Create the perfect JPA entity

Question

I ve been working with JPA  implementation Hibernate  for some time now and each time I need to create entities I find myself struggling with issues as AccessType  immutable properties  equals hashCode        So I decided to try and find out the general best practice for each issue and write this down for personal use  I would not mind however for anyone to comment on it or to tell me where I m wrong   Entity Class   implement Serializable    Reason  The specification says you have to  but some JPA providers do not enforce this  Hibernate as JPA provider does not enforce this  but it can fail somewhere deep in its stomach with ClassCastException  if Serializable has not been implemented    Constructors   create a constructor with all required fields of the entity   Reason  A constructor should always leave the instance created in a sane state    besides this constructor  have a package private default constructor    Reason  Default constructor is required to have Hibernate initialize the entity  private is allowed but package private  or public  visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation    Fields Properties   Use field access in general and property access when needed    Reason   this is probably the most debatable issue since there are no clear and convincing  arguments for one or the other  property access vs field access   however  field access seems to be general favourite because of clearer code  better encapsulation and no need to create setters for immutable fields  Omit setters for immutable fields  not required for access type field  properties may be private Reason  I once heard that protected is better for  Hibernate  performance but all I can find on the web is  Hibernate can access public  private  and protected accessor methods  as well as public  private and protected fields directly  The choice is up to you and you can match it to fit your application design    Equals hashCode   Never use a generated id if this id is only set when persisting the entity By preference  use immutable values to form a unique Business Key and use this to test equality if a unique Business Key is not available use a non-transient UUID which is created when the entity is initialised  See this great article for more information  never refer to related entities  ManyToOne   if this entity  like a parent entity  needs to be part of the Business Key then compare the ID s only  Calling getId   on a proxy will not trigger the loading of the entity  as long as you re using property access type    Example Entity   Entity  Table name    ROOM   public class Room implements Serializable        private static final long serialVersionUID   1L        Id      GeneratedValue      Column name    room id       private Integer id        Column name    number        private String number    immutable       Column name    capacity       private Integer capacity        ManyToOne fetch   FetchType LAZY  optional   false       JoinColumn name    building id       private Building building    immutable      Room                default constructor            public Room Building building  String number               constructor with required field         notNull building   Method called with null parameter  application             notNull number   Method called with null parameter  name              this building   building          this number   number              Override     public boolean equals final Object otherObj            if   otherObj    null       otherObj instanceof Room                 return false                       a room can be uniquely identified by it s number and the building it belongs to  normally I would use a UUID in any case but this is just to illustrate the usage of getId           final Room other    Room  otherObj          return new EqualsBuilder   append getNumber    other getNumber                     append getBuilding   getId    other getBuilding   getId                     isEquals              this assumes that Building id is annotated with  Access value   AccessType PROPERTY              public Building getBuilding             return building              public Integer getId             return id             public String getNumber             return number              Override     public int hashCode             return new HashCodeBuilder   append getNumber    append getBuilding   getId    toHashCode               public void setCapacity Integer capacity            this capacity   capacity               no setters for number  building nor id      Other suggestions to add to this list are more than welcome     UPDATE  Since reading this article I have adapted my way of implementing eq hC    if an immutable simple business key is available  use that in all other cases  use a uuid

User · Answer

The JPA 2 0 Specification states that         The entity class must have a no-arg constructor  It may have other constructors as well  The no-arg constructor must be public or protected    The entity class must a be top-level class  An enum or interface must not    be   designated as an entity    The entity class must not be final  No methods or persistent instance variables of the entity class may be final    If an entity instance is to be passed by value as a detached object  e g   through a remote interface   the entity class must implement the Serializable interface    Both abstract and concrete classes can be entities  Entities may extend non-entity classes as well as entity classes  and non-entity classes may extend entity classes       The specification contains no requirements about the implementation of equals and hashCode methods for entities  only for primary key classes and map keys as far as I know

User · Answer

Entity interface  public interface Entity lt I gt  extends Serializable            return entity identity     I getId             return HashCode of entity identity     int identityHashCode             param other               Other entity     return true if identities of entities are equal     boolean identityEquals Entity lt   gt  other       Basic implementation for all Entities  simplifies Equals Hashcode implementations   public abstract class AbstractEntity lt I gt  implements Entity lt I gt      Override public final boolean identityEquals Entity lt   gt  other        if  getId      null            return false            return getId   equals other getId         Override public final int identityHashCode         return new HashCodeBuilder   append this getId    toHashCode        Override public final int hashCode         return identityHashCode        Override public final boolean equals final Object o        if  this    o            return true            if   o    null      getClass      o getClass               return false             return identityEquals  Entity lt   gt   o       Override public String toString         return getClass   getSimpleName            identity           OR         return ReflectionToStringBuilder reflectionToString this  ToStringStyle MULTI LINE STYLE         Room Entity impl     Entity  Table name    ROOM   public class Room extends AbstractEntity lt Integer gt     private static final long serialVersionUID   1L    Id  GeneratedValue strategy   GenerationType AUTO   Column name    room id   private Integer id    Column name    number    private String number    immutable   Column name    capacity   private Integer capacity    ManyToOne fetch   FetchType LAZY  optional   false   JoinColumn name    building id   private Building building    immutable  Room            default constructor    public Room Building building  String number           constructor with required field     notNull building   Method called with null parameter  application         notNull number   Method called with null parameter  name          this building   building      this number   number     public Integer getId        return id     public Building getBuilding         return building     public String getNumber         return number      public void setCapacity Integer capacity        this capacity   capacity       no setters for number  building nor id     I don t see a point of comparing equality of entities based on business fields in every case of JPA Entities  That might be more of a case if these JPA entities are thought of as Domain-Driven ValueObjects  instead of Domain-Driven Entities  which these code examples are for

