Why is my Spring @Autowired field null?

[R Vicky]

I am using @Autowired for using dependency of a different class but getting NPE. My code is a simple java spring code

Code :

@Component
public class MainClass {

	@Autowired
	private static KeyLogger obj;
	
	public static void main(String[] args) {
		
		System.out.println("HI");
		System.out.println(obj.withField("key", "89899"));
	}
	
}

Output :
HI
Exception in thread "main" java.lang.NullPointerException
at test.MainClass.main(MainClass.java:24

[Rob Spoor]

Welcome to the Ranch!

@Autowired only works on fields of bean instances. It does not work on static fields. It also does not work if you create bean instances yourself (using new). You need to let both the bean, and the object you want autowired, be managed by Spring. In your example Spring isn't even involved yet.

[R Vicky]

@Rob, if I remove the static from @Autowired, it gives "Cannot make a static reference to the non-static field obj"

Below is the method withField() that I want to invoke to log messages in key value pair. COuld you please correct what changes I need to make. Thanks in advance.

public class KeyLogger {

    
    private final Map<String,Object> _fieldsMap = new LinkedHashMap<>();
    
    public TestLogger withField(String key, Object value) {
        _fieldsMap.put(key, value);
        return this;
    }
    
    private KeyLogger(String message) {
        withField(MESSAGE_FIELD_KEY, message);
    }
       @Override
        public String toString() {
            final StringBuilder sb = new StringBuilder();
            boolean firstIteration = true;
            for (Map.Entry<String, Object> field : _fieldsMap.entrySet()) {
                String key = field.getKey();
                String value;
               
                    value = Objects.toString(field.getValue(), StringUtils.EMPTY);
                
                // escape all double quotes
                value = StringUtils.replace(value, DOUBLE_QUOTES, ESCAPED_DOUBLE_QUOTES);

                // surround the value in double quotes if it contains a space
                if (value.contains(StringUtils.SPACE)) {
                    value = DOUBLE_QUOTES + value + DOUBLE_QUOTES;
                }

                if (firstIteration) {
                    firstIteration = false;
                } else {
                    sb.append(KEY_VALUE_PAIR_SEPARATOR);
                }
                sb.append(key).append(KEY_VALUE_DELIMITER).append(value);
            }
            return sb.toString();
        }
        
}

[Rob Spoor]

R Vicky wrote:@Rob, if I remove the static from @Autowired, it gives "Cannot make a static reference to the non-static field obj"

You need to:
* Create a Spring context.
* Get an instance of MainClass from the context.
* Access the instance's field.

I'm not sure if your KeyLogger can actually be auto-wired. It's not a Spring bean, and even if it were, it does not have a constructor that Spring can call. For each bean constructor, Spring must be able to find proper values for the parameters. Either because they are annotated (e.g. @Value), or because it's another Spring bean.
How are you even planning on creating KeyLogger instances?

[Ankit Garg]

I think you might want to start by looking at a basic Spring program, few example project tutorials I found are listed below:

https://www.tutorialspoint.com/spring/spring_hello_world_example.htm
https://www.vogella.com/tutorials/SpringDependencyInjection/article.html
https://dzone.com/articles/dependency-injection-in-spring

There are multiple ways to make Spring work, java config, xml config, component-scan to automatically find the beans (or a combination of these). Dependency injection can also be explicit or auto-wiring. You should start with whatever approach you find easiest to understand first.

[Tim Holloway]

Rob Spoor wrote:
I'm not sure if your KeyLogger can actually be auto-wired. It's not a Spring bean, and even if it were, it does not have a constructor that Spring can call. For each bean constructor, Spring must be able to find proper values for the parameters. Either because they are annotated (e.g. @Value), or because it's another Spring bean.
How are you even planning on creating KeyLogger instances?

As far as I can recall, Spring doesn't care. Although the ideal components for Spring are JavaBeans (and especially POJOs), I'm pretty sure that any class that Spring can instantiate via newInstance() can be served by the BeanFactory.

So you can't autowire completely static stuff, but with proper Spring configurations, any instantiable class should be fair game.

[Himai Minh]

Hi, R Vicky,
Maybe, put @Component to annotate KeyLogger class.
Also, make sure you have @ComponentScan in your "runner" class , the one with the main method.
@ComponentScan  is to scan the packages and sub packages of the "runner" class.
Make sure your KeyLogger is in the same package as the runner class or in the subpackages.

The reason why I suggest this because there should be a KeyLogger  bean in order to autowire it.
By annotating KeyLogger as @Component, the Spring container will create a bean out of KeyLogger class.
Reference:
https://www.baeldung.com/spring-component-scanning