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

The main problem is that:

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

_{*obviously excluding Constructor Injection}

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Real-world 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     } } 

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IMPORTANT: @PostConstruct and @PreDestroy have been completely removed in Java 11.

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'