Spring boot @Value NullPointerException

Question

I'm writing a Spring Boot application and am trying to load some values from a properties file using the @Value annotation. However, the variables with this annotation remain null even though I believe they should get a value.

The files are located in src/main/resources/custom.propertes and src/main/java/MyClass.java .

(I have removed parts of the code that I believe are irrelevant from the snippets below)

MyClass.java

@Component
@PropertySource("classpath:custom.properties")
public class MyClass {
    @Value("${my.property:default}")
    private String myProperty;

    public MyClass() {
        System.out.println(myProperty); // throws NullPointerException
    }
}

custom.properties

my.property=hello, world!

What should I do to ensure I can read the values from my property file?

Thanks!

Answer 1

@value will be invoked after the object is created. Since you are using the property inside the constructor hence it is not available.

You should be using constructor injection anyway. It makes testing your class easier.

public MyClass(@Value("${my.property:default}") String myProperty) {
    System.out.println(myProperty); // doesn't throw NullPointerException
}

Answer 2

You are getting this error because you are initializing the class with new keyword. To solve this, first you need to create the configuration class and under this class you need to create the bean of this class. When you will call it by using bean then it will work..

My code:

        @Component
        @PropertySource("db.properties")
        public class ConnectionFactory {
            @Value("${jdbc.user}")
            private String user;
            @Value("${jdbc.password}")
            private String password;
            @Value("${jdbc.url}")
            private String url;
            Connection connection;

            @Bean
            public String init(){

                return ("the value is: "+user);
            }

        My Config.class:

        @Configuration
        @ComponentScan
        public class Config {

            @Bean
            public Testing testing() {
                return new Testing();
            }

            @Bean
            public ConnectionFactory connectionFactory() {
                return new ConnectionFactory();
            }
        }

        Calling it:

        public static void main(String[] args) {
                AnnotationConfigApplicationContext context= new AnnotationConfigApplicationContext(Config.class);
                ConnectionFactory connectionFactory= context.getBean(ConnectionFactory.class);
                System.out.println(connectionFactory.init());
            }