Null pointer exception for autowired class method

Question

I am new to Apache Camel and trying to build an api. It has three classes.

Class A - this is for reading application.properties file as below

@Configuration
@ConfigurationProperties(prefix="api")
public class A{

private String username;

public String getUsername(){
    return username
}

Class B - Apache Camel Route.

Class C - Bean class where class A is autowired.

@Component
public class b extends RouteBuilder{
    @Autowired
    ClassC customC;

    public void configure{
        from("direct:start")
            .bean(customC)
            .to("log:first-timer")
    }
}

@Component
class C{
    @Autowired
    ClassA config;

    String username=config.getUsername();
}

Whenever IDE executes the api, it throws Null pointer exception in the line String username=config.getUsername(); .I have verified that spring boot is able to load application.properties.

I am not sure why calling method of autowired class is throwing null pointer exception.

Answer 1

The problem is in your C class.

@Component
class C{
    @Autowired
    A config;

    String username = config.gtUsername();
}

The sequence during Spring's initialization was like this:

1. Create an instance of C
   (by calling new C())
   This constructor calls username = config.gtUsername() and fails with a NullPointerException because config is still null.
2. Process the @Autowired
   (initialize your config property)
   But of course it doesn't get this far because the exception already happened in step 1.

---

You can fix this problem by removing the initialization of your username property from the constructor and instead put it into a separate method (let's call it init()) and annotate it with @PostConstruct. See also Spring PostConstruct and PreDestroy Annotations.

@Component
class C{
    @Autowired
    A config;

    String username;
    
    @PostConstruct
    void init() {
        username = config.getUsername();
    }
}

Then the sequence during Spring's initialition will be like this:

1. Create a C instance
   (by calling new C())
2. Process the @Autowired.
   (This initializes your config property)
3. Call the @PostConstruct-annotated method.
   (This initializes your username property using the already initialized config property)

---

Another even simpler way to fix your problem, is to use constructor injection (as @Ferry already suggested in his comment):

Instead of relying on the implicit default constructor, you provide an @Autowired-annotated constructor which can take an A config parameter. Furthermore: By doing so, you will not need the A config property anymore.

@Component
class C{ 
    String username;

    @Autowired
    C(A config) {
        username = config.getUsername();
    }
}

Then the sequence during Spring's initialization will be like this:

1. Process the @Autowired
   (create an instance of C by calling new C(config) where config is an A instance previously created during the autowiring process)
   This initializes your username property using the config received by the constructor.