Spring 基于其他bean列表的bean创建

Question

Anyone can explain how Spring defines the bean creation mechanism when create a bean which depends on a list of other beans? It would be good to show the part of Spring specification on how it's defined.

Code like:

public interface Test {
}

@Service
public class TestImpl1 implements Test{
}

@Service
public class TestImpl2 implements Test{
}

public class TestContainer {
    List<Test> testList;
    TestContainer() {
        testList = new ArrayList<>();
    }

    public void addTest(Test test) {
        testList.add(test);
    }
}

then

@Bean
public TestContainer testContainer(List<Test> testList) {
    TestContainer testContainer = new TestContainer();
    for (Test test : testList) {
        testContainer.addTest(test);
    }
    return testContainer;
}

Question is really: when creating bean for TestContainer, how does Spring figure out what should be in List testList?

Answer 1

This looks like what you are looking for:

https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#beans-factory-class

You haven't specified what exactly do you want to know about bean creation but here is the minimum you should know. By default all beans are singleton scoped (only created once during the container life-cycle and for all the subsequent request the same instance is returned). All singleton scoped beans are created eagerly. If the singleton bean is dependent on some other beans (needs them for instantiation) then those other beans will be instantiated with it/right before it, doesn't matter whether they are singletons or not, marked as lazy or not.

Answer 2

This is part of the spring documentation I think on their website. Look for it. The whole spring documentation is worth to read, even if it is a long read ( https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#spring-core )

But mostly, spring would first read all the XML, anotations and other source of config data and get a list of beans to init. Then it would get a list of dependencies between beans constructing kind of a tree. As the dependencies need to be initialized first, there an obvious orderining for initialization.

On top, you can set your own priorities for beans so that you can get some beans initialized first or last for example.

For your specific case, spring will inspect the code source as well as use the Java reflection API to figure out you want a collection of interface Test implementations. So spring would look for ALL the defined bean that match and return them that not more complex than that.

Answer 3

It's mostly using the concepts of graph data structure where the beans become the nodes of the graph and they are resolved using topological sort.