spring 知识整理（四）：spring bean初始化过程

接着上文，来看下DefaultListableBeanFactory的getBean过程。

BeanFactory有几个getBean的重载方法。但在DefaultListableBeanFactory中，这些重载方法最终会调用AbstractBeanFactory类的一个doGetBean方法。

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
    return doGetBean(name, requiredType, null, false);
}
@Override
public Object getBean(String name, Object... args) throws BeansException {
    return doGetBean(name, null, args, false);
}

追踪doGetBean方法

下面的缩减的代码

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    final String beanName = transformedBeanName(name);
    Object bean;
    //从缓存中获取单例
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }else {
        try {
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);
            // 创建单例的bean
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        return createBean(beanName, mbd, args);
                    }
                    catch (BeansException ex) {
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }else if (mbd.isPrototype()) {
                // 创建原型的bean
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }else{
                //其他类型
            }
        }catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }
    return (T) bean;
}

从上面代码可以看出spring在getBean的时候主要做了两个操作：

• 从缓存中获取Bean对象
• 若缓存中不存在Bean对象，则执行创建bean的操作

为了弄清流程，我们还是先弄懂spring单例的创建过程。

spring单例的创建过程

从上面的代码可以抽取一下创建单例的代码。

sharedInstance = getSingleton(beanName, () -> {
    try {
        return createBean(beanName, mbd, args);
    }
    catch (BeansException ex) {
        destroySingleton(beanName);
        throw ex;
    }
});

从上面可以看出，创建单例的时候主要是调用了getSingleton的方法，该方法第一个参数是beanName，第二个参数是一个ObjectFactory，从名字上就可以看出来，ObjectFactory主要是用来创建对象的。而这个ObjectFactory主要也是调用AbstractAutowireCapableBeanFactory 的createBean方法。createBean把beanName、RootBeanDefinition、arg传输进去。好，先看看getSingleton的代码

getSingleton方法

getSingleton方法是AbstractBeanFactory的父类DefaultSingletonBeanRegistry中的方法。DefaultSingletonBeanRegistry从名字可以看出主要是为了单例而创建的。

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    synchronized (this.singletonObjects) { //加锁
        //从缓存中获取对象
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            //缓存中获取不到对象，则开始创建对象
            //判断容器是否在销毁，若是在销毁，则直接跑一次。
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,"");
            }
            //创建前校验，主要是检查排除的bean和把beanName放到singletonsCurrentlyInCreation的map中去
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                //通过ObjectFactory去创建Object
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                //创建后主要检查排除的bean和把beanName从singletonsCurrentlyInCreation的map中移除
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                //如果是创建则把生成的对象添加到缓存中去
                addSingleton(beanName, singletonObject);
            }
        }
        return singletonObject;
    }
}

从上面代码中可以看出，getSingleton方法主要任务是确保并发，确保只创建一个单例，而且具体创建对象还是交给ObjectFactory创建，等对象创建完成后，缓存对象。

protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
}

上面是缓存对象的方法，主要也是把对象放到singletonObjects和registeredSingleton两个Map中去。

再回头看看createBean方法。

createBean

追踪下AbstractAutowireCapableBeanFactory 的createBean方法

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
    throws BeanCreationException {
    RootBeanDefinition mbdToUse = mbd;
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }
    try {
        //准备prepare方法
        //主要是处理的lookup-method、replace-method方法，若找到相应的的方法，则进行标记
        mbdToUse.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                        beanName, "Validation of method overrides failed", ex);
    }
    try {
        // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
        //嗯，上面描述很明确了，给机会BeanPostProcessors去返回一个代理类，如果不返回代理类则继续
        //BeanPostProcessors是只实现InstantiationAwareBeanPostProcessor接口的类，实现接口的类可以自定义生成自己的bean
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse); //标记1️⃣
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                                   "BeanPostProcessor before instantiation of bean failed", ex);
    }
    try {
        //嗯，这里就开始真正创建类了
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(),
                           beanName, "Unexpected exception during bean creation", ex);
    }
}

跟踪下标记1️⃣中的方法，看看里面到底是怎么回事

resolveBeforeInstantiation

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        // Make sure bean class is actually resolved at this point.
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            Class<?> targetType = determineTargetType(beanName, mbd);
            if (targetType != null) {
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        mbd.beforeInstantiationResolved = (bean != null);
    }
    return bean;
}

再追踪下applyBeanPostProcessorsBeforeInstantiation和applyBeanPostProcessorsAfterInitialization

protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
            if (result != null) {
                return result;
            }
        }
    }
    return null;
}

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;
}

原来是给机会BeanPostProcessor和它的子接口InstantiationAwareBeanPostProcessor的机会，也就是说我们实现这两个接口的bean就可以处理自己的对象生成规则了，666的。

好了，在深究下doCreateBean方法，听说java中do开都的方法都是真正做事的方法

doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
            throws BeanCreationException {
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        //创建实例的BeanWrapper
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    //从beanWrapper里面获取包装类对象
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }
    // Allow post-processors to modify the merged bean definition.
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                //调用MergedBeanDefinitionPostProcessor
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                                      isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        //属性注入
        populateBean(beanName, mbd, instanceWrapper);
        //进行 init-method初始化，主要是通知各个BeanPostProcessor
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    catch (Throwable ex) {
        if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
            throw (BeanCreationException) ex;
        }
        else {
            throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
        }
    }
    if (earlySingletonExposure) {
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                        actualDependentBeans.add(dependentBean);
                    }
                }
                if (!actualDependentBeans.isEmpty()) {
                    throw new BeanCurrentlyInCreationException(beanName,  "");
                }
            }
        }
    }
    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }
    return exposedObject;
}

doCreateBean方法主要是做了以下事情：

• createBeanInstance 生成对象，并返回包装类
• popoulateBean 进行属性注入
• instantiateBean 进行init-method初始化和后置等处理，主要是通知各个BeanPostProcessor

总结

贴的代码有点多，创建对象的一些逻辑还没深究，继续下去就怕很难闭合了。倒不如先来个总结。

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