What is lazy loading in Hibernate

Question

What is lazy loading in Java  I don t understand the process  Can anybody help me to understand the process of lazy loading

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In layman s language  it is like you are making a cake and you will need 5-10 ingredients from fridge  You have two options  get all ingredients from fridge and put it on your kitchen platform  or bring the item you want when you need   Similarly  in eager loading  you fetch all information about bean and its related classes  not child or is-a relation but has a relationship  i e  cake has flour  has milk  has cream etc   and in case of lazy loading  first you bring only its identifier and values that are coming from same table  necessary ingredients that first you will need in your bowl in case of cake   All information that is coming from other tables will be fetched as and when required used

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Lazy Loading  Well  it simply means that child records are not fetched immediately  but automatically as soon as you try to access them

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Lazy loading allows you to defer the association retrieval or to have a better control over the fetching strategy   When you use EAGER loading  you define a global fetch plan which cannot be overridden at query time  meaning you are limited to a decision you took while designing your entity model  The EAGER fetching is a code smell  because the fetching strategy is a query-time policy and it might differ from a business use case to another   The fetching strategy is a very important aspect  as too much EAGER fetching can cause serious performance related issues

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Lazy setting decides whether to load child objects while loading the Parent Object You need to do this setting respective hibernate mapping file of the parent class Lazy   true  means not to load child By default the lazy loading of the child objects is true

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Martin Fowler defines the Lazy Load pattern in Patterns of Enterprise Application Architecture as such      An object that doesn t contain all of the data you need but knows how to get it    So  when loading a given object  the idea is to not eager load the related object s  that you may not use immediately to save the related performance cost  Instead  the related object s  will be loaded only when used    This is not a pattern specific to data access and Hibernate but it is particularly useful in such fields and Hibernate supports lazy loading of one-to-many associations and single-point associations  one-to-one and many-to-one  also under certain conditions   Lazy interaction is discussed in more detail in Chapter 19 of the Hibernate 3 0 Reference Documentation

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Bydefault lazy loading is true Lazy loading means when the select query is executed it will not hit the database  It will wait for getter function i e when we required then  it will fetch from the datbase  for example  You are a parent who has a kid with a lot of toys  But the current issue is whenever you call him  we assume you have a boy   he comes to you with all his toys as well  Now this is an issue since you do not want him carrying around his toys all the time  So being the rationale parent  you go right ahead and define the toys of the child as LAZY  Now whenever you call him  he just comes to you without his toys

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Lazy loading is a design pattern   commonly used in computer programming   to defer initialization of an object   until the point at which it is needed    It can contribute to efficiency in the   program s operation if properly and   appropriately used    Wikipedia  Link of Lazy Loading from hibernate org

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Lazy loading  means that an entity will be loaded only when you actually accesses the entity for the first time   The pattern is like this   public Entity getEntity         if  entity    null            entity   loadEntity              return entity      This saves the cost of preloading prefilling all the entities in a large dataset beforehand while you after all actually don t need all of them   In Hibernate  you can configure to lazily load a collection of child entities  The actual lazy loading is then done inside the methods of the PersistentSet which Hibernate uses  under the hoods  to assign the collection of entities as Set   E g   public class Parent       private Set lt Child gt  children       public Set lt Child gt  getChildren             return children               public void doSomething         Set lt Child gt  children   parent getChildren       Still contains nothing          Whenever you call one of the following  indirectly           Hibernate will start to actually load and fill the set      children size        children iterator

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Lazy fetching decides whether to load child objects while loading the Parent Object   You need to do this setting respective hibernate mapping file of the parent class   Lazy   true  means not to load child   By default the lazy loading of the child objects is true    This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild   method on parent In this case hibernate issues a fresh database call to load the child when getChild   is actully called on the Parent object   But in some cases you do need to load the child objects when parent is loaded   Just make the lazy false and hibernate will load the child when parent is loaded from the database   Example    If you have a TABLE   EMPLOYEE mapped to Employee object and contains set of Address objects   Parent Class   Employee class  Child class   Address Class   public class Employee    private Set address   new HashSet       contains set of child Address objects  public Set getAddress       return address      public void setAddresss Set address     this  address   address           In the Employee hbm xml file    lt set name  address  inverse  true  cascade  delete  lazy  false  gt    lt key column  a id    gt    lt one-to-many class  beans Address   gt    lt  set gt     In the above configuration   If lazy  false    - when you load the Employee object that time child object Address is also loaded and set to setAddresss   method   If you call employee getAdress   then loaded data returns No fresh database call    If lazy  true   - This the default configuration  If you don t mention then hibernate consider lazy true   when you load the Employee object that time child object Adress is not loaded  You need extra call to data base to get address objects   If you call employee getAdress   then that time database query fires and return results  Fresh database call

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Well it simply means loading data you need currently instead of loading whole bunch of data at once which you won t use now  Thereby making application load time faster than usual

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Lazy setting decides whether to load child objects while loading the Parent Object You need to do this setting respective hibernate mapping file of the parent class Lazy   true  means not to load child By default the lazy loading of the child objects is true  This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild   method on parent In this case hibernate issues a fresh database call to load the child when getChild   is actully called on the Parent object But in some cases you do need to load the child objects when parent is loaded  Just make the lazy false and hibernate will load the child when parent is loaded from the database Exampleslazy true  default Address child of User class can be made lazy if it is not required frequently lazy falseBut you may need to load the Author object for Book parent whenever you deal with the book for online bookshop

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Say you have a parent and that parent has a collection of children  Hibernate now can  lazy-load  the children  which means that it does not actually load all the children when loading the parent  Instead  it loads them when requested to do so  You can either request this explicitly or  and this is far more common  hibernate will load them automatically when you try to access a child   Lazy-loading can help improve the performance significantly since often you won t need the children and so they will not be loaded   Also beware of the n 1-problem  Hibernate will not actually load all children when you access the collection  Instead  it will load each child individually  When iterating over the collection  this causes a query for every child  In order to avoid this  you can trick hibernate into loading all children simultaneously  e g  by calling parent getChildren   size

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Hiberante supports the feature of lazy initialization for both entities and collections  Hibernate engine loads only those objects that we are querying for does not other entites or collections         lazy  false  by default loading initialization mention for the only child is lazy in case of true that is parent is loading does not support child

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Surprisingly  none of answers talk about how it is achieved by hibernate behind the screens   Lazy loading is a design pattern that is effectively used in hibernate for performance reasons which involves following techniques     1  Byte code instrumentation    Enhances the base class definition with hibernate hooks to intercept all the calls to that entity object   Done either at compile time or run load  time  1 1 Compile time   Post compile time operation Mostly by maven ant plugins   1 2 Run time   If no compile time instrumentation is done  this is created at run time    Using libraries like javassist   2  Proxies  The entity object that Hibernate returns are proxy of the real type   See also  Javassist  What is the main idea and where real use

[java] What is lazy loading in Hibernate?

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