spring Dependency Injection (DI) and Inversion of Control (IoC) Autowiring a dependency through XML configuration
Example
Dependencies can be autowired when using the component scan feature of the Spring framework. For autowiring to work, the following XML configuration must be made:
<context:annotation-config/>
<context:component-scan base-package="[base package]"/>
where, base-package is the fully-qualified Java package within which Spring should perform component scan.
Constructor injection
Dependencies can be injected through the class constructor as follows:
@Component
class Bar { ... }
@Component
class Foo {
private Bar bar;
@Autowired
public Foo(Bar bar) { this.bar = bar; }
}
Here, @Autowired is a Spring-specific annotation. Spring also supports JSR-299 to enable application portability to other Java-based dependency injection frameworks. This allows @Autowired to be replaced with @Inject as:
@Component
class Foo {
private Bar bar;
@Inject
public Foo(Bar bar) { this.bar = bar; }
}
Property injection
Dependencies can also be injected using setter methods as follows:
@Component
class Foo {
private Bar bar;
@Autowired
public void setBar(Bar bar) { this.bar = bar; }
}
Field injection
Autowiring also allows initializing fields within class instances directly, as follows:
@Component
class Foo {
@Autowired
private Bar bar;
}
For Spring versions 4.1+ you can use Optional for optional dependencies.
@Component
class Foo {
@Autowired
private Optional<Bar> bar;
}
The same approach can be used for constructor DI.
@Component
class Foo {
private Optional<Bar> bar;
@Autowired
Foo(Optional<Bar> bar) {
this.bar = bar;
}
}
