Dependency Injection
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 constructor of the class A - this is then called
construction injection
-
a setter - this is then called setter injection
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 simplifies the use of dependency injection
by providing a standard way of providing the configuration
and by managing the reference to the created objects.
The Spring Framework is a very comprehensive framework.
The fundamental functionality provided by the Spring Container
is dependency injection. Spring provides a light-weight container,
e.g. the Spring core container, for dependency injection (DI).
This container lets you inject required objects into other
objects. This results in a design in which the Java class are not
hard-coupled. The injection in Spring is either done via setter
injection of via construction injection.
These classes which are managed by Spring must conform to
the JavaBean standard.
In the context of Spring classes are also referred to as beans
or as spring beans.
The Spring core container:
-
handles the configuration, generally based on
annotations or on an XML file (XMLBeanFactory)
-
manages the selected Java classes via the BeanFactory
The core container uses the so-called bean factory to create
new objects. New objects are generally created as Singletons if not
specified differently.
