JAVA类装载的三个基本原则

1. 委托原则：当加载一个类时，首先会委托父加载器来加载

2. 可视化原则：子类加载器能看到父类加载器加载的所有类，但是反过来是不行的。同时，兄弟类加载器也看不到对方加载的类

3. 唯一性原则：如果一个类加载器加载了某个类，那么它的子类加载器是不会再加载该类了的。

Delegation Principle: If a class is not loaded already, the classloaders delegate the request to load that class to their parent classloaders.

Visibility Principle: Classes loaded by parent classloaders are visible to child classloaders but not vice versa.

Uniqueness Principle: When a classloader loads a class, the child classloaders in the hierarchy will never reload that class.

参考： http://www.objectsource.com/j2eechapters/Ch21-ClassLoaders_and_J2EE.htm