Bean的实例化
结构器的抉择
SimpleInstantiationStrategy
Bean实例化有两种策略，一种是CGLIB，另一种是通过BeanUtil，即用JVM反射生成对象。
如果Bean中有@LookUp注解，或者xml中指定了lookup-method，那么就会采纳CGLIB来实例化对象，因为须要重写@LookUp标记的办法，这种状况下很容易想到应用代理，应用代理，被代理类又不需实现接口，无疑CGLB形式适合。
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) { // Don't override the class with CGLIB if no overrides. if (!bd.hasMethodOverrides()) { Constructor<?> constructorToUse; synchronized (bd.constructorArgumentLock) { constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod; if (constructorToUse == null) { final Class<?> clazz = bd.getBeanClass(); if (clazz.isInterface()) { throw new BeanInstantiationException(clazz, "Specified class is an interface"); } //获取无参结构器去实例化Bean 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JVM反射性能生成实例化对象 return BeanUtils.instantiateClass(constructorToUse); } else { // 通过CGLIB来生成实例化对象 return instantiateWithMethodInjection(bd, beanName, owner); }}