5.3. Bijection
Dependency injection or inversion of control (IoC) allows one component to reference another by having the container "inject" the component into a setter method or instance variable. In previous dependency injection implementations, injection occurred at component construction, and the reference did not change for the lifetime of the component instance. This is reasonable for stateless components — from the client's perspective, all instances of a particular stateless component are interchangeable. However, Seam emphasizes the use of stateful components, so traditional dependency injection as a construct is less useful. Seam introduces the notion of bijection as a generalization of injection. In contrast to injection, bijection is:
- contextual
- Bijection is used to assemble stateful components from various different contexts. A component from a wider context can even refer to a component from a narrower context.
- bidirectional
- Values are injected from context variables into attributes of the invoked component, and returned (via outjection) to the context, allowing the invoked component to manipulate contextual variable values simply by setting its own instance variables.
- dynamic
- Since the value of contextual variables changes over time, and since Seam components are stateful, bijection takes place every time a component is invoked.
In essence, bijection lets you alias a context variable to a component instance variable, by specifying that the value of the instance variable is injected, outjected, or both. Annotations are used to enable bijection.
The
@In annotation specifies that a value should be injected, either into an instance variable:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
@In User user;
...
}
or into a setter method:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
User user;
@In
public void setUser(User user) { this.user=user; }
...
}
By default, Seam performs a priority search of all contexts, using the name of the property or instance variable being injected. You may wish to specify the context variable name explicitly, using, for example,
@In("currentUser").
If you want Seam to create an instance of the component, where there is no existing component instance bound to the named context variable, you should specify
@In(create=true). If the value is optional (it can be null), specify @In(required=false).
For some components, specifying
@In(create=true) each time it is used can be repetitive. In such cases, annotate the component @AutoCreate. This way, it will always be created whenever required, even without the explicit use of create=true.
You can even inject the value of an expression:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
@In("#{user.username}") String username;
...
}
Injected values are disinjected (that is, set to
null) immediately after method completion and outjection.
(More information about component lifecycle and injection can be found in the next chapter.)
The
@Out annotation specifies that an attribute should be outjected, either from an instance variable:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
@Out User user;
...
}
or from a getter method:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
User user;
@Out
public User getUser() {
return user;
}
...
}
An attribute may be both injected and outjected:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
@In
@Out User user;
...
}
or:
@Name("loginAction")
@Stateless
public class LoginAction implements Login {
User user;
@In
public void setUser(User user) {
this.user=user;
}
@Out
public User getUser() {
return user; }
...
}