spring_annotation_day_07 | 解忧杂货店

spring_annotation

发布日期: 2018-10-16

文章字数: 841

阅读时长: 3 分

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Spring注解开发

今天学习BeanPostProcessor的生命周期流程

BeanPostProcessor的生命周期流程

我们知道BeanPostProcessor是后置处理器，是在Bean初始化前后执行一些列操作的，底层到底是怎么执行的呢？现在debug一下。

1）设置断点，在com.liuzhuo.bean包下的MyBeanPostProcessor类中，给postProcessBeforeInitialization方法设置断点。

2) debug一下。发现调用栈如下：

程序是从test01开始：

1）创建AnnotationConfigApplicationContext对象

2）执行AnnotationConfigApplicationContext的构造方法：

    public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
        this();
        register(annotatedClasses);
        refresh();
    }

3) 调用refresh()方法：完成一系列资源配置操作。最后调用finishBeanFactoryInitialization()方法，初始化容器。

    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();
            }
        }
    }

4）进入finishBeanFactoryInitialization：
调用
beanFactory.preInstantiateSingletons();

5) 最终调用doCreateBean()方法：
我们主要看其中的：

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            populateBean(beanName, mbd, instanceWrapper);
            if (exposedObject != null) {
                exposedObject = initializeBean(beanName, exposedObject, mbd);
            }
        }

populateBean: 给Bean的属性赋值等操作
initializeBean: Bean初始化操作。

6）点击initializeBean方法:

        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }

        try {
            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()) {
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }
        return wrappedBean;

其中：
applyBeanPostProcessorsBeforeInitialization：完成初始化之前的操作。

invokeInitMethods：初始化是操作(@PostConstruc、init-method等)

applyBeanPostProcessorsAfterInitialization：完成初始化之后的操作。

7）点击applyBeanPostProcessorsBeforeInitialization：

    @Override
    public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {

        Object result = existingBean;
        for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
            result = beanProcessor.postProcessBeforeInitialization(result, beanName);
            if (result == null) {
                return result;
            }
        }
        return result;
    }

发现：就是使用for循环来调用一系列BeanPostProcessor接口的实现类，包括Spring容器自身的和我们自定义的实现类。

注意:如果其中的某一个BeanPostProcessor实现类返回了null，那么后面的BeanPostProcessor实现类就不执行了！！！

大概的流程就走完了~~~

Spring底层对BeanPostProcessor的使用

ps：在idea中：类名查找类:Ctrl+Shift+Alt+N;

1) 找到BeanPostProcessor接口：

public interface BeanPostProcessor {


    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;


    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

2) 在BeanPostProcessor上Ctrl+T：找到所有的实现类：

我们能看到各种BeanPostProcessor的实现类

3）ApplicationContextAwareProcessor

我们使用一下ApplicationContextAwareProcessor这个处理器。这个处理器是给我们的自定义Bean注入上下文容器的。

4）在com.liuzhuo.bean包下，随便找个一个Bean，并实现ApplicationContextAware接口：
ps:这里使用的Dog类

@Component
public class Dog implements ApplicationContextAware

5）需要实现：setApplicationContext方法

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