inside when() you don't call method on mock but on some other object
错误原因:调用静态方法,要事先引入静态类,否则mock的时候会默认为测试的类
解决方法:@PrepareForTest({SecurityContextHolder.class})引入静态类
注:@PrepareForTest在运行测试用例时,会创建一个新的org.powermock.core.classloader.MockClassLoader实例,然后加载该测试用例使用到的类(系统类除外)