Difference between FetchType LAZY and EAGER in Java Persistence API

Question

I am a newbie to Java Persistence API and Hibernate    What is the difference between FetchType LAZY and FetchType EAGER in Java Persistence API

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By default  for all collection and map objects the fetching rule is FetchType LAZY and for other instances it follows the FetchType EAGER policy  In brief   OneToMany and  ManyToMany relations does not fetch the related objects  collection and map  implicictly but the retrieval operation is cascaded through the field in  OneToOne and  ManyToOne ones    courtesy  - objectdbcom

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Both FetchType LAZY and FetchType EAGER are used to define the default fetch plan  Unfortunately  you can only override the default fetch plan for LAZY fetching  EAGER fetching is less flexible and can lead to many performance issues  My advice is to restrain the urge of making your associations EAGER because fetching is a query-time responsibility  So all your queries should use the fetch directive to only retrieve what s necessary for the current business case

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The main difference between the two types of fetching is a moment when data gets loaded into a memory  I have attached 2 photos to help you understand this  Eager fetch  Lazy fetch

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The Lazy Fetch type is by default selected by Hibernate unless you explicitly mark Eager Fetch type  To be more accurate and concise  difference can be stated as below   FetchType LAZY   This does not load the relationships unless you invoke it via the getter method    FetchType EAGER   This loads all the relationships   Pros and Cons of these two fetch types   Lazy initialization improves performance by avoiding unnecessary computation and reduce memory requirements   Eager initialization takes more memory consumption and processing speed is slow    Having said that  depends on the situation either one of these initialization can be used

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Sometimes you have two entities and there s a relationship between them  For example  you might have an entity called University and another entity called Student and a University might have many Students   The University entity might have some basic properties such as id  name  address  etc  as well as a collection property called students that returns the list of students for a given university     public class University      private String id     private String name     private String address     private List lt Student gt  students         setters and getters     Now when you load a University from the database  JPA loads its id  name  and address fields for you  But you have two options for how students should be loaded     To load it together with the rest of the fields  i e  eagerly   or  To load it on-demand  i e  lazily  when you call the university s getStudents   method    When a university has many students it is not efficient to load all of its students together with it  especially when they are not needed and in suchlike cases you can declare that you want students to be loaded when they are actually needed  This is called lazy loading   Here s an example  where students is explicitly marked to be loaded eagerly    Entity public class University         Id     private String id       private String name       private String address        OneToMany fetch   FetchType EAGER      private List lt Student gt  students          etc          And here s an example where students is explicitly marked to be loaded lazily    Entity public class University         Id     private String id       private String name       private String address        OneToMany fetch   FetchType LAZY      private List lt Student gt  students          etc

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Book java          import java io Serializable          import javax persistence Column          import javax persistence Entity          import javax persistence GeneratedValue          import javax persistence GenerationType          import javax persistence Id          import javax persistence ManyToOne          import javax persistence Table            Entity          Table name  Books           public class Books implements Serializable           private static final long serialVersionUID   1L           Id          GeneratedValue strategy GenerationType IDENTITY           Column name  book id           private int id           Column name  book name           private String name            Column name  author name           private String authorName            ManyToOne         Subject subject           public Subject getSubject                 return subject                    public void setSubject Subject subject                this subject   subject                     public int getId                 return id                    public void setId int id                this id   id                    public String getName                 return name                    public void setName String name                this name   name                    public String getAuthorName                 return authorName                    public void setAuthorName String authorName                this authorName   authorName                         Subject java      import java io Serializable      import java util ArrayList      import java util List      import javax persistence CascadeType      import javax persistence Column      import javax persistence Entity      import javax persistence FetchType      import javax persistence GeneratedValue       import javax persistence GenerationType      import javax persistence Id      import javax persistence OneToMany      import javax persistence Table        Entity      Table name  Subject       public class Subject implements Serializable       private static final long serialVersionUID   1L       Id      GeneratedValue strategy GenerationType IDENTITY       Column name  subject id       private int id       Column name  subject name       private String name              Observe carefully i have mentioned fetchType EAGER  By default its is fetchType LAZY for  OneToMany i have mentioned it but not required  Check the Output by changing it to fetchType EAGER              OneToMany mappedBy  subject  cascade CascadeType ALL fetch FetchType LAZY  orphanRemoval true      List lt Books gt  listBooks new ArrayList lt Books gt          public List lt Books gt  getListBooks             return listBooks            public void setListBooks List lt Books gt  listBooks            this listBooks   listBooks            public int getId             return id            public void setId int id            this id   id            public String getName             return name            public void setName String name            this name   name                 HibernateUtil java  import org hibernate SessionFactory  import org hibernate boot registry StandardServiceRegistryBuilder  import org hibernate cfg Configuration  public class HibernateUtil     private static SessionFactory sessionFactory    static       Configuration configuration   new Configuration        configuration addAnnotatedClass  Com OneToMany Books class       configuration addAnnotatedClass  Com OneToMany Subject class       configuration setProperty  connection driver class   com mysql jdbc Driver        configuration setProperty  hibernate connection url    jdbc mysql   localhost 3306 hibernate                                        configuration setProperty  hibernate connection username    root             configuration setProperty  hibernate connection password    root        configuration setProperty  dialect    org hibernate dialect MySQLDialect        configuration setProperty  hibernate hbm2ddl auto    update        configuration setProperty  hibernate show sql    true        configuration setProperty   hibernate connection pool size    10        configuration setProperty   hibernate cache use second level cache    true        configuration setProperty   hibernate cache use query cache    true        configuration setProperty   cache provider class    org hibernate cache EhCacheProvider        configuration setProperty  hibernate cache region factory class    org hibernate cache ehcache EhCacheRegionFactory           configuration     StandardServiceRegistryBuilder builder   new StandardServiceRegistryBuilder   applySettings configuration getProperties         sessionFactory   configuration buildSessionFactory builder build        public static SessionFactory getSessionFactory         return sessionFactory         Main java      import org hibernate Session      import org hibernate SessionFactory       public class Main        public static void main String   args            SessionFactory factory HibernateUtil getSessionFactory            save factory           retrieve factory                private static void retrieve SessionFactory factory            Session session factory openSession            try              session getTransaction   begin                Subject subject  Subject session get Subject class  1               System out println  subject associated collection is loading lazily as  OneToMany is lazy loaded                 Books books  Books session get Books class  1               System out println  books associated collection is loading eagerly as by default  ManyToOne is Eagerly loaded                  Books b1  Books session get Books class  new Integer 1                 Subject sub session get Subject class  1               sub getListBooks   remove b1               session save sub               session getTransaction   commit               catch Exception e               e printStackTrace             finally              session close                                 private static void save SessionFactory factory           Subject subject new Subject            subject setName  C               Books books new Books            books setAuthorName  Bala            books setName  C   Book            books setSubject subject            subject getListBooks   add books           Session session factory openSession            try          session beginTransaction             session save subject            session getTransaction   commit             catch Exception e               e printStackTrace             finally              session close                             Check the retrieve   method of Main java  When we get Subject  then its collection listBooks  annotated with  OneToMany  will be loaded lazily  But  on the other hand  Books related association of collection subject  annotated with  ManyToOne  loads eargerly  by  default  1  for  ManyToOne  fetchType EAGER   We can change the behaviour by placing fetchType EAGER on  OneToMany Subject java or fetchType LAZY on  ManyToOne in Books java

