问题
I'm using the @Autowired annotation under a @Configuration class constructor.
@Configuration
public class MyConfiguration {
private MyServiceA myServiceA;
private MyServiceB myServiceB
@Autowired
public MyConfiguration(MyServiceA myServiceA, MyServiceB myServiceB){
this.myServiceA = myServiceA;
this.myServiceB = myServiceB;
}
}
As the Spring documentation sais, I'm able to declare whether the annotated dependency is required.
If I mark the @Autowired annotation under the constructor as required=false, I'm saying that the two services to be autowired are not required (as the Spring documentation sais):
@Autowired(required = false)
public MyConfiguration(MyServiceA myServiceA, MyServiceB myServiceB){
this.myServiceA = myServiceA;
this.myServiceB = myServiceB;
}
From Spring documentation:
In the case of multiple argument methods, the 'required' parameter is applicable for all arguments.
How can I set the required attribute to each constructor parameter individually? Is necessary to use @Autowired annotation under every field?
Regards,
回答1:
If you're using Java 8 and Spring Framework 4, you can use Optional.
@Autowired
public MyConfiguration(Optional<MyServiceA> myServiceA, Optional<MyServiceB> myServiceB){
myServiceA.ifPresent(service->{this.myServiceA = service});
myServiceB.ifPresent(service->{this.myServiceB = service});
}
回答2:
Explicit approach
Basically, you have a bean which have some required and optional dependencies. The recommended way of handling this scenario, not only configuration beans but any other, is to create a constructor only for mandatory dependencies and use setter injection for optional ones.
public class MyConfiguration {
private final MyServiceA myServiceA;
private MyServiceB myServiceB
@Autowired
public MyConfiguration(MyServiceA myServiceA){
this.myServiceA = myServiceA;
}
@Autowired(required = false)
public void setMyServiceB(MyServiceB myServiceB) {
this.myServiceB = myServiceB;
}
}
With this approach you can easily unit test the class without necessity for any mocking library. You can create an object in testing state using the constructor and optional setters.
Putting @Autowired(required = false) directly on the field and removing the setter will also work, but since you are using the constructor injection, I assume you want to state dependencies more explicitly.
Additional idea
You can also consider using the Optional type to wrap not mandatory dependencies. It is common among developers to assume that if a class has a property, it should be set, which is obviously not right in your scenario. To mark the possibility of absence for particular dependencies more clear you probably can use Optional:
public class MyConfiguration {
private final MyServiceA myServiceA;
private Optional<MyServiceB> myServiceB
@Autowired
public MyConfiguration(MyServiceA myServiceA){
this.myServiceA = myServiceA;
this.myServiceB = Optional.empty();
}
@Autowired(required = false)
public void setMyServiceB(MyServiceB myServiceB) {
this.myServiceB = Optional.ofNullable(myServiceB);
}
}
Some people are against using the Optional type for class properties (mainly because of this answer from Brian Goetz), but at the end of the day it should be the decision made by the whole team that is going to work on the project.
回答3:
Since Spring 4.3.0.RC1 you can do this:
public MyConfiguration(MyServiceA myServiceA, @Autowired(required = false) MyServiceB myServiceB){
this.myServiceA = myServiceA;
this.myServiceB = myServiceB;
}
As ElementType.PARAMETER was added as annotation target.
From Spring 5.0 @Autowired(required = false) can also be replaced by @Nullable or a Kotlin nullable type can be used without any annotation, e.g. MyServiceB?
回答4:
As of Spring Framework 5.0, you can also use a @Nullable annotation (of any kind in any package — for example, javax.annotation.Nullable from JSR-305):
@Configuration
public class MyConfiguration {
private MyServiceA myServiceA;
private MyServiceB myServiceB
@Autowired
public MyConfiguration(@Nullable MyServiceA myServiceA, MyServiceB myServiceB){
this.myServiceA = myServiceA;
this.myServiceB = myServiceB;
}
}
来源：https://stackoverflow.com/questions/42135102/how-to-set-autowired-constructor-params-as-required-false-individually