每日一结——Spring初始化Bean几种方式

Spring初始化Bean的几种方式

1. 实现InitializingBean接口，重写afterPropertiesSet方法
2. <Bean>元素上添加init-method初始化
3. 使用@PostConstruct注解

执行顺序：Constructor > @PostConstruct > InitializingBean > init-method

源码位置

AbstractAutowireCapableBeanFactory#initializeBean

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
    // ...

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        // 初始化前置处理
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        // 调用初始化方法
        invokeInitMethods(beanName, wrappedBean, mbd);
    } catch (Throwable ex) {
        // ...
    }
    if (mbd == null || !mbd.isSynthetic()) {
        // 初始化后置处理
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

这里主要包含了初始化的前置处理、调用初始化方法、初始化后置处理

@PostConstruct

@PostConstruct注解将在applyBeanPostProcessorsBeforeInitialization这个前置处理

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

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

跟进InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization方法

@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;
    }

findLifecycleMetadata方法会解析元数据，所以@PostConstruct注解的初始化方法在这里找到了。

再跟进findLifecycleMetadata方法

@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;
    }

再进findLifecycleMetadata

private LifecycleMetadata findLifecycleMetadata(Class<?> clazz) {
        if (this.lifecycleMetadataCache == null) {
            // Happens after deserialization, during destruction...
            return buildLifecycleMetadata(clazz);
        }
        // Quick check on the concurrent map first, with minimal locking.
        LifecycleMetadata metadata = this.lifecycleMetadataCache.get(clazz);
        if (metadata == null) {
            synchronized (this.lifecycleMetadataCache) {
                metadata = this.lifecycleMetadataCache.get(clazz);
                if (metadata == null) {
                    metadata = buildLifecycleMetadata(clazz);
                    this.lifecycleMetadataCache.put(clazz, metadata);
                }
                return metadata;
            }
        }
        return metadata;
    }

再进buildLifecycleMetadata

private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {
        final boolean debug = logger.isDebugEnabled();
        LinkedList<LifecycleElement> initMethods = new LinkedList<>();
        LinkedList<LifecycleElement> destroyMethods = new LinkedList<>();
        Class<?> targetClass = clazz;

        do {
            final LinkedList<LifecycleElement> currInitMethods = new LinkedList<>();
            final LinkedList<LifecycleElement> currDestroyMethods = new LinkedList<>();

            ReflectionUtils.doWithLocalMethods(targetClass, method -> {
                if (initAnnotationType != null && method.isAnnotationPresent(initAnnotationType)) {
                    LifecycleElement element = new LifecycleElement(method);
                    currInitMethods.add(element);
                    if (debug) {
                        logger.debug("Found init method on class [" + clazz.getName() + "]: " + method);
                    }
                }
                if (destroyAnnotationType != null && method.isAnnotationPresent(destroyAnnotationType)) {
                    currDestroyMethods.add(new LifecycleElement(method));
                    if (debug) {
                        logger.debug("Found destroy method on class [" + clazz.getName() + "]: " + method);
                    }
                }
            });

            initMethods.addAll(0, currInitMethods);
            destroyMethods.addAll(currDestroyMethods);
            targetClass = targetClass.getSuperclass();
        }
        while (targetClass != null && targetClass != Object.class);

        return new LifecycleMetadata(clazz, initMethods, destroyMethods);
    }

可以看到有个判断是否被initAnnotationType注释注解，然后添加到集合中。

initAnnotationType位于CommonAnnotationBeanPostProcessor类中

public CommonAnnotationBeanPostProcessor() {
        setOrder(Ordered.LOWEST_PRECEDENCE - 3);
        setInitAnnotationType(PostConstruct.class);
        setDestroyAnnotationType(PreDestroy.class);
        ignoreResourceType("javax.xml.ws.WebServiceContext");
    }

可以看到CommonAnnotationBeanPostProcessor和InitDestroyAnnotationBeanPostProcessor的关系是继承关系

实现InitializingBean接口afterPropertiesSet方法

继续initializeBean方法，这次来看invokeInitMethods方法

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
        .....
        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;
    }

invokeInitMethods方法

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.isDebugEnabled()) {
                logger.debug("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);
            }
        }
    }

可以看出首先会检查是否是InitializingBean，如果是调用afterPropertiesSet()方法

init-method

在invokeInitMethods方法中，我们会发现以下这段代码，它是执行init-method配置的内容

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);
            }
        }

postProcessAfterInitialization方法

再次回到initializeBean方法，可以看到最后调用了applyBeanPostProcessorsAfterInitialization方法

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

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

总结

根据上述源码可以得出以下执行顺序：postProcessBeforeInitialization->InitializingBean（afterPropertiesSet）->init-method->postProcessAfterInitialization

BeanPostProcessor后置处理器

如果一个Bean实现了BeanPostProcess接口，将会调用postProcessBeforeInitialization(Object bean, String beanName)方法，
经常用于对Bean内容的更改，并且由于这个在Bean初始化结束时调用，也常被应用与缓存或者内存技术
postProcessBeforeInitialization方法在bean初始化之前执行， postProcessAfterInitialization方法在bean初始化之后执行