User · Answer

EAGER loading of collections means that they are fetched fully at the time their parent is fetched  So if you have Course and it has  List lt Student gt   all the students are fetched from the database at the time the Course is fetched   LAZY on the other hand means that the contents of the List are fetched only when you try to access them  For example  by calling course getStudents   iterator    Calling any access method on the List will initiate a call to the database to retrieve the elements  This is implemented by creating a Proxy around the List  or Set   So for your lazy collections  the concrete types are not ArrayList and HashSet  but PersistentSet and PersistentList  or PersistentBag

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As per my knowledge both type of fetch depends your requirement   FetchType LAZY is on demand  i e  when we required the data    FetchType EAGER is immediate  i e  before our requirement comes we are unnecessarily fetching the record

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public enum FetchType   extends java lang Enum   Defines strategies for fetching data from the database  The EAGER strategy is a requirement on the persistence provider runtime that data must be eagerly fetched  The LAZY strategy is a hint to the persistence provider runtime that data should be fetched lazily when it is first accessed  The implementation is permitted to eagerly fetch data for which the LAZY strategy hint has been specified       Example        Basic fetch LAZY       protected String getName     return name      Source

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I may consider performance and memory utilization  One big difference is that EAGER fetch strategy allows to use fetched data object without session  Why  All data is fetched when eager marked data in the object when session is connected   However  in case of lazy loading strategy  lazy loading marked object does not retrieve data if session is disconnected  after session close   statement   All that can be made by hibernate proxy  Eager strategy lets data to be still available after closing session

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LAZY  It fetches the child entities lazily i e at the time of fetching parent entity it just fetches proxy created by cglib or any other utility  of the child entities and when you access any property of child entity then it is actually fetched by hibernate   EAGER  it fetches the child entities along with parent   For better understanding go to Jboss documentation or you can use hibernate show sql true for your app and check the queries issued by the hibernate

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drop-shadow if you re using Hibernate  you can call Hibernate initialize   when you invoke the getStudents   method   Public class UniversityDaoImpl extends GenericDaoHibernate lt University  Integer gt  implements UniversityDao                  Override     public University get final Integer id            Query query   getQuery  from University u where idUniversity  id   setParameter  id   id  setMaxResults 1  setFetchSize 1           University university    University  query uniqueResult               Hibernate initialize university getStudents                return university

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From the Javadoc      The EAGER strategy is a requirement on the persistence provider runtime that data must be eagerly fetched  The LAZY strategy is a hint to the persistence provider runtime that data should be fetched lazily when it is first accessed     E g   eager is more proactive than lazy  Lazy only happens on first use  if the provider takes the hint   whereas with eager things  may  get pre-fetched

User · Answer

I want to add this note to what said above  Suppose you are using Spring Rest with this simple architect   Controller  lt - gt  Service  lt - gt  Repository  And you want to return some data to the front-end  if you are using FetchType LAZY  you will get an exception after you return data to the controller method since the session is closed in the Service so the JSON Mapper Object can t get the data  There is three common options to solve this problem  depends on the design  performance and the developer   The easiest one is to use FetchType EAGER  So that the session will still alive at the controller method  but this method will impact the performance  Anti-patterns solutions  to make the session live until the execution ends  it is make a huge performance issue in the system  The best practice is to use FetchType LAZY with mapper like MapStruct to transfer data from Entity to another data object DTO and then send it back to the controller  so there is no exception if the session closed

User · Answer

Basically   LAZY   fetch when needed EAGER   fetch immediately

[java] Difference between FetchType LAZY and EAGER in Java Persistence API?

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