Spring的BeanUtils原理

在Spring内部，很多地方都不使用new关键字来创建对象，而是使用BeanUtils工具来实例化一个类型，来看看其中一个方法：

public static  T instantiateClass(Class clazz) throws BeanInstantiationException {
	Assert.notNull(clazz, "Class must not be null");
	if (clazz.isInterface()) {
		throw new BeanInstantiationException(clazz, "Specified class is an interface");
	}
	try {
		Constructor ctor = (KotlinDetector.isKotlinType(clazz) ?
				KotlinDelegate.getPrimaryConstructor(clazz) : clazz.getDeclaredConstructor());
		return instantiateClass(ctor);
	}
	catch (NoSuchMethodException ex) {
		throw new BeanInstantiationException(clazz, "No default constructor found", ex);
	}
	catch (LinkageError err) {
		throw new BeanInstantiationException(clazz, "Unresolvable class definition", err);
	}
}

其中clazz.getDeclaredConstructor()获取到无参构造方法，然后来看看instantiateClass()方法：

public static  T instantiateClass(Constructor ctor, Object... args) throws BeanInstantiationException {
	Assert.notNull(ctor, "Constructor must not be null");
	try {
		ReflectionUtils.makeAccessible(ctor);
		return (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());
	}
}

然后直接调用ctor.newInstance(args)生成对象。所以粗略看来BeanUtils.instantiateClass(Class clazz)的功能就是调用通过无参构造函数实例化类型。