Java components / classes should be as independent as possible of other Java classes. This increases the possibility to reuse these classes and to test them independently of other classes(Unit Testing). To decouple Java components from other Java components the dependency to a certain other class should get injected into them rather that the class itself creates / finds this object.

A class A has a dependency to class B if class uses class B as a variable.

If dependency injection is used then the class B is given to class A via

The general concept between dependency injection is called Inversion of Control. A class should not configure itself but should be configured from outside.

A design based on independent classes / components increases the re-usability and possibility to test the software. For example if a class A expects a Dao (Data Access object) for receiving the data from a database you can easily create another test object which mocks the database connection and inject this object into A to test A without having an actual database connection.

A software design based on dependency injection is possible with standard Java.

Spring just add some simplifications in using dependency injection by providing a standard way of providing the configuration and by managing the reference to the created objects.