先看API解释：
For ClassNotFoundException:
Thrown when an application tries to load in a class through its string name using:
* The forName method in class Class.
* The findSystemClass method in class ClassLoader.
* The loadClass method in class ClassLoader.
but no definition for the class with the specified name could be found.
For NoClassDefFoundError:
Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.
The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.
简单的说，就是如果使用new SomeClass()的方式创建对象的，而这时ClassLoader装载SomeClass失败（可能因为权限不够，JNI错误,etc），就会抛出NoClassDefFoundError。
而如果使用反射来装载一个对象，但是SomeClass不在CLASSPATH中，这时就会抛出ClassNotFoundException。