Spring getBean实例化Bean的过程

前面一章关于

refresh()方法的调用过程当中提到最终会实例化非懒加载的单例bean，今天就来看一下bean初始化的整个过程。

对

refresh()方法感兴趣的能够去看下前面一章内容： juejin.im/post/5cef2e…

1.首先refresh()方法最后调用finishBeanFactoryInitialization(beanFactory)执行初始化bean工做,而后会调用DefaultListableBeanFactory.preInstantiateSingletons()。部分源码以下：

这里会获取全部的单例的非懒加载的Beandefinition遍历执AbstractBeanFactory.getBean()。

2.doGetBean方法执行过程：

        2.1 先调用getSingleton()先去缓存中查询有没有，若是存在就获取缓存中的对象。这里有三个缓存对象，分别是：singletonObjects(单例对象的cache)、earlySingletonObjects(提早暴光的单例对象的Cache)、singletonFactories(单例对象工厂的cache)。

Object sharedInstance = getSingleton(beanName);
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		synchronized (this.singletonObjects) {
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
					singletonObject = singletonFactory.getObject();
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return singletonObject;
}
复制代码

        2.2 调用父容器的getBean若是存在直接返回。

        2.3 判断是否配置了depends-on属性，若是有先建立依赖的bean。

        2.4 判断是单例仍是原型，调用createBean()建立Bean实例。

   

3.createBean的处理过程：

3.1 校验override方法

// Prepare method overrides.
try {
	mbdToUse.prepareMethodOverrides();
}

复制代码

3.2 这里最关键的代码代理类的生成。

try {
	// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
	//处理实现了InstantiationAwareBeanPostProcessor接口的类。
        //扩展点：能够建立代理对象并返回
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
	if (bean != null) {
		return bean;
	}
}复制代码

3.3 继续调用doCreateBean方法建立bean实例

4. AbstractAutowireCapableBeanFactory类的doCreateBean方法

4.1 建立bean实例

if (instanceWrapper == null) {
	instanceWrapper = createBeanInstance(beanName, mbd, args);
}

复制代码

//createBeanInstance的主要实现
// Candidate constructors for autowiring?
//获取全部构造器，匹配构造器，反射调用构造器生成实例
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
		mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
	return autowireConstructor(beanName, mbd, ctors, args);
}

// Preferred constructors for default construction? 这个还不清楚怎么用
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
	return autowireConstructor(beanName, mbd, ctors, null);
}
//调用无参构造器
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);复制代码

无参构造器：调用SimpleInstantiationStrategy的方法instantiate进行校验，

若是没有构造器抛出异常；

throw new BeanInstantiationException(clazz, "No default constructor found", ex);复制代码

若是是接口抛出异常。

throw new BeanInstantiationException(clazz, "Specified class is an interface");复制代码

最后反射调用无参构造器初始化Bean实例。

BeanUtils

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
	Assert.notNull(ctor, "Constructor must not be null");
	try {
		ReflectionUtils.makeAccessible(ctor);
		return (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
				KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
	}
	catch (InstantiationException ex) {
		throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
	}
	catch (IllegalAccessException ex) {
		throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
	}
	catch (IllegalArgumentException ex) {
		throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
	}
	catch (InvocationTargetException ex) {
		throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
	}
}复制代码

有参数的构造器： 这里代码就不贴了比较多就简单介绍下。主要是对参数的校验，若是有引用就经过getBean获取引用对象（也就是从新走一遍流程）。最终调用SimpleInstantiationStrategy的重载方法instantiate实例化Bean。

4.2 到此为止Bean实例已经建立完毕，接下来就是设置属性，初始化等工做。

   ①：解决属性循环依赖问题，为循环引用依赖提早缓存单例Bean

//是否提前暴露单例bean实例
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
		isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
	if (logger.isTraceEnabled()) {
		logger.trace("Eagerly caching bean '" + beanName +
				"' to allow for resolving potential circular references");
	}
        //放到singletonFactories中
	addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}复制代码

②：populateBean方法

执行全部实现了InstantiationAwareBeanPostProcessor接口的实例,实例化Bean的后置处理

boolean continueWithPropertyPopulation = true;
//实现了InstantiationAwareBeanPostProcessor的实例将被在Bean实例化以后被执行
//postProcessAfterInstantiation方法。
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		if (bp instanceof InstantiationAwareBeanPostProcessor) {
			InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
			if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
				continueWithPropertyPopulation = false;
				break;
			}
		}
	}
}复制代码

设置属性

//获取配置的的属性列表<property>PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//对设置了autowire的处理，根据类型和名称从新获取属性列表
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
	MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
	// Add property values based on autowire by name if applicable.
	if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
		autowireByName(beanName, mbd, bw, newPvs);
	}
	// Add property values based on autowire by type if applicable.
	if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
		autowireByType(beanName, mbd, bw, newPvs);
	}
	pvs = newPvs;
}

boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
	if (pvs == null) {
		pvs = mbd.getPropertyValues();
	}
    // 设置属性以前对属性经行前置处理
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		if (bp instanceof InstantiationAwareBeanPostProcessor) {
			InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
			PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
			if (pvsToUse == null) {
				if (filteredPds == null) {
					filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
				}
				pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
				if (pvsToUse == null) {
					return;
				}
			}
			pvs = pvsToUse;
		}
	}
}

if (needsDepCheck) {//是否开启依赖校验，默认false
	if (filteredPds == null) {
                //排除在忽略的依赖接口上定义的忽略的依赖类型或属性
		filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
	}
        //检查是否全部属性已被设置
	checkDependencies(beanName, mbd, filteredPds, pvs);
}

if (pvs != null) {
        //设置属性
	applyPropertyValues(beanName, mbd, bw, pvs);
}复制代码

③：initializeBean方法

执行织入方法invokeAwareMethods

if (bean instanceof Aware) {
	if (bean instanceof BeanNameAware) {
		((BeanNameAware) bean).setBeanName(beanName);
	}
	if (bean instanceof BeanClassLoaderAware) {
		ClassLoader bcl = getBeanClassLoader();
		if (bcl != null) {
			((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
		}
	}
	if (bean instanceof BeanFactoryAware) {
		((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
	}
}复制代码

执行BeanPostProcessor的postProcessBeforeInitialization方法，执行init-method以前对进行Bean处理。

if (mbd == null || !mbd.isSynthetic()) {
	wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}复制代码

执行配置的初始化方法（init-method）

try {
       //invokeInitMethods方法执行配置的初始化方法，
        //若是bean是InitializingBean实例还会执行afterPropertiesSet方法。
	invokeInitMethods(beanName, wrappedBean, mbd);
}复制代码

执行BeanPostProcessor的postProcessAfterInitialization方法，执行init-method以后对bean进行处理。此处能够返回代理对象

if (mbd == null || !mbd.isSynthetic()) {
	wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}复制代码

对scope进行处理（这部分代码没有去研究，有兴趣的能够本身设置不一样的scope，而后debug跟一下）。

registerDisposableBeanIfNecessary(beanName, bean, mbd);复制代码

5.小结

上面就是getBean的整个过程。下面盗用别人的图来看一下整个过程的流程：

其中有些没讲有些只是简单带过，感兴趣的本身看跟源码的时候能够去仔细看看。

还有一点就是BeanFactoryPostProcessor和BeanPostProcessor这两个接口的实现类贯穿整个容器初始化的各个过程，了解不一样实如今容器初始化过程当中的做用对于本身扩展Spring会有很大的帮助，强烈建议去了解一下不一样子类的做用。