c == null
URLClassLoader.findClass()
class = c
parent == null
class == null
return c
parent != null
defineClass()实际进行类加载5个步骤加载--验证--准备--解析--初始化
c = findBootstrapClassOrNull(name)
class == URLClassLoader.findClass()(AppClassLoader对象执行findClass()）
Class c = findLoadedClass
ExtClassLoader.loadClass()
return class
c = defineClass()(实际 c = null)return c
c = parent.loadClass(name)
class = findBootstrapClassOrNull(name)return null
双亲委派机制源码分析
c = URLClassLoader.findClass()（ExtClassLoader对象执行findClass()）
Class class = findLoadedClass
AppClassLoader.loadClass()
