Give me an example:

我的一个类里面引入了：

import org.apache.commons.lang3.StringUtils;

At this time, my class can use the methods in the StringUtils class:

boolean isEmpty = StringUtils.isNotEmpty(str);

That’s right, you can directly use the method in it.

Then suppose that I want to use the methods in StringUtils provided by spring in this class, but I do not import spring's StringUtils class. What should I do at this time? We can do this (provided there is a spring jar):

boolean isEmpty = org.springframework.util.StringUtils.isEmpty(str);

That’s right, it’s the fully qualified name to access spring’s StringUtils class.

This is my understanding, any corrections are welcome.

Just check JLS for this kind of problem
6.7. Fully Qualified Names and Canonical Names