Why use @PostConstruct?

Question

In a managed bean, @PostConstruct is called after the regular Java object constructor.

Why would I use @PostConstruct to initialize by bean, instead of the regular constructor itself?

Answer 1

• because when the constructor is called, the bean is not yet initialized - i.e. no dependencies are injected. In the @PostConstruct method the bean is fully initialized and you can use the dependencies.

• because this is the contract that guarantees that this method will be invoked only once in the bean lifecycle. It may happen (though unlikely) that a bean is instantiated multiple times by the container in its internal working, but it guarantees that @PostConstruct will be invoked only once.

Answer 2

You always should prefer constructor injection, but having that said, if for any reason you have to use field injection, in that case the main problem is that:

in a constructor, the injection of the dependencies has not yet occurred

---

Example

public class Foo {

    @Inject
    Logger LOG;
        
    @PostConstruct
    public void fooInit(){
        LOG.info("This will be printed; LOG has already been injected");
    }

    public Foo() {
        LOG.info("This will NOT be printed, LOG is still null");
        // NullPointerException will be thrown here
    }
}

---

Important

@PostConstruct and @PreDestroy have been completely [removed in Java 11](https://jaxenter.com/jdk-11-java-ee-modules-140674.html).

To keep using them, you'll need to add the javax.annotation-api JAR to your dependencies.

Maven

<!-- https://mvnrepository.com/artifact/javax.annotation/javax.annotation-api -->
<dependency>
    <groupId>javax.annotation</groupId>
    <artifactId>javax.annotation-api</artifactId>
    <version>1.3.2</version>
</dependency>

Gradle

// https://mvnrepository.com/artifact/javax.annotation/javax.annotation-api
compile group: 'javax.annotation', name: 'javax.annotation-api', version: '1.3.2'

Answer 3

Also constructor based initialisation will not work as intended whenever some kind of proxying or remoting is involved.

The ct will get called whenever an EJB gets deserialized, and whenever a new proxy gets created for it...

Answer 4

If your class performs all of its initialization in the constructor, then @PostConstruct is indeed redundant.

However, if your class has its dependencies injected using setter methods, then the class's constructor cannot fully initialize the object, and sometimes some initialization needs to be performed after all the setter methods have been called, hence the use case of @PostConstruct.

Answer 5

There is only really one reason - because you want to take an action that requires the object to be fully constructed, like passing the "this" reference to an executor.

All the other reasons only occur if you fail to write code according to best practices (e.g. using field injections instead of constructor injection, etc).

But, since Java is free to re-order instructions in constructors, you cannot depend on any one field being fully constructed before handing off a reference to the object to an executor (i.e, the this reference should not escape until the constructor is finished)