《spring设计思想》10-反射调用实现bean的实例化

上面一节简单的介绍了InstantiationAwareBeanPostProcessor可以在bean的实例化之前，调用回调的接口-postProcessorBeforInstantiation方法打断spring bean的默认实现。那本节简单的看下spring bean的默认实例化方法。

回到createBean(String beanName,RootBeanDefinition mbd,Objec[] args)方法。

	/**
	 * Central method of this class: creates a bean instance,
	 * populates the bean instance, applies post-processors, etc.
	 * @see #doCreateBean
	 */
	@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

	
		RootBeanDefinition mbdToUse = mbd;

		//beanClass : String -> Class
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}
        // 仙履奇缘，给个机会
		try {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			if (bean != null) {
				return bean;
			}
		}
		// Spring bean的默认实例化方法
		try {
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			
			return beanInstance;
		}

	}

继续跟踪方法调用 doCreateBean(beanName,mbdToUse,args);

	/**
	 * Actually create the specified bean. Pre-creation processing has already happened
	 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
	 * <p>Differentiates between default bean instantiation, use of a
	 * factory method, and autowiring a constructor.
	 * @param beanName the name of the bean
	 * @param mbd the merged bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a new instance of the bean
	 * @throws BeanCreationException if the bean could not be created
	 * @see #instantiateBean
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 */
	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) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}
        //省略若干方法
        .......

	}

方法说明里讲：该方法是真正的创建bean的方法，仙履奇缘的故事已经发生过，大家啥也没发生，继续默认实现。

提炼出来一段代码 BeanWrapper instanceWrapper = createBeanInstance(beanName,mbd,args)；

final Object bean = instanceWrapper.getWrappedInstance();

可以看到bean已经在createBeanInstance(beanName,mbd,args)方法中创建完成，继续追踪代码：

	/**
	 * Create a new instance for the specified bean, using an appropriate instantiation strategy:
	 * factory method, constructor autowiring, or simple instantiation.
	 * @param beanName the name of the bean
	 * @param mbd the bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a BeanWrapper for the new instance
	 * @see #obtainFromSupplier
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 * @see #instantiateBean
	 */
	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		// Make sure bean class is actually resolved at this point.
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
        //最简单的bean的实例化代码
		// No special handling: simply use no-arg constructor.
		return instantiateBean(beanName, mbd);
	}

简单的看到，instantiateBean(beanName,mbd)方法实例化了bean

继续最终代码：beanInstance  = getInstantiationStrategy().instantiate(mbd,beanName,parent);

InstantiationStrategy接口定义了实例化bean的接口instantiate(mbd,beanName,parent);

默认实现 SimpleInstantiationStrategy:

方法实现：

Constructor<?> constructorToUse =  clazz.getDeclaredConstructor();
return BeanUtils.instantiateClass(constructorToUse);

继续看BeanUtils的方法instantiateClass(constrictorToUse);

            ReflectionUtils.makeAccessible(ctor);
			return (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
					KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));

其实就是调用反射，使用Constructor.newInstance(args)方法获取实例化的bean。

调用堆栈：