Spring beans are classes. Instead of instantiating a class (using new), you get an instance as a bean cast to your class type from the application context, where the bean is what you configured in the application context configuration. This way, the whole application maintains singleton-scope instance throughout the application. All beans are initialized following their configuration order right after the application context is instantiated. Even if you don't get any beans in your application, all beans instances are already created the moment after you created the application context.

Spring核心技术参考文档描述了bean是什么。

根据Spring IoC容器和bean的介绍部分(其中“IoC”表示“控制反转”):

在Spring中，构成应用程序主干并由Spring IoC容器管理的对象称为bean。bean是由Spring IoC容器实例化、组装和管理的对象。否则，bean只是应用程序中的众多对象之一。bean以及它们之间的依赖关系反映在容器使用的配置元数据中。

Bean和作用域在Bean作用域部分中描述:

When you create a bean definition, you create a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, as with a class, you can create many object instances from a single recipe. You can control not only the various dependencies and configuration values that are to be plugged into an object that is created from a particular bean definition but also control the scope of the objects created from a particular bean definition. This approach is powerful and flexible, because you can choose the scope of the objects you create through configuration instead of having to bake in the scope of an object at the Java class level. Beans can be defined to be deployed in one of a number of scopes.

Spring的XML配置由bean组成，而bean基本上是类。它们只是我们在ApplicationContext中使用的pojo。定义bean可以看作是替换关键字new。所以无论你在应用程序中使用关键字new，比如:

MyRepository myRepository =new MyRepository ();

当你使用关键字new时你可以删除配置并将其放入XML文件中。 所以我们将像这样编码:

<bean name="myRepository " 
      class="com.demo.repository.MyRepository " />

现在我们可以简单地使用Setter注入/构造函数注入。我正在使用Setter注入。

public class MyServiceImpl implements MyService {
    private MyRepository myRepository;
    public void setMyRepository(MyRepository myRepository)
        {
    this.myRepository = myRepository ;
        }
public List<Customer> findAll() {
        return myRepository.findAll();
    }
}

Spring bean只是由Spring容器管理的实例对象，也就是说，它们是由框架创建和连接的，并放入一个“对象袋”(容器)中，您以后可以从那里获取它们。

“连接”部分是依赖注入的全部内容，它的意思是你可以说“我将需要这个东西”，框架将遵循一些规则来为你获得适当的实例。

对于那些不习惯Spring的人来说，我认为Wikipedia Spring的文章有一个很好的描述:

Central to the Spring Framework is its inversion of control container, which provides a consistent means of configuring and managing Java objects using reflection. The container is responsible for managing object lifecycles of specific objects: creating these objects, calling their initialization methods, and configuring these objects by wiring them together. Objects created by the container are also called managed objects or beans. The container can be configured by loading XML files or detecting specific Java annotations on configuration classes. These data sources contain the bean definitions which provide the information required to create the beans. Objects can be obtained by means of either dependency lookup or dependency injection. Dependency lookup is a pattern where a caller asks the container object for an object with a specific name or of a specific type. Dependency injection is a pattern where the container passes objects by name to other objects, via either constructors, properties, or factory methods.

For Spring, all objects are beans! The fundamental step in the Spring Framework is to define your objects as beans. Beans are nothing but object instances that would be created by the spring framework by looking at their class definitions. These definitions basically form the configuration metadata. The framework then creates a plan for which objects need to be instantiated, which dependencies need to be set and injected, the scope of the newly created instance, etc., based on this configuration metadata. The metadata can be supplied in a simple XML file, just like in the first chapter. Alternatively, one could provide the metadata as Annotation or Java Configuration.

书:Just Spring

Spring beans are classes. Instead of instantiating a class (using new), you get an instance as a bean cast to your class type from the application context, where the bean is what you configured in the application context configuration. This way, the whole application maintains singleton-scope instance throughout the application. All beans are initialized following their configuration order right after the application context is instantiated. Even if you don't get any beans in your application, all beans instances are already created the moment after you created the application context.