using Spring property when initializing a class

Question

The following produces a null error ("A" is null), but I am not sure why. Is the bean instantiated before the property value is set?

package org.ets.readtogether.queuing;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component("Abean")
public class Test {

    @Value("${send.timeout.secs}")
    public Integer A;

    public int B = A * 1000;
}

Well yeah, how do you set a field without there being an object? — Sotirios Delimanolis, Nov 21 '18 at 14:53
You can also use SpEL in the `@Value` annotation: `@Value("#{systemProperties['send.timeout.secs'] * 1000}")` — daniu, Nov 21 '18 at 15:07

Essex Boy · Answer 1 · 2018-11-21T15:05:22.137

3

Try this:

@Component("Abean")
public class Test {

    @Value("${send.timeout.secs}")
    public Integer A;

    public int B;

    @PostConstruct
    public void init() {
       B = A * 1000;
    }
}

Good example here

edited Nov 21 '18 at 15:05

answered Nov 21 '18 at 14:55

Essex Boy

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score 2 · Answer 2 · answered Nov 21 '18 at 15:03

Spring adds all the dependencies injected in an object after instantiating it. Object instantiation precedes any @Autowired dependency injection or @Value value assignment done by Spring.

The instantiation of your class's object fails even before Spring has the object to inject dependencies because the statement public int B = A * 1000; is called during object instantiation.

In order to assign the variable B a value after Spring has finished all injections, perform the operation in either a @PostConstruct method or in an @AutoWired constructor.

   public int B; // remove the assignment here.

   @PostConstruct
   public void postConstruct () {
       this.B = A * 1000;
   }

The above method would be called after the object has been instantiated and after Spring has done its work.

using Spring property when initializing a class

2 Answers2