SpringREST控制器中的java实例化模型

Question

我只需要简短地解释一下我的spring web应用程序中我不完全理解的一部分

我有一个简单的mvc spring web应用StudentController。对于数据，我有一个类StudentDataModelStub，它实现了IStudentDataModel接口，它有一些简单的函数来处理StudentDataModelStub类中List存储的数据

在{}课上我有

@Autowired private IStudentDataModel model;

当我删除@Autowired注释时，我得到了预期的NullPointerException，因为我没有实例化StudentDataModelStub。但是spring如何知道用哪个类实例化IStudentDataModel，因为多个类可以实现IStudentDataModel接口，以及为什么model必须是@Autowired

这是StudentDataModelStub类的一部分

public class StudentDataModelStub implements IStudentDataModel {

private final Map<Integer, Student> data = new HashMap<>();

@Override
public List<Student> getStudents() {
    // TODO Auto-generated method stub
    return new ArrayList<>(data.values());
}

@Override
public Student getStudent(int id) throws IdNotFoundException {
    // TODO Auto-generated method stub
    if(!data.containsKey(id)){
        throw new IdNotFoundException("...");
    }

    return data.get(id);

}

Answer 1

When I delete @Autowired annotation I get NullPointerException which is expected, because I don't instantiate StudentDataModelStub. But how does the spring knows which class to instantiate IStudentDataModel with, because multiple classes can implement IStudentDataModel interface?

Spring容器扫描@Componentscan（或<context:component-scan>内xml）中指定的包内的依赖项，并将它们注入bean

现在，关于注入哪个实现，您可以从Spring文档中找到以下文本（也可以查看here）（我的重点）

@Primary is an effective way to use autowiring by type with several instances when one primary candidate can be determined. When more control over the selection process is required, Spring’s @Qualifier annotation can be used. You can associate qualifier values with specific arguments, narrowing the set of type matches so that a specific bean is chosen for each argument.

简而言之，如果为同一个interface定义了多个实现，那么需要通过指定@Primary或@Qualifier来告诉容器要注入哪个bean

Why model must be @Autowired?

你的StudentDataModelStub类实际上不是一个模型（实体）类，它实际上是在缓存学生的数据，并根据id返回数据