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

User · Answer

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.

User · Answer

Consider the following scenario   public class Car      Inject   private Engine engine       public Car         engine initialize                    Since Car has to be instantiated prior to field injection  the injection point engine is still null during the execution of the constructor  resulting in a NullPointerException   This problem can be solved either by JSR-330 Dependency Injection for Java constructor injection or JSR 250 Common Annotations for the Java  PostConstruct method annotation     PostConstruct  JSR-250 defines a common set of annotations which has been included in Java SE 6      The PostConstruct annotation is used on a method that needs to be   executed after dependency injection is done to perform any   initialization  This method MUST be invoked before the class is put   into service  This annotation MUST be supported on all classes that   support dependency injection        JSR-250 Chap  2 5 javax annotation PostConstruct   The  PostConstruct annotation allows for the definition of methods to be executed after the instance has been instantiated and all injects have been performed   public class Car      Inject   private Engine engine        PostConstruct   public void postConstruct         engine initialize                     Instead of performing the initialization in the constructor  the code is moved to a method annotated with  PostConstruct   The processing of post-construct methods is a simple matter of finding all methods annotated with  PostConstruct and invoking them in turn   private  void processPostConstruct Class type  T targetInstance      Method   declaredMethods   type getDeclaredMethods       Arrays stream declaredMethods         filter method - gt  method getAnnotation PostConstruct class     null          forEach postConstructMethod - gt             try              postConstructMethod setAccessible true              postConstructMethod invoke targetInstance  new Object                 catch  IllegalAccessException   IllegalArgumentException   InvocationTargetException ex                    throw new RuntimeException ex                           The processing of post-construct methods has to be performed after instantiation and injection have been completed

User · Answer

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

User · Answer

The main problem is that       in a constructor  the injection of the dependencies has not yet occurred     obviously excluding Constructor Injection    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             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   lt  -- https   mvnrepository com artifact javax annotation javax annotation-api -- gt   lt dependency gt       lt groupId gt javax annotation lt  groupId gt       lt artifactId gt javax annotation-api lt  artifactId gt       lt version gt 1 3 2 lt  version gt   lt  dependency gt    Gradle     https   mvnrepository com artifact javax annotation javax annotation-api compile group   javax annotation   name   javax annotation-api   version   1 3 2

User · Answer

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

[java] Why use @PostConstruct?

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