApplicationContextAware) {
                //调用实现类的setApplicationContext方法把applicationContext set进去
                ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
            }
        }
    }

    //bean实例化之后
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        return bean;
    }


    private static class EmbeddedValueResolver implements StringValueResolver {

        private final ConfigurableBeanFactory beanFactory;

        public EmbeddedValueResolver(ConfigurableBeanFactory beanFactory) {
            this.beanFactory = beanFactory;
        }

        public String resolveStringValue(String strVal) {
            return this.beanFactory.resolveEmbeddedValue(strVal);
        }
    }

}

3.4 我们都知道了实现了BeanPostProcessor处理器，可以在bean初始化前、后根据自己业务做一些事情(具体可参考拙作BeanPostProcessor处理器)；可以看到postProcessAfterInitialization只是返回bean而已，可以忽略它；关键在于postProcessBeforeInitialization方法里面，里面invokeAwareInterfaces方法是怎样都会走的,里面有这样一段代码((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);这就是去调用具体实现类的setApplicationContext方法把applicationContext传进去了。其他Aware基本上也是这个道理。

到这儿基本结束了，另外文章代码或者我理解有误的地方,希望能批评指出。

ApplicationContextAware原理

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