Spring dependency injection got NullPointException when calling its class method

Question

I have the following Java class that has a field dao which is injected by constructor args:

public class FeatureFormationFromRaw {

    private MyDAOImpl dao;   //field to be injected from beam.xml

    public FeatureFormationFromRaw(MyDAOImpl dao) {
        //do a fresh clean and save all data, dependency injection by constructor args
        dao.removeAll();          //This is fine.
        dao.saveDataAll();        // This is fine.
    }


    public float calcuFeatures(String caseName) {

        List<JSONObject> rawData = dao.getData(caseName);    //This line throws NullPointException because dao=null here.
                .........
    }

    public static void main(String[] args) {

        ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        FeatureFormationFromRaw featureForm = (FeatureFormationFromRaw) context.getBean("featureFormation");

        float value = featureForm.calcuFeatures("000034");

    }

}

The bean configuration file bean.xml injects the MyDAOImpl object to the class via constructor args:

    <bean id="featureFormation" class="com.example.FeatureFormationFromRaw">
        <constructor-arg ref="myDaoImpl" />
    </bean>

    <bean id="myDaoImpl" class="com.example.MyDAOImpl">
    </bean>

I debugged my application and found that when the constructor FeatureFormationFromRaw(MyDAOImpl dao) is executed, dao gets the proper value from Spring bean injection. However, when the method calcuFeatures() is called, the variable dao is null at the first line. Why is that? Why does the variable dao disappear and become null after constructor call?

did you set private MyDAOImpl dao; this in your constructor..i.e this.dao = dao; — Jayaram
– Jayaram, Commented Sep 11, 2013 at 15:11
You aren't assigning the variable in your constructor. Also judging from the name you should be programming to interfaces MyDAO not implementations MyDAOImpl. — M. Deinum
– M. Deinum, Commented Sep 11, 2013 at 15:11
@M.Deinum: Thanks for your suggestion. I could use MyDAO rather than the implementing class MyDAOImpl in the constructor arg, but then I need to do a cast like this.dao = (MyDAOImpl) dao. Is it always a good practice to use interface than implementing class in constructor args? Any guidelines on using interface vs. concrete class in Spring DI constructor args? — tonga
– tonga, Commented Sep 11, 2013 at 15:32
Ideally you should use the interface both for the constructor arg and for the actual variable. This way you get all of the benefit (ability to hide implementation details, ability to easily test) of coding to the interface and of dependency injection. — Jacob Mattison
– Jacob Mattison, Commented Sep 11, 2013 at 15:58
@JacobM: Thanks for your comments. This reminds me the fundamentals of OO :). — tonga
– tonga, Commented Sep 11, 2013 at 16:10

Jacob Mattison · Accepted Answer · 2013-09-11 15:09:25Z

5

In your constructer, having had the dao passed in, you must assign the dao to your private variable. Otherwise you can't call it anywhere else.

add this.dao = dao; to your constructor.

In other words, when you call dao.removeAll() within the constructor, that works because it's using the parameter dao. But when you call dao.getData() in another method, it fails because it's using the private MyDAOImpl dao; which hasn't been initialized. The injection puts it into the constructor, but doesn't put it into the private variable. You have to do that.

answered Sep 11, 2013 at 15:09

Jacob Mattison

51.2k11 gold badges109 silver badges128 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

tonga Over a year ago

Thanks. This was what caused the NullPointException. I forgot this line after I get used to DI via setting method.

Jayaram · Accepted Answer · 2013-09-11 15:13:10Z

1

private MyDAOImpl dao;   //field to be injected from beam.xml

    public FeatureFormationFromRaw(MyDAOImpl dao) {
        //do a fresh clean and save all data, dependency injection by constructor args
        dao.removeAll();          //This is fine.
        dao.saveDataAll();        // This is fine.
    }

add this.dao = dao; inside your constructor ..which has not be assigned so when you are using with other method that time its null so you end of with NPE

answered Sep 11, 2013 at 15:13

Jayaram

1,72518 silver badges31 bronze badges

Collectives™ on Stack Overflow

Spring dependency injection got NullPointException when calling its class method

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related