@PostConstruct

In this article we will talk about @PostConstruct. Let’s start with simple example

@Service
@Slf4j
public class SomeServiceImpl implements SomeService{
    public SomeServiceImpl(){
        log.info("From constructor");
    }
    @PostConstruct
    public void postConstruct(){
        log.info("From PostConstruct");

    }
}

If we run our application we will see output displayed on following sequence

From constructor
From PostConstruct

From the output we can see that method annotated with @PostConstruct is called after the Constructor.

@PostConstruct is an annotation which can be used on a method that needs to be executed after dependency injection is done to perform any initialization. So method annotated with @PostConstruct are called after the Constructor and after all the dependency of the class are resolved too. This kind of behaviour is not much important when we are doing Constructor Injection. But if we are using Setter or Field Injection then dependency of the class may not be resolved when Constructor is invoked. For example

@Service
@Slf4j
public class SomeServiceImpl implements SomeService{

    @Autowired
    SomeRepository someRepository;

    public SomeServiceImpl(){
        someRepository.someMethod();
        log.info("From constructor");
    }

    @PostConstruct
    public void postConstruct(){
        log.info("From PostConstruct");

    }
}

In above code we are Autowiring (Field Injection) SomeRepository and we are calling the method of someRepository on Constructor. If we try to run this program then we will get a NullPointerException on Line Number 9. When Constructor is invoked SomeRepository is not yet resolved and is still null.

If we move the method call from Constructor to @PostConstruct like below

@Service
@Slf4j
public class SomeServiceImpl implements SomeService{

    @Autowired
    SomeRepository someRepository;

    public SomeServiceImpl(){
        log.info("From constructor");
    }

    @PostConstruct
    public void postConstruct(){
        someRepository.someMethod();
        log.info("From PostConstruct");

    }
}

all the dependencies will be resolve and instance of someRepository will not be null when @PostConstruct is called.

Some Key Points #

• @PostConstruct is called after Constructor
• By the time @PostConstruct is called all the dependencies of the class will already be resolved
• Can be annotated for one method per class
• The method on which @PostConstruct is applied MAY be public, protected, package private or private.
• The method MUST NOT be static except for the application client.
• The method MAY be final.

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