Why does JPA have a Transient annotation

Question

Java has the transientkeyword  Why does JPA have  Transient instead of simply using the already existing java keyword

User · Answer

In laymen s terms  if you use the  Transient annotation on an attribute of an entity  this attribute will be singled out and will not be saved to the database  The rest of the attribute of the object within the entity will still be saved   Im saving the Object to the database using the jpa repository built in save method as so  userRoleJoinRepository save user2

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As others have said   Transient is used to mark fields which shouldn t be persisted  Consider this short example   public enum Gender   MALE  FEMALE  UNKNOWN     Entity public Person       private Gender g      private long id        Id      GeneratedValue strategy GenerationType AUTO      public long getId     return id        public void setId long id    this id   id         public Gender getGender     return g            public void setGender Gender g    this g   g          Transient     public boolean isMale             return Gender MALE equals g               Transient     public boolean isFemale             return Gender FEMALE equals g             When this class is fed to the JPA  it persists the gender and id but doesn t try to persist the helper boolean methods - without  Transient the underlying system would complain that the Entity class Person is missing setMale   and setFemale   methods and thus wouldn t persist Person at all

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Java s transient keyword is used to denote that a field is not to be serialized  whereas JPA s  Transient annotation is used to indicate that a field is not to be persisted in the database  i e  their semantics are different

User · Answer

Because they have different meanings  The  Transient annotation tells the JPA provider to not persist any  non-transient  attribute  The other tells the serialization framework to not serialize an attribute  You might want to have a  Transient property and still serialize it

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For Kotlin developers  remember the Java transient keyword becomes the built-in Kotlin  Transient annotation  Therefore  make sure you have the JPA import if you re using JPA  Transient in your entity  import javax persistence Transient

User · Answer

Purpose is different   The transient keyword and  Transient annotation have two different purposes  one deals with serialization and one deals with persistence  As programmers  we often marry these two concepts into one  but this is not accurate in general  Persistence refers to the characteristic of state that outlives the process that created it  Serialization in Java refers to the process of encoding decoding an object s state as a byte stream    The transient keyword is a stronger condition than  Transient   If a field uses the transient keyword  that field will not be serialized when the object is converted to a byte stream  Furthermore  since JPA treats fields marked with the transient keyword as having the  Transient annotation  the field will not be persisted by JPA either    On the other hand  fields annotated  Transient alone will be converted to a byte stream when the object is serialized  but it will not be persisted by JPA  Therefore  the transient keyword is a stronger condition than the  Transient annotation   Example  This begs the question  Why would anyone want to serialize a field that is not persisted to the application s database  The reality is that serialization is used for more than just persistence  In an Enterprise Java application there needs to be a mechanism to exchange objects between distributed components  serialization provides a common communication protocol to handle this   Thus  a field may hold critical information for the purpose of inter-component communication  but that same field may have no value from a persistence perspective    For example  suppose an optimization algorithm is run on a server  and suppose this algorithm takes several hours to complete  To a client  having the most up-to-date set of solutions is important  So  a client can subscribe to the server and receive periodic updates during the algorithm s execution phase  These updates are provided using the ProgressReport object     Entity public class ProgressReport implements Serializable       private static final long serialVersionUID   1L        Transient     long estimatedMinutesRemaining      String statusMessage      Solution currentBestSolution       The Solution class might look like this    Entity public class Solution implements Serializable       private static final long serialVersionUID   1L       double     dataArray      Properties properties      The server persists each ProgressReport to its database  The server does not care to persist estimatedMinutesRemaining  but the client certainly cares about this information  Therefore  the estimatedMinutesRemaining is annotated using  Transient  When the final Solution is located by the algorithm  it is persisted by JPA directly without using a ProgressReport

User · Answer

I will try to answer the question of  why   Imagine a situation where you have a huge database with a lot of columns in a table  and your project system uses tools to generate entities from database   Hibernate has those  etc     Now  suppose that by your business logic you need a particular field NOT to be persisted   You have to  configure  your entity in a particular way  While Transient keyword works on an object - as it behaves within a java language  the  Transient only designed to answer the tasks that pertains only to persistence tasks

User · Answer

If you just want a field won t get persisted  both transient and  Transient work  But the question is why  Transient since transient already exists    Because  Transient field will still get serialized    Suppose you create a entity  doing some CPU-consuming calculation to get a result and this result will not save in database  But you want to sent the entity to other Java applications to use by JMS  then you should use  Transient  not the JavaSE keyword transient  So the receivers running on other VMs can save their time to re-calculate again

[java] Why does JPA have a @Transient annotation?

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