Spring-IOC加载Bean【十三】bean对象的实例化-创建BeanWrapper实例之InstantiationStrategy

在AbstractAutowireCapableBeanFactory的createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) 的方法中，
获取新实例BeanWrapper有四种方式：

obtainFromSupplier回调方式
没有用到InstantiationStrategy
instantiateUsingFactoryMethod工厂方法方式
使用了InstantiationStrategy中的instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
@Nullable Object factoryBean, Method factoryMethod, Object… args)
autowireConstructor自动注入方式
使用了InstantiationStrategy中的instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
Constructor<?> ctor, Object… args)
instantiateBean默认构造函数方式
使用了InstantiationStrategy中的instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner)

1、InstantiationStrategy接口

负责创建与root bean definition对应的实例。这是一种策略，因为各种方法都是可能的，包括使用CGLIB动态创建子类来支持方法注入。

public interface InstantiationStrategy {
//在此工厂中返回具有给定名称的bean实例。
    Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner)
            throws BeansException;
//在这个工厂中返回具有给定名称的bean实例，并通过给定的构造函数创建它。
    Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
                       Constructor<?> ctor, Object... args) throws BeansException;
//在此工厂中返回具有给定名称的bean实例，并通过给定的工厂方法创建它。
    Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
                       @Nullable Object factoryBean, Method factoryMethod, Object... args)
            throws BeansException;

}

下面逐个分析这个接口的实现类SimpleInstantiationStrategy中的方法实现。

1.1、instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner)

@Override
	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
		// 如果没有覆盖，不要使用CGLIB覆盖该类。
		if (!bd.hasMethodOverrides()) {
			Constructor<?> constructorToUse;
			//构造函数锁
			synchronized (bd.constructorArgumentLock) {
			    //解析构造方法
				constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
				//如果没有构造方法
				if (constructorToUse == null) {
					final Class<?> clazz = bd.getBeanClass();
					//指定的类是一个接口
					if (clazz.isInterface()) {
						throw new BeanInstantiationException(clazz, "Specified class is an interface");
					}
					//从clazz中获取方法：getDeclaredConstructor包括私有方法
					try {
						if (System.getSecurityManager() != null) {
							constructorToUse = AccessController.doPrivileged(
									(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
						}
						else {
							constructorToUse = clazz.getDeclaredConstructor();
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Throwable ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			//通过反射获取对象
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// 必须生成CGLIB子类。Method Injection not supported in SimpleInstantiationStrategy
            // CglibSubclassingInstantiationStrategy extends SimpleInstantiationStrategy
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}

1.2、instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,final Constructor<?> ctor, Object… args)

@Override
	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
			final Constructor<?> ctor, Object... args) {
        // 如果没有覆盖，不要使用CGLIB覆盖该类。
		if (!bd.hasMethodOverrides()) {
			if (System.getSecurityManager() != null) {
				// use own privileged to change accessibility (when security is on)
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					ReflectionUtils.makeAccessible(ctor);
					return null;
				});
			}
            //通过反射获取对象
			return BeanUtils.instantiateClass(ctor, args);
		}
		else {
		    //CGLIB
			return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
		}
	}

1.3、instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,

        @Nullable Object factoryBean, final Method factoryMethod, Object... args)

@Override
	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
			@Nullable Object factoryBean, final Method factoryMethod, Object... args) {

		try {
		    //安全管理器
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					ReflectionUtils.makeAccessible(factoryMethod);
					return null;
				});
			}
			else {
				ReflectionUtils.makeAccessible(factoryMethod);
			}

			//从ThreadLocal中获取先前调用的工厂方法
			Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
			try {
			    //工厂方法塞到ThreadLocal
				currentlyInvokedFactoryMethod.set(factoryMethod);
				// 反射
				Object result = factoryMethod.invoke(factoryBean, args);
				if (result == null) {
					result = new NullBean();
				}
				return result;
			}
			finally {
				if (priorInvokedFactoryMethod != null) {
					currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
				}
				else {
					currentlyInvokedFactoryMethod.remove();
				}
			}
		}
		//各种catch
        
		catch (IllegalArgumentException ex) {
			throw new BeanInstantiationException(factoryMethod,
					"Illegal arguments to factory method '" + factoryMethod.getName() + "'; " +
					"args: " + StringUtils.arrayToCommaDelimitedString(args), ex);
		}
		catch (IllegalAccessException ex) {
			throw new BeanInstantiationException(factoryMethod,
					"Cannot access factory method '" + factoryMethod.getName() + "'; is it public?", ex);
		}
		catch (InvocationTargetException ex) {
			String msg = "Factory method '" + factoryMethod.getName() + "' threw exception";
			if (bd.getFactoryBeanName() != null && owner instanceof ConfigurableBeanFactory &&
					((ConfigurableBeanFactory) owner).isCurrentlyInCreation(bd.getFactoryBeanName())) {
				msg = "Circular reference involving containing bean '" + bd.getFactoryBeanName() + "' - consider " +
						"declaring the factory method as static for independence from its containing instance. " + msg;
			}
			throw new BeanInstantiationException(factoryMethod, msg, ex.getTargetException());
		}
	}