Spring Bean 初始化之InitializingBean, init-method 和 PostConstruct

雨点打透心脏的1/2处 2021-11-09 13:52 323阅读 0赞

InitializingBean从接口的名字上就可以知道，它的作用就是在bean初始化后执行定制化的操作。

Spring容器中的Bean是有生命周期的，Spring允许在Bean初始化完成后以及Bean销毁前执行特定的操作，长用的设定方式有以下三种：

1、通过实现 InitializingBean/DisposableBean 接口来定制初始化之后/销毁之前的操作方法；
        2、通过 <bean> 元素的 init-method/destroy-method 属性指定初始化之后 /销毁之前调用的操作方法；
        3、在指定方法上加上@PostConstruct 或@PreDestroy注解来制定该方法是在初始化之后还是销毁之前调用。

initializingBean接口的源码如下：

/*
     * Copyright 2002-2018 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
     *
     *      https://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.beans.factory;
    
    /**
     * Interface to be implemented by beans that need to react once all their properties
     * have been set by a {@link BeanFactory}: e.g. to perform custom initialization,
     * or merely to check that all mandatory properties have been set.
     *
     * <p>An alternative to implementing {@code InitializingBean} is specifying a custom
     * init method, for example in an XML bean definition. For a list of all bean
     * lifecycle methods, see the {@link BeanFactory BeanFactory javadocs}.
     *
     * @author Rod Johnson
     * @author Juergen Hoeller
     * @see DisposableBean
     * @see org.springframework.beans.factory.config.BeanDefinition#getPropertyValues()
     * @see org.springframework.beans.factory.support.AbstractBeanDefinition#getInitMethodName()
     */
    public interface InitializingBean {
    
    	/**
    	 * Invoked by the containing {@code BeanFactory} after it has set all bean properties
    	 * and satisfied {@link BeanFactoryAware}, {@code ApplicationContextAware} etc.
    	 * <p>This method allows the bean instance to perform validation of its overall
    	 * configuration and final initialization when all bean properties have been set.
    	 * @throws Exception in the event of misconfiguration (such as failure to set an
    	 * essential property) or if initialization fails for any other reason
    	 */
    	void afterPropertiesSet() throws Exception;
    
    }

接口只有一个方法afterPropertiesSet，此方法的调用入口是负责加载spring bean的AbstractAutowireCapableBeanFactory，源码如下：

/**
    	 * Give a bean a chance to react now all its properties are set,
    	 * and a chance to know about its owning bean factory (this object).
    	 * This means checking whether the bean implements InitializingBean or defines
    	 * a custom init method, and invoking the necessary callback(s) if it does.
    	 * @param beanName the bean name in the factory (for debugging purposes)
    	 * @param bean the new bean instance we may need to initialize
    	 * @param mbd the merged bean definition that the bean was created with
    	 * (can also be {@code null}, if given an existing bean instance)
    	 * @throws Throwable if thrown by init methods or by the invocation process
    	 * @see #invokeCustomInitMethod
    	 */
    	protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
    			throws Throwable {
    
    		boolean isInitializingBean = (bean instanceof InitializingBean);
    		if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
    			if (logger.isTraceEnabled()) {
    				logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
    			}
    			if (System.getSecurityManager() != null) {
    				try {
    					AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
    						((InitializingBean) bean).afterPropertiesSet();
    						return null;
    					}, getAccessControlContext());
    				}
    				catch (PrivilegedActionException pae) {
    					throw pae.getException();
    				}
    			}
    			else {
    				((InitializingBean) bean).afterPropertiesSet();
    			}
    		}
    
    		if (mbd != null && bean.getClass() != NullBean.class) {
    			String initMethodName = mbd.getInitMethodName();
    			if (StringUtils.hasLength(initMethodName) &&
    					!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
    					!mbd.isExternallyManagedInitMethod(initMethodName)) {
    				invokeCustomInitMethod(beanName, bean, mbd);
    			}
    		}
    	}

从这段代码可以得到以下结论：

1、spring为bean提供了两种初始化bean的方式，实现InitializingBean接口，实现afterPropertiesSet方
    法，或者在配置文件中通过init-method指定，两种方式可以同时使用
    2、实现InitializingBean接口是直接调用afterPropertiesSet方法，比通过反射调用init-method指定的方
    法效率相对来说要高点。但是init-method方式消除了对spring的依赖
    3、先调用afterPropertiesSet，再执行 init-method 方法，如果调用afterPropertiesSet方法时出错，则
    不调用init-method指定的方法

@PostConstruct，该注解被调用类`InitDestroyAnnotationBeanPostProcessor`

@Override
    	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    		LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
    		try {
    			metadata.invokeInitMethods(bean, beanName);
    		}
    		catch (InvocationTargetException ex) {
    			throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
    		}
    		catch (Throwable ex) {
    			throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
    		}
    		return bean;
    	}

从命名上，我们就可以得到某些信息——这是一个BeanPostProcessor。想到了什么？在也谈Spring容器的生命周期中，提到过BeanPostProcessor的postProcessBeforeInitialization是在Bean生命周期中afterPropertiesSet和init-method之前被调用的。另外通过跟踪，@PostConstruct方法的调用方式也是通过发射机制。

总结一下

1、spring bean的初始化执行顺序：构造方法 --> @PostConstruct注解的方法 --> afterPropertiesSet方
    法 --> init-method指定的方法。具体可以参考例子
    2、afterPropertiesSet通过接口实现方式调用（效率上高一点），@PostConstruct和init-method都是通过反
    射机制调用