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Why does the nested class in a ConfigurationProperties class need to be static?

I am using an application.yml file as the external configuration in my spring boot application which is bound to AppConfig.java using the #ConfigurationProperties annotation. Inside AppConfig.java I have nested classes based on the hierarchy in application.yml. When I use static to declare the nested classes, everything works fine. But recently in a project I missed static for one of the nested classes and it led to a NullPointerException. Through online resources, I have read when to and when not make nested classes static. However, I need to understand how the binding of application.yml and AppConfig.java occurs in spring boot and why the nested classes need to be static.
application.yml
spring:
foo:
host: localhost
bar:
endpoint: localhost
AppConfig.java
#Component
#ConfigurationProperties("spring")
public class AppConfig {
private Foo foo;
private Bar bar;
public static class Foo {
private String host;
// getter, setter
}
public class Bar {
private String host;
// getter, setter
}
//getters, setters
}
When I Autowire AppConfig.java in other classes, appConfig.getFoo() works fine but appConfig.getBar() leads to a NullPointerException.

I have not found why it must be static particularly.
However, I have found other comment in the documentation. It is related to the validation of #ConfigurationProperties bean.
Here it is:
The configuration properties validator is created very early in the application’s lifecycle, and declaring the #Bean method as static lets the bean be created without having to instantiate the #Configuration class.
It was about inner class.
More details : https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-external-config-validation

Spring is using a Binder to apply the properties to ConfigurationProperty beans.
The Binder, that Spring uses, can create instances, if a getX() doesn't return a class. This however is limited to simple constructors. A class, which is not static, can only be instantiated with a reference to the instance, in which it is contained.
If this is REALLY, what you need, you could create an instance of the class during bean creation.
#Component
#ConfigurationProperties("spring")
public class AppConfig {
private Foo foo = new Foo();
private Bar bar = new Bar();
public static class Foo {
private String host;
// getter, setter
}
public class Bar {
private String host;
// getter, setter
}
//getters, setters
}

The inner class has to be static as spring while instantiating the main class, won't be able to set the properties for the inner class because there is no instance yet. So the inner static class is frist of all instantiated by the classloader then, spring will be able to set its properties.

#Autowired a class member in Java #Configuration file

in these days i'm studying how to define beans with Spring Core, in the XML file and/or the Java notation. These methods seem equivalent, but are they still so if we add the following condition?
It is not possible to change the class file.
In this case, everything written in the XML file should be written in the AppConfig file (the one in which the class has the #Configuration annotation).
Sadly i found a counterexample: setting a class member (an attribute) with the autowire. While it is possible to precede the class member definition with the #Autowired annotation, that's not the case when using only the Java configuration file.
#Component
public class Foo{
#Autowired
private Bar bar;
}
The following code in fact is wrong:
public class AppConfig{
#Bean
Foo foo(){
Foo foo = new Foo();
#Autowired
Bar foo.bar ?? ;
return foo;
}
}
I've already searched every documentation and every question, but I'm still in confusion about this topic.

Let the method Foo foo() accept the Bar bar as the parameter. Spring framework recognizes the bean instance available by its type to be injected. Beware of more bean instances of Bar, then the #Qualifier must be used to differentiate them and inject the correct one. There is no need to use #Autowired again.
#Bean
Foo foo(Bar bar){
Foo foo = new Foo();
foo.method(bar); // Or whatsoever...
return foo;
}
Read more at D-Zone article about injecting beans in Spring, it might help.

Spring Inheritance - Annotation

I came across this link which explains how a bean can be inherited. Assuming that HelloWorld class in this example is exposed as a bean using #Component annotation , how can create another bean which inherits this bean? Can I use extends to inherit HelloWorld bean and add #Component to the new class in order to extend the existing bean expose it as a new bean with additional features?

First you make your abstract configuration, which is achieved by not marking it as #Configuration, like this:
// notice there is no annotation here
public class ParentConfig {
#Bean
public ParentBean parentBean() {
return new ParentBean();
}
}
An then you extend it, like this:
#Configuration
public class ChildConfig extends ParentConfig {
#Bean
public ChildBean childBean() {
return new ChildBean();
}
}
The result will be exactly the same as if you did this:
#Configuration
public class FullConfig {
#Bean
public ParentBean parentBean() {
return new ParentBean();
}
#Bean
public ChildBean childBean() {
return new ChildBean();
}
}
Edit: answer to the follow-up question in the comment.
If Spring picks up both classes, parent and child, there will be problems with duplicated beans, so you cannot extend it directly. Even if you override methods, the beans from the super-class will also be instantiated by the ParentConfig.
Since your parent class is already compiled, you have 2 options:
Talk to the author of the parent class and kindly ask him to change it.
Change the #ComponentScan packages.
To clarify on solution 2:
If the parent class is in the package com.parent.ParentConfig and the child class is the package com.child.ChildConfig, you can configure the component scanning so that only classes under com.child get picked up.
You can specify the component scanning packages using the #ComponentScan("com.child") annotation on your main configuration file (think application class).

What is the purpose of the Spring Bean annotation name attribute?

I'm just learning spring, and something struck me as very odd about the annotation configurations using the name attribute as a string.
#Bean(name = "com.my.injected.Service")
public InjectedService injectedService() {
return injectedService;
}
Is this name similar to the Spring Bean XML configuration id and class attributes?
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
Why isn't this simply
#Bean(clazz = com.my.injected.Service.class)
public InjectedService injectedService() {
return injectedService;
}
instead?
You're fully qualifying the path in both cases and actually using the class makes it way easier for your IDE to tell you when you've screwed it up. I understand that the XML configuration came first, and naturally it was always looking up things by string, so is this just a holdover? Is there some advantage to using strings or major disadvantage to using .class?
Question was originally based on a false premise. I edited it to spell out what this premise was and make it less confusing for new people who come along. Hopefully I did this such that the given answers are still exactly applicable; apologies if not.