User · Answer

My 2 cents addition to the answers here are    With reference to Field or Property access  away from performance considerations  both are legitimately accessed by means of getters and setters  thus  my model logic can set get them in the same manner   The difference comes to play when the persistence runtime provider  Hibernate  EclipseLink or else  needs to persist set some record in Table A which has a foreign key referring to some column in Table B  In case of a Property access type  the persistence runtime system uses my coded setter method to assign the cell in Table B column a new value  In case of a Field access type  the persistence runtime system sets the cell in Table B column directly   This difference is not of importance in the context of a uni-directional relationship  yet it is a MUST to use my own coded setter method  Property access type  for a bi-directional relationship provided the setter method is well designed to account for consistency  Consistency is a critical issue for bi-directional relationships refer to this link for a simple example for a well-designed setter  With reference to Equals hashCode  It is impossible to use the Eclipse auto-generated Equals hashCode methods for entities participating in a bi-directional relationship  otherwise they will have a circular reference resulting in a stackoverflow Exception  Once you try a bidirectional relationship  say OneToOne  and auto-generate Equals   or hashCode   or even toString   you will get caught in this stackoverflow exception

User · Answer

I ll try to answer several key points   this is from long Hibernate  persistence experience including several major applications   Entity Class  implement Serializable   Keys needs to implement Serializable  Stuff that s going to go in the HttpSession  or be sent over the wire by RPC Java EE  needs to implement Serializable  Other stuff  not so much  Spend your time on what s important   Constructors  create a constructor with all required fields of the entity   Constructor s  for application logic  should have only a few critical  foreign key  or  type kind  fields which will always be known when creating the entity  The rest should be set by calling the setter methods -- that s what they re for   Avoid putting too many fields into constructors  Constructors should be convenient  and give basic sanity to the object  Name  Type and or Parents are all typically useful   OTOH if application rules  today  require a Customer to have an Address  leave that to a setter  That is an example of a  weak rule   Maybe next week  you want to create a Customer object before going to the Enter Details screen   Don t trip yourself up  leave possibility for unknown  incomplete or  partially entered  data   Constructors  also  package private default constructor   Yes  but use  protected  rather than package private  Subclassing stuff is a real pain when the necessary internals are not visible   Fields Properties  Use  property  field access for Hibernate  and from outside the instance  Within the instance  use the fields directly  Reason   allows standard reflection  the simplest  amp  most basic method for Hibernate  to work   As for fields  immutable  to the application -- Hibernate still needs to be able to load these  You could try making these methods  private   and or put an annotation on them  to prevent application code making unwanted access   Note   when writing an equals   function  use getters for values on the  other  instance  Otherwise  you ll hit uninitialized  empty fields on proxy instances   Protected is better for  Hibernate  performance   Unlikely   Equals HashCode   This is relevant to working with entities  before they ve been saved -- which is a thorny issue  Hashing comparing on immutable values  In most business applications  there aren t any   A customer can change address  change the name of their business  etc etc -- not common  but it happens  Corrections also need to be possible to make  when the data was not entered correctly   The few things that are normally kept immutable  are Parenting and perhaps Type Kind -- normally the user recreates the record  rather than changing these  But these do not uniquely identify the entity   So  long and short  the claimed  immutable  data isn t really  Primary Key  ID fields are generated for the precise purpose  of providing such guaranteed stability  amp  immutability   You need to plan  amp  consider your need for comparison  amp  hashing  amp  request-processing work phases when A  working with  changed  bound data  from the UI if you compare hash on  infrequently changed fields   or B  working with  unsaved data   if you compare hash on ID   Equals HashCode -- if a unique Business Key is not available  use a non-transient UUID which is created when the entity is initialized  Yes  this is a good strategy when required  Be aware that UUIDs are not free  performance-wise though -- and clustering complicates things   Equals HashCode -- never refer to related entities   If related entity  like a parent entity  needs to be part of the Business Key then add a non insertable  non updatable field to store the parent id  with the same name as the ManytoOne JoinColumn  and use this id in the equality check   Sounds like good advice   Hope this helps

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