Reverse injection is to automatically assign values to some properties in your class through the framework. Generally speaking, it is a spring frame.

In 1, such as Action, Service and Dao, if you use the Spring framework, you can let the framework automatically inject values into the attributes in these classes, that is, initialize that attribute.

2. Spring is equivalent to a big container with everything in it. You must copy a property and get it from it, but you must first configure it in the Spring configuration file.