#Bean annotation is meant to provide a spring bean. The type of the bean to provide will be the same type of the class/interface you define in the return method. So, instead of declaring to return a concrete class in the method, return the top (abstract) class/interface instead.
Imagine this case:
public interface MyEntityDao {
MyEntity get(String id);
}
#Repository
public class MyEntityDaoDatabaseImpl implements MyEntityDao {
#Override
public MyEntity get(String id) {
/* implementation that goes to database every time */
}
}
#Repository
public class MyEntityDaoCacheImpl implements MyEntityDao {
#Override
public MyEntity get(String id) {
/* implementation that looks the data
up in cache, never in database */
}
}
#Configuration
public class MyAppConfiguration {
#Bean
public MyEntityDaoDatabaseImpl method1() {
return new MyEntityDaoDatabaseImpl();
}
#Bean
public MyEntityDaoCacheImpl method2() {
return new MyEntityDaoCacheImpl();
}
}
#Service
public class MyEntityService {
#Autowired //what to inject here?
MyEntityDao dao;
}
In case above, there are two implementations of the proposed interface. How the framework may be able to understand which implementation to use except for the name?
#Service
public class MyEntityService {
#Autowired
#Qualifier("properBeanNameToInject")
MyEntityDao dao;
}

Bean name is not necessarily related to its class or even any of interfaces it implements. It is a name and nothing more. When you use the annotation configuration, Spring figures out what the exact class or interface the #Bean provides like the rest of java code would: either through the fully qualified name in the code or through the imports specified in the file. In your case, you presumably have an import com.my.injected.Service; statement at the top of the java file.
Your example is using the fully qualified class name as the bean name. It is your choice. You could use any other identifier. Using the fully qualified name could be useful if your code is providing an object that is named exactly like another 3rd party #Bean object that your code must include or consume. However, you could just as easily use name = "myService".
The bean name helps Spring (and application programmer) to distinguish between multiple instances of of the same bean class because you can deploy the same class as bean several times. If only one instance of bean type appear you event do not have to give it name manually: spring does this by default.
If you have several beans that have the same type or implement the same interface and you want to refer specific bean use #Qualifier annotation.

Spring Autowire Annotation with Several Interface Implementations

Suppose you have one interface
public interface A {
public void doSomething();
}
and two implementation classes
#Component(value="aImpl1")
public class AImpl1 implements A {
}
#Component(value="aImpl2")
public class AImpl2 implements A{
}
And finally a class that will use an "A" implementation:
#Component
public class MyClass {
#Autowire
A a;
}
Now if I want to inject AImpl1 I add the #Qualifier("aImpl1") while if I want to inject AImpl2 I add #Qualifier("aImpl2")
The question is: Is it possible to instruct spring somehow to look up all implementations of "A" in this case AImpl1 and AImpl2 and use some application specific conventions to choose the most appropriate implementation? for example in this case my convention could be use the implementation with the greatest suffix (i.e. AImpl2)?
EDIT: the class MyClass should not be aware at all about the implementation lookup logic, it should just find its property "a" set with an object of AImpl2.

You can inject all implentations as List:
#Autowired
List<A> as;
or as Map with bean name as key:
#Autowired
Map<String, A> as;
and then choose proper implementation manually (perhaps, in a setter method):
#Autowired
public void setAs(Map<String, A> as) {
this.a = ...;
}

Assuming you already have hundreds of interfaces and implementations (as you said in a comment), and you do not want to refactor all the code... then is a tricky problem... and this is a tricky solution:
You could create a custom BeanDefinitionRegistryPostProcessor and implement either the method postProcessBeanDefinitionRegistry or postProcessBeanFactory.
This way you have access to all bean definitions before they are instantiated and injected. Do your logic to find which is the preferred implementation for each one of your interfaces, and then, set that one as primary.
#Component
public class CustomBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
#Override
public void postProcessBeanDefinitionRegistry(
BeanDefinitionRegistry registry) throws BeansException {
// this method can be used to set a primary bean, although
// beans defined in a #Configuration class will not be avalable here.
}
#Override
public void postProcessBeanFactory(
ConfigurableListableBeanFactory beanFactory) throws BeansException {
// here, all beans are available including those defined by #configuration, #component, xml, etc.
// do some magic to somehow find which is the preferred bean name for each interface
// you have access to all bean-definition names with: beanFactory.getBeanDefinitionNames()
String beanName = "aImpl2"; // let's say is this one
// get the definition for that bean and set it as primary
beanFactory.getBeanDefinition(beanName).setPrimary(true)
}
}
The hard part is to find the bean name, it depends of the specifics of your application. I guess that having a consistent naming convention will help.
Update:
It seems that both methods in the interface BeanDefinitionRegistryPostProcessor can be used for this purpose. Having in mind that in the postProcessBeanDefinitionRegistry phase, beans configured through #configuration classes are not yet available, as noted in the comments below.
On the other hand they are indeed available in postProcessBeanFactory.

If you have a Configuration class you could use a method in that to make the decision of which implementation of A to return. Then the autowired will inject the appropriate instance for that class.
#Configuration
public class ApplicationConfiguration {
#Bean
A getA() {
// instantiate the implementation of A that you would like to have injected
// or you could use reflection to find the correct class from the classpath.
// return the instance
}
}
This assumes you always want to use the same instance everywhere you are injecting A. If not, then you could have different #Bean annotated methods with names to get different versions.

You can try to use Spring Profiles.