Spring does not inject beans in my java application-CodePudding

I have two classes and I want to inject one class into another using Spring

public class A {
    private B b;
    ...
}

@Component
public class B {
    ...
}

But when I try to call b object method i cath NullPointerException. And I didn't understand why Spring didn't inject bean in A class. Can someone explain to me what's wrong?

I have read all the Spring documentation in the world and have not found a solution.

CodePudding user response：

Note that according to your code A is not managed by Spring (it is not annotated by it), and therefore, nothing instantiates B

To solve this you should mark A as a component as well:

@Component
public class A {
    private B b;
    ...
}

@Component
public class B {
    ...
}

Moreover, it's always better to use constructor injection over property injection and/or setter-getter injections, so long story short I would suggest this approach:

@Component
public class A {
    private final B b;

    public A(final B b) {
        this.b = b;
    }
}

@Component
public class B {
        ...
}

CodePudding user response：

You only have allowed your Class B to be treated as Spring Bean (Managed by Spring Bean Lifecycle) by annotating it with @Component. That's fine. But you haven't done same with Class A. Spring does not know about this class and does not manage it, so it will not provide you with a reference of B into A.

You can use constructor based injection

@Component
public class A {
    private final B b;

    @Autowired
    public A(B b){
     this.b = b; 
    }

    // other methods
}

@Component
public class B {
    ...
}

CodePudding user response：

You should use the @Autowiring annotation on the b field. That fix your